Buttressing effect in carbene chemistry. Effect of 3-alkyl groups on the reactions of 2-alkoxydiphenylcarbenes

Abstract

Irradiation of 2-methoxydiphenyldiazomethane 1a in diethyl ether at 10 °C gave 3-phenyldihydrobenzofuran 3a along with a small amount of the ether adduct 4a, which became the major product when the irradiation was carried out in the ether matrix at –196 °C. The reaction patterns were dramatically changed as an alkyl group was introduced into the 3-position of substrates 1. Thus, irradiation of 2-methoxy-3-alkyldiphenyldiazomethanes 1b–d in diethyl ether either at 10 °C or at –196 °C afforded the corresponding benzofurans 3b–d at the complete expense of the ether adducts 4b–d. The results are nicely explained in terms of the buttressing effect of the 3-alkyl group, which prevents the 2-methoxy group from lying in the plane of the phenyl ring in the precursor molecules and assists the methoxy group in rotating around the C–O bond toward the carbene centre after elimination of N₂. Generation of carbenes 2 in methanol at 10 °C produced O–H-insertion adducts 5 as the major product at the expense of benzofurans 3. The ether 5a was formed as a major product in the reaction of carbene 2a with the alcohol matrix at –196 °C, but the benzofurans 3 became the major product in the irradiation of substrates 1b–d in methanol matrix at –196 °C. The results are again explicable in terms of the buttressing effect of 3-alkyl substituents on the relative populations of the rotational isomers of the carbenes 2.

The effect of the 3-alkyl group on the reaction of 2,2′,5′-trimethoxydiphenylcarbenes 7 which form two kinds of intramolecular C–H-insertion products 8 and 9 were studied and the results are again discussed in terms of the buttressing effect.