Metal-catalyzed rearrangements of allylic esters

Abstract

It has been shown that Pd(CH3CN)2Cl2 catalyzes the rearrangement of allylic esters. Carbamates rearrange more rapidly than acetates which in turn rearrange more rapidly than carbonates. Allylic carbamates are more rapidly isomerized by Pd(CH3CN)2Cl2 than by Hg(OCOCF3)2. The rearrangement proceeds well except in cases where the central carbon of the allylic system is fully substituted.In contrast to the reported rearrangement of allylic carbamates by Hg2+ and previous reports of Pd2+-catalyzed rearrangements of allylic esters, this work showed the latter are extremely E specific; thus, E allylic acetates yield equilibrium mixtures free of Z allylic isomers, under conditions in which Z allylic acetates are unreactive. This E specificity is applied to stereospecific 1,3-diene synthesis from bis E,Z-allylic acetates.

Improved Synthesis of Cyclic Tertiary Allylic Alcohols by Asymmetric 1,2‐Addition of AlMe3 to Enones

“…In contrast, the Ireland protocol proceeds under alkaline conditions. For this transformation, allylic alcohol 2 a was acetylated with Ac 2 O to afford compound 7 in 80 % yield (Scheme ) 26. Formation of the respective trimethylsilyl enol ether at −78 °C, [3,3]‐sigmatropic rearrangement in refluxing toluene and hydrolysis of the initially formed silyl ester furnished desired carboxylic acid 8 in 66 % yield 27.…”

“…For this transformation, allylic alcohol 2 a was acetylated with Ac 2 O to afford compound 7 in 80 % yield (Scheme 3). [26] Formation of the respective trimethylsilyl enol ether at À78 8C, [3,3]-sigmatropic rearrangement in refluxing toluene and hydrolysis of the initially formed silyl ester furnished desired carboxylic acid 8 in 66 % yield. [27] Compound 8 is a versatile chiral building block and racemic derivatives have previously been prepared either by related rearrangements [28a, b] or by a Tsuji-Trost reaction that starts from diethyl malonate and 3-methylcyclohex-2-enyl acetate (the regioisomer of compound 7).…”

Enzymatic cleavage and formation of cyanohydrins: a reaction of biological and synthetic relevance

“…The butyrate was then easily converted into the unsaturated nitrile 12 using a [3,3]-sigmatropic rearrangement catalysed by bis(acetonitrile)dichloropalladium 13 (Scheme 7). This rearrangement proceeded, as previously described, 44,45 in a stereospecific manner with complete retention of configuration and enantiomeric excess. Hydrolysis of the butyrate yielded the alcohol 14, which was then protected with the tert-butyldiphenylsilyloxy functionality to yield the nitrile 15.…”

Palladium(II)‐Catalyzed Isomerization of (Z)‐1,4‐Diacetoxy‐2‐Butene: Solvent Effects

“…The [3,3] sigmatropic rearrangement of allylic acetates through an 1,3-acetoxonium ion type intermediate is usually admitted with Pd II catalysts, [3,[5][6][7] especially when PdCl 2 (RCN) 2 is used with solvents such as THF, PhMe, and CH 2 Cl 2 . [8][9][10][11] Another mechanism was proposed for when PdCl 2 in AcOH or Pd(OAc) 2 alone was used; this mechanism involves the addition of Pd II and acetate across the double bond followed by the regeneration of the catalyst from β-elimination of the acetate, specifically, an acetoxypalladation-deacetoxypalladation sequence. [9,12] It was also envisaged that the isomerization of linalyl acetate could imply the cleavage of the C-OAc bond to afford an intermediate having an allylic cation moiety and the 6,7-C=C bond coordinated to (AcOPdCl 2 ) -.…”

“…[8][9][10][11] Another mechanism was proposed for when PdCl 2 in AcOH or Pd(OAc) 2 alone was used; this mechanism involves the addition of Pd II and acetate across the double bond followed by the regeneration of the catalyst from β-elimination of the acetate, specifically, an acetoxypalladation-deacetoxypalladation sequence. [9,12] It was also envisaged that the isomerization of linalyl acetate could imply the cleavage of the C-OAc bond to afford an intermediate having an allylic cation moiety and the 6,7-C=C bond coordinated to (AcOPdCl 2 ) -. [13] To the best of our knowledge, the Pd II -catalyzed isomerization of 1,2-diacetoxy-3-ene and 1,4-diacetoxy-2-ene type compounds has only been reported from 6,7-diacetoxy-8-tetradecene [11] and 1-substituted-1,4-diacetoxy-4-phenyl-2-butenes.…”