Aspects of Strain in Organic Chemistry

Abstract

Approximately 100 methylidene-1H-cyclopropa[b]naphthalenes (25) have been prepared from disilanes (45) and (72) by use of five methods devised to provide otherwise scarcely accessible derivatives. In many cases, the procedures now give increased product yield, e.g. olefin (141) from (45) and acetophenone is now available in 85% yield, cf. 39% previously, and nitrile (129l) is obtained in 67% yield rather than 22%. However, parent methylidenecyclopropanaphthalene (132) was not attained by any procedure. The ketocyanine dyes (166) give rise to novel alkylidenecycloproparenes (167) with extended conjugation. The cis,cis to cis,trans rotameric ratios of the products (167b, e, g) have been determined by comparison of their measured dipole moments, relative to those of their ring-locked analogues (170b, e, g). Attempts to prepare p-trifluoromethylphenylmethylidene derivative (129k) fail and instead a Tishchenko-like mechanism is operative in which intervention by, and subsequent ejection of, the 3,6-dimethoxy-e-silyl anion (73) is proposed in the formation of p-trifluoromethylbenzyl p-trifluoromethylbenzoate (183). The structure of (183) has been confirmed by X-ray analysis and base-hydrolysis to acid (186) and alcohol (187). An analogous product, ester (198) is obtained from 2-thiophenecarboxaldehyde under the same conditions. With tetracyclone (206), disilane (45) provides silanol (207) from protonation of the Peterson oxyanion (208) and not the exocyclic olefin (204) as steric factors dictate the product outcome. Use of Ag(I) to effect dimerisation of diphenylmethylidenecycloproparene (127a) is unsuccessful and provides ethoxystyrene (218) or ethanone (219) from solvent-dependent reactions, instead. With the monophenyl derivative (126a), alkyne (223) and ethanone (224) are attained in yields of 62 and 21%, respectively. These results negate the use of this reagent to give cross-conjugated pentacenes by analogy with cyclopropanaphthalene (5). Disilane (45) also does not dimerise under analogous conditions. Attempted oxidative demethylation of aryl 1,4-diethers (129)-(131) with cerium(IV) fails to provide the alkylidenecyclopropanaphthoquinones (76). A comprehensive dipole moment study employing data collected from c. 100 alkylidenecycloproparenes has been performed, for which the electron-donating character of the cycloproparenyl moiety is unequivocally established. Crystal structure data for cycloheptatrienylidene diether (163) show the seven-membered ring double bond to be bent out of the plane of the cycloproparene even more (45°) than that of its less polar benzo analogue (249) (28°). The molecular polarisation, predicted by theory to be in the direction of the cycloheptatrienylidene is confirmed as such and the relative twisting is proposed as a function of resistance to an 8π7C antiaromatic subunit. The polarity of aryl-substituted alkylidenecycloproparenes is a function of exocyclic substitution pattern, molecular planarity, and the relative inductive vs. mesomeric properties of remote substituent(s) R. Use of Hammett σp+ constants with µ indicates when the ambiphilic cycloproparenyl nucleus changes from electron sink to electron source. The n.m.r. spectroscopic features of the alkylidenecycloproparenes have been examined and the impact of the remote substituent evaluated. Plots of σp+ against δc for each range of p-aryl-substituted derivatives (126)-(131) and (167) show a systematic shift in δc for each of the cycloproparenyl carbon atoms in accord with changes in R. Conversely, these molecules show no linear shift of σp+ with δH for H8 and H2/7. Finally, the first olefin derivative of cyclobuta[a]cyclopropa[d]benzene "rocketene" (14) has been prepared. The recently published synthesis of stannylcyclobutabenzene (279) fails under the conditions specified. Detailed studies have led to a now reproducible synthesis of this compound. The cyclobutabenzene (279) is transformed into rocketene in accord with the prescribed preparation, that itself, has been converted into the 4,4-disilane (292). When treated with benzophenone and base, (292) provides the first methylidenerocketene (295).