An enzyme-metal combo reaction has been developed for the dynamic kinetic resolution of allylic alcohols in which racemic substrates are transformed by a lipase and a ruthenium complex in the presence of an acyl donor to allylic acetates of high optical purity in over 80% yield.

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Electrophilic pincer-ligated Pt(II)-dications are efficient catalysts for the cycloisomerization of 1,6-dienes, initiated by alkene activation. The tridentate ligands inhibit beta-hydride elimination and thus enable cationic mechanisms that turnover by Pt(II) extrusion. PPP ligands lead to cyclopropane products, while PNP ligands provide cyclohexene products; mechanistic issues are also discussed.

Biomimetic polyolefin cascade reactions [1,2] represent some of the most challenging problems in reaction design, and their products are ubiquitous in the natural world.[3] Since the Brønsted-Lewis acids (BLAs) of Yamamoto et al. are the only known synthetic catalysts for asymmetric catalytic initiation of cation-olefin cyclizations, [4,5] [7] and Toyota, Ihara et al. [8] demonstrated that nucleophilic enols lead to carbocyclic products by b-hydride elimination [9] or protonation. [10] Although substituent effects [11] and stereochemical studies [6c] support a cationic and cyclic intermediate, respectively, the exact nature of this intermediate is unclear, and direct evidence for this cation is lacking.[12] We therefore initiated a plan that first gathered evidence supporting the intermediacy of the carbocyclic cation, while also determining whether it could function as a point for initiating new metal-mediated reactivity. The key to our pursuit was a recent report by Vitagliano et al. of a dicationic Pt complex of a pyridyl bisphosphane pincer ligand [Pt(PNP)](BF 4 ) 2 (PNP = 2,6-bis(diphenylphosphanylmethyl)pyridine) that catalyzes the dimerization of ethylene and 2-methyl-2-butene. [13,14] Intermolecular nucleophilic addition of 2-methyl-2-butene to coordinated ethylene was proposed, with turnover by a sequence of 1,2-hydride shifts. Most importantly, b-hydride elimination did not occur, since no open cis coordination sites were available, and this suggests that this complex might be capable of initiating cation formation while preserving the stereochemistry of the MÀC bond.Our first approach to trapping the putative cation utilized the Pd analogue of the Vitagliano complex 1 and dienylphenol 3. Unlike direct Brønsted acid [4] and Hg II [15] activation/ polycyclization, Pd II prefers to coordinate and activate the least substituted alkene, [16] ensuring activation at the terminus. As shown in Scheme 1, the isolable C 6 F 5 CN adduct [17] efficiently (1 h, RT, CH 2 Cl 2 ) converted 3 to a new metalcontaining product devoid of alkene resonances ( 1 H NMR spectroscopy) as a 93:7 mixture of isomers according to 31 P NMR spectroscopy. This compound is stable up to 100 8C, though demetalation with NaBH 4 at room temperature rapidly [14b] yielded tricycle 4 in 90 % yield (two steps) and d.r. > 99:1. [18] Since a single diastereomer was observable in the crude demetalation product, we surmised that the isomer mixture for the Pd alkyl complex must result from epimers at the metal-containing stereocenter, a situation suggestive of competing chair-chair and boat-chair transition states, [19] each with a degree of concertedness in the CÀC/CÀO bondforming event, that is, a free carbocation is not likely formed [cf. Eq. (1)]. [1,20] Pincer complex [Pt(PPP)](BF 4 ) 2 (2, PPP = bis(2-diphenylphosphanylethyl)phenylphosphane), [21] derived from commercially available triphos, provided the intermediate alkyl complex with a slightly higher diastereoselectivity (96:4), and 4 in similarly high yield (87 %) and selectivity (> 99:1...