Platinum Metals in the Catalytic Asymmetric Isomerization of Allylic Alcohols

Abstract

Past and recent advances in the metal-catalyzed asymmetric isomerization of allylic alcohols into carbonyl compounds are discussed in the present Highlight. Emphasis is placed on rhodium, ruthenium, and iridium; the only three metals that have proven successful to date for this most challenging transformation. Ç IntroductionThe metal-catalyzed asymmetric isomerization of primary and secondary allylic alcohols into the corresponding aldehydes and ketones®a reaction of both academic and industrial relevance®has attracted renewed interest in recent years. 1Because such transformations undergo refunctionalization upon isomerization, they are often described as internal redox processes and from a synthetic viewpoint, legitimately placed at the very top of the atom-economy scale (Scheme 1). 2Although a variety of transition metals displays interesting catalytic activity for the non-asymmetric isomerization of some allylic alcohols with TOF up to 62500, 3 the substrate generality remains a major limitation. In most cases, the catalytic activity tends to decrease as a function of the degree of substitution of the olefinic double bond and thus limits investigation of substrates with a prochiral double bond. Catalysts that are effective for the isomerization of primary allylic alcohols are less active for secondary allylic alcohols as is the converse. Furthermore, the lack of supporting organometallic chemistry and the limited number of thorough mechanistic studies using achiral catalysts constitute severe bottlenecks in the rationalized development of highly active and selective chiral catalysts for the related asymmetric isomerization reactions.The focus of the present Highlight is placed exclusively on chiral catalysts elaborated around rhodium, iridium, and ruthenium, three of the Platinum Metals which have been applied with various success to the asymmetric isomerization of primary and secondary allylic alcohols. Ç Primary Allylic AlcoholsThe asymmetric isomerization of primary allylic alcohols into chiral aldehydes is historically intimately linked to the related isomerization of allylic amines into chiral enamines. 46 The degree of refinement attained by this rhodium-catalyzed process in terms of activity, selectivity, generality, and practicality is exceptional and has helped defining standards of excellence; very few other catalytic asymmetric transformations have reached to date. In contrast, the asymmetric isomerization of allylic alcohols does not belong to this elite category. Because of the apparent similarity between both isomerization reactions, and the influence of the success story of the cationic [(binap)Rh] system for the isomerization of allylic amines, most of the subsequent catalysts have been designed using chiral bidentate phosphines and rhodium for the isomerization of primary allylic alcohols. The initial studies conducted in the early 80s' showed that [(binap)Rh(cod)]ClO 4 1 (cod: 1,5-cyclooctadiene) was also catalytically competent for the isomerization of primary allylic alcohols i...

Deep Eutectic Solvents: Environmentally Friendly Media for Metal-Catalyzed Organic Reactions

“…Catalyst 2 was also efficient for the isomerization of a family of mono-and di-substituted allylic alcohols (see Scheme 4). As previously observed in this metal-catalyzed isomerization (90)(91)(92)(93)(94)(95)(96)(97), a strong dependence upon the substitution grade on the olefin was observed, thereby higher ruthenium charges (5-10 mol% Ru) and reaction time (8-24 h) were required to promote the isomerization of di-substituted allylic alcohols. Finally, the mentioned conditions allowed for the recycling of the catalytic system during 4 consecutive cycles, with no significant observation of loss of activity during the two first runs.…”

“…Isomerization reactions are typical examples of atom economic processes in which no by-products are generated. Thus, the redox isomerization of readily accessible allylic alcohols catalyzed by transition-metal complexes is a useful and straightforward synthetic route to saturated carbonyl compounds, which are very important raw materials in organic synthesis (90)(91)(92)(93)(94)(95)(96)(97). In this sense, García-Álvarez et al described the unique example reported to date for the redox isomerization of allylic alcohols in DESs catalyzed by the Ru(IV)-complex [Ru(η 3 :η 3 -C 10 H 16 )Cl 2 (κ 1 -N-benzimidazole)] (2) (98).…”

Iridium‐Catalyzed 1,3‐Hydrogen Shift/Chlorination of Allylic Alcohols

“…[14] Previous studies support that the formation of a-functionalized carbonyl compounds from allylic alcohols occurs through: 1) a transition-metal-catalyzed 1,3-hydrogen shift forming an enolate or an enol intermediate, and 2) subsequent nucleophilic attack on the electrophile (Scheme 4). [6][7][8][9] In a cross-over experiment using deuterium-labeled [D 1 ]-2 f, we have confirmed that the isomerization follows an intramolecular 1,3-hydrogen shift (Scheme 5 and the Supporting Information). [7b, 8] Although this result is consistent with the intermediacy of enol(ate)s, the details of the mechanism, and in particular of the C À Cl bond formation step, remain to be elucidated.…”

“…A considerable advantage of using allylic alcohols as enol equivalents [6,7] is that the new bond (to the electrophile) is formed exclusively at the alkenylic carbon atom of the allylic alcohol [RCH(OH) À CH = CHR; Scheme 1b]. This type of transformation has almost exclusively been investigated using carbon electrophiles (e.g.…”

Nanocatalysts for Rearrangement Reactions

“…[69][70][71][72] The process involves the spontaneous tautomerization of the intermediate enol formed during the C=C bond migration. In this context, the magnetic nanoparticles 1, decorated on the surface with a RAPTA-type complex (see Scheme 8.1), have proven to be an efficient catalytic system for the redox isomerization of allylic alcohols (Scheme 8.5).…”