Isomerization metal catalysis

Vkiyl aHenes (44) are rearranged with heat or metal catalysis and photosensitized isomerization to produce the vitamin D triene (156—160). 

The metal catalysis method has been used for the preparation of simple enols, for example, by isomerization of allylic alcohols. These enols are stable enough for isolation (see p. 75), but slowly tautomerize to the aldehyde or ketone, with half-lives ranging from 40-50 min to several days. ... 

The mechanism for hydrosilylation in Figs. 6 and 7 clearly has much in common with suggestions regarding homogeneous transition metal catalysis for other processes involving olefins, such as hydrogenation, isomerization, the oxo reaction, and oligo- and polymerization. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

The metal catalysis method has been used for the preparation of simple enols, by isomerization of allylic alcohols, e.g.,71a... 

Enantioselective isomerization of olefins for preparation of optically active olefins has great synthetic potential with a long tradition [1] and the non-stereoselective version is probably the most intensively investigated reaction in transition metal catalysis [2]. In particular, stereoselective hydrogen migration in a-functionalized olefins, for example allyl alcohols and allylamines [3], affording optically active aldehydes, ketones and amines, is part of the standard repertoire of enantioselec-... 

The first example of fully aqueous metal catalysis of olefin isomerization was reported by Grubbs et al. in 1994 [2]. These authors adopted [Ru(H20)6](tos)2 (tos = p-toluenesulfonate) [8] as a catalyst, which is highly active for the ring-opening polymerization of strained cyclic olefin. Both allylic alcohol and allylic ethers undergo isomerization in the presence of [Ru(H20)6](tos)2. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

There has recently been an upsurge in the research into the role of metals in the isomerization of oxiranes. This research is in part of a synthetic nature and in part of theoretical significance in that the aim is the understanding of the stereochemistry and mechanism of metal catalysis. 

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). 

Camurati, I. Fait, A. Piemontesi, F. Resconi, L. Tartarini, S. Transfer and Isomerization Reactions in Propylene Polymerization with the Isospecific, Highly Regiospecific raz-Me2C(3-/-Bu-l-Ind)2ZrC12/MAO Catalyst. In Transition Metal Catalysis in Macromolecular Design ACS Symposium Series Boffa, L. S., Novak, B. M., Eds. American Chemical Society Washington, DC, 2000 Vol. 760, 174. 

The first successful achievements using asymmetric homogeneous transition metal catalysis were obtained in the asymmetric hydrogenation of alkenes24 25, This method has been successfully used in many synthetic applications (Section D.2.5.1.)26-29. In addition, chirally modified versions of the transition metal catalyzed hydrosilylation of olefins and carbonyl compounds (Sections D.2.3.1. and 2.5.1.) and olefin isomerization (Section D.2.6.2.) have been developed. Transition metal catalyzed asymmetric epoxidation constitutes one of the most powerful examples of this type (Section D.4.5.2.). 

Catalysts which contain reduced transition metal clusters besides acid sites are able to catalyze reactions that are not observed on catalysts exposing one type of site only. The reaction network is inadequately described by models which assume only additivity of catalytic functions and shuttling of intermediates between sites. There is strong evidence that metal clusters and Bronsted sites form metal-proton adducts. These act as "collapsed bifunctional sites" all alkane isomerization steps can take place on such sites during one single residence of the adsorbed molecule. At low temperature, adsorption in a mode reminiscent of a carbenium ion can suppress pure metal catalysis. 

---