Rate constants for solvolysis of tertiary chlorides and p-nitrobenzoates are compared and converted to Bentley’s unified reactivity scale. For the majority of structures consistent behavior is observed. Deviations are interpreted in terms of F-strain dr, to a minor extent, of leaving group hindrance. The reactivity of the tricyclic compound 31-OPNB can be rationalized by strain calculations if F-strain due to the presence of the OPNB substituent is taken into consideration

Blanc, Jacky

Mareda, Jiri

Müller, Paul

CHIMIA International Journal for Chemistry

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CHIMIA

Steric Effects on Reaction Rates Correlation Analysis of Solvolysis Data for Tertiary Carbon Substrates

Paul Müller

Jacky Blanc

Jiri Mareda

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FORSCHUNG ______________________399CHIMIA 41 (1987) Nr. 11 (November)Steric Effects on Reaction Rates Correlation Analysis of Solvolysis Data for Tertiary Carbon Substrates*** Correspondence : Prof. Dr. P. Müller Département de Chimie Organique Université de Genève30, quai Ernest Ansermet, CH 1211 Genève 4** Part X of a series of papers. This work was sup­ported by the Swiss National Science Foundation (grant No 2.805-0.85). The authors are indebted to T. W. Bentley, M. Saunders, and P. von R. Schleyer for many helpful comments, discussions, and suggestions. - For Part IX of this series see Ref. ^.Paul Müller*,  Jacky Blanc, and Jiri MaredaAbstract: Rate constants for solvolysis of tertiary chlorides and p-nitrdbenzoates are compared and converted to Bentley’s unified reactivity scale. For the majority of struc­tures consistent behavior is observed. Deviations are interpreted in terms of F-strain dr, to a minor extent, of leaving group hindrance. The reactivity of the tricyclic compound 31-OPNB can be rationalized by strain calculations if F-strain due to the presence of the OPNB substituent is taken into consideration.The solvolytic reactivity of organic sub­strates is dependent from a variety of fac­tors, some of them steric, some electronic, while yet others are due to environmental causes. In the series of bridgehead sub­strates steric effects predominate111 while all others are remarkably constant. In par­ticular, relative rates of bridgehead deriva­tives are essentially independent from leav­ing groups and solvents. On these grounds a unified reactivity scale for solvolysis of bridgehead derivatives under standard conditions (OTs leaving group, solvolysis in 80% EtOH, 70°C) was recently pro­posed by Bentley PI. The mechanistic sim­plicity of these substrates provides a unique test ground for mechanistic con­cepts. Molecular-mechanics calculations have been applied to the problem of solvo­lytic bridgehead reactivity as early as 1967131 and repeatedly since then11,41. Bent­ley’s unified reactivity scale121 can be ratio­nalized in terms of strain changes between the bridgehead substrate (R-X) and the respective carbenium ion (R®)I51.The existence of a similar set of unified reactivity data is a prerequisite for wider application of molecular mechanics to sol­volysis of non-bridgehead tertiary sub­strates. The existence of such a reactivity scale is note self-evident; tertiary sub­strates do not exhibit the same mechanistic uniformity as do bridgehead derivatives. The solvolysis mechanism may involvesolvent participation161, anchimeric assis­tance171, F-strain|4 S,<'1, or leaving group hin­drance1101 in addition to release or build-up of steric strain in the molecular skeleton.Table 1. Kinetic data for solvolysis of chloride and p -nitrobenzoate derivatives, R-^ (X = Cl? OPNB);No SubstrateR =lg* ”X = C1lg* b)X = OPNBk . c’d) ^calc .X = OTs from ClIg w’X= OTs from OPNB1 /-Butyl -2.82° -7.22®' 2.38 2.462 t -Butyl(dimethyl)methyl -2.7511' -6.61®’ 2.45 3.123 Di-Z-butyl(methyl)methyl -1.721’ -5.11k> 3.48 4.944 Tri-1 -butylmethyl-1.40”-3.52h) 6.815 1 -Methylcÿclopentyl -6.55®' 3.80 3.256 1,2,2,5,5-Pentamethylcyclopentyl 0.33m)-7.29®>5.537 l-Methylcyclohexyl -3.15” 2.05 2.388 1 -Methylcycloheptyl -1.43” -5.71®’ 3.77 4.249 ds-l -Hydrindanyl -1.82“' -6.16°' 3.38 3.7110 trans -1 -Hydrindanyl -0.94"' -5.56°' 4.26 4.4111 cw-l-Decalinyl -3.13p) -7.13°' 2.07 2.5712 trans-l -Decalinyl -3.24p’ -7.75” 1.96 1.8313 2-Methyl-2*e«i/o  -norbornyl -2.79“’ -7.97®' 2.49 1.5814 2-Methyl-2-exo -nofbornyl -0.62” -5.43®' 4.58 4.5715 2-1 -Butyl-2-endo -norbornyl -3.87®' 6.4016 2-1 - But y 1-2-exo -nôrbornyl-6.86s>-1.92” 8.6917 7-Methyl-7-norbornyl-7.07“'-1.66 -2.42x)18 1,2-Dime thy 1-2-ewJo -norbornyl -1.83” 3.37 2.6419 1,2-Dimethyl-2-exo -norbornyl 0.12” -4.67“' 5.32 5.4620 2,3,3-Trimethyl-2-ew Jo -norbornyl0.78m'-7.64®' 1.9721 2,3,3-Trimethyl-2-exo-norbornyl -4.45®’ 5.98 5.7222 2-r-Butyl-3,3-dimethyl-2-enrfo-norbornyl -2.42®’ 8.1123 9-Methyl-9-bicyclo[3.3. l]nonyl -6.69®’ 3.0824 9-Z-Butyl-9-bicyclo[3.3. l]nonyl-5.61”-2.33®' 8.2125 1-Adamantyl -9.52” -0.41 -0.2526 2-Methyl-2-adamantyl . -1.99” -6.86®' 3.21 2.8827 2-f-Butyl-2-adamantyl-5.00v>-2.33®’ 8.2128 1 - Homoadamanty 1 -9.38’’ 0.20 -0.0829 3-Homoadamantyl -3.29w’ -7.72” 1.91 1.8730 t, t, t-l-Tricyclo[7.3.1.05,13]-tridecanyl -3.71” -8.07” 1.49 1.4631 Z, Z, r -13-T ricyclo[7.3.1.05,13]-tridecanyl -4.36” -5.08” 0.84 4.9832 c, c, ^ -13-Tricyclo[7.3.1.05,13]-tridecanyl -3.12” -7.93” 2.08 1.62“'Converted to 80% EtOH, 70°C from other temperatures and, in part, other solvents. b'Converted to 80% acetone, 70°C. “'Conversion factor 1.6 x 105. d’Ref.121. e'OPNB rates converted to chloride rates with Eq. (1), the latter converted according to c'. ”Ref.191. ®’Ref.lnl. h'Ref.1121. ‘'Ref.1131. k'Converted from 70% acetone 1141 via fc(Z-BuOPNB) = 6.81 x 10~6 s-1 (100°C)1151. ‘'From data relative to /-BuCl1161 and rate of z-BuCl reported in Ref.pl.m' From relative rates in EtOH, 25 °C1171. ”' Ref.118!. °' Extrapolated from 60 % acetone, 100 °C1191. p' Ref.1201. “'Calculated from rate of 14-C1 in EtOH and exo/endo rate ratio of l,2-dimethyl-2-norbornyl chlorides1211. ”Ref.[221. “'Extrapolated from 80% acetone1231, ‘'in EtOH1211. “'in 80% acetone1211. ’'Extrapolated from 70% CH3CN, 150°C (X = OPNB) and from 80% EtOH, 25°C (X = Br)1241; relative rates of perfluorobutyrates are slightly different1251. ’'Ref.1'1. “'From rate of p-toluenesulfonate in AcOH, 70°C1261; conversion factor to 80% EtOH = 3.5 pl.Some of these complicating effects may be dependent upon the leaving group, others on the solvent or even on both. In this communication we compare rate data for solvolysis of tertiary p-nitrobenzoates (OPNB’s) and chlorides under standard conditions. The objective is to establish consistent behavior, wherever it exists or on the contrary, identify and if possible rationalize deviations from «normal» sol­volytic reactivity.The data are collected in Table 1. For chlorides all rate constants are converted to standard conditions (80% EtOH, 70 °C) as follows: Data were extrapolated from other temperatures to 70°C by means of the Arrhenius equation if rate constants at two or more temperatures were available; otherwise the rate constant relative to t- BuCl or another suitable reference com­pound was determined and extrapolated to 70 °C assuming constant A factors. The same procedure was adopted for conver­sion of rate constants from other solvents. Details are given in the footnotes of Table 1. The rate constants of chlorides are con­verted to Bentley’S unified reactivity scale (1g ^cak (OTs)) using the conversion factor: 1g 1.6 * 105 ( = 5d0)121. For OPNB deriva­tives the standard conditions are 80% acetone, 70°C. The data available from theFORSCHUNG ________________400literature were converted to these condi­tions in the same way as the chloride data. The last column of Table 1 contains calcu­lated rate constants for standard condi­tions (80% EtOH, X=OTs) calculated from OPNB reactivities via conversion to chlorides (see below).Comparison of Chlorides with OPNB Derivatives (Fig. 1):The rate constants for solvolysis of chlo­rides (80% EtOH) and />-nitrobenzoates (OPNB’s, 80% acetone) are in general remarkably consistent as revealed by Fig. 1. Only two compounds (3 and 31) fall out of the correlation. The remaining data correlate by means of Equation (1):1g k(OPNB) = 0.839 1g k(Cl) -4.95;r = 0.966 (1)If, for reasons discussed below, 2, 13, and 18 are excluded from the plot, the relation­ship becomes:lgk(OPNB) = 0.851gk(Cl) -4.89;r = 0.984 (2)CHIMIA 41 (1987) Nr. 11 (November)Fig.l. Plot of rate constants for solvolysis ofp-nitrobenzoates (80% acetone, 70°C) vs. chlorides (80% ethanol, 70°C). Data from Table 1.We note that contrary to the situation in the restricted series of bridgehead com­pounds121 the slope of the correlation line is different from the expected value of 1.0. The OPNB rates are slightly compressed in comparison to the chloride rates, but we believe that this discrepancy should not be mechanistically significant. It may well be due to our conversion of the rate data. The unreactive OPNB’s are extrapolated from 125-150°C to 70°C, the very reactive ones from 25 °C, but very often data were avail­able only at two temperatures, sometimes even only at one. Such extrapolations could lead to systematic distortions. In the case of chloride solvolysis the extrapo­lations involve a smaller temperature range (25-70°C). We believe therefore that the chloride data are more reliable and they are used in preference for conversion to Bentley's unified reactivity scale121 (X = OTs. 80% EtOH, 70°C; second last column of Table 1). The OPNB rates are converted to standard conditions via the chlorides by means of Equation (2). We are rather surprised to find that mechanisti­cally so different compounds like ada- mantyl, tert-butyl or exo-norbornyl re­spond similarly to solvent and leaving group change, but it is clear that this com­mon behavior must break down in limiting solvents161.Upwards deviations from the plot in Fig. 1 can be attributed to F-strain. This effect has been recognized in the case of 31-OPNB191 and 3 I12], but it could also play some role in 2-OPNB. Since the steric re­quirements of Cl and OPNB groups are obviously different, some of the small scat­ter in the plot should be due to this differ­ence. Nevertheless, in a first approxi­mation F-strain is almost negligible in un­crowded substrates.10 20AEst(R®-RX)Fig. 2. Plot of rate constants for solvolysis of bridgehead halides and p-nitrobenzoates vs. AESI (R® — RCl) with correction for 31-OPNB and 32-OPNB (see text and Table 2). Correlation equation: Ig kcalc = — 0.36 AEst (R® — RCl) + 1.32; r = 0.9833.The most significant downwards devia­tions in Fig. 1 occur with the endo -norbor­nyl derivatives 13 and 18. Following Brown’s suggestion, these should be ascribed to leaving group hindrance1271 which is expected to be more pronounced with OPNB’s than with chlorides because of the higher steric requirements of the former. However, the effect is disappoint­ingly small.A Model for Molecular-Mechanics Treat­ment of F-Strain in p-Nitrobenzoates:The recognition of different types of strain effects (B-strain, F-strain, I-strain) represented a significant step for the devel­opment of the concept of strain, but their nature is in principle identical. F-strain is due to steric interaction between the leav­ing group and the rest of the molecule; this interaction can be evaluated with molec-401CHIMIA 41 (1987) Nr. Il (November)FORSCHUNGular mechanics if the corresponding leav­ing group or an appropriate surrogate is used for the calculations.One of the classical examples of rate en­hancement in solvolysis owing to F-strain is the t,/, t-9b-phenalenyl derivative 31P1. The reactivity of the chloride 31-Cl like that of bridgehead chlorides in general is satisfactorily correlated with the steric energy difference AE„ (R® — RC1) (Fig. 2). If the chloro substituent of 31 is replaced by OPNB a significant rate enhancement occurs; the rate constants extrapolated from 31-Cl and 31-OPNB differ by ca. 4 log units (see Table 1). The rate enhance­ment of 31-OPNB can be rationalized by force-field calculations using a simplified leaving group model (acetate) instead of OPNB. This simplification is justified, because in the most stable conformations of31-OAc(and also of 32-OAc) the principal F-strain interactions occur between the carbonyl group and the molecular skeleton, while the bulkier CH3 group points away from the latter. In a first approximation, extra strain due to the presence of the group R (R = CH, or C6H4NO2) may be neglected.Steric energies of chlorides are not di­rectly comparable with those of acetates, but comparable numbers may be obtained as follows: The strain difference between 31-Cl and 31-OAc is — 7.78 kcal/mol. Adding this difference to AE& (R® — RC1) ( — 1.34) we arrive at — 9.12 kcal/mol for AEst (R® - ROPNB) (Table 2). The same procedure applied to the c, c, t derivative 32Table 2. Steric energy differences and solvolytic reactivity of bridgehead halides and p-notrobenzoates, R-X (X = Cl, OPNB).______________________________________________________________No CompoundR =AEsl (R® - RCl)a> lg*b>25 1-Adamantyl 7.17 -0.4029 3-Homoadamantyl -1.69 1.9731-Cl t, t, Z-13-Tricyclo[7.3.1.05’13]tridecanyl -1.34 0.8431-OPNB t,t,t-l 3-Tricyclo[7.3.1,05’ 13]tridecanyl -9.12c) 4.91®32-C1 c, c, t-l3-Tricyclo[7.3.1,05,13] tridecanyl -1.77 2.0832-OPNB c,c, t-13-Tricyclo[7.3.1.05,13tridecanyl -1.29c) 1.62dl33 1-Bicyclo[2.2.2]octyl 13.58 4.0034 1-Bicyclo[3.3.1]nonyl 3.50 0.5135 1 -Bicyclo[3,2.2]nonyl 3.74 -0.1336 1-Bicyclo[3.3.2]decyl 4.66 3.0837 1-Bicyclo[3.3.3]undenyl -12.13 5.45^Calculated with force-field UNICAT f4'Sl ^Calculated for X = OTs, 80% EtOHpl c)Calculated for R-ococh3. see text. ® Calculated from rate of OPNB derivative (Table 1).[1] R. C. Bingham, P. v. R. Schleyer, J. Am. Chem. Soc. 93 (1971) 3189; W. Parker, R.L. Tranter, C.I.F. Watt, L.W.K. Chang, P.v.R. Schleyer, ibid. 96 (1974) 7121.[2] T.W. Bentley, K. Roberts, J. Org. Chem. 50 (1985) 5852.[3] G. J. Gleicher, P. v. R. Schleyer, J. Am. Chem. Soc. 89 (1967) 582.[4] P. Mtiller, J. Blanc, J. Mareda, Chimia 38 (1984) 389; Helv. Chim Acta 69 (1986) 635.[5] P. Müller, J. Mareda, Helv. Chim. Acta 70 (1987) 1017.[6] T.W. Bentley, C.T. Bowen, W. Parker, C.I.F. Watt, J. Am. Chem. 101 (1979) 2486; T.W. Bent­ley, G. E. Carter, ibid. 104 (1982) 5741.[7] C. A. Grob, Acc. Chem. Res. 16 (1983) 426; G.A. Olah, G. K. Surya Prakash, M. Saunders, ibid. 16 (1983) 440; H.C. Brown: The Non-classical Ion Problem, Plenum Press, New York (1977).[8] T.T. Tidwell, J. Org. Chem. 39 (1974) 3533.[9] J. Slutsky, R. C. Bingham, P. v. R. Schleyer, W.C.gives a strain difference between 32-C1 and 32-OAc of 0.48 and J£st (R® - ROPNB) increases to — 1.29 kcal/mol. Fig. 2 shows that the data for OPNB solvolysis fit al­most perfectly the strain-reactivity correla­tion of bridgehead chlorides. It remains to be seen if this approach is applicable to OPNB solvolysis in general.Received: May 21, 1987 [FC 112]Dickason, H.C. Brown, J. Am. Chem. Soc. 96 (1974) 1969.[10] S. Ikegami. D.L. Vander Jagt, H.C. Brown, J. Am. Chem. Soc. 90 (1968) 7124.[11] E.N. Peters, H.C. Brown, J. Am. Chem. Soc. 97 (1975) 2892; H.C. Brown, F.J. Chloupek, K. Ta­keuchi, J. Org. Chem. 50 (1985) 826.[12] P.D. Bartlett, T.T. Tidwell, J. Am. Chem. Soc. 90 (1968)4421.[13] V.J. Shiner Jr., G. F. Meier, J. Org. Chem. 31 (1966) 137.[14] H. Tanida, H. Matsumura, J. Am. Chem. Soc. 95 (1973) 1586.[15] C. F. Wilcox, M. E. Mesirov, J. Am. Chem. Soc. 84 (1962) 2755.[16] H. C. Brown, M. Borkowski, J. Am. Chem. Soc. 74 (1952) 1894.[17] H.C. Brown, F.J. Chloupek, J. Am. Chem. Soc. 85 (1963) 2322.[18] K. B. Becker, A. F. Böschung, C. A. Grob, Helv. Chim. Aeta 56 (1973) 2733.[19] R.C. Fort Jr., R.E. Hornish, G.A. Liang, J. Am. Chem. Soc. 92 (1970) 7558.[20] K.B. Becker, A.F. Böschung, M. Geisel, C.A. Grob, Helv. Chim. Acta 56 (1973) 2747.[21] H.C. Brown, M. Ravindranathan, C.G. Rao, F.J. Chloupek, M.H. Rei, J. Org. Chem. 43(1978) 3667.[22] E.N. Peters, H.C. Brown, J. Am. Chem. Soc. 96 (1974) 265.[23] R.K. Lustgarten, J. L’homme, S. Winstein, J. Org. Chem. 37 (1972) 1075.[24] J. Mergelsberg, C. Rüchardt, Tetrahedron Lett. 23 (1982) 3189.[25] D. Farcasiu, J. Jähme, C. Rüchardt, J. Am. Chem. Soc. 707(1985) 5717.[26] R.D. Fisher. R.C. Seib, V.J. Shiner Jr., I. Szele, M. Tomic, D.E. Sunko, J. Am. Chem. Soc. 97 (1975) 2408.[27] H. C. Brown, Acc. Chem. Res. 16 (1983) 432.

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Steric Effects on Reaction Rates Correlation Analysis of Solvolysis Data for Tertiary Carbon Substrates

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