Carbenes Wolff rearrangments

The rearrangement of acylcarbenes to ketenes is called the Wolff rearrangement (18-8). A few rearrangements in which carbenes rearrange... 

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-... 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

The main synthetic application of the Wolff rearrangement is for the one-carbon homologation of carboxylic acids.242 In this procedure, a diazomethyl ketone is synthesized from an acyl chloride. The rearrangement is then carried out in a nucleophilic solvent that traps the ketene to form a carboxylic acid (in water) or an ester (in alcohols). Silver oxide is often used as a catalyst, since it seems to promote the rearrangement over carbene formation.243... 

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. 

Photoelimination of nitrogen from diazoketones is complicated by Wolff rearrangement of the intermediate carbene, as shown below for diazoaceto-phenone<35) ... 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Apart from the widely studied silver(i) A-heterocyclic carbenes, Stoltz and Beauchamp made the first report on the gas-phase synthesis of silver(i) Fischer carbenes from the loss of N2 in various diazo malonates upon electrospray ionization and subsequent collisional activation.118 The carbenes generated were capable of undergoing multiple Wolff rearrangements and loss of CO (Scheme 18). 

The one-step mechanism, depicted in path a, consists simply of a 1,2-shift of an ortho carbon. While this process is an all-carbon version of the Wolff rearrangement, the bond order of the migrating bond is substantially greater than 1.0. Hence this would represent an unprecedented reaction of carbenes. 

The sensitized reaction yields the cyclopropanes 67 as the only product, whereas direct photolysis results in a Wolff-rearrangement yielding the cyclo-pentaneester 77 AWolff-rearrangement is only given by the singlet carbene... 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

A density functional study has been made of the competition between Wolff rearrangement and [1,2]-H shift in /S-oxy-a-diazocarbonyl compounds. Silver-catalysed decomposition of a-diazoketones (88 n = 0), derived from A-tosyl a-amino acids in methanol, gave rise to mixtures of products of Wolff rearrangement (89) and direct insertion of the carbene into the NH bond (90). The -amino acid derived species (88 n = 1) gave rise to products of Wolff rearrangement. 

Figure 18. Diazonaphthoquinone-novolac resist. The novolac (Novolak) matrix resin is prepared by acid catalyzed copolymerization of cresol and formaldehyde. The base insoluble sensitizer, a diazohaphthoquinone, undergoes photolysis to produce a carbene which then undergoes Wolff rearrangement to form a ketene. The ketene adds water which is present in, the film, to form a base soluble, indenecarboxylic acid photoproduct.

Also, the Wolff rearrangement of diazo Meldmm s acid 33, studied by DFT at the B3PW91/6-311+G(3df,2p) level of theory, proved to be a concerted process because the product of the photochemical or thermal decomposition in methanolic solution was the ketoester 34 (Scheme 3) while the expected products of the singlet carbene 35, for example 36, were not detected <2003JA14153, 2005CJC1382>. 

During the photolysis of a-diazophosphonium salts, the skeletons of the reactants are preserved. Excluding a plausable Wolff rearrangement, only the O/H insertion product of the carbene with the solvents can be obtained. Diazophosphonium tetrafluoroborate (6), formed by UV irradiation in methanol under a nitrogen atmosphere and subsequent anion exchange with sodium tetraphenylborate, yields the stable (2-methoxy-2-oxopropyl)-triphenylphosphonium tetraphenylborate (7) (equation 4)31. 

---