Reaktionen vinyloger Oxycarbene mit Methanol

Abstract

Reactions of Vinylogous Oxycarbenes with Methanol The carbenes 16 and 30 were generated in methanol/sodium methoxide by photolyses of 2‐methylchromone tosylhydrazone (14) and 3,5‐dimethoxy‐4,4‐dimethyl‐2,5‐cyclohexadienone tosylhydrazone (28), respectively. Neither 16 nor 30 accept methanol at the carbenic site to give products of a formal O – H insertion. Carbene 16 affords 2‐methoxy‐2‐methyl‐2H‐1‐benzopyran (19) by way of the benzopyrylium ion 17. The same ether was obtained from the independently prepared perchlorate of 17. Protonation of 30 was followed by demethylation of the delocalized carboxonium ion 31 to give 5‐methoxy‐6,6‐dimethyl‐2,4‐cyclohexadienone (33) as the only product.

Δ3-1,3,4-Oxadiazolines.† Versatile sources of reactive intermediates

“…42 Insertion into OH bonds is probably stepwise also, at least for nucleophilic carbenes, with proton transfer from the hydroxy compound to the carbene as the first step, followed by ion-pair collapse. [43][44][45] The mechanism is a consequence of delocalization of electron density from the heteroatoms to the dicoordinate carbon of the carbene and to the oxy-or dioxymethyl cation developed at the transition states of such reactions (Scheme 12) for example. Oxycarbenes are nucleophilic 34,38-41,46-54 and the transition state (modelled with the product in Scheme 12) is stabilized by charge dispersal, relative to one from a more electrophilic carbene.…”

Novel Products from Thermolysis of 5,5-Dimethyl-2,2-diphenoxy-Δ3-1,3,4- oxadiazoline in the Presence of DMAD

“…The chemistry of nucleophilic carbenes continues to attract attention. − Nucleophilic carbenes are those in which one or more donor substituents at the carbene carbon feed electron density into the formally vacant orbital of the carbene singlet. Such donation can stabilize a carbene to the extent that it becomes persistent 6-8 and can give it nucleophilic properties, − Scheme . We were interested in replacing the substituents of the nucleophilic dialkoxycarbenes with aryloxy groups, not only to tune the nucleophilicity of the carbenes but also to look for new chemistry that might arise from reactions at the aromatic ring.…”