Mayr, Herbert

Roth, Michael

Schade, Christian

English

Open Access LMU

J. Org. Chem. 1994,59,169-172 169 Kinetics of Carbenium Ion Additions to Methylenecycloalkanes: Cycloalkyl Cation Stabilities Are Not Predominantly Controlled by Strain Michael Roth, Christian Schade, and Herbert Mayr' Institut fiir Organische Chemie, Technische Hochachule Darmstadt, Petersenstrcrcree 22, 64287 D a r m t a d t ,  Germany Received Augwt  3, 1993. Kinetic investigations of the reactions of the @-anisybphenylcarbenium tetrachloroborate (1 BCL) with methylenecycloalkanes (ring size 3-12 and 15) have been performed. The second-order rate constants correlate with the solvolysis rates of the corresponding cycloalkyl derivatives, though none of the ring carbons is rehybridized in the rate-determining step (electrophilic attack of 1 at the CC double bond). A simple explanation for the observed rate effects cannot be given, and it is claimed that the common rationalization of cycloalkyl solvolysis rates by differences of internal strain has to be modified. Introduction In 1952, Brown reported the solvolysis rate constants of 1-chloro-1-methylcycloalkanes to be strongly dependent on ring size (Figure 1).l The reactivity maxima for the five- and eight-membered rings and the reactivity min- imum for the six-membered ring were explained by the change of internal strain during rehybridization of a ring carbon from sp3 to sp2. This interpretation seemed to be widely accepted, when Pock and Mayr reported compe- tition experiments which showed that methylenecyclo- pentane is 50 times more reactive toward diarylcarbenium ions than methylenecyclohexane.2 The reactivity order of the addition reactions was thus similar to that observed for cycloalkyl solvolyses, though none of the ring carbons is rehybridized in the rate-determining step. In order to examine whether the coincidence of relative rates for these three compounds is accidental or whether there is a general relationship between these two types of reaction, we have now investigated the reactivities of an extended series of methylenecycloalkanes (ring size 3-12,15) toward the @- anisy1)phenylcarbenium ion [@-MeOC&)PhCH+f which has previously been used as a reference electrophile for comparing nucleophilicities of alkenes.= Methods p-Methoxybenzhydryl chloride is completely ionized when added to a solution of BC4 in CHzC12 and gives a colored solution of 1 BCb-, as described previ~usly.~ This solution is completely decolorized upon addition of the methylenecyclodkanea R3-Rl6, and the second-order rate constants for these reactions, which proceed via rate determining attack of 1 at the methylenecycloalkanes (Scheme 11, can be determined photometrically as de- scribed b e f ~ r e . ~  * Ahtract published m Advance ACS Abatracta, December 1,1993. ( 1 )  Brown, H. C.; Borkomki, M. J. Am. Chem. SOC. 1962, 74,1894- (2) Mayr, H.; Pock, R. Chem. Ber. 1986,119,2473-2496. (3) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J. Am. (4)  Mayr, H.; Schneider, R.; Irrgang, B.; Schade, C. J. Am. Chem. SOC. (6) Mayr, H.; Schneider, R.; Grabis, U. J. Am. Chem. SOC. 1990,112, 1902. Chem. SOC. 1990,112,4446464. 1990,112,4454-4459. 44604467. ~ ~ .  .~. (6) Mayr, H. Angew. Chem. 1990,102,1416-1428; Angew. Chem., Znt. Ed. Engl. 1990,29,1371-1384. 1 1  as . 0 .  4 5  . -1 . -15 . -2 - -25 . 3 4 5 6 7 8 9 K l l l U U M U 1 6 1 7 1 8  number of ring carbons - Figure 1. Effect of ring size on the solvolysie ratas of l-chloro- 1-methylcycloalkanee (80% aqueous EtOH, 25 OC).' Results Product studies show that the adducts P4-Pl6 are produced in almost quantitative yield when the methyl- enecycloalkanes R4-Rl6 are combined with 1 BC4- in dichloromethane at -78 OC. A full characterization of the products is given in the supplementary material. In analogy to the corresponding solvolytic reactions, a ring- opening reaction7 t+es place when methylenecyclopro- pane (R3) is attacked by the carbenium ion 1, and the allylic chloride P3 is isolated instead of a cyclopropyl derivative. The rate constants determined in this work (Table 1, Figure 2), which show reactivity maxima for the five- and eighbmembered ring, are in qualitative agreement with the reactivity pattern observed for solvolysis reactions of 1-chloro-1-methylcycloalkanes in 80% ethanol (Figure 1). Table 2 shows that the directly measured rata constanfa are in good agreement with the results of competition (7) Ekperimentalobom (a )S l iwineLy ,W.F . ;SUT.M.;~ ,  P. v. R. J. Am. Chem. SOC. 1972,94,133-146. (b) Howell, B. A; Jewatt, J. G. J. Am. Chem. SOC. 1971,93,79&900. (c) Roberta, J. D.; Chamben, V. C. J. Am. Chem. SoC. 19S1, 73, 6034-6040. CdculatioM: (a) Raghavachari, K.; Whiteaide, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. SOC. 1981,103,6649-6667. (e) Lien, M. H.; Hopkinron, A. C. J. Mol. Struct. 1985,121,l. (0 Lien, M. H.; Hopkinron, A. C. J. Phyr. Chem. 1984,88,1613-1517. (g) Otto, A. H. J. Prakt. Chem. lW2, W, bse-602. 0022-3263/94/1959-0169$04.50/0 Q 1994 American Chemical Society 170 J. Org. Chem., Vol. 59, No. 1, 1994 Roth et al. Table 1. Rate Constants (-70 "C) and Activation Parametera for the Beactionr of (pAniry1)phenylcarbenium Tetrachloroborate with ~hylenecycloalkaner in CHfllt k2, AH*, As*, alkene L mol-' s-' kJ mol-' J mol-' K-I T. O C  R3 R4 Its R6 R7 R8 R9 R10 R11 R12 RlB 2-methyl-l- pentane 0.12 40.1 46.9 1760 426 6086 1598 2260 783 133 88.4 25.8 20.6 0.3 11.6 f 0.4 19.8 f 0.1 14.2 & 0.1 11.2 0.3 10.7 f 0.1 11.7 f 0.3 12.0 0.2 12.6 * 0.3 16.7 f 0.1 19.6 1.2 -110 * 1 -123 * 2 -112 * 1 -121 * 1 -116 2 -128 f 1 -120 f 1 -127 f 1 -139 f 1 -122 f 1 -119 f 6 Scheme 1 1 kz S~OW I n=3:  R3 I n =  15: R15 n = f  CH, hCH- CHjjC: Ph I C&Cl P3 Bcld -70 -70 to -20 -70 to -20 -70 to -20 -70 to -20 -80 to -60 -70 to -30 -70 to -30 -70 to -30 -70 to -20 -70 to -20 -70 to -30 I Ph n = 4 :  PI n-  15: P1S I h =pMeoCsHq- experiments performed with the bis@-methylpheny1)- carbenium ion (Tol&H+) several years Discussion Table 1 shows that the observed reactivity order reflects a complex interplay between enthalpic and entropic effects. For the methylenecycloalkanes with n = 4-7, both AH* and A S  are variable, but the differences in AH* are more pronounced. In contrast, the reactivity differences of the medium-size rings (n = 8-12) are predominantly due to entropic effects. We do not have an explanation for the regular alternation of AS* for n = 4-11 (Figure 3). While comparison of the plots shown in Figures 1 and 2 suggests a close relationship between solvolysis rates of cycloaulyl derivatives and addition rates to methylenecy- cloalkanea, the correlation in Figure 4 is only of moderate quality (r  = 0.89). We stress this point since plots as shown in Figurea 1 and 2 have often been employed for comparing cycloalkane reactivities. I '9 4 35 . 3 -  25 . 2 .  L5 . 1 .  (15 . 0 -  45. Figure 2. Second-order rate constants for the reactions of methylenecycloalkanes with 1 BCL- aa a function of ring size (CH2C12, -70 "C). Table 2 1.00 L 1.00 1.86 1.64 96.1 1.91 -0 67.8 1.81 a From competition experiments, ref 2. Direct rate measurements, this work. ~- I I -I' " " " " " " ' 3 4 5 6 7 8 9 10 11 I2 13 14 15 16 numbefofIhgurboM -Figure 3. Activation entropy for the reactions of 1 BCL- with methylenecycloalkanes as a function of ring size. Since changes of strain had been suggested to account for the different rates of cycloalkyl solvolyses, one has to consider whether changes of strain can also account for the different reactivities of the methylenecycloalkanes. As diecussed in the Introduction, this explanation appeared improbable to us since in the rate-determining step of the addition reaction the hybridization of the ring carbons is not altered. In order to exclude the possibility that rehybridization of the exocyclic carbon was responsible for the different reaction rates of methylenecycloalkanes, we have per- formed force field calculations (PIMM88)* of the methyl- enecycloalkanes R4-RlO and the corresponding l-meth- Carbenium Ion Additions to Methylenecycloalkanes 4 8' a 6  15 6 5  4 -55 -5 4 5  4 -35 -3 '8.tl (25 "C) -Figure 4. Correlation between the rate constants of the addition of 1 BCL- to methylenecycloalkanes (CHzC12, -70 "C) and the solvolysis rate constants of l-chloro-l-methylcyclodkanes (80 % aqueous EtOH, 25 OC)' [m = 0.65, r = 0.891. ylcycloalkyl cations assuming the same force constants for the sp2-hybridized ring carbon in methylenecycloal- kanes and in methylcycloalkyl cations. While protonation was calculated to cause identical enthalpy changes for the compounds with n = 610,  the protonations of methyl- enecyclopentane R5 and methylenecyclobutane R4 were calculated to be 4 and 10 kJ-mol-' less exothermic, re- spectively. Thus, as intuitively expected, no correlation between methylenecycloalkane reactivities and calculated changes of strain between methylenecycloalkanes and their protonated analogues was found.g There is also no correlation between the calculated ionization potentials (MINDO/B and AM1) of R5-R8 and the reaction rates of RS-Rs toward 1, in accord with previous observations that the rates of carbenium additions toward alkenes do not correlate with the ionization potentials of the  olefin^.^?^?^ We had observed a good correlation between the reactivities of alkenes, dienes, and allylsilanes toward benzhydryl cations and the solvolysis rates of those alkyl chlorides that give the same types of carbenium inter- mediates,4J0 however, and concluded that the relative reactivities of *-systems toward benzhydryl cations are controlled by the relative stabilities of the carbenium ions produced during electrophilic attack. By analogy, we now infer that the relative reactivities of the methylenecy- cloalkanes also reflect the relative stabilities of the cor- responding cycloalkyl cations. For l- ferrocenylcycloalkyl cations, a relationship between solvolysis rates and ther- modynamic stabilities of the carbenium ions had been demonstrated by Ortaggi (Figure 51." The correlation between the rates of electrophilic additions to methylenecycloalkanes and cycloalkyl sol- volysis rates (Figure 4) raises the question whether the (8) Smith, A. E.; Lindner, H. J. J. Comput. Aided Mol. Des. 1991,5, 236-262. (9) One of the reviewers haa questioned the appropriateness of force field calculations to explain small differences of reactivity. This point is well taken, but it also questions the significance of the correlations in Figures 6 and 7 which providethe basis for the I-strain theory of cycloalkyl solvolysee. It does not affect the essential conclusions of this work, however, which are exclusively based on correlations between experimental data (Figure 4). (10) Hagen, G.; Mayr, H. J.  Am. Chem. SOC. 1991, 113, 4954-4961. (11) Ortaggi, G. Gaze. Chim. Ital. 1987,117, 75-81. (12) Richey, H. G., Jr.; Fletcher, R.; Overmoyer, R. G. Tetrahedron Lett. 1970, 3703-3706. J. Org. Chem., Vol. 59, No. 1, 1994 171 -L5 1 Figure 5. Correlation between the rates of the solvolyses of ferrocenyl cycloalkylacetates (aqueous HzSOI, 25 "C) and the PKR+ values of the corresponding carbocationsll [m = 1.06, r = 0.9991, Scheme 2 ODNB' 0-DNB 80% aq. acetone 100 O C  n ( 4  5 6 7 ~~ I 1.0 13.7 2.7 5.3 differences in solvolysis rates are really caused by variable changes of strain and not by the same factors that control the relative reactivities of the methylenecycloalkanes. Already in 1956 the limitations of the I-strain theory have been stressed by Brown, when the poor correlation between the ethanolysis rates of l-methylcycloalkyl chlo- rides and the rates of BH4- reductions of cycloalkanones was noted.13-16 Another important argument against the dominating role of strain was Richey's report that the solvolysis rates of the allyl dinitrobenzoates depicted in Scheme 2 show a similar dependence on ring size as the cycloalkyl solvolyses, though none of the ring carbons becomes rehybridized in the rate-determining step.12 In spite of these discrepancies, the application of the I-strain theory has been continued,16 and force field calculations even seemed to provide a quantitative basis for this theory. Schneider reported that cycloalkyltosylate solvolyses in trifluoroethanol (TFE) and hexafluoro-2- propanol (HFIP), but not in ethanol, correlate with the calculated differences in strain energy.l7-l9 Remarkably, (13) Brown, H. C. J. Chem. SOC. 1966, 1248-1268. (14) Brown, H. C.; Ichikawa, K. Tetrahedron 1967,1, 221-230. (16) Brown, H. C.; Ham, G. J.  Am. Chem. SOC. 1966, 78,2736-2739. (16) McManus, S. P.; Pittman, C. U., Jr. In Organic Reactive Intermediates; McManus, 5. P., Ed.; Academic Press: New York and London, 1973; pp 273-275. (17) Schneider, H.-J.; Thomas, F. J. Am. Chem. SOC. 1980,102,1424- 1425. - (18) Schneider, H.-J.; Schmidt, G.; Thomas, F. J. Am. Chem. SOC. (19) Schneider, H.-J.; Becker, N.; Schmidt, G.; Thomas, F. J. Org. 1983,105, 3556-3563. Chem. 1986,51, 3602-3607. 172 J. Org. Chem., Vol. 59, No. 1, 1994 -IS -2s . -3 - -15 . 4 .  Igkl 45 . e 6  -5 t Figure 6. Correlation between the solvolysis rate constanta of cycloalkyl tmylates (TFE, 25 OC)I8 and the differences in strain energies (cycloalkanones-methylcycloalkanes),'B kJ/mol [(2.303RT)m = -0.52, r = 0.701. the unusual high reactivity of the cyclopentyl compound, the starting point of Brown's qualitative I-strain theory, has not been observed in the fluorinated alcohols. This is surprising, since in solvolyses (80% acetone) of l-(aryl- cycloalky1)-p-nitrobenzoates (ring size 4-8) reactivity maxima have also been found for the five- and eight- membered rings,zo though for these compounds nucleo- philic solvent participation cannot play a dominant role. Correlations of high quality between rates of cycloalkyl solvolyses and differences in strain (r = 0.96, 7 points, TFE1s r = 0.96, 7 points, HFIPI9) have been obtained when certain ring systems were eliminated. Figures 6 and 7 show, however, that the correlations giver values of 0.70 (9 points, TFE) and 0.84 (8points, HFIP) when all available data are considered. These correlations are, therefore, worse than the correlation between solvolysis rate con- stants and the rates of electrophilic additions to meth- ylenecycloalkanes (Figure 4) which are not controlled by strain (see also ref 9). Strain effects may, therefore, account for part of the reactivity differences of cycloalkyl derivatives, but other factors (e.g., ring size dependent hyperconjugative effects or transannular hydride b r i d g b ~ g l ~ * ~ ~  ) that stabilize the intermediate cycloalkyl cations must be at least equally important. Conclusion While the correlation between strain effects and rate as well as equilibrium constants for nucleophile additions to (20) Brown, H. C.; Ravindranathan, M.; Peters, E. N.; Rao, C. G.; Rho, M. M. J. Am. Chem. SOC. 1977,99,5373-5378. (21) (a) Schneider, H.J.; Heiske, D. J. Am. Chem. SOC. 1981, 103, 3601-3605. (b) Kirchen, R. P.; Soremen, T. S. J.  Am. Chem. SOC. 1979, 101,3240-3244. (c) Nordlander, J. E.; Owuor, P. 0.; Cabral, D. J.; Haky, J. E. J. Am. Chem. SOC. 1982,104,201-206. (d) Parker, W.; Watt, C. I. F. J.  Chem. Soc., Perkin Trans. 2 1976,1647-1651. (e) Allinger, N. L.; Szkrybalo, W. Tetrahedron 196&24,4699-4711. Arguments that trans- annular hydride bridging does not occur in the rata-determining step of solvolytic reactions have been published: (0 F'relog, V.; Raynham, J. G. InMoleculorRearrangements; de Mayo, P., Ed.; Wiley-Intsrscience: New York, 1963; Part 1, Chapter 9, pp 593-616. (g) f i n g e r ,  N. L.; Greenberg, S. J. Am. Chem. SOC. 1962,84,2394-2402. (h) Roberta, A. A.; Anderson, C. B. Tetrahedron Lett. 1969,3883-3885. (i) Alliiger, N. L.; Neumann, C. L.; Sugiyama, H. J. Org. Chem. 1971,36,1360-1365. (i) Harris, J. M.; Mount, D. L.; Smith, M. R.; Neal, W. C., Jr.; Dukes, M. D.; Raber, D. J. J. Am. Chem. SOC. 1978,100,8147-8166. 4s -1 -15 -2 -2s -1s 4 lg kl -3 4 5  I -Z? -19 -16 -I3 -10 -7 4 -1 2 5 8 Astrain Figure 7. Correlation between the solvolyeis rate constanta of cycloalkyl toeylates (HFP, 25 "C) and the differences in strain energies (cycloalkanone~-methylcycloalkane~),*s kJ/mol L(2.303RT)m = -0.72, r = 0.841. cycloalkanones is well sub~tant ia ted ,2~~~~ variable strain effects are not the major reason for the differences of cycloalkyl solvolysis rates. The correlation between cy- cloalkyl solvolysis rates and electrophilic additions to methylenecycloalkanes (Figure 4) stresses the importance of electronic effects. Since acomplexinterplayof enthalpic and entropic factors was found to be responsible for the relative reactivities of methylenecycloalkanes, an analysis of cycloalkyl solvolyses that is based solely on strain enthalpies appears to be too simplistic. Experimental Section Details of the kinetic procedure and preparation of the carbenium ion precursors have been published The methylenecycloalkanes have been synthesized from the cycloal- kanones according to a modified procedure of Takai (for details see supplementary material).% Addition Products P3-PLS. Three equiv of R3-RlS was added dropwise to a colored solution of 1 equiv of 1 BCL- in CHzClz (100 mL) at -78 "C. The mixture was allowed to stand at -78 OC until it was completely decolorized (approximately 2 h). The mixture was washed with concentrated hydrochloric acid and dried over MgSOd. The solvent was evaporated, and the remaining colorless oil was fiitered over neutral alumina before recrystallization from EtOH. Acknowledgment. We thank Prof. A. Maercker for bringing this problem to our attention, Monika Deters for experimental assistance, Prof. H.-J. Schneider for critical comments, Prof. H. J. Lindner for providing his force field programs, and the Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie for financial support. Supplementary Material Available: Data tables containing concentrations, rate constanta, and activation parameters of the kinetic experiments, experimental procedures for RI-R16, and full characterization of R7-RlS and P3-PlS (12 pages). This material is contained in libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. (22) Milller, P.; Blanc, J. Hela Chim. Acta 1980,69, 1759-1766. (23) MWer, P.; Mareda, J. In Physical Organic Chemistry 1986; Kobayaehi, M., Ed.; Studies in Organic Chemistry, Elsevier Science Publishers B. V., Amsterdam, 1987; Vol. 31, pp 529-636. (24) Takai, K.; Hotta, Y.; Oshima, K.; Nozaki, H. Tetrahedron Lett. 1978,2417-2420. 

Kinetics of carbenium ion additions to methylenecycloalkanes. Cycloalkyl cation stabilities are not predominantly controlled by strain

Michael Roth

Christian Schade

Herbert Mayr

Crossref

Kinetics of carbenium ion additions to methylenecycloalkanes: cycloalkyl cation stabilities are not predominantly controlled by strain

Universität München: Elektronischen Publikationen

J. Org. Chem. 1994,59,169-172 169 
Kinetics of Carbenium Ion Additions to Methylenecycloalkanes: 
Cycloalkyl Cation Stabilities Are Not Predominantly Controlled by 
Strain 
Michael Roth, Christian Schade, and Herbert Mayr' 
Institut fiir Organische Chemie, Technische Hochachule Darmstadt, Petersenstrcrcree 22, 
64287 D a r m t a d t ,  Germany 
Received Augwt  3, 1993. 
Kinetic investigations of the reactions of the @-anisybphenylcarbenium tetrachloroborate (1 BCL) 
with methylenecycloalkanes (ring size 3-12 and 15) have been performed. The second-order rate 
constants correlate with the solvolysis rates of the corresponding cycloalkyl derivatives, though none 
of the ring carbons is rehybridized in the rate-determining step (electrophilic attack of 1 at the CC 
double bond). A simple explanation for the observed rate effects cannot be given, and it is claimed 
that the common rationalization of cycloalkyl solvolysis rates by differences of internal strain has 
to be modified. 
Introduction 
In 1952, Brown reported the solvolysis rate constants 
of 1-chloro-1-methylcycloalkanes to be strongly dependent 
on ring size (Figure 1).l The reactivity maxima for the 
five- and eight-membered rings and the reactivity min- 
imum for the six-membered ring were explained by the 
change of internal strain during rehybridization of a ring 
carbon from sp3 to sp2. This interpretation seemed to be 
widely accepted, when Pock and Mayr reported compe- 
tition experiments which showed that methylenecyclo- 
pentane is 50 times more reactive toward diarylcarbenium 
ions than methylenecyclohexane.2 The reactivity order 
of the addition reactions was thus similar to that observed 
for cycloalkyl solvolyses, though none of the ring carbons 
is rehybridized in the rate-determining step. In order to 
examine whether the coincidence of relative rates for these 
three compounds is accidental or whether there is a general 
relationship between these two types of reaction, we have 
now investigated the reactivities of an extended series of 
methylenecycloalkanes (ring size 3-12,15) toward the @- 
anisy1)phenylcarbenium ion [@-MeOC&)PhCH+f which 
has previously been used as a reference electrophile for 
comparing nucleophilicities of alkenes.= 
Methods 
p-Methoxybenzhydryl chloride is completely ionized 
when added to a solution of BC4 in CHzC12 and gives a 
colored solution of 1 BCb-, as described previ~usly.~ This 
solution is completely decolorized upon addition of the 
methylenecyclodkanea R3-Rl6, and the second-order rate 
constants for these reactions, which proceed via rate 
determining attack of 1 at the methylenecycloalkanes 
(Scheme 11, can be determined photometrically as de- 
scribed b e f ~ r e . ~  
* Ahtract published m Advance ACS Abatracta, December 1,1993. 
( 1 )  Brown, H. C.; Borkomki, M. J. Am. Chem. SOC. 1962, 74,1894- 
(2) Mayr, H.; Pock, R. Chem. Ber. 1986,119,2473-2496. 
(3) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J. Am. 
(4)  Mayr, H.; Schneider, R.; Irrgang, B.; Schade, C. J. Am. Chem. SOC. 
(6) Mayr, H.; Schneider, R.; Grabis, U. J. Am. Chem. SOC. 1990,112, 
1902. 
Chem. SOC. 1990,112,4446464. 
1990,112,4454-4459. 
44604467. ~ ~ .  .~. 
(6) Mayr, H. Angew. Chem. 1990,102,1416-1428; Angew. Chem., Znt. 
Ed. Engl. 1990,29,1371-1384. 
1 1  
as . 
0 .  
4 5  . 
-1 . 
-15 . 
-2 - 
-25 . 
3 4 5 6 7 8 9 K l l l U U M U 1 6 1 7 1 8  
number of ring carbons - 
Figure 1. Effect of ring size on the solvolysie ratas of l-chloro- 
1-methylcycloalkanee (80% aqueous EtOH, 25 OC).' 
Results 
Product studies show that the adducts P4-Pl6 are 
produced in almost quantitative yield when the methyl- 
enecycloalkanes R4-Rl6 are combined with 1 BC4- in 
dichloromethane at -78 OC. A full characterization of the 
products is given in the supplementary material. In 
analogy to the corresponding solvolytic reactions, a ring- 
opening reaction7 t+es place when methylenecyclopro- 
pane (R3) is attacked by the carbenium ion 1, and the 
allylic chloride P3 is isolated instead of a cyclopropyl 
derivative. 
The rate constants determined in this work (Table 1, 
Figure 2), which show reactivity maxima for the five- and 
eighbmembered ring, are in qualitative agreement with 
the reactivity pattern observed for solvolysis reactions of 
1-chloro-1-methylcycloalkanes in 80% ethanol (Figure 1). 
Table 2 shows that the directly measured rata constanfa 
are in good agreement with the results of competition 
(7) Ekperimentalobom (a )S l iwineLy ,W.F . ;SUT.M.;~ ,  
P. v. R. J. Am. Chem. SOC. 1972,94,133-146. (b) Howell, B. A; Jewatt, 
J. G. J. Am. Chem. SOC. 1971,93,79&900. (c) Roberta, J. D.; Chamben, 
V. C. J. Am. Chem. SoC. 19S1, 73, 6034-6040. CdculatioM: (a) 
Raghavachari, K.; Whiteaide, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. 
Chem. SOC. 1981,103,6649-6667. (e) Lien, M. H.; Hopkinron, A. C. J. 
Mol. Struct. 1985,121,l. (0 Lien, M. H.; Hopkinron, A. C. J. Phyr. Chem. 
1984,88,1613-1517. (g) Otto, A. H. J. Prakt. Chem. lW2, W, bse-602. 
0022-3263/94/1959-0169$04.50/0 Q 1994 American Chemical Society 
170 J. Org. Chem., Vol. 59, No. 1, 1994 Roth et al. 
Table 1. Rate Constants (-70 "C) and Activation 
Parametera for the Beactionr of 
(pAniry1)phenylcarbenium Tetrachloroborate with 
~hylenecycloalkaner in CHfllt 
k2, AH*, As*, 
alkene L mol-' s-' kJ mol-' J mol-' K-I T. O C  
R3 
R4 
Its 
R6 
R7 
R8 
R9 
R10 
R11 
R12 
RlB 
2-methyl-l- 
pentane 
0.12 
40.1 
46.9 
1760 
426 
6086 
1598 
2260 
783 
133 
88.4 
25.8 
20.6 0.3 
11.6 f 0.4 
19.8 f 0.1 
14.2 & 0.1 
11.2 0.3 
10.7 f 0.1 
11.7 f 0.3 
12.0 0.2 
12.6 * 0.3 
16.7 f 0.1 
19.6 1.2 
-110 * 1 
-123 * 2 
-112 * 1 
-121 * 1 
-116 2 
-128 f 1 
-120 f 1 
-127 f 1 
-139 f 1 
-122 f 1 
-119 f 6 
Scheme 1 
1 
kz S~OW I 
n=3:  R3 
I 
n =  15: R15 
n = f  
CH, 
hCH- CHjjC: 
Ph I C&Cl 
P3 
Bcld 
-70 
-70 to -20 
-70 to -20 
-70 to -20 
-70 to -20 
-80 to -60 
-70 to -30 
-70 to -30 
-70 to -30 
-70 to -20 
-70 to -20 
-70 to -30 
I 
Ph 
n = 4 :  PI 
n-  15: P1S 
I 
h =pMeoCsHq- 
experiments performed with the bis@-methylpheny1)- 
carbenium ion (Tol&H+) several years 
Discussion 
Table 1 shows that the observed reactivity order reflects 
a complex interplay between enthalpic and entropic effects. 
For the methylenecycloalkanes with n = 4-7, both AH* 
and A S  are variable, but the differences in AH* are more 
pronounced. In contrast, the reactivity differences of the 
medium-size rings (n = 8-12) are predominantly due to 
entropic effects. We do not have an explanation for the 
regular alternation of AS* for n = 4-11 (Figure 3). 
While comparison of the plots shown in Figures 1 and 
2 suggests a close relationship between solvolysis rates of 
cycloaulyl derivatives and addition rates to methylenecy- 
cloalkanea, the correlation in Figure 4 is only of moderate 
quality (r  = 0.89). We stress this point since plots as shown 
in Figurea 1 and 2 have often been employed for comparing 
cycloalkane reactivities. 
I 
'9 4 
35 . 
3 -  
25 . 
2 .  
L5 . 
1 .  
(15 . 
0 -  
45. 
Figure 2. Second-order rate constants for the reactions of 
methylenecycloalkanes with 1 BCL- aa a function of ring size 
(CH2C12, -70 "C). 
Table 2 
1.00 L 1.00 
1.86 1.64 
96.1 
1.91 -0 
67.8 
1.81 
a From competition experiments, ref 2. Direct rate measurements, 
this work. 
~- I I 
-I' " " " " " " ' 
3 4 5 6 7 8 9 10 11 I2 13 14 15 16 
numbefofIhgurboM -
Figure 3. Activation entropy for the reactions of 1 BCL- with 
methylenecycloalkanes as a function of ring size. 
Since changes of strain had been suggested to account 
for the different rates of cycloalkyl solvolyses, one has to 
consider whether changes of strain can also account for 
the different reactivities of the methylenecycloalkanes. 
As diecussed in the Introduction, this explanation appeared 
improbable to us since in the rate-determining step of the 
addition reaction the hybridization of the ring carbons is 
not altered. 
In order to exclude the possibility that rehybridization 
of the exocyclic carbon was responsible for the different 
reaction rates of methylenecycloalkanes, we have per- 
formed force field calculations (PIMM88)* of the methyl- 
enecycloalkanes R4-RlO and the corresponding l-meth- 
Carbenium Ion Additions to Methylenecycloalkanes 
4 
8' 
a 6  
15 
6 5  4 -55 -5 4 5  4 -35 -3 
'8.tl (25 "C) -
Figure 4. Correlation between the rate constants of the addition 
of 1 BCL- to methylenecycloalkanes (CHzC12, -70 "C) and the 
solvolysis rate constants of l-chloro-l-methylcyclodkanes (80 % 
aqueous EtOH, 25 OC)' [m = 0.65, r = 0.891. 
ylcycloalkyl cations assuming the same force constants 
for the sp2-hybridized ring carbon in methylenecycloal- 
kanes and in methylcycloalkyl cations. While protonation 
was calculated to cause identical enthalpy changes for the 
compounds with n = 610,  the protonations of methyl- 
enecyclopentane R5 and methylenecyclobutane R4 were 
calculated to be 4 and 10 kJ-mol-' less exothermic, re- 
spectively. Thus, as intuitively expected, no correlation 
between methylenecycloalkane reactivities and calculated 
changes of strain between methylenecycloalkanes and their 
protonated analogues was found.g 
There is also no correlation between the calculated 
ionization potentials (MINDO/B and AM1) of R5-R8 and 
the reaction rates of RS-Rs toward 1, in accord with 
previous observations that the rates of carbenium additions 
toward alkenes do not correlate with the ionization 
potentials of the  olefin^.^?^?^ 
We had observed a good correlation between the 
reactivities of alkenes, dienes, and allylsilanes toward 
benzhydryl cations and the solvolysis rates of those alkyl 
chlorides that give the same types of carbenium inter- 
mediates,4J0 however, and concluded that the relative 
reactivities of *-systems toward benzhydryl cations are 
controlled by the relative stabilities of the carbenium ions 
produced during electrophilic attack. By analogy, we now 
infer that the relative reactivities of the methylenecy- 
cloalkanes also reflect the relative stabilities of the cor- 
responding cycloalkyl cations. For l- ferrocenylcycloalkyl 
cations, a relationship between solvolysis rates and ther- 
modynamic stabilities of the carbenium ions had been 
demonstrated by Ortaggi (Figure 51." 
The correlation between the rates of electrophilic 
additions to methylenecycloalkanes and cycloalkyl sol- 
volysis rates (Figure 4) raises the question whether the 
(8) Smith, A. E.; Lindner, H. J. J. Comput. Aided Mol. Des. 1991,5, 
236-262. 
(9) One of the reviewers haa questioned the appropriateness of force 
field calculations to explain small differences of reactivity. This point is 
well taken, but it also questions the significance of the correlations in 
Figures 6 and 7 which providethe basis for the I-strain theory of cycloalkyl 
solvolysee. It does not affect the essential conclusions of this work, however, 
which are exclusively based on correlations between experimental data 
(Figure 4). 
(10) Hagen, G.; Mayr, H. J.  Am. Chem. SOC. 1991, 113, 4954-4961. 
(11) Ortaggi, G. Gaze. Chim. Ital. 1987,117, 75-81. 
(12) Richey, H. G., Jr.; Fletcher, R.; Overmoyer, R. G. Tetrahedron 
Lett. 1970, 3703-3706. 
J. Org. Chem., Vol. 59, No. 1, 1994 171 
-L5 1 
Figure 5. Correlation between the rates of the solvolyses of 
ferrocenyl cycloalkylacetates (aqueous HzSOI, 25 "C) and the 
PKR+ values of the corresponding carbocationsll [m = 1.06, r = 
0.9991, 
Scheme 2 
ODNB' 
0-DNB 
80% aq. acetone 
100 O C  
n ( 4  5 6 7 
~~ I 1.0 13.7 2.7 5.3 
differences in solvolysis rates are really caused by variable 
changes of strain and not by the same factors that control 
the relative reactivities of the methylenecycloalkanes. 
Already in 1956 the limitations of the I-strain theory 
have been stressed by Brown, when the poor correlation 
between the ethanolysis rates of l-methylcycloalkyl chlo- 
rides and the rates of BH4- reductions of cycloalkanones 
was noted.13-16 
Another important argument against the dominating 
role of strain was Richey's report that the solvolysis rates 
of the allyl dinitrobenzoates depicted in Scheme 2 show 
a similar dependence on ring size as the cycloalkyl 
solvolyses, though none of the ring carbons becomes 
rehybridized in the rate-determining step.12 
In spite of these discrepancies, the application of the 
I-strain theory has been continued,16 and force field 
calculations even seemed to provide a quantitative basis 
for this theory. Schneider reported that cycloalkyltosylate 
solvolyses in trifluoroethanol (TFE) and hexafluoro-2- 
propanol (HFIP), but not in ethanol, correlate with the 
calculated differences in strain energy.l7-l9 Remarkably, 
(13) Brown, H. C. J. Chem. SOC. 1966, 1248-1268. 
(14) Brown, H. C.; Ichikawa, K. Tetrahedron 1967,1, 221-230. 
(16) Brown, H. C.; Ham, G. J.  Am. Chem. SOC. 1966, 78,2736-2739. 
(16) McManus, S. P.; Pittman, C. U., Jr. In Organic Reactive 
Intermediates; McManus, 5. P., Ed.; Academic Press: New York and 
London, 1973; pp 273-275. 
(17) Schneider, H.-J.; Thomas, F. J. Am. Chem. SOC. 1980,102,1424- 
1425. - 
(18) Schneider, H.-J.; Schmidt, G.; Thomas, F. J. Am. Chem. SOC. 
(19) Schneider, H.-J.; Becker, N.; Schmidt, G.; Thomas, F. J. Org. 
1983,105, 3556-3563. 
Chem. 1986,51, 3602-3607. 
172 J. Org. Chem., Vol. 59, No. 1, 1994 
-IS 
-2s . 
-3 - 
-15 . 
4 .  
Igkl 45 
. 
e 6  
-5 t 
Figure 6. Correlation between the solvolysis rate constanta of 
cycloalkyl tmylates (TFE, 25 OC)I8 and the differences in strain 
energies (cycloalkanones-methylcycloalkanes),'B kJ/mol 
[(2.303RT)m = -0.52, r = 0.701. 
the unusual high reactivity of the cyclopentyl compound, 
the starting point of Brown's qualitative I-strain theory, 
has not been observed in the fluorinated alcohols. This 
is surprising, since in solvolyses (80% acetone) of l-(aryl- 
cycloalky1)-p-nitrobenzoates (ring size 4-8) reactivity 
maxima have also been found for the five- and eight- 
membered rings,zo though for these compounds nucleo- 
philic solvent participation cannot play a dominant role. 
Correlations of high quality between rates of cycloalkyl 
solvolyses and differences in strain (r = 0.96, 7 points, 
TFE1s r = 0.96, 7 points, HFIPI9) have been obtained 
when certain ring systems were eliminated. Figures 6 and 
7 show, however, that the correlations giver values of 0.70 
(9 points, TFE) and 0.84 (8points, HFIP) when all available 
data are considered. These correlations are, therefore, 
worse than the correlation between solvolysis rate con- 
stants and the rates of electrophilic additions to meth- 
ylenecycloalkanes (Figure 4) which are not controlled by 
strain (see also ref 9). 
Strain effects may, therefore, account for part of the 
reactivity differences of cycloalkyl derivatives, but other 
factors (e.g., ring size dependent hyperconjugative effects 
or transannular hydride b r i d g b ~ g l ~ * ~ ~  ) that stabilize the 
intermediate cycloalkyl cations must be at least equally 
important. 
Conclusion 
While the correlation between strain effects and rate as 
well as equilibrium constants for nucleophile additions to 
(20) Brown, H. C.; Ravindranathan, M.; Peters, E. N.; Rao, C. G.; Rho, 
M. M. J. Am. Chem. SOC. 1977,99,5373-5378. 
(21) (a) Schneider, H.J.; Heiske, D. J. Am. Chem. SOC. 1981, 103, 
3601-3605. (b) Kirchen, R. P.; Soremen, T. S. J.  Am. Chem. SOC. 1979, 
101,3240-3244. (c) Nordlander, J. E.; Owuor, P. 0.; Cabral, D. J.; Haky, 
J. E. J. Am. Chem. SOC. 1982,104,201-206. (d) Parker, W.; Watt, C. I. 
F. J.  Chem. Soc., Perkin Trans. 2 1976,1647-1651. (e) Allinger, N. L.; 
Szkrybalo, W. Tetrahedron 196&24,4699-4711. Arguments that trans- 
annular hydride bridging does not occur in the rata-determining step of 
solvolytic reactions have been published: (0 F'relog, V.; Raynham, J. G. 
InMoleculorRearrangements; de Mayo, P., Ed.; Wiley-Intsrscience: New 
York, 1963; Part 1, Chapter 9, pp 593-616. (g) f i n g e r ,  N. L.; Greenberg, 
S. J. Am. Chem. SOC. 1962,84,2394-2402. (h) Roberta, A. A.; Anderson, 
C. B. Tetrahedron Lett. 1969,3883-3885. (i) Alliiger, N. L.; Neumann, 
C. L.; Sugiyama, H. J. Org. Chem. 1971,36,1360-1365. (i) Harris, J. M.; 
Mount, D. L.; Smith, M. R.; Neal, W. C., Jr.; Dukes, M. D.; Raber, D. J. 
J. Am. Chem. SOC. 1978,100,8147-8166. 
4s 
-1 
-15 
-2 
-2s 
-1s 
4 
lg kl -3 
4 5  
I 
-Z? -19 -16 -I3 -10 -7 4 -1 2 5 8 
Astrain 
Figure 7. Correlation between the solvolyeis rate constanta of 
cycloalkyl toeylates (HFP, 25 "C) and the differences in strain 
energies (cycloalkanone~-methylcycloalkane~),*s kJ/mol 
L(2.303RT)m = -0.72, r = 0.841. 
cycloalkanones is well sub~tant ia ted ,2~~~~ variable strain 
effects are not the major reason for the differences of 
cycloalkyl solvolysis rates. The correlation between cy- 
cloalkyl solvolysis rates and electrophilic additions to 
methylenecycloalkanes (Figure 4) stresses the importance 
of electronic effects. Since acomplexinterplayof enthalpic 
and entropic factors was found to be responsible for the 
relative reactivities of methylenecycloalkanes, an analysis 
of cycloalkyl solvolyses that is based solely on strain 
enthalpies appears to be too simplistic. 
Experimental Section 
Details of the kinetic procedure and preparation of the 
carbenium ion precursors have been published The 
methylenecycloalkanes have been synthesized from the cycloal- 
kanones according to a modified procedure of Takai (for details 
see supplementary material).% 
Addition Products P3-PLS. Three equiv of R3-RlS was 
added dropwise to a colored solution of 1 equiv of 1 BCL- in 
CHzClz (100 mL) at -78 "C. The mixture was allowed to stand 
at -78 OC until it was completely decolorized (approximately 2 
h). The mixture was washed with concentrated hydrochloric 
acid and dried over MgSOd. The solvent was evaporated, and 
the remaining colorless oil was fiitered over neutral alumina before 
recrystallization from EtOH. 
Acknowledgment. We thank Prof. A. Maercker for 
bringing this problem to our attention, Monika Deters for 
experimental assistance, Prof. H.-J. Schneider for critical 
comments, Prof. H. J. Lindner for providing his force field 
programs, and the Deutsche Forschungsgemeinschaft and 
Fonds der Chemischen Industrie for financial support. 
Supplementary Material Available: Data tables containing 
concentrations, rate constanta, and activation parameters of the 
kinetic experiments, experimental procedures for RI-R16, and 
full characterization of R7-RlS and P3-PlS (12 pages). This 
material is contained in libraries on microfiche, immediately 
follows this article in the microfilm version of the journal, and 
can be ordered from the ACS; see any current masthead page for 
ordering information. 
(22) Milller, P.; Blanc, J. Hela Chim. Acta 1980,69, 1759-1766. 
(23) MWer, P.; Mareda, J. In Physical Organic Chemistry 1986; 
Kobayaehi, M., Ed.; Studies in Organic Chemistry, Elsevier Science 
Publishers B. V., Amsterdam, 1987; Vol. 31, pp 529-636. 
(24) Takai, K.; Hotta, Y.; Oshima, K.; Nozaki, H. Tetrahedron Lett. 
1978,2417-2420. 


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Kinetics of carbenium ion additions to methylenecycloalkanes. Cycloalkyl cation stabilities are not predominantly controlled by strain

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