Hydroalkoxylation cycles

Mechanistic hypotheses play an important role in developing new catalytic and selective heterofunctionalizations of alkenes. Two basic reaction cycles for metal-catalyzed hydroalkoxylation (and hydration, for R = H) of alkenes can be postulated (Scheme 2). One pathway leads to Markovnikov products via activation of the nucleophile, oxy-metallation, and protonolysis (hydro-de-metallation) (Scheme 2a). Alternatively to the inner sphere syn-oxymetallation depicted in Scheme 2a, external anti-attack of the nucleophUe to coordinated olefin is plausible. The oxidation state of the metal remains constant in this cycle. The alternative hydrometallation pathway (Scheme 2b) proceeds via oxidative addition of the H-OR bond, hydrometallation of the olefin, and reductive elimination to the anti-Markovnikov addition product [3,4]. 

Mechanistic investigation of the Au -catalysed SiO)"-exo-trig hydroalkoxylation of allene (240) revealed a rapid and reversible C-0 bond formation to generate (241), followed by the turnover-limiting protodeauration producing the vinyl tetrahydrofuran (242). This pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex (243). ... 

Scheme 14 Catalytic cycle for Ln[N(TMS)2]3-catalyzed intramolecular hydroalkoxylation of y-hydroxyalkyne (Ln = La, Nd, Sm, Y, and Lu)...

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