Lewis acid-catalysed reaction of 1,1-dimethyl-3-phenylprop-2-ynyl chloride with cyclopentadiene yields a 2 : 1 addition product via[2 + 2]-cycloaddition of an intermediate allenyl cation

Kaliba, Claus

Mayr, Herbert

Wilhelm, Eberhard

English

Open Access LMU ( Ludwig-Maximilians-Univ. München)

J.C.S. CHEM. COMM., 1981 683 [2+2] -Cycloaddition of a Stabilised Vinyl Cation with Cyclopentadiene ; X-Ray Crystal Structure of the 2:l Addition Product By HERBERT MAYR,* EBERHARD WILHELM, and CLAUS KALIBA (Institut fur Organische Chernie der Friedrich-A lexander- Universitat Erlangen-Niirnberg, Henkestrasse 42, D-8520 Erlangen, Federal Republic of Germany) Szcmmary Lewis acid-catalysed reaction of 1,l-dimethyl-3- phenylprop-2-ynyl chloride with cyclopentadiene yields a 2 :  1 addition product via [2 + 21-cycloaddition of an intermediate allenyl cation. ACCORDING to Woodward and Hoffmann, vinyl cations are ',2a-components par excellence' for concerted [,2, + n2a]- cyc1oadditions.l In agreement with this concept, four- membered rings are formed from vinyl cations and alkynes, or a l k e n e ~ . ~  We now find that cyclopentadiene also under- goes [2 + 21-cycloadditions with vinyl cations of type (2), even though the formation of 1-ess-strained products [equa- tion ( l ) ]  seems to be feasible with 1,3-dienes. We reported recently that ZnC1,-catalysed reactions of propargyl chlorides with cyclopentadiene yield bicyclic vinyl chlorides. These arise from initial attack of cyclopentadiene at C-3 of the intermediate allenyl cations (2) .4 Ally1 chlorides derived from (1) and (3) were not detected, in accord with our postulate that Lewis acid-catalysed addition reactions yield 1 : 1 products only if the adducts dissociate faster than the products.6 If allenyl cations (2) undergo vinyl cation type cycloadditions [equation (l)], the resulting allyl cations (1) and (3) are predicted5 to react with further cyclo- pentadiene to give higher addition products, since allyl chlorides solvolyse faster than their precursor propargyl chlorides.6 Me 4- product I 1 Ph-C=C-C-CI + 0 Ma In  fact, in addition to the previously reported 1: 1 product (5),4 the reaction of (4) with cyclopentadiene afforded a fraction of high molecular weight from which the 2 : 1 product 4-chloro-2,2-dimethyl-7-phenylpentacyclo- [6.3. ll$*. lS p 8 . 0 1  ~ 7 . 0 ~  $I2] tridec- 1 0-ene (6) (m.p. 106.5-108.0 "C) was isolated chromatographically. N.m.r. spectra show two vinylic carbon and hydrogen atoms, in accord with a variety of structures arising from two cycloaddition pro- cesses and a n-cyclization. Conclusive structural evidence was obtained from X-ray analysis. Crystal data : (6), C,,H2,C1, monoclinic, space group P2,/n, a = 8.187(2), b = 15.405(3), c = 25.826(4) A, 18 = 91.38(3)", 2 = 8. A total of 9056 reflections was measured on a Hilger & Watts Y290 diffractometer using Mo-K, radiation. Averaging of symmetrically equivalent measurements yielded 3052 independent reflections. As 2 = 8, there are two molecules in the asymmetric unit. The structure was solved automatically with the program system 'CRY STAN" and refined by the block-diagonal least- squares method. The final R was 0.08 for 2253 reflections with 8 < 1 8 O . t  The two molecules in the asymmetric unit are enantiomers. There are no significant geometrical differences between these two molecules. $ Ll FIGURE. Molecular structure of C2,H2,C1 (one enan tiomer) A reaction mechanism for the formation of (6)  is suggested in the Scheme. [,2, + ,2,]-Cycloaddition of the initially formed allenyl cation with cyclopentadiene yields allyl cation (7), which undergoes a [3 + 41-cycloaddition* with cyclopentadiene. A 1,2-alkyl shift and n-cyclisation afford carbenium ion (lo), which is finally trapped by C1- to give (6).  7 Data with 8 > 18", for which not all equivalents could be measured, were of poorer quality. $ The atomic co-ordinates for this work are available on request from the Director of the Cambridge Crystallographic Data Centre, Any request should be accompanied by the full literature University Chemical Laboratory, Lensfield Rd., Cambridge CB2 IEW. citation for this communication. 684 J.C.S. CHEM. COMM., 1981 Me &+ (10) Me Me [CL - 1 - (6) SCHEME Me Me + i,PhLi ii,HCI in pentane I The suggested mechanism is supported by the observation that when (7) is generated from (11) in the presence of cyclopentadiene, it yields (6) as the only isolable compound Therefore, the formation of (6) from (4) and cyclopenta- diene is considered to involve the first [2 + 21-cycloaddition of a vinyl cation with a 1,3-diene. A concerted cyclo- addition mechanism is most likely, since the potential inter- mediate (12) is supposed to yield the less-strained ion (1) rather than (3).s This work was supported by the Deutsche Forschungs- gemeinschaft and the Fonds der Chemischen Industrie. We thank U. Kobold and A. Erbe for experimental assistance and Prof. H. Burzlaff for permission to use his diffracto- meter and PDP 11/45 minicomputer. (52%)  * (Received, 13th April 1981; Corn. 429.) R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1969, 8, 781. K. Griesbaum, Angew. Chem., Int. Ed. Engl., 1969, 8, 933. G. Hammen and M. Hanack, Angew. Chem., Int. Ed. Engl., 1979, 18, 614; M. Hanack, I. Harder, and K.-R. Bofinger, Tetrahedron H. Mayr and I. K. Halberstadt, Angew. Chew., Int. Ed. Engl., 1980, 19, 814; H. Mayr and F. Schiitz, Tetrahedron Lett., 1981, 926. H. Mayr, Angew. Chern., Int. E d .  Engl., 1981, 20, 184. H. G. Richey, Jr., and J .  M. Richey in 'Carbonium Ions,' Vol. 11, eds. G. A. Olah and P. v. R. Schleyer, Wiley-Interscience, New H. Burzlaff, R. Bohme, and M. Gomm, 'CRYSTAN - A Crystallographic Program System for Minicomputers,' Universitat Erlangen- The exclusive formation of cyclobutanones from diphenylketen and 1,3-dienes has previously been considered an argument for Lett., 1981, 553. York, 1970, p. 944. Nurnberg, 1977. a concerted cycloaddition mechanism: R. Huisgen and P. Otto, Chem. Ber., 1969, 102, 3475. * H .  M. R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1973, 12, 819. 

[2 + 2]-Cycloaddition of a stabilised vinyl cation with cyclopentadiene; X-ray crystal structure of the 2 : 1 addition product

Herbert  Mayr

Eberhard Wilhelm

Claus Kaliba

Crossref

Universität München: Elektronischen Publikationen

J.C.S. CHEM. COMM., 1981 683 
[2+2] -Cycloaddition of a Stabilised Vinyl Cation with Cyclopentadiene ; 
X-Ray Crystal Structure of the 2:l Addition Product 
By HERBERT MAYR,* EBERHARD WILHELM, and CLAUS KALIBA 
(Institut fur Organische Chernie der Friedrich-A lexander- Universitat Erlangen-Niirnberg, 
Henkestrasse 42, D-8520 Erlangen, Federal Republic of Germany) 
Szcmmary Lewis acid-catalysed reaction of 1,l-dimethyl-3- 
phenylprop-2-ynyl chloride with cyclopentadiene yields 
a 2 :  1 addition product via [2 + 21-cycloaddition of an 
intermediate allenyl cation. 
ACCORDING to Woodward and Hoffmann, vinyl cations are 
',2a-components par excellence' for concerted [,2, + n2a]- 
cyc1oadditions.l In agreement with this concept, four- 
membered rings are formed from vinyl cations and alkynes, 
or a l k e n e ~ . ~  We now find that cyclopentadiene also under- 
goes [2 + 21-cycloadditions with vinyl cations of type (2), 
even though the formation of 1-ess-strained products [equa- 
tion ( l ) ]  seems to be feasible with 1,3-dienes. 
We reported recently that ZnC1,-catalysed reactions of 
propargyl chlorides with cyclopentadiene yield bicyclic vinyl 
chlorides. These arise from initial attack of cyclopentadiene 
at C-3 of the intermediate allenyl cations (2) .4 Ally1 chlorides 
derived from (1) and (3) were not detected, in accord with 
our postulate that Lewis acid-catalysed addition reactions 
yield 1 : 1 products only if the adducts dissociate faster than 
the products.6 If allenyl cations (2) undergo vinyl cation 
type cycloadditions [equation (l)], the resulting allyl cations 
(1) and (3) are predicted5 to react with further cyclo- 
pentadiene to give higher addition products, since allyl 
chlorides solvolyse faster than their precursor propargyl 
chlorides.6 
Me 
4- product 
I 
1 
Ph-C=C-C-CI + 0 
Ma 
In  fact, in addition to the previously reported 1: 1 
product (5),4 the reaction of (4) with cyclopentadiene 
afforded a fraction of high molecular weight from which 
the 2 : 1 product 4-chloro-2,2-dimethyl-7-phenylpentacyclo- 
[6.3. ll$*. lS p 8 . 0 1  ~ 7 . 0 ~  $I2] tridec- 1 0-ene (6) (m.p. 106.5-108.0 "C) 
was isolated chromatographically. N.m.r. spectra show two 
vinylic carbon and hydrogen atoms, in accord with a 
variety of structures arising from two cycloaddition pro- 
cesses and a n-cyclization. Conclusive structural evidence 
was obtained from X-ray analysis. 
Crystal data : (6), C,,H2,C1, monoclinic, space group 
P2,/n, a = 8.187(2), b = 15.405(3), c = 25.826(4) A, 
18 = 91.38(3)", 2 = 8. A total of 9056 reflections was 
measured on a Hilger & Watts Y290 diffractometer using 
Mo-K, radiation. Averaging of symmetrically equivalent 
measurements yielded 3052 independent reflections. As 
2 = 8, there are two molecules in the asymmetric unit. 
The structure was solved automatically with the program 
system 'CRY STAN" and refined by the block-diagonal least- 
squares method. The final R was 0.08 for 2253 reflections 
with 8 < 1 8 O . t  The two molecules in the asymmetric 
unit are enantiomers. There are no significant geometrical 
differences between these two molecules. $ 
Ll 
FIGURE. Molecular structure of C2,H2,C1 (one enan tiomer) 
A reaction mechanism for the formation of (6)  is suggested 
in the Scheme. [,2, + ,2,]-Cycloaddition of the initially 
formed allenyl cation with cyclopentadiene yields allyl 
cation (7), which undergoes a [3 + 41-cycloaddition* with 
cyclopentadiene. A 1,2-alkyl shift and n-cyclisation afford 
carbenium ion (lo), which is finally trapped by C1- to 
give (6).  
7 Data with 8 > 18", for which not all equivalents could be measured, were of poorer quality. 
$ The atomic co-ordinates for this work are available on request from the Director of the Cambridge Crystallographic Data Centre, 
Any request should be accompanied by the full literature University Chemical Laboratory, Lensfield Rd., Cambridge CB2 IEW. 
citation for this communication. 
684 J.C.S. CHEM. COMM., 1981 
Me 
&+ 
(10) 
Me Me 
[CL - 1 - 
(6) 
SCHEME 
Me Me 
+ 
i,PhLi ii,HCI in 
pentane I 
The suggested mechanism is supported by the observation 
that when (7) is generated from (11) in the presence of 
cyclopentadiene, it yields (6) as the only isolable compound 
Therefore, the formation of (6) from (4) and cyclopenta- 
diene is considered to involve the first [2 + 21-cycloaddition 
of a vinyl cation with a 1,3-diene. A concerted cyclo- 
addition mechanism is most likely, since the potential inter- 
mediate (12) is supposed to yield the less-strained ion (1) 
rather than (3).s 
This work was supported by the Deutsche Forschungs- 
gemeinschaft and the Fonds der Chemischen Industrie. We 
thank U. Kobold and A. Erbe for experimental assistance 
and Prof. H. Burzlaff for permission to use his diffracto- 
meter and PDP 11/45 minicomputer. 
(52%)  * 
(Received, 13th April 1981; Corn. 429.) 
R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1969, 8, 781. 
K. Griesbaum, Angew. Chem., Int. Ed. Engl., 1969, 8, 933. 
G. Hammen and M. Hanack, Angew. Chem., Int. Ed. Engl., 1979, 18, 614; M. Hanack, I. Harder, and K.-R. Bofinger, Tetrahedron 
H. Mayr and I. K. Halberstadt, Angew. Chew., Int. Ed. Engl., 1980, 19, 814; H. Mayr and F. Schiitz, Tetrahedron Lett., 1981, 926. 
H. Mayr, Angew. Chern., Int. E d .  Engl., 1981, 20, 184. 
H. G. Richey, Jr., and J .  M. Richey in 'Carbonium Ions,' Vol. 11, eds. G. A. Olah and P. v. R. Schleyer, Wiley-Interscience, New 
H. Burzlaff, R. Bohme, and M. Gomm, 'CRYSTAN - A Crystallographic Program System for Minicomputers,' Universitat Erlangen- 
The exclusive formation of cyclobutanones from diphenylketen and 1,3-dienes has previously been considered an argument for 
Lett., 1981, 553. 
York, 1970, p. 944. 
Nurnberg, 1977. 
a concerted cycloaddition mechanism: R. Huisgen and P. Otto, Chem. Ber., 1969, 102, 3475. 
* H .  M. R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1973, 12, 819. 


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[2 + 2]-Cycloaddition of a stabilised vinyl cation with cyclopentadiene; X-ray crystal structure of the 2 : 1 addition product

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