Nucleophilic Abstraction in Hydrides, Alkyls, and Acyls

Wacker chemistry normally gives branched products via Mar-kovnikov olefin hydration, yet linear alcohols are highly desirable. Grubbs has now found conditions that give the long-sought anti-Markovnikov olefin hydration, based on a multistep scheme in which a pair of catalysts cooperate. 

Nucleophilic addition of H2O to coordinated alkyne is the key step of catalytic alkyne hydration with traditional Hg(II) or Au(I) catalysts (Eq. 8.26) that convert terminal alkynes RC=CH to the methyl ketones RCOMe. As part of a general trend associated with the rise of green chemistry (Section 1.1), toxicity and expense concerns have led to the advent of base metal catalysts, such as a water-soluble Co(III) porphyrin.  

Alkyl groups can be exchanged between metals, typically with inversion at carbon. This transmetalation reaction provides a route for the race-mization of a metal alkyl during the early stages of an oxidative addition reaction, while there is still some of the low-valent metal left in the reaction mixture. In Eq. 8.28, exchange of a (CRa) fragment between the metals turns the Pd(0) partner into Pd(II), and the Pd(II) into Pd(0). The stereochemical outcome of an OA can be clouded by exchange reactions such as these. 

Acyls undergo abstraction by nucleophiles in the last steps of Eqs. 8.22 and 8.23. As in the abstraction of Eq. 8.28, the reaction goes with reduction of the metal by two units, so a Pd(II) acyl is ideal because the Pd(0) state is easily accessible. 

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