Isomerization hydrometallation

The isomerization of internal alkenes to terminal ones before hydrometalation or the isomerization during hydrometalation results in the formation of terminal prod-... 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

The ruthenium-catalyzed cycloisomerization of a variety of <5-enallenes was also achieved, forming cyclic 1,3-dienes or 1,4-dienes depending on the substrates and reaction conditions [32] (Eq. 22). This intramolecular coupling of the C=C bond and allenes can be envisioned by the initial hydrometallation of the allene moiety followed by intramolecular olefin insertion and isomerization. 

Usually, the reaction displays m-stereoselectivity giving (-E)-organomctallics. However, isomerization occurs very often leading to mixtures of (Z)- and (/. )-vinyl organometallics. The accepted general mechanism for the hydrometallation reaction involves the initial coordination of the alkyne to a vacant orbital of the metal, followed by insertion of the hydrogen-metal bond to a 7T-bond of the alkyne (Scheme 34).132... 

Spectacular enantioselection has been observed in hydrogenation (cf. Section 2.2) [3] and hydrometallation of unsaturated compounds (cf. Section 2.6) [4], olefin epoxidation (cf Section 2.4.3) [5] and dihydroxylation (cf Section 3.3.2) [6], hydrovinylation (cf Section 3.3.3) [7], hydroformylation (cf Section 2.1.1) [4a, 8], carbene reactions [9] (cf Section 3.1.10), olefin isomerization (cf Section 3.2.14) [10], olefin oligomerization (cf Section 2.3.1.1) [11], organometallic addition to aldehydes [12], allylic alkylation [13], Grignard coupling reactions [14], aldol-type reactions [15], Diels-Alder reactions [12a, 16], and ene reactions [17], among others. This chapter presents several selected examples of practical significance. 

Thermal isomerization of t-Bu2Be to i-Bu2Be, which occurs slowly even at RT, proceeds by elimination and recombination by hydrometallation of 2-methylpropene with an intermediate beryllium hydride a similar mechanism accounts for the racemiza-tion of (-h)-Be[(R)—CH2CH(Me)Et]2. 

The synthesis of (7-alkylcobalts from the reaction of Co hydrides and alkenes is relevant to the use of Co hydrides in alkene hydroformylation and is of interest because of the biological significance of carbon-cobalt bonds in vitamin B12 chemistry. The reaction of HCo(CO)4 with simple alkenes involves hydrometalation as a key step in hydrogenation, isomerization or hydroformylation of alkenes . ff-Alkylcobalt compounds are detectable intermediates in reactions of HCo(CO)4 and alkenes by Ir, but... 

The RhI-catalysed rearrangement of A-allylaziridines to (Z)-A-alkenylaziridines has been investigated by computational methods and the reaction occurs via a hydrometalation//3-hydride elimination. Alkylidene cycloheptadienes have been prepared via a rhodium(I)-catalysed tandem isomerization of cyclopropylenynes through a [1,5] carbon-carbon migration via an allene intermediate with high Z E selectivities (Scheme 152). ... 

Subsequent to Brookhart s findings, Jun el al. [101] developed a rhodium-catalyzed alkylation of a,P-unsaturated carbonyl compounds in the presence of diethylamine (Scheme 19.70). The proposed mechanism involves the in silu formation of an enamine, followed by C-H activation directed by this enam-ine, olefin hydrometallation, and reductive ehmination. In situ enamine hydrolysis regenerates the carbonyl group. The authors demonstrated that the formation of product 32 originates from the isomerization of initially formed compound 31 with the rhodium catalyst. 

The ability of Pd to participate in both hydrometallation and dehydrometallation snggests that Pd and its complexes can serve as catalysts for isomerization of alkenes and alkynes via a series of hydropalladation-dehydropalladation, that is 1,2-H shift. On the other hand, Pd-induced aUylic rearrangement provides a mechanism whereby alkenes can iso-merize via 1,3-shift. In either of these processes, the crucial reqnirement is the presence or ready availability of an empty coordination site. As amply demonstrated throughout this Handbook, Pd is imminently capable of serving as the catalyst center for isomerization of alkenes and alkynes. 

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