J. Am. Chem. SOC.1981, 103, 2310-2317
2310
+ +
Substituent Effects in [2a 2a 201 Thermal Decarbonylation of Cage Ketones. Remarkably Effective Elongation of Strained C-C Bond by Through-Bond Coupling Kazunobu Harano," Takashi Ban,lPMasami Yasuda,la Eiji 6sawa,lb and Ken Kanematsu*la Contribution from the Institute of Synthetic Organic Chemistry, Faculty of Pharmaceutical Sciences, Kyushu University, Fukuoka 81 2, and the Department of Chemistry, Faculty of Sciences, Hokkaido University, Sapporo 060, Japan. Received July 28, 1980
Abstract: Strained pentacyclic ketones (5a-d) were prepared in high yields by photoinduced [4 + 217~cycloadduct of N-(ethoxycarbony1)azepine with substituted cyclopentadienones. The structure of one of the cage ketones (Sa) was confirmed by X-ray analysis to be 1,2-diphenyl-5,7-bis(methoxycarbonyl)-ll-(ethoxycarbonyl)- 1 l-azapentacyclo[5.5.0.02~5.03~'2.04~8]dodeca-9-en-6-one. The C,(Ph)-C,(Ph) bond of this molecule is extraordinarily long (1.657 (5) A). Neither mechanical strain nor conventional through-bond interaction of phenyl T systems was satisfactory to interpret this elongation. We propose that the heightened u level and lower u* level of the prestrained C& bond, originally caused by the deformed carbon skeleton of 5, lead to efficient mixing with the phenyl ?r orbitals. The enhanced "through-bond" interactions involving strained u bond were demonstrated with combined molecular mechanics and MNDO calculations of model structures including 1,4-diphenylbicyclo[2.2.0]hexane. Also unusual in the observed structure of Sa is the conformationof methoxycarbonyl group adjacent to keto group, wherein three carbonyl dipoles align in nearly syn-parallel fashion. Compounds S a 4 decarbonylate rapidly at 180 O C to give tricyclic trienes 6a-d in quantitative yields. The thermally labile, long CI-C2 bond of 5 is suggested to play a key role in the ring opening. Decarbonylation rate increased 100-fold by changing the methyl group adjacent to carbonyl of 5a to alkoxycarbonyls (5d,e). This rate enhancement was explained in terms of frontier orbital and steric theories.
In their pioneering work on the utilization of solar energy through valence isomerization between the tricyclic diene 2 (Ar = p-tolyl; R = CH,; X = (CH& n = 1-3) and the strained-cage molecule 3, Mukai and his co-workers2 prepared 2 by novel thermal decarbonylation of pentacyclic ketone 1 (Scheme I). They recorded two remarkable features of the decarbonylation reaction: (1) one of the broken u bonds must be doubly substituted with Ar groups and (2) the reaction proceeds smoothly when X is a three-carbon bridge. The role of Ar group appeared to stabilize the biradical transition state and the longer X bridge to induce higher strain in the C(Ar)-C(Ar) bond.* However, both molecular mechanic3 and MIND0/32a calculations agree in that the effect of inserting a three-carbon bridge upon the strain of the CI-C2 bond of unsubstituted 1 (Ar = H ) as a model is minimal. In the course of our studies on the synthetic design by logical assembling of molecules in the frontier-orbital-controlled pericyclic reactions of h e t e r ~ p i n e s we , ~ ~prepared a series of pentacyclic ketones 5a-e related to 1. They also undergo rapid decarbonylation to afford 6. X-ray analysis revealed an unusually long bond in one of the ketones. This observation led to the recognitioq of a general bond-lengthening effect by the "through-bond" mechanism in strained C(Ph)-C(Ph) systems related to 5.
Results Synthesis of Strained Pentacyclic Ketones. Irradiation of benzene solutions of the anti-endo [2 + 4]a cycloadducts (4a-e), which were obtained from 2,5-disubstituted-3,4-diphenyl-cyclopentadienones and (N-ethoxycarbonyl)azepine," gave pentacyclic ketones (5a-e) in high yields (Scheme 11). The results of photochemical synthesis of 5 are summarized in Table I and the N M R spectra of 5 in Tables I1 and 111. (1) (a) Kyushu University. (b) Hokkaido University. (2) (a) Mukai, T.; Yamashita, Y. Tetrahedron Lett. 1978, 357. (b) Tezuka, T.; Yamashita, Y.; Mukai, T. J. Am. Chem. SOC.1976, 98, 6051. (3) Osawa, E.; Aigami, K.; Inamoto, Y. J . Chem. Soc., Perkin Trans. 2
1979, 181. (4) (a) Harano, K.; Yasuda, M.; Ban, T.; Kanematsu, K. J . Org. Chem. 1980,45,4455. (b) Mori, M; Hayamizu, A.; Kanematsu, K. J . Chem. SOC.,
Perkin Trans. I, in the press.
Scheme I
1
2
3
4a-e
Sa-e
6a-e
Scheme I1
a, R=CH,;b, R=CH,CH,;c, R=CH,CH,CH,;d, R=CO,CH,; e, R = CO,CH,CH,
Table I. Photolyses of Anti-Endo [ 4 + 21 n Cycloadducts IR v(C=O), uva compd Sa
Me
mp,"C 209-210
5b
Et
171-172
5c
Pr
175-177
5d
C0,Me 218-219
5e
C0,Et
a
R
206-209
yield, cm-' 9% CHC1, Nujol 98 1702 1700 1758 1753 96 1700 1703 17'52 1750 96 1702 1703 1752 1750 98 1708 1700 1740 1730 1788 1799 97 1712 1700 1740 1730 1788 1799
%m" nm emX 255 (sh) 4300 256(~h) 4400 256(sh) 4600 256(sh) 5200 258(sh) 5500
Ethanol.
The four saturated and two olefinic protons in the azepine fragment of 5 appear well resolved, and their coupling constants were determined by the double-resonance technique. The keto groups of 5d and 5e are more congested than those of Sa-c; the
0002-7863/81/1503-2310$01.25/00 1981 American Chemical Society
'I'hermal Decarbortylation of Cage Ketones Table 11.
' H NMR" Spectra of Cage Ketones
chem shift, 6 (J,Hz) compd ___- -1.08, 1.14 (s, 2CH,, 6 H), 1.32 (t,CH,, 3 H), 2.20 5a (dd, H,, 1 H , J = 8.0, 4.4), 3.14 (dd, H,, 1 H , J = 7.6), 4.19 (dd, H,, 1 H, J = 5.2), 4.23 ((1, CIl,, 2 €I), 4.72 (d, H,, 1 H , J = 7.6), 5.54 (bs, HI,, 1 H), 6.78 (dd, H,,, 1 H), 6.92-7.06 (m, Ph-H, 10 H) 0.72-0.96 (In, 2CII,, 6 H), 1.25-1.88 (m, 2CH,, 4 H), 5b 1.32 (t, CH,, 3 H),2.38 (dd, H,, 1 H , J = 4.8, 8.8), 3.37 (dd, H,, 1 H , J = 7.0), 4.06 (dd, H,, 1 H , J = 6.0), 4.25 (q, CH,, 2 H), 4.80 (t, H,, 1 H , J = 9 . 2 ) , 5.52 (bs, HI,, 1 H), 6.80 (ni, HI,, 1 H), 6.92 -7.08 (In, Ph-H, 10 H) 0.81 (m, 2CH,, 6 H), 1.00-1.70 (m, 4CH,, 8 H), 1.34 5c (t,CH3,3H),2.37(dd,H,,1H,J=4.8,8.8),3.36 (dd,I14,1H,J=6.9),4.07(dd,H,,11i,J=5.2),4.25 (q,CH2,2H),4.78(t,H,, 1H,J=9.2),5.52(bs,Hl,, 1 H), 6.80 (m, HI,, 1 H), 6.92-7.10 (m, Ph-H, 10 H) 5d 1.32(t,CH3,3H),2.96(dd,H,,1H,J=4.8,8.0),3.52, 3.54(s,2CH,,6H),3.91(dd,H4,1H,J=7.6),4.23 ((1,CH,, 2 H), 4.36 (dd, H,, 1 H , J = 6.4), 4.70 (t, H,, 1H,J~7.7),5.76(bd,Hl,,1H),6.78(d,H,,,11-I), 6.80-7.20 (m, Ph-H, 1 0 H) 5e 0.85, 0.92 (t, 2CH,, 6 11), 1.31 (t, CH,, 3 H), 2.96 (dd, H,, 1 H , J = 4.9, 8.4), 3.90 (dd, H,, 1 H), 3.98 ((1, 2CH,, 4 H), 4.24 (q, CH,, 2 H), 4.37 (dd, 11,, 1 H), 4.62 (t, H,, 1 H , J = 8.8), 5.72 (b d, HI,, 1 H!, 6.78 (d, HI,,1 H), 6.92-7.18 (m, Ph-H, 10 H)
" CDCI,.
Abbreviations: d , doublet; s, singlet; m, multiplet;
b, broad; q, quartet.
Table 111. 13CNMR Spectra" of Cage Ketones compd
chem shift, ppm
5a
9.49, 12.01 (CH,), 14.47 (CH,), 43.42 (C-4), 45.23 (C-3), 45.89 (C-8), 48.52, 50.86 (C-1, C-2), 53.85 (C-12), 57.48,66.97 (C-5, C-7), 62.52 (CH,), 105.35 (C-9), 153.28 (CO,), 214.10 (C-6) 8.44, 9.55 (CH,), 14.47 (CH,), 16.58, 19.69 (CH,), 39.90 (C-2), 41.31 (C-3), 45.59 (C-8), 50.04,53.67 (C-1, C-2), 54.32 (C-12), 60.76, 67.27 (C-5, C-7), 62.52 (CH,), 105.64 (C-9), 153.28 (CO,), 214.10 ((2-6) 14.47 (CH,), 14.59, 14.88 (CH,), 17.52, 18.46, 25.84. 29.12 (CH,), 40.49 (C-4), 41.84 (C-3), 45.53 (('-8), 50.21, 53.03 (C-1, C-2), 54.32 (C-12), 60.41,67.21 (C-5, C-7), 62.52 (CH,), 105.76 (C-g), 153.28 (CO,), 214.10 (C-6) 14.41 (CH,), 41.78 (C-4), 43.36 (C-3), 45.70 (C-8), 51.97 (CH,), 52.44 (C-12), 53.77,57.83 (C-I, C-2), 62.81 (CH,), 65.21, 68.03 (C-5, C-7), 101.13 (C-9), 153.22 (CO,), 166.57, 167.63 (CO,), 197.58 ('2-6) 13.48, 13.71 (CH,), 14.41 (CH,), 41.43 (C-4), 43.30 (C-3), 45.47 (C-8), 51.86, 57.54 (C-1, C-2), 52.44 (C-12), 60.29, 60.76 (CH,), 62.64 (CH,), 65.10, 67.62 (C-5, C-7), 102.42 (C-9), 153.10 (CO,), 165.99, 167.11 (CO.). 197.69 (C-6)
5b
5C
5d
5e
J . Am. Chem. Soc., Vol. 103, No. 9, 1981 2311 Table IV. Final Positional Parameters (X l o 4 )of Nonhydrogen Atoms with Estimated Standard Deviations in Parentheses X
Y
7049 (2) 7947 (2) 8570 (2) 7783 (3) 7365 (2) 6099 (2) 6137 (2) 6701 (2) 6921 (3) 7439 (3) 7901 (2) 8072 (2) 5321 (2) 6562 (2) 5622 (3) 5206 (3) 5706 (3) 6623 (3) 7051 (3) 8465 (3) 7860 (3) 8336 (4) 9406 (4) 10011 (3) 9547 (3) 7714 (3) 7287 (2) 8563 (2) 8950 (4) 5013 (2) 4114 (2) 5168 (2) 4195 (3) 8254 (2) 8728 (2) 801 1 (2) 8482 (4) 8465 (7)
7994 (3) 7315 (3) 8042 (3) 6817 (3) 6004 (3) 5713 (3) 6988 (3) 6928 (3) 7988 (3) 9231 (3) 9802 (2) 9025 (3) 4808 (2) 8377 (3) 7463 (3) 7786 (4) 9019 (4) 9932 (4) 9613 (3) 7443 (3) 6620 (3) 6761 (4) 7712 (4) 8526 (4) 8398 (4) 4940 (3) 4134 (3) 4984 (2) 3968 (4) 7073 (3) 6175 (2) 8295 (2) 8501 (4) 11094 (3) 11650 (2) 11659 (2) 13064 (4) 13523 (5)
4623 (4) 4457 (4) 3706 (4) 1896 (4) 2626 (4) 2444 (4) 28 10 (4) 1369 (4) 1156 (4) 2364 (4) 4125 (3) 4673 (4) 2194 (3) 6139 (4) 6081 (5) 7508 (5) 8989 (5) 9057 (4) 7643 (4) 6008 (4) 6417 (4) 7887 (5) 8956 (5) 8566 (5) 7091 (4) 2162 (4) 2385 (4) 1439 (3) 907 (5) 2546 (4) 1621 (3) 3361 (3) 2906 (6) 5370 (4) 6874 (3) 4679 (3) 5834 (6) 4825 (7)
Table VI. Interatomic Distances (A) with Estimated Standard Deviations in Parentheses 1.657 (5) 1.558 (5) 1.552 (5) 1.505 (5) 1.557 (5) 1.560 (5) 1.480 ( 5 ) 1.550 (5) 1.530 (5) 1.566 (5) 1.528 (5) 1.5 19 (5) 1.493 (6) 1.549 (5) 1.193 (5) 1.566 (5) 1.500 (5) 1.493 (6) 1.310 (6) 1.402 ( 5 ) 1.464 (5) 1.370 (5)
C(14)-C(15) C(14)-C(19) C(15)-C(16) C(16)-C(17) C(t7)-C(18) C(18)-C(19) C(20)C(21) C(20)-C(25) C(21)
