Studies on the origin of dihydrofurans from .alpha.-diazocarbonyl compounds. Concerted 1,3-dipolar cycloaddition vs. nonsynchronous coupling in the copper chelate catalyzed reactions of .alpha.-diazodicarbonyl compounds with electron-rich olefins

Abstract

The copper chelate catalyzed thermolysis of alkyl 2-diazo-3-oxobutyrate (1) and of 3-diazo-2,4-pentanedione (2) in the presence of several vinyl ethers to give 4-(alkoxycarbonyl)-and 4-acyl-2,3-dihydrofurans is used to probe the mechanism of this transformation in terms of the concerted 1,3-dipolar cycloaddition of the metaloxocarbene complex vs. the initial formation of the cyclopropane followed by 1,3 sigmatropic rearrangement to the heterocycle. Evidence is presented in favor of a third possibility, namely, that of a nonsynchronous stereospecific addition of the metal-carbene to the olefinic substrate to account for the formation of cyclopropanes, dihydrofiirans, and products of apparent allylic C-H insertion and cyclopropane structural isomerization from a common intermediate. This mechanism is supported by a study of the addition of lb to benzo[6]furan.The chemistry of photolytically and thermally generated a-ketocarbenes involves the processes of cyclopropane formation, insertion, rearrangement, and dimerization.1,2 The reactions of alleged metal-carbene complexes derived from reaction of an a-diazocarbonyl system with a transition-metal salt or chelate resemble those of the free carbene, which has given rise to such interpretations as singlet-like and triplet-like "carbenoid species".1 2*In addition to the usual cyclopropanes, such heterocyclic compounds as oxazoles,9"6 dioxoles,6® lactones,6b,c furans,7-10 and(1) (a) Kirmse, W. "Carbene Chemistry", 2nd ed; Academic Press:

Dihydrofurans from α-diazoketones due to facile ring opening – cyclization of donor–acceptor cyclopropane intermediates

“…This influence is usually only observed with a-diazoketones, as a-diazoesters are less nucleophilic. Dihydrofurans have also been obtained from copper-catalyzed reactions of diazomalonates and related diazoketones with various vinyl ethers (27,28). These subtle effects are illustrated by the finding that dihydrofuran 63 (9%) was the minor product (cyclopropane 83%) in the Rh,(OAc), catalyzed reaction of a-diazopropiophenone (60) with ethyl vinyl ether, but 64 was the major product (40%) with 2-methoxypropene (6).…”

Compostos alfa-diazo carbonílicos: uma estratégia atraente para a síntese orgânica

“…A preparação do diazomalonaldeído 25 (55) foi desenvolvida por Arnold e colaboradores a partir da diazotação do α-amino malonaldeído (54). Este único método de obtenção do diazo composto 55, envolve várias etapas (Esquema 12).…”

“…Os únicos produtos isoláveis nesta síntese são os sais diperclorato 52 e monoperclorato 53. Na hidrólise alcalina de 53 obtém-se o α-aminomalonaldeído (54) in situ. Esta substância, além de não ser um composto comercial, somente é estável em solução alcalina.…”

“…Esta substância, além de não ser um composto comercial, somente é estável em solução alcalina. A etapa de diazotação do α-aminomalonaldeído (54) é realizada na própria solução alcalina. No complexo mecanismo proposto pelos autores 26 para as obtenções dos sais 52 e 53, a formamida N,N-dissubstituída 50 reage com oxicloreto de fósforo levando inicialmente a um complexo eletrofílico de Vilsmeier-Haack.…”

“…Em algumas reações não é possível isolar-se o ciclopropano intermediário obtendo-se diretamente o diidrofurano 3q,r . No Esquema 22 estão apresentados vários diidrofuranos 83 49 , 85 50 , 87 51 , 89 52 , 91 53 e 93 54 obtidos a partir de reações com enóis éteres cíclicos e os diidrofuranos 95 52 , …”