Styrenes hydrometallation

The mixed coupling of two different alkenes allows the formation of new functional unsaturated products but requires high regioselectivity. A ruthenium hydride complex, generated in situ from the reaction of RuHCl(CO)(PCy3)2 with HBF4.OEt2, was found to be an effective catalyst for the hydrovinylation of alkenes [8]. The reaction of styrene with ethylene produced the hydrovinylation compound 10 in 93% yield (Eq. 5). Initial hydrometallation of the alkene and insertion of ethylene seemed to be a plausible mechanism. 

This is in fact the result found for hydrometallation of substituted styrenes by [RhH2Cl(PR3)3] (equation 9). K and k vary inversely as the substituent X is changed, so the overall rates are fairly constant. 

A quite different case is presented by the bent metallocene systems, [Cp2MH(alkene)] where M = Nb or Ta. Here the metal-alkene complex is relatively quite stable, so much so that the kinetics of formation of a stable alkyl metal complex cannot be easily studied. Nonetheless it is possible to get a handle on the hydrometallation kinetics by means of dynamic NMR methods (Table 1). The increased rate for propene versus ethylene results from both steric destabilization of the ground state and electronic stabilization of the transition state for the former. The first is typical of alkene complexes the second implies that some partial positive charge develops at the -carbon during the hydrometallation process, also seen in the trend for substituted styrenes. 

Before leaving this section we briefly examine hydrometallation mechanisms completely different from the classic one depicted in Scheme 2. One class involves metal hydrides which do not have a readily available vacant site and requires conjugated alkenes — 1,3-dienes, styrenes these proceed via a... 

Styrene also reacts with HMn(CO)5 to form 1-phenethylmanganese pentacarbonyl as a detectable intermediate in styrene hydrogenations using HMn(CO). Hydrometalation of a chelated alkene by a hydridomanganese species is also involved in ring contractions of Mn phosphino heterocycles . 

Hydrometallations of alkenes and acetylenes such as hydroboration, hydrosila-tion, hydroalumination, hydrostannation and hydrozirconation are useful synthetic reactions. Rhodium-phosphine complexes catalyze hydroboration of substituted styrenes with a catecolborane (Scheme 68). An extensive survey of the phosphine... 

The basic features of the reaction may be summarized as follows (Scheme 3-92). Nickel 7t-allyl dimers are often used as pre-catalysts. Treatment with monodentate phosphines and Lewis acids likely generates a cationic nickel hydride species, which serves as the active catalyst for the reaction. Regioselective hydrometallation of the styrene generates intermediate 26, with the regiochemistry being driven by benzylic stabilization. Insertion of ethylene followed by p-hydride elimination produces the hydrovinylation product and regenerates the active cationic nickel hydride species. 

The examples of hydrometalations are hydroboration [64,72-77], hydrosilylation [78], etc. [78,79] of unsaturated compounds such as olefins, acetylenes, carbonyls and imines. For example, styrene reacts with catecholboran in the presence of a rhodium catalyst to afford stereoselectively the R isomer of 2-phenylethanol as shown in eq. (18.34) [72]. 

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