Wolff rearrangement competing reactions

The key step of the Amdt-Eistert Homologation is the Wolff-Rearrangement of the diazoketones to ketenes, which can be accomplished thermally (over the range between r.t. and 750°C, photochemically or by silver(I) catalysis. The reaction is conducted in the presence of nucleophiles such as water (to yield carboxylic acids), alcohols (to give alcohols) or amines (to give amides), to capture the ketene intermediate and avoid the competing formation of diketenes. 

Dirhodium(II)-catalysed reaction of a 3-indolyl a-diazo-/5-keto ester in the presence of hexanamide results in competing metal carbene N-H insertion and Wolff rearrangement (Scheme 85).128... 

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). 

The migratory aptitude of R in (11) varies widely with its structure (see Section 3.9.2.1), the shift of an alkoxy group being among the slowest. The formation of alkoxyketenes in the photolysis of alkyl diazoacetates is a fairly recent discovery. The major competing reactions of the carbene precursor are insertions into the C—H and O—H bonds of alcohols employed as solvents and ketene traps. The extent of Wolff rearrangement varies with structure ethyl diazoacetate (20-25%), phenyl diazoacetate (45-60%), and A -methyldiazoacetamide (30%). These reactions are of limited synthetic interest at present. 

For photochemically generated (2-biphenylcarbonyl)phenylcarbene, several competing intramolecular reactions are observed, namely cyclopropanation of an aromatic nucleus (followed by norcaradiene to cycloheptatriene tautomerization), Wolff rearrangement, C-H insertion, and a carbene-to-carbene rearrangement (see Houben-Weyl, Vol. E19b, pl282). 

Photolysis of a-diazo-jS-diketones showed initial formation of an a-oxo-ketene, which decarboxylated to an oxocarbene, then underwent a conventional Wolff rearrangement. a-Diazocarbonyl compounds in which the carbonyl group is part of an ester function can undergo the Wolff rearrangement, but conventional carbene reactions can compete more readily. 

Insertion into C—H bonds has also been reported using rhodium(II) catalysis, but this reaction competes with a Wolff rearrangement. 

Davis, J.R., Kane, P.D., Moody, C.J. and Slawin, A.M.Z. (2005) Control of competing N-H insertion and Wolff rearrangement in dirhodium(II)-catalysed reactions of 3-indolyl diazo-ketoesters. Synthesis of a potential precursor to the marine 5-(3-indolyl)oxazole martefragin A. The Journal of Organic Chemistry, 70, 5840-5851. 

This reaction was first reported by Semmler in 1892 and subsequently studied by Wolff in 1902 and Schroeter in 1911. It is the rearrangement of an oxime of an a,p unsaturated cyclohexenone (or known as the o, )0-unsaturated cyclohexenyl ketoxime) to an aromatic amine in a mixture of acetic anhydride and acetic acid saturated with HCl or HBr. Therefore, this reaction is generally known as the Semmler-Wolff aromatization," Semmler-Wolff rearrangement, or Semmler-Wolff reaction. Occasionally, it is also referred to as the Semmler-Wolff-Schroeter reaction. It should be pointed out that the Beckmann Rearrangement,and the fragmentation to nitriles often compete with the Semmler-Wolff aromatization. In addition, the Semmler-Wolff aromatization is obstructed by the substituents close to the reaction center. ... 

---