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Mechanism of the Cycloaddition of Bicyclo[2.1.0]pent-2-ene a Non-concerted Homo-Diels-Alder Reaction

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Mechanism of the Cycloaddition of
Bicyclo(2.1.0lpent-2-ene: a Non-concerted
Homo-Diels-Alder Reaction **
By Frank-Gerrit Kliirner * and Manfred Naumann
Dedicated to Professor Wolfgang R. Roth
on the occasion of his 60th birthday
Roth and Martin
have interpreted the endo-stereospecific course of the [,2 + r2]-cycloaddition of 4-phenyl-4H1,2,4-triazole-3,5-dione
9 a to the bicyclo[2.1.O]pentane derivative 1as characteristic of a two-step process. As first step,
the authors postulate, in line with MO considerations,['] an
electronically favored backside attack of 9 a at the strained,
central cyclopropane bond of 1 with formation of the reactive diradical intermediate 2, which then closes in the second
step to give the stable cycloadduct 3. An analogous endostereospecificity is also observed in the addition of olefins
such as maleic anhydride, fumaronitrile and maleonitrile to
of 9 a to tricycl0[]heptaneJ4] and of 9 b to bicyclo[l .I .O]b~tane.[~1
In contrast, exostereospecificity is observed in the case of the homo-DielsAlder reaction of tetracyanoethylene 10, N-phenylmaleimide
or fumaronitrile with the 6-methylhomofurans 4a, b,'61 in
which a concerted [(,2, + n2s)+ n2,]-cycloaddition could be
involved due to the stereospecific cis-addition of the dienophiles (fumaro- and maleonitrile). Bicyclo[2.1 .O]pent-2-ene6
reacts with 10 predominantly in the sense of a homo-DielsAlder reaction to give an adduct of type 7l7] and with 9a, b
in the sense of a [,2 + x2]-cycloaddition to give adducts of
type 8.Ls1We report here on the surprising stereochemical
We have solved the problem of which side the dienophile
attacks the bicyclo[2.1 .O]pentenesystem by using the known
bicyclopentene esters 6a, bCg1
as model compounds. The separated diasteromers 6 a and 6 b each react quantitatively and
stereospecifically with 10, even at 3 "C (overnight in toluene),
to give the formal homo-Diels-Alder adducts 7 a and 7 b
respectively. The structure and the stereochemical arrangement of the substituents on C-4 in the diastereomers 7a. b
could be determined 'H-NMR spectroscopically. With the
the hydrogen atom 5-H cis to the
methyl group on C-4 of 7 b absorbs at higher field (6 = 3.82)
than the corresponding trans hydrogen atom 5-H of 7 a
(6 = 4.30), in agreement with the hydrogen atoms 1,4-H, cis
and trans to the methyl group on C-5 of 6 b (6 = 2.03) and 6 a
(6 = 2.38).['] This assignment can be checked independently
by means of NOE 'H-NMR difference spectra. Upon irradiation of the resonance frequency of the 4-methyl signal, the
increase in intensity of the signal of 5-H (relative to 3-H) in
the spectrum of 7 b brought about by the nuclear Overhauser
effect is, as a result of the smaller steric {CH,..-H}distance
(2.90 A), about double that of the corresponding signal in
the spectrum of 7a, in which 5-H and the 4-CH3 grou are
trans to each other, and their steric separation is 3.84 .I' 'I
lla, b
7a. b
a . X = C H , , Y=CO,CH,; b. X=CO,CH,. Y = C H ,
+x=x *
9a: R = C6HS
4a,5a R'=CH,, Rz=H;4b,5b- R'=H,RZ=CH,
course of the bicyclo[2.1.O]pentene cycloaddition, which
shows that the homo-Diels-Alder reaction also takes place in
two steps in this case.
[*] Prof. Dr F.-G. Kllmer. Dip].-Chem. M. Naumann
Fakultit fur Chemie der Universitdt
Postfach 102 148. D-4630 Bochum 1 (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
Ciwm. In[. Ed. EngI. 29 (1990) No. 9
The cycloaddition of 10 to the bicyclo[2.1.O]pentenes 6a,
b proceeds stereospecifically endo with respect to the cyclopropane ring. Thus, the stereochemical course of this reaction is exactly the opposite to that of the homo-Diels-Alder
reaction of the homofurans 4a, b and consistent with that of
the [,2 + z2]-cycloaddition of the bicyclo[2.1 .O]pentane system (e.g. I). It may be assumed therefore that the formal
homo-Diels-Alder reaction of the bicyclo[2.1.O]pentene proceeds, just like the cycloaddition of the bicyclo[2.1.O]pentane, in two steps via a diradical intermediate 11a, b.['']
To check this assumption we have investigated the stereochemical course of the reaction with respect to the olefinic
components using the dicyanofumaric and maleic acid esters
12a, c [ ' ~ as
] model compounds. Reaction of a 93:7 mixture
of bicyclopentene 6 and 1,3-cyclopentadiene with 12a (3 h,
O T , THF) led to the formation of three products in the
'H-NMR spectroscopically determined ratio of 5 5 : 30: 15.
The by-product formed in 15 YOyield is identical with the
Diels-Alder adduct 14a of 1,3-cyclopentadiene and 12a. The
two main products, of which the isomer formed in 55 % yield
could be enriched by fractional crystallization to 80 %,
could, according to a comparison of their spectral data["]
with those of the tetracyanoethylene adduct 7,17]be the bicyclo[3.2.0]heptene derivatives 13a, b. An unequivocal stereochemical assignment of the substituents on C-6 and C-7 is
not possible here. The reaction of6(purity > 99%) with 12c
leads at 0°C (48% conversion of 12c after 15 min) to a
mixture of seven products in the ratio 31:22:19:11:10:5:3.
Of the three main products, two, which are formed in 31 and
1 9 % yield, are identical with 13a and 13b, the products of
the reaction of 6 with 12a. The spectral data of three byproducts (formed in 5, 10 and 3 % yield) are identical with
those of 14a, c and d, the Diels-Alder adducts of 1,3-cy-
VCH Vrrlu~.~gesellsrhafr
mhH. 0-6940 Wrinheim,I990
S 3.50+.25/0
clopentadiene and 12a and 12c re~pectively."~~
We assign to
the remaining products (22 and 11 %) the structures of the
still missing cis-diastereomers 13c, d. No significant cis-trans
isomerization 12c -+ 12a ( < S o / , ) takes place under the
reaction conditions." It is demonstrated therefore, that
the cycloaddition of bicyclo[2.1 .O]pentene 6 proceeds nonstereospecifically, as expected in the case of a two-step process.
b: R1=R4=C0,CH,, R Z = R 3 = C N ;
a: R ~ = R ' + = C NRZ=R3=C0,CH,;
c: R' = R3 =CO,CH,, R2 = R4 =CN; d: R1= R3 = CN, R2 =R4 =C02 C H 3
Finally, there remains the question of which factors are
responsible for the different course of the homo-Diels-Alder
reaction of homofuran 4 and bicyclopentene 6 . The two systems have very different strain energies. In the homofuran 4,
the strain energy is essentially determined by the cyclopropane ring, whereas in the bicyclopentene 6 it is composed
of the three- and four-membered ring strain as well as the
negative resonance energy, which is attributed to the antiaromatic character of 6 as homocyclobutadiene.r'61 In the
transition state of the two-step process, the central cyclopropane bond could be opened further than in the concerted
homo-Diels-Alder reaction. The reaction of 6 profits in particular from this bond opening, since the total strain of the
bicyclopentene is released in one step.
Received: April 4, 1990 [Z 3894 IE]
German version: Angew Chem. 102 (1990) 1097
CAS-Registry numbers6, 5164-35-2; 6a, 71215-51-5; 6b, 71276-51-2;7a, 128575-90-6; 7b, 128657.249; IZa, 35234-87-8; 12c, 101342-44-3; 13a, 128575-91-7; 13b. 128657-25-0;
13c. 128657-26-1; 13d, 128657-27-2; 14a, 128575-92-8; 14c. 128657-28-3; 14d,
128657-29-4; 1,3 . cyclopentadiene, 542-92-7.
2,3-(CO,CH,),), 6.33 (m, 2 H ; 5,6-H2); of 14d: 6 = 1.81, 2.40 (2m. 2 H ;
7,7-H2).3.72(m,2H; 1,4-H,),3.85(s.6H;2,3-(CO,CH3),),6.67(m,2H;
1111 The average CH, ...H distances were taken from MMX force field calculations: J. J. Gajewski, K. E. Gilbert, Serena Software 1988.
[12] Owing tothehigh polarityoftheolefins lOand 12a,c thereactlveintermediates of type 11 could also have dipolar structures.
[13] C. 1. Ireland, K. Jones, J. S. Pizey, S . Johnson, Synrh. Commun. 6 (1976)
185: T. Gotoh. A. B. Padias, H. K . Hall, Jr., J. Am. Chem. Soc. 108 (1986)
I141 Since the Diels-Alder reaction of 1,3-cyclopentadiene with 12c ( O T , 3 h,
THF) yields only the cis-adducts 14c and 14d in the ratio 87:13, the
rruns-adduct 14a must be formed directly from 6 in the reaction of 6 with
12c. This result indicates that the [,2 +.2]-cycloaddition also proceeds
non-stereospecifically and thus, as expected, in two steps.
[lS] After 2 h the ratio of unchanged 12c to 12a is 90:10 and after 3 h 77:23.
In a control experiment the extent of cis-trans isomerization 12c -+ 12a
without addition of 6 was determined as 28% after 3 h at O'C. It can
therefore be ruled out that 6 catalyzes the cis-trans isomerization
I 2 c - 12a. From the time-dependence of seven measurements (between 15 and 180 min) the product ratio of the reaction o f 6 with 12c was
extrapolated to the time t = 0. [13a:13b:13~:13d:14a:14~:14d],-,
29- 18:24: 12: 3.1 1 :4.
[16] W. R. Roth. F.-G. Klirner, H.-W. Lennartz. Chem. Ber. 113 (1980) 1818.
Nickel-Catalyzed Cyclodimerization of
Hexapentaenes: [4]Radialenes and [S]Radialenones
with Cumulated Double Bonds **
By Masahiko Iyoda,* Yoshiyuki Kuwatani, Masaji Oda,
Yasushi Kai,* Nobuko Kaneshisa, and Mobutami Kasai *
Although cumulenic double bonds"] have a wide variety
of potential reactive sites, which can lead to novel cychc
dimers, trimers, and oligomers, great difficulty in controlling
the reactivity has prevented the utilization of cumulenes in
organic synthesis. We recently reported the nickel-catalyzed
cyclooligomerization of [3]cumulenes (butatrienes) IZ1 and
the cyclodimerization of [5]cumulenes (hexapentaenes) 13] to
give various types of radialenes, and demonstrated the synthetic utility of this type of reaction. We report here
the nickel-mediated synthesis of novel [4]radialenes and
based on the same synthetic strategy.
Tetramethyl- and tetra-tert-butyl[5]cumulenes dimerize
thermally to give octamethylcyclododeca-1,3,7,9-tetrayne
and tetrakis(di-ter~-butylvinylidene)cyclobutane.~71However, nickel-catalyzed cyclodimerization of tetraarylhexapentaene yields 1,2-bis(diarylmethylene)-3,4-bis(diarylpropadieny1idene)cyclobutane as the head-to-head dimer.13]
Thus, the known thermal and nickel-catalyzed dimerizations
of [5]cumulenes occur regioselectively. In order to investigate
the difference in the reactivity of these double bonds, we
carried out reactions of the [S]cumulenes 5 with nickel catalysts.
The [S]cumulenes 5 a-c were prepared according to the
procedure outlined in Scheme 1. Ethynylation of the ketones
W. R. Roth, M. Martin, Tetrahedron Lett. 1967, 4695.
F. S. Collins, J. K. George, C. Trindle, J. Am. Chem. Soc. 94 (1972) 3732.
Review: P. G. Gassman. Acc. Chem. Res. 4 (1971) 128.
W. R. Roth, F:G. Klarner, W. Grimme, H. G. Koser, R. Busch, B.
Muskulus, R. Breuckmann, B. P. Schoiz, H.-W. Lennartz, Chem Ber. 116
(1983) 2717.
[ S ] M. H. Chang, D. A. Dougherty, J. Am. Chem. Soc. 104 (1982) 1131.
[6] E-G. Klarner, D. Schroer, Chem. Ber. 122 (1989) 179.
[7] J. E. Baldwin, R. K. Pinschmidt, Jr., Tetrahedron Left. 1971, 935.
[S] W. Adam, A. Beinhauer, 0 . De Lucchi, R. J. Rosenthal, Tetrahedron Lett.
24 (1983) 5727.
[9] E-G. Klarner, F. Adamsky, Chem. Ber. 116 (1983) 299.
[lo] 'HNMR(400MHz,CDC13)of7a:6 = 1.77(s,3H;4-CH3),3.75(s,3H;
OCH3),4.15(dt, lH;l~H.5(1,5)=6.0,3(1,2)=J(1,3) =2.0Hz),4.30(d,
1 H ; 5-H), 6.12 (dd, 1 H ; 2-H, J(2,3) = 5.5 Hz), 6 29 (ddd, 1 H; 3-H,
43.5) i1 Hz);of 7 b : 6 = 1.40 (s, 3 H ; 4-CH3), 3.81 (dt, 1 H ; 5-H,
I-H, J(1,2) = J(1,3) = 2.0 Hz), 5.98 (ddd, 1 H ; 2-H, J(2,3) = 6.0 Hz), 6.58
(ddd, 1 H ; 3-H). 'H NMR (400 MHz, [DJacetone) of 13a: d = 2.78 (m,
2 H ; 4,4-H,), 3.90. 3.94 (2s. 6 H ; 6,7-(CO,CH,),), 3.96 (m, 2 H ; l,5-H2),
5.89, 6.14 (21% 2 H ; 2,3-H2); of 13b: 6 = 2.78 (m, 2H: 4,4-H,), 3.78 (td,
1 H; 5-H, J(1,5)= 4 4 , s ) = 7.6, J ( 4 , S ) = 2.6 Hz). 3.89, 3.97 (2s. 6 H ; 6,7(CO,CH,),), 4.21 (m, 1 H ; I-H), 5.79, 6.11 (2m 2 H ; 2,3-H2); in the 'HNMR spectrum ofthe mixture in the reaction of 6 with 12c. the new signals
at 6 = 6.25, 5.91 and 6.05, 6.03 can be assigned to the olefinic hydrogens
2,3-H; of 14a: 6 = 1.92,2.20 (2m, 2 H , 7,7-H,, J = 10.0 Hz), 3.70 (m, 2 H ,
1,4-H2), 3.88,3.96(2~,6 H ; 2.3-(C0,CH3),). (2dd. 2 H : 5,6-H2);
of14c:6 = 1.91,2.01(2m,2H;7,7-H,).3.75(m,2H;1,4-H,),3.77(~,6H.
@> VCH VerlagsgeselkchofimhH, D-6940 Weinheim. 1990
Prof. Dr. M. Iyoda, Y Kuwatani, Prof. Dr. M. Oda
Department of Chemistry, Faculty of Science
Osaka University
Toyonaka, Osaka 560 (Japan)
Prof. Dr. Y Kai, Prof. Dr. N. Kasai, N Kanehisa
Department of Applied Chemistry, Faculty of Engineering
Osaka University
Yamadaoka, Suita, Osaka 565 (Japan)
This work was supported by CIBA-GEIGY Foundation (Japan) for the
Promotion of Science and by the Ministry of Education, Science and
Culture, Japan. (Grant-in-Aid for Scientific Research No. 63540396).
0570-OX33190jO909-1062S 3 50+ 2.510
Angew. Chem. Int. Ed. Eng/ 29 (1990) N o . 9
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