Diphenylketene (2) undergoes [4 + 2] cycloadditions with the s-cis fixed diene (1) to give the dihydropyran (3) and the cyclohexenone (4)

Heigl, Ulrich W.

Mayr, Herbert

English

Herbert Mayr

Ulrich W. Heigl

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[4 + 2] Cycloadditions of diphenylketene with a highly substituted 1,3-diene

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1804 J. CHEM. SOC., CHEM. COMMUN., 1987 [4 + 21 Cycloadditions of Diphenylketene with a Highly Substituted 1,3-Diene Herbert Mayr* and Ulrich W. Heigl lnstitut fur Chemie, Medizinische Universitat zu Liibeck, Ratzeburger Allee 160, 0-2400 Liibeck I, Federal Republic of Germany Diphenylketene (2) undergoes [4 + 21 cycloadditions with the s-cis fixed diene (1) to give the dihydropyran (3) and the cyclohexenone (4). Ketenes undergo [2 + 21 cycloadditions with alkenes or 1,3-dienes to give cyclobutanones.1J Stepwise [4 + 21 cycloaddition reactions across the C=O double bond have been observed when donor (alkoxy and trimethylsiloxy) substituted 1,3-dienes react with alkyl-, aryl-, or halogeno- ketenes ,3  and when the electron deficient bis(trifluoromethy1)- ketene reacts with buta-1,3-diene.4 In some cases, the C=C double bond of diphenylketene has been found to act as a Ph dienophile towards a,P-unsaturated ketones, imines, and benzoquinone diimines.5 We now report the first Diels-Alder reaction of the C=C bond of a ketene with an alka-1,3-diene in which the usually occurring [2 + 21 process is sterically disfavoured. Compound (1) , prepared by heating 172-bis- (bromomethyl)-3,3,4,4,5,5-hexamethylcyclopentane~ with magnesium in tetrahydrofuran (THF) , has previously been reported to give 174-adducts with carbenes.7 When compounds (1) and (2) were heated in cyclohexane at 55 "C for 15 days, a 1 : 1 mixture (70%) of (3) and (4) was obtained, which was separated by medium pressure liquid chromatography [silica gel, hexane-ether (95 : 5)l.t In the t Compound (3): m.p. 128.5-129.5"C; 'H n.m.r.: 6 0.80 (s, 6H), 0.83 (s, 6H), 0.95 (s, 6H), 2.68 (m, 2H, 5-H2), 4.37 (m, 2H, 2-H2), 6.9-7.3 (m, 10 H); I3C n.m.r. (CDC13): 6 24.41 (t, C-5), 65.48 (t, C-2). Compound (4): m.p. 97.5-98.5 "C; 'H n.m.r. : 6 0.75 (s, 6H), 0.78 (s, 6H), 1.03 (s, 6H), 2.78 (s, 2H), 3.02 (s, 2H), 7.12-7.33 (m, 10H); l3C n.m.r. (CDCL,): 6 36.25,36.94 ( 2  t ,  C-2,5), i .r.  (KBr): 1705 cm-1 (C=O). J .  CHEM. soc., CHEM. COMMUN., 1987 1805 Ph .O (5) (6 )  presence of acid, compound (3) readily isomerises to give the conjugated diene ( 5 ) ,  and the solvents used for the n.m.r. spectroscopy of (3) have to be neutralised carefully. In several runs, the reaction of (1) with (2) yielded the 2 : 1 product (6) instead of (3). The structure of (6) was elucidated using the 1 T - W  coupling constants determined by the INADEQUATE pulse sequence.$ Compound (3) did not react with (2) in the presence of triethylamine or azoisobutyro- nitrile at 80°C but gave (6) when catalytic amounts of ZnC12 were present. Electrophilic catalysis, therefore, appears to be responsible for the formation of (6). When the reaction of (1) with (2) was carried out in j: An analogous structure was postulated for the [2 : 11 product from bis(trifluoromethy1)ketene and buta-l,3-diene (Yu. A. Cheburkov, N. Mukhamadeliev, and I.  L. Knunyautis, Tetrahedron, 1968, 24, 1341, but discounted later.4 benzonitrile instead of cyclohexane, the product ratio (3) : (4) increased from 1 : 1 to 3 : 1 while the reaction rate increased by a factor of ca. 6. Since diphenylketene (2), like (l), has only a small dipole moment, the small k(PhCN)/k(CbHl2) ratio argues against the formation of a dipolar intermediate in the rate determining step. We thank R. Koschinsky for carrying out the n.m.r. experiments, and the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financial support. Received, 6th July 1987; Corn. 938 References 1 R. Huisgen and P. Otto, Chem. Ber., 1969, 102, 3475. 2 R. B. Woodward and R. Hoffmann, Angew. Chem., lnt. Ed. Engl., 1969, 8,781. 3 J. C. Martin, P. G. Gott, V. W. Goodlett, and R. H. Hasek,J. Org. Chem., 1965, 30, 4175; J .  P. Gouesnard, Tetrahedron, 1974, 30, 3113; W. T. Brady and M. 0. Agho, J .  Heterocycl. Chem., 1983,20, 501. 4 D. C. England and C. G. Krespan, J .  Org. Chem., 1970,35, 3300. 5 R. Gompper, Angew. Chem., Int. Ed.  Engl., 1969, 8, 312; W. 6 H.  Mayr, E. Will, U. W. Heigl, and C. Schade, Tetrahedron, 1986, 7 H. Mayr and U. W. Heigl, Angew. Chem., lnt. Ed. Engl., 1985,24, Friedrichsen and H.-G. Oeser, Chern. Ber., 1975, 108, 31. 42, 2519. 579. 

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1804 J. CHEM. SOC., CHEM. COMMUN., 1987 
[4 + 21 Cycloadditions of Diphenylketene with a Highly Substituted 1,3-Diene 
Herbert Mayr* and Ulrich W. Heigl 
lnstitut fur Chemie, Medizinische Universitat zu Liibeck, Ratzeburger Allee 160, 0-2400 Liibeck I, Federal Republic of 
Germany 
Diphenylketene (2) undergoes [4 + 21 cycloadditions with the s-cis fixed diene (1) to give the dihydropyran (3) and 
the cyclohexenone (4). 
Ketenes undergo [2 + 21 cycloadditions with alkenes or 
1,3-dienes to give cyclobutanones.1J Stepwise [4 + 21 
cycloaddition reactions across the C=O double bond have 
been observed when donor (alkoxy and trimethylsiloxy) 
substituted 1,3-dienes react with alkyl-, aryl-, or halogeno- 
ketenes ,3  and when the electron deficient bis(trifluoromethy1)- 
ketene reacts with buta-1,3-diene.4 In some cases, the C=C 
double bond of diphenylketene has been found to act as a 
Ph 
dienophile towards a,P-unsaturated ketones, imines, and 
benzoquinone diimines.5 We now report the first Diels-Alder 
reaction of the C=C bond of a ketene with an alka-1,3-diene in 
which the usually occurring [2 + 21 process is sterically 
disfavoured. Compound (1) , prepared by heating 172-bis- 
(bromomethyl)-3,3,4,4,5,5-hexamethylcyclopentane~ with 
magnesium in tetrahydrofuran (THF) , has previously been 
reported to give 174-adducts with carbenes.7 
When compounds (1) and (2) were heated in cyclohexane at 
55 "C for 15 days, a 1 : 1 mixture (70%) of (3) and (4) was 
obtained, which was separated by medium pressure liquid 
chromatography [silica gel, hexane-ether (95 : 5)l.t In the 
t Compound (3): m.p. 128.5-129.5"C; 'H n.m.r.: 6 0.80 (s, 6H), 
0.83 (s, 6H), 0.95 (s, 6H), 2.68 (m, 2H, 5-H2), 4.37 (m, 2H, 2-H2), 
6.9-7.3 (m, 10 H); I3C n.m.r. (CDC13): 6 24.41 (t, C-5), 65.48 (t, 
C-2). Compound (4): m.p. 97.5-98.5 "C; 'H n.m.r. : 6 0.75 (s, 6H), 
0.78 (s, 6H), 1.03 (s, 6H), 2.78 (s, 2H), 3.02 (s, 2H), 7.12-7.33 (m, 
10H); l3C n.m.r. (CDCL,): 6 36.25,36.94 ( 2  t ,  C-2,5), i .r.  (KBr): 1705 
cm-1 (C=O). 
J .  CHEM. soc., CHEM. COMMUN., 1987 1805 
Ph .O 
(5) (6 )  
presence of acid, compound (3) readily isomerises to give the 
conjugated diene ( 5 ) ,  and the solvents used for the n.m.r. 
spectroscopy of (3) have to be neutralised carefully. 
In several runs, the reaction of (1) with (2) yielded the 2 : 1 
product (6) instead of (3). The structure of (6) was elucidated 
using the 1 T - W  coupling constants determined by the 
INADEQUATE pulse sequence.$ Compound (3) did not 
react with (2) in the presence of triethylamine or azoisobutyro- 
nitrile at 80°C but gave (6) when catalytic amounts of ZnC12 
were present. Electrophilic catalysis, therefore, appears to be 
responsible for the formation of (6). 
When the reaction of (1) with (2) was carried out in 
j: An analogous structure was postulated for the [2 : 11 product from 
bis(trifluoromethy1)ketene and buta-l,3-diene (Yu. A. Cheburkov, 
N. Mukhamadeliev, and I.  L. Knunyautis, Tetrahedron, 1968, 24, 
1341, but discounted later.4 
benzonitrile instead of cyclohexane, the product ratio (3) : (4) 
increased from 1 : 1 to 3 : 1 while the reaction rate increased by 
a factor of ca. 6. Since diphenylketene (2), like (l), has only a 
small dipole moment, the small k(PhCN)/k(CbHl2) ratio 
argues against the formation of a dipolar intermediate in the 
rate determining step. 
We thank R. Koschinsky for carrying out the n.m.r. 
experiments, and the Deutsche Forschungsgemeinschaft and 
the Fonds der Chemischen Industrie for financial support. 
Received, 6th July 1987; Corn. 938 
References 
1 R. Huisgen and P. Otto, Chem. Ber., 1969, 102, 3475. 
2 R. B. Woodward and R. Hoffmann, Angew. Chem., lnt. Ed. Engl., 
1969, 8,781. 
3 J. C. Martin, P. G. Gott, V. W. Goodlett, and R. H. Hasek,J. Org. 
Chem., 1965, 30, 4175; J .  P. Gouesnard, Tetrahedron, 1974, 30, 
3113; W. T. Brady and M. 0. Agho, J .  Heterocycl. Chem., 1983,20, 
501. 
4 D. C. England and C. G. Krespan, J .  Org. Chem., 1970,35, 3300. 
5 R. Gompper, Angew. Chem., Int. Ed.  Engl., 1969, 8, 312; W. 
6 H.  Mayr, E. Will, U. W. Heigl, and C. Schade, Tetrahedron, 1986, 
7 H. Mayr and U. W. Heigl, Angew. Chem., lnt. Ed. Engl., 1985,24, 
Friedrichsen and H.-G. Oeser, Chern. Ber., 1975, 108, 31. 
42, 2519. 
579. 


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[4 + 2] Cycloadditions of diphenylketene with a highly substituted 1,3-diene

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