Claisen rearrangement over the past nine decades.

Mercury(II) and palladium(II) salts have found broad applications as catalysts for [3,3]-sigmatropic rearrangements leading to formation of CO, CN, CS, and CC σ bonds. Increases in reaction rate are often very large (1010 – 1014 at 1 M catalyst concentration) and allow many previously difficult transformations to be conducted at or near room temperature, often with attendant increases in stereoselectivity and decreases in by-product formation. The mechanism of these catalyzed transformations is briefly discussed, and evidence is summarized to suggest that many follow a cyclization-induced rearrangement mechanism.

Mercury(II)‐ and Palladium(II)‐Catalyzed [3,3]‐Sigmatropic Rearrangements [New Synthetic Methods (46)]

Asymmetric O- and C-Alkylation of Phenols

Catalysis of the Claisen rearrangement

This article surveys variants of the Claisen rearrangement that are of general use in stereoselective organic synthesis. Recent results concerning the influence of steric and electronic parameters on transition state geometries are highlighted. The impact of the Claisen methodology in modern synthetic strategy and its application for the preparation of highly functionalized derivatives is exemplified.

7.2 – Claisen Rearrangements

Allyl aryl ethers which have no strongly electron attracting substituents undergo a charge-induced [3 s, 3 s] sigmatropic rearrangement in the prescence of 0.7 mole boron trichloride in chlorobenzene at low temperature, to give after hydrolysis the corresponding o-allyl phenols (Tables 1 and 2). The charge induction causes an increase in the reaction rate relative to the thermal Claisen rearrangement of ∼1010. With the exception of allyl 3-methoxyphenyl ether (5), m-substituted allyl aryl ethers show similar behaviour (with respect to the composition of the product mixture) to that observed in the thermal rearrangement (Table 3). The rearrangement of allyl aryl ethers with an alkyl group in the o-position, in the prescence of boron trichloride, yields a mixture of o- and p-allyl phenols, where more p-product is present than in the corresponding product mixture from the thermal rearrangement (Table 4). This ‘para-effect’ is especially noticeable for o-alkylated α-methylallyl aryl ethers (Table 5 ).



With boron trichloride, 2,6-dialkylated allyl aryl ethers give reaction products which arise, in each case, from a sequence of an ortho-Claisen rearrangement followed by a [1,2]-, [3,3]- or [3,4]-shift of the allyl moiety (Tables 6 and 7). Ally1 mesityl ether (80), with boron trichloride, gives pure 3-ally1 mesitol (95). From phenol, penta-ally1 phenol (101) can be obtained by a total of five O-allylations followed by three thermal and two boron trichloride-induced rearrangements.



The sigmatropic rearrangements of the ethers studied, using D- and 14C-labelled compounds, are collected in scheme 2; only the reaction steps indicated by heavy arrows are of importance.



With protic acids, there is a [3,3]-shift of the allyl group in 6-allyl-2,6-disubstituted cyclohexa-2,4-dien-l-ones, while with boron trichloride the [3,3]-reaction is also observed along with the much less important [1,2]- and [3,4]-transformations (Table 8). 4-Allyl-4-alkyl-cyclohexa-2,5-dien-1-ones give only [3,3]-rearrangements with boron trichloride (Table 9). As expected, the naphthalenone 112, which is formed by allowing boron trichloridc to react for a short time with allyl (1-methyl-2-naphthyl) ether (111), undergoes only a [3,4] rearrangement (Scheme 3).



Representations of how, in our opinion, the complex behaviour of allyl aryl ethers and allyl cyclohexadienones under the influence of boron trichloride, can be rationalized are collected together in Schemes 4 and 5.



In the last part of the discussion section, the steric factors leading to the appearance of the ‘para-effect’, are dealt with (Scheme 6).

Umlagerung von Allyl‐aryläthern und Allyl‐cyclohexadienonen mittels Bortrichlorid

Die Umwandlung von Allyl-arylathern in Allylphenole last sich nicht nur thermisch (Claisen-Umlagerung), sondern auch mittels Bortrichlorid in Chlorbenzol (O,7-l ,O Mol BCl3 pro l Mol Ather) durchfuhren.

R. Madeja

Papers

Umlagerung von Allyl‐aryläthern und Allyl‐cyclohexadienonen mittels Bortrichlorid

Umlagerung von allyl‐arylaethern und allyl‐cyclohexadienonen mittels bortrichlorid