摘要:

SECTION C Organic Chemistry Overcrowded Molecules. Part V.l Stereochemistry and Stereospecif ic Photochemical Rear rangernent Reaction of I ,2- B isd i phenylrnet hylene- 3-methylindane By H. G. Heller" and K. Salisbury, Edward Davies Chemical Laboratory, University College of Wales, The stereochemistry of 1.2-bisdiphenylmethylene-3-methylindane is discussed, and evidence is presented that this severely overcrowded molecule undergoes photochemical ring closure by only one of the possible conrotatory modes, followed by a [I ,5] hydrogen shift, to give 4b.5-dihydro-I 1 -methyl-5,5,1 O-triphenylbenzo[b]fluorene having the methyl group anti to the 4b-hydrogen. Aberystwyth THE n.m.r. spectra of 1,2-bisdiphenylmethylene-3,3-di- methylindane (I; R = Me) and one of its photochemical rearrangement products, 4b,5-dihydro-l1,1 l-dimethyl- 5,5,10-triphenylbenzo[b]fluorene (I1 ; R = Me) a sug- gested a method not only of investigating the stereo- chemistry of 1.,2-bisdiphenylmethylene-3-methylindane (I; R = H), but also of determining whether its photo- chemical ring closure is concerted, and, if so, of ascertain- ing the influence of steric effects on the mode of cyclisa- tion.We have suggested 3 that molecular overcrowding in diene (I ; R = Me) is relieved by out-of-plane bending and twisting of the exocyclic double bonds, as well as by in-plane bending of these bonds away from each other, since the strong steric interaction between phenyl groups B and C cannot be relieved to any appreciable extent by their rotation or by puckering of the rigid indane ring.When phenyl group B overlaps phenyl group C, bond distortions cause phenyl group A to shield the methyl group projecting forward from the plane of the system [see (I; R = Me)] and to deshield the other, as demonstrated by n.m.r. studies which show methyl group absorptions at z 9.0 and 8.3. l,Z-Bisdiphenylmethylene-3-methyIindane (I ; R = H) was synthesised in the same manner as the dimethyl diene (I; R = Me).2 Its n.m.r. spectrum shows the methyl group absorption as a doublet centred at z 84, indicating that phenyl group A projects towards the 3-hydrogen and deshields the methyl group, i.e. the arrangement is as expected, with minimum interaction between phenyl group A and the 3-methyl substituent. A Dreiding molecular model of this diene shows that phenyl groups B and C overlap and lie in planes almost parallel to that of the indane nucleus, and that in the preferred conformation phenyl groups A and D are at about 45" to the plane of the indane nucleus, if steric interactions between (i) phenyl group A and the 3-hydro- Part IV, H.G. Heller and K. Salisbury, J . Clzem. SOC. (C), H. G. Heller, D. Auld, and K. Salisbury, J . Chem. SOC. (C), 1970, 399. 1967, 1084. gen and an ortho-hydrogen of phenyl group B and (ii) phenyl group D and the 7-hydrogen and an ortho- hydrogen of phenyl group C, are to be at a minimum. The proposed conformation of phenyl group D is sup- ported by the n.m.r. spectrum of the diene, which shows a one-proton doublet centred at T 3.6, attributed to the 7-hydrogen. This high-field absorption is characteristic of other 1,2-bi~diphenylmethyleneindanes,~~~ and similar shielding of the 1- and 8-hydrogens of 9-diphenylmethyl- enefluorenes has been rep~rted.~ Ph \ \ / 11,511 'h2 tm, qp Ph Ring closure of 1,2-bisdiphenylrnethylene-3-methyl- indane (I; R = H), followed by a [1,5] hydrogen shift, could give rise to two 4b,5-dihydro-l l-methyl-5,5,10-tri- phenyl-1 lH-benzo[b]fhorenes, one having the methyl group mati to the 4b-hydrogen (IIa; R = H), and 3 H.G. Heller, D. Auld, and K. Salisbury, J . Chem. SOC. (C), 1967, 2457. 4 E. D. Bergmann, M. R. Rabinowitz, and I. Agranat, J . Chem. SOC. (B), 1967, 1281.874 J. Chem. SOC. (C), 1970 the other having a syrt arrangement (IIb; R = H). A Dreiding model shows that the methyl group is shielded by one of the 5-phenyl groups only when it is anti to the 4b-hydrogen.The anti-isomer (IIa; R = H), m.p. 209-210", is the sole product from the photochemical rearrangement of the diene (I; R = H). Its structure follows from the close similarity of its characteristic U.V. spectrum to that of the dimethyl derivative (11; R = Me), and its n.m.r. spectrum, which shows the strongly shielded methyl group as a doublet centred at 7 9.85. The n.m.r. spectrum of the dimethyl derivative (11; R = Me) shows signals for the shielded methyl group anti to the 4b-hydrogen (T 9.6), and the syn-methyl group (T 8~8).~ A Dreiding molecular model of diene (I; R = H) indicates that several modes of ring closure are not feasible because of the severe steric interactions between phenyl groups B and C which would be involved (e.g.disrotatory mode v, and conrotatory modes x and y). Other modes of ring closure (e.g. disrotatory mode u and conrotatory modes w and z) are comparatively free from steric interactions. Cyclisation by mode w, followed by a [1,5] hydrogen shift, gives the artti-isomer (IIa; R = H), the sole product of the reaction. This photochemical conrotatory ring closure is in accord with the Woodward- Hoffmann rules for electrocyclic reactions.5 As the syfi-isomer (IIb; R = H) could not be detected in the reaction, it is assumed that ring closure by disrotatory mode zt does not occur. The photochemical reaction of diene (I; R = Me) yields the dimethyltriphenylbenzofluorenes (I1 ; R = Me) and (111; R = Me) in 70 and 20% yields respec- tively, after chromatographic separation.2 The reason why ring closure by conrotatory mode a is less easy for diene (I; R = H) than for diene (I; R = Me) is of interest.We tentatively suggest that the energy barrier t o rotation of phenyl group A past a 3-methyl substituent should be approximately the same for both dienes, but because of the greater steric interaction between phenyl group A and a 3-methyl substituent in dime (I; R = Me), the ground state energy of this compound is higher, and hence its activation energy for ring closure by mode z is lower than that for diene (I; R = H). As dienes (I; R = H) and (I; R = Me) are stable up to 300°, their thermal rearrangement reactions were not investi- gated. EXPERIMENTAL U.V. spectra were measured for solutions in n-hexane with a Unicam SP 500 spectrometer and n.m.r.spectra for solutions in deuteriochloroform with a Perkin-Elmer R12 (60 MHz) spectrometer. 1,2-BisdiphenyEmeihy~ene-3-upzebhylindane (I ; R = H) .- Diphenylketen (1.6 g.) and 2-diphenylmethylene-3-methyl- indan-l-one (1-2 g.) were heated under nitrogen at 140" for 14 hr. The resulting yellow glass, in benzene, was chroma- tographed on alumina (Camag 507-C-1) and developed with light petroleum in the dark to give a yellow band which yielded 1,2-bisdiphenyZunethyEene-3-methyZiazdane (1.5 g.) , yellow crystals, m.p. 199-201" (from ethanol) (Found: C, 93.4; H, 6-3. C,,H,, requires C, 93.9; H, 6.1%). Its U.V. spectrum [Lz 278, 335, and 342 nm. (log E 4.41 , 4.00, and 4-00)] resembled that of 1, Z-bisdiphenylmethylene- 3,3-dimethylindane.2 The n.m.r.spectrum showed T 2.5- 3.3 (23H, complex m, aromatic), 3.55 (lH, d, J 8 Hz, shielded 7-proton), 6-5 (lH, q, J 7 Hz, 3-proton), and 8.4 (3H, d, J 7 Hz, Me). The diene (I; R = H) (0.5 g.) in light petroleum (600 ml.) was exposed (20 hr.) to 366 nm. radiation from a 125 w mercury vapour discharge lamp, until the yellow colour of the solution disappeared. The product, after removal of solvent, showed only one spot on t.1.c. and had a U.V. spectrum almost identical with that of the pure anti-isomer of 4b,6-dihydro-l l-methyl-5,5,1 O-triphenyl- 1 1H-benzo[b]- fluorene (IIa; R = H) m.p. 209-210°, which was obtained by recrystallisation of the product from benzene and light petroleum (Found: C, 93.75; H, 6-3. C,,H,, requires C, 93.9; H, 6.1%). Its U.V. spectrum [A,, 272, 290, and 294 nm. (log E 4-09, 3-79, and 3-81)] closely resembled those of 4b, 5-dihydro-5,5,1O-triphenyl-l1H-benzo[b] fluorene and its 11,ll-dimethyl derivative (IIa; R = Me).2 The n.m.r. spectrum showed T 24-3.5 (23H, complex rn, aromatic), 4.55 (lH, s, 4b-proton), 5.9 (lH, q, J 7 Hz, ll-proton), and 9-85 (3H, d, J 7 Hz, Me). [9/1702 Received, October 7th, 19691 6 R. B. Woodward and R. Hoffmann, Accounts Chem. Res., 1968, 1, 17.

ISSN:0022-4952    

DOI:10.1039/J39700000873

出版商:RSC    

年代:1970

数据来源: RSC