Active transmetalating species

Mechanistic studies investigating the role of additives and the nature of the active transmetalating species... 

In THF, in contrast to DMI or NMP, its solvation is not favored and the Schlenk equilibrium shifts to ZnBt2 and wBu2Zn, which is not an active transmetallation species [271]. 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

In analogy to the Au(I) and Pd(II) systems of Hayashi and Shibasaki, respectively, this process is proposed to proceed through a metalloenolate intermediate. The catalytically active metalloenolate species is generated upon desilylative metallation of the enol silane by the cupric fluoride complex. In support of the hypothesis that a soft-metal enolate is an intermediate in the reaction, the investigators have observed that the reaction can be successfully executed under conditions that directly promote transmetallation of the enol silane in the absence of fluoride (Scheme 22). When a solution of enol silane is successively treated with 10 mol % of either MeLi or (Bu4N)Ph3Sip2 at 0 °C, followed by 5 mol % of (S)-BINAP Cu(OTf)2 at -78 C and benzaldehyde, the aldol adduct was isolated... 

This remarkable reaction is catalyzed by early and middle metals, primarily complexes of Mo, W, and Ru. The active catalytic species in all cases is an alkylidene complex of the metal, (M=CR2). In the case of the W complex, the active catalyst is made by transmetallation from Et4Pb to give (Ar0)9W(0)Et2 followed by a-abstraction to give the alkylidene complex ((Ar0)2W(0)=CHMe). 

What is the reaction mechanism of the Pd-catalyzed formation of PPEs and PAEs While there has not been a full force mechanistic investigation of the Pd-catalyzed formation of PAEs, a working model is at hand (Scheme 6.3 and Scheme 6.4). If a Pd compound is used in the reaction, it has to be reduced into the catalytically active zerovalent species. In step I, an alkyne is transformed into a cuprate by the cocatalyst Cul in the presence of an amine. Then L2PdX2 reacts under transmetallation with the cuprate to give a dialkynylated tetrahedral intermediate that reductively eliminates to furnish a diyne byproduct... 

The overall transformation presumably proceeds through the initial formation of a catalytically active alkylrhodium species 389 capable of carbometaUative addition across the unsaturated carbon-carbon multiple bond of the substrate. TTie corresponding alkenylrhodium intermediates 391 undergo a transmetallation, furnishing the corresponding alkenylzinc entities 390 (Scheme 10.132). 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

The catalytic process is also achieved in the Pd(0)/Cr(II)-mediated coupling of organic halides with aldehydes (Scheme 33) [74], Oxidative addition of a vinyl or aryl halide to a Pd(0) species, followed by transmetallation with a chromium salt and subsequent addition of the resulting organo chromate to an aldehyde, leads to the alcohol 54. The presence of an oxophile [Li(I) salts or MesSiCl] allows the cleavage of the Cr(III) - 0 bond to liberate Cr(III), which is reduced to active Cr(II) on the electrode surface. 

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

Thus, the involvement of one of the following three possible mechanisms has been suggested (i) nucleophilic addition of the acylzirconocene chloride to the Lewis acid activated aldehyde, (ii) nucleophilic addition of the cationic species of the acylzirconocene chloride formed by an Ag(I) salt or a Lewis acid, or (iii) transmetalation of the acylzirconocene chloride with the Lewis acid and subsequent nucleophilic addition. 

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