Transition metal catalysis rearrangements

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

Thermal rearrangement of XII (90-120°) gave primarily the benzene product (XIV) and acid-catalyzed isomerization gave approximately equal quantities of Dewar-benzene (XIII) and benzene (XIV). Transition-metal catalysis, in contrast, operated selectively at —30° on the cyclobutane ring, yielding mainly Dewar-benzene (XIII) (Table I). 

Investigations which concern the mechanisms of skeletal rearrangements of saturated hydrocarbons induced by heterogeneous transition metal catalysis are of great interest for industrial applications, e.g. for petroleum reforming processes The developments in this field were reviewed recently by Hejtmanek, and by Maire and Garin , who focussed on the probable reaction mechanisms which include bond-shift and cyclic mechanisms for the skeletal isomerization of acyclic alkanes. Scheme 1 summarizes the... 

In 2009, Lambert reported a mild, efficient, and stereoselective synthesis of fused cyclopentenes via Mgl2-promoted isomerization of activated VCPs. The VCPs were prepared by Pd-catalyzed cyclopropanation of 1,3-dienyl p-ketoesters in moderate to high yields and good diastereoselectivities. Typical standard reaction conditions for the VCP-CP rearrangement (i.e., pyrolysis, transition metal catalysis, standard Lewis acid catalysis) were found to be ineffective for these substrates. However, the use of 1.5 equiv. of Mgl2 led to conplete conversion, providing bicyclic cyclopentenes in high yields fScheme 11.15L... 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

The transition metal catalysis of this tandem (cascade) reaction involving [3,3]-sigmatropic rearrangement (since it would be the carbonyl oxygen of the acetate forming a new bond to the p-acetylenic atom) followed by the formal Myers-Saito cyclization of allene 3.524 results in the formation of aromatic ketones 3.525 in up to 94% yield (Scheme 3.29) [265]. 

Under transition metal catalysis, different types of skeletal rearrangement can take place indeed, where the original connectivity along the enyne chain can be maintained or not, as a function of the substrate and the catalyst used [1-4]. High selectivity and efficiency are often realized. The field has been exhaustively reviewed in recent papers [5-8], Surprisingly, asymmetric variants of these reactions [9] are comparatively underdeveloped, in spite of their potential synthetic utility for the construction of chiral cyclic moieties. 

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