Ketene-carbene mechanism, rearrangements

The actual product of the reaction is thus the ketene, which then reacts with water (15-3), an alcohol (15-5), or ammonia or an amine (15-8). Particularly stable ketenes (e.g., Ph2C=C=0) have been isolated and others have been trapped in other ways (e.g., as P-lactams, 16-64). The purpose of the catalyst is not well understood, though many suggestions have been made. This mechanism is strictly analogous to that of the Curtius rearrangement (18-14). Although the mechanism as shown above involves a free carbene and there is much evidence to support this, it is also possible that at least in some cases the two steps are concerted and a free carbene is absent. 

Fig. 14.26. Mechanisms of the photochemicatty initiated and Ag(I)-catatyzed Wotff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazota-tion of the diazoketene D in the presence of catalytic amounts ofAg(I). E and F are converted into G via a [1,2]-shift of the alkyl group R1. N2 and a carbene C are formed in the photochemically initiated reaction. The carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclo-propene A. The [l,2-]-shift of B also leads to the ketene G.

The ester group in II is suggestive—although it is not a proof—of the intermediacy of a ketene, and ketene production in diazocarbonyl chemistry usually implies a Wolff rearrangement. The construction of a three-carbon chain on the other side of the ketone is a confirmation of this prediction. In turn, the Wolff rearrangement requires an a-keto carbene precursor that is the fate of diazo compounds exposed to ultraviolet light (wavelength lower than 3200 A). All this is translated into the mechanism depicted in Scheme 43.1. 

If the ketene was produced via a Wolff rearrangement at low temperature then the carboxylic acid c.ster. should certainly form as a result of the room temperature exposure. In view of this we conclude that the mechanism for the vacuum electron beam exposure docs not involve a Wolff rearrangement. Instead we propose that-a carbene formed by loss of Aj from the diazoketone plays a central role in the chemistry that ensues after electron beam exposure. The propo.sed mechanism for the electron beam induced chemistry discu.ssed thus far is summarized in the scheme below. 

By 1950 five distinct mechanisms had been suggested to account for the formation of the major products of the Favorskii rearrangement. Four involved epoxide, ketene, enol, and carbene intermediates. A fifth mechanism related to the benzylic acid rearrangement was also proposed. Then, in 1951 Loftfield isolated two esters with identical isotope distributions at their a and P carbons from treatment of a radiolabeled, cyclic a-chloroketone with an alkoxide. These two products suggested a symmetrical intermediate, leading Loftfield to postulate the existence of a cyclopropanone along the reaction pathway. ... 

The mechanism of the Wolff rearrangement has been proposed as either the concerted loss of nitrogen and rearrangement to a ketene, or the stepwise loss of nitrogen to give an intermediate carbene (or oxirene) that then rearranges to give ketene (Scheme 1). Carbenes have been detected both directly and indirectly, but the oxirene intermediate has been much more elusive and only indirect evidence is available. 

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