Phosphine ligands transmetallation

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. 

Palladium(0)-catalyzed transformations generally involve three steps oxidative addition, insertion or transmetallation (really a special type of insertion), and reductive elimination. Together they comprise a pathway for the formation of new carbon-carbon bonds. Oxidative addition takes place when a coordinatively unsaturated Pd(0) species cleaves a covalent bond to give a new complex in which die palladium is oxidized to Pd(II). Typically dissociation of two phosphine ligands to a 14-electron complex is file first step followed by oxidative addition to give a 16-electron Pd(II) complex. 

Various phosphine ligands are effective in stabilizing the palladium(O) species, but the stoichiometry of phosphine to palladium and the bulkiness or donating ability of phosphine ligands change the reactivity of catalysts toward oxidative addition and transmetalation (Scheme 5). Pd(PPh3)4 and other... 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

Other transmetallation reactions of silver halides or their complexes with phosphine ligands (e.g. [AgIP(Bu-w)3]) with alkylhthium and alkylmagnesium reagents confirm the fleeting stability of the alkylsilver compounds. In each case the composition of the AgR intermediates has been derived from the analysis of the products formed in their thermal decomposition. So far, representatives of the class of simple alkylsilver complexes have not been isolated in pure form. 

The mechanism follows that of a normal Stille coupling except that the carbon monoxide first exchanges for one of the phosphine ligands and then very rapidly inserts to produce an acyl palladium(II) complex. Transmetallation with the vinyl stannane in the usual way forms trimethylstannyl iodide and the key palladium complex carrying two carbon ligands. Transmetallation is always the slow step in these coupling reactions, allowing time for the carbon monoxide insertion. The final step—reductive elimination—releases the Pd(0) catalyst for the next cycle. 

C.ii.a. Silyl Derivatives. A Japanese gronp recently reported an original Pd-catalyzed reaction with vinyl- or aryl-fluoro silyl derivatives and vinyl epoxides. " A mechanism involving a zwitterionic TjLallylpalladinm silicate intermediate has been proposed (Scheme 5). Intramolecular transmetallation can give two 17 -allylorganopalladium complexes in equilibrium. The steric bulkiness of the phosphine ligand as well as its electronic affinity are responsible for the 1,2/1,4-product distribution. 

Stille coupling of alkyl halides with alkenyl stannanes can be achieved using electron-rich alkyl phosphines ligands, combined with the addition of a fluoride source as a nucleophilic promoter (Scheme 2.69). ° This probably works by generating a stannate complex 2.201 that participates in transmetallation more readily than the original stannane, in a similar manner to the addition of Lewis bases to Suzuki reactions (Section 2.6). 

A study of palladium-catalysed conjugate addition of diorganozincs to various enone types indicated that both Pd(0) and Pd(II) complexes could catalyse the reaction. Phosphine ligands such as PPhj or PBuj were effective, but only at a 1 1 Pd P ratio a 1 2 ratio caused yields to collapse. The observation is consistent with a mechanism computed for the Pd(0) case, in which the enone is simultaneously coordinated to Pd(0) and R2Zn this undergoes oxidative addition to palladium with simultaneous transmetalation from Zn to Pd, followed by reductive elimination. 

Fig. 4.6 Optimized structures for the transition state and all the intermediates involved in the transmetalation with ZnMeCl leading to the trans product (3) via a concerted mechanism. Phenyl rings of the phosphine ligands have been simplified, and the second THF molecule of the organozinc reactant is not shown for clarity. Distances are shown in A...

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