Carbenes ketocarbenes

In this review an attempt is made to discuss all the important interactions of highly reactive divalent carbon derivatives (carbenes, methylenes) and heterocyclic compounds and the accompanying molecular rearrangements. The most widely studied reactions have been those of dihalocarbenes, particularly dichlorocarbene, and the a-ketocarbenes obtained by photolytic or copper-catalyzed decomposition of diazo compounds such as diazoacetic ester or diazoacetone. The reactions of diazomethane with heterocyclic compounds have already been reviewed in this series. ... 

When the Wolff rearrangement is carried out photochemically, the mechanism is basically the same, but another pathway can intervene. Some of the ketocarbene orieinallv formed can undergo a carbene-carbene rearrangement, through an oxi-... 

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

Insertion of a ketocarbene moiety into a C—O bond of orthoesters is normally performed with catalysis by BF3 Et20. Copper(II) trifiouromethanesulfonate was found to be a similarly efficient catalyst also, at least in some cases, whereas Rh2(OAc)4 was much less suited to promote this transformation l60). Besides the C/O insertion product 343, the alcohol insertion product 344 and, in reactions with ethyl diazoacetate, the formal carbene dimers were obtained. In agreement with BF3 EtzO, Cu(OTf)2 did not bring about insertion into a C—O bond of trimethyl... 

Ketocarbenes generated from diazoketones give two main type of reactions. The first one is the conventional carbene reaction, i.e. intramolecular insertion into a C—H or C—C bond, as applied in the synthesis of a... 

The linear polymerization of Scheme 7.15 represents an unusual case of diazoacetophenone oxidation. For instance, on the action of copper oxide, diazoacetophenone gives ketocarbene, which is involved in typical carbene reactions such as dimerization, addition to olefins, and insertion in the 0-H bonds of alcohols. If the amine cation-radical is used as an oxidant instead of copper oxide, only the polymer is formed. The ketocarbene was not observed despite careful searches (Jones 1981). 

Fig. 9.1 Generating an oxo carbene (a ketocarbene ) from a labelled diazo ketone sometimes leads to a ketene in which the label is scrambled. This indicates that a species with the symmetry of oxirene is formed...

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 transient ketocarbene is also able to effect both intramolecular and intermolecular H-abstraction. The fact that a triplet state of the carbene is often implicated follows from observations that sensitized photolyses frequently enhance the abstraction pathway. Abstraction and radical combination results in the formal insertion of the carbene into the C—H (or S—H, N—H, etc.) bond. Insertions may also occur into C—C, C—O, C—S, C—Hal, etc. bonds the mechanisms are often not known, but a singlet state of the ketocarbene is probably involved in many cases. Reactions at soft basic centers (e.g. —S or —Br ) generally proceed by the preliminary formation of an ylide. The multiplicity of the possible pathways is illustrated in Scheme 7, equation (14) (photolysis in dioxane affords only 27, 57% and 28, 43%), and Scheme 8. ... 

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