Wolff rearrangement initiation

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

Different rearrangements were observed in other cases. Thus, Maas22 reported that when photolyzed in benzene the polysilyldiazoketone 180 gave the isomeric ketene 181, the product of a Wolff rearrangement (a 1,2 carbon-to-carbon rearrangement) of the initially formed carbene 182 (Eq. 57). The isomeric bis-silylketene 183 was not observed, but the siloxa-tene 184 was also a product of the reaction. 

The formal relationship between cyclopropenone and an a,a -biscarbene of a ketone (R—C—CO—C—R ) initiated investigations on photolytic and Ag-catalyzed decomposition of a, a -bisdiazo dibenzyl ketone (49) (Trost50 ). Indeed, diphenyl-cyclopropenone was formed in addition to other products (52 and tolane) derived from it furthermore, products resulting from solvent insertion and Wolff rearrangement of the monocarbene 50 were isolated (51) ... 

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

Fig. 14.29. Preparation of an a-diazoketone (compound E) from a ketone (A) and subsequent Wolff rearrangement of the a-diazoketone. Initially, A is transformed to give the enolate B of its a-formyl derivative. In a Regitz diazo group transfer reaction, this will then be converted into the a-diazoketone E. Ring contraction via Wolff rearrangement occurs and the 10-membered cyclic diazoketone C rearranges in aqueous media to give the nine-membered ring carboxylic acid E via the ketene D.

Fig. 11.24. Mechanisms of the photochemically initiated and Ag(I)-catalyzed Wolff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazotation 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 an excited carbene C are formed in the photochemically initiated reaction. The excited carbene usually relaxes to the normal ketocarbene E, and this carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclopropene A. The [l,2-]-shift of B also leads to the ketene G.

The same ketene G is obtained if the Wolff rearrangement is initiated photochemically. 

Yniujnm hrtfrfqii )in f tutifltar fra carbonyl group. The initial product of what is known as the Wolff rearrangement is a ketene, which " - " - cannot be isolated but is hydrolysed to the ester in the work-up. Wolff rearrangement is a typical... 

This ester is one carbon atom short of the full side chain of grandisol, so an Arndt-Eistert reaction was used to lengthen the chain by one atom. First, the ester was converted into the diazoketone with diazomethane and, then, the Wolff rearrangement was initiated by formation of the carbene with a Amdt-Eistert chain extension of ester silver compound at the Ag(II) oxidation state. 

Wolff rearrangements, involving shifts of alkyl groups, are effectively intramolecular insertions into C-C bonds. Carbenes will also insert into other bonds, especially O-H and N-H bonds, though the mechanism in these cases involves initial attack on the lone pair of the heteroatom. 

Cyclobutanones. Irradiation of 1 in the presence of cyclohexene produces the cyclobutanone 2 rather than the cyclopropane 3, as originally believed. This reaction involves a Wolff rearrangement of the initially formed carbene (a) to a ketene (b), which undergoes regio- and stereoselective cycloaddition to the olefin. 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

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