Cross-coupling reactions silicon compounds

In contrast to the transition metals, where there is often a change in oxidation level at the metal during the reaction, there is usually no change in oxidation level for boron, silicon, and tin compounds. The synthetically important reactions of these three groups of compounds involve transfer of a carbon substituent with one (radical equivalent) or two (carbanion equivalent) electrons to a reactive carbon center. Here we focus on the nonradical reactions and deal with radical reactions in Chapter 10. We have already introduced one important aspect of boron and tin chemistry in the transmetallation reactions involved in Pd-catalyzed cross-coupling reactions, discussed... 

First, coordinatively unsaturated active palladium catalyst, PdL2, is produced via dissociation of the ligands, which then reacts with acyl halide to give the acylpalladium intermediate. Since deinsertion of CO of the acylpalladium derivatives may occur simul-taneously, the next step, transmetallation (so-called metathesis), is the most crucial for the efficiency of the overall reaction. A variety of organometallic compounds, such as boron, aluminum, copper, zinc, mercury, silicon, tin, lead, zirconium, and bismuth, are used as the partner in this coupling reaction without loss of CO. In this section, the important features of the cross-coupling reactions of a variety of organometallic compounds with acyl halides and related electrophiles are discussed. 

B.iv.d. Cross-Coupling Reactions Involving Silicon Compounds. A very recent example is reported for the cross-coupling of aryl(fluoro)silanes with aryl iodides attached to solid support. 

Direct Silylation of Heteroarylcarbonyl Compounds. Under ruthenium catalysis vinyltrimethylsilane reacts to ortho silyl-ate heteroaryl carbonyl compounds directly in good yields (eqs 32 and 33). The reaction only works with heteroaromatic systems. The resulting aryltrimethylsilanes can be used to introduce electrophiles regioselectively through electrophilic desilyla-tion. The reaction also works with vinyltriethoxysilane, opening the possibility of silicon-based cross-coupling reactions. 

The reaction of certain palladium-heteroatom complexes to alkenes and alkynes is a versatile tool for the synthesis of alkanes and alkene having heteroatoms attached. In particular, the various B-B, B-Si, and B-Sn compounds can be used for palladium-catalyzed borylation of alkenes and alkynes (Scheme 5-2). Borostannylation takes place at ambient temperature, whereas silylboration " only proceeds at a temperature above 80 °C due to the slow oxidative addition of a B-Si bond to a palladium(O) catalyst. Both reactions selectively provide cz j-products via addition of silicone or tin to the internal carbon and boron. The reactions are compatible with various functional groups for both terminal and internal alkynes. Cross-coupling reaction of boranes with organic halides selectively occurs at the terminal C-B bonds to provide regiodefined and stereodefined alkenylboron, alkenylsilicon, and alkenyltin compounds. 

The use of 2-silyl-substituted 1,3-dienes in Diels-Alder and cross-coupling reactions was reported in [246]. Among other silicon compounds the authors utilized silatranyl (102) and catecholato-chelated silyl derivatives (103) with penta-coordinated Si atoms for the reactions shown in Scheme 30. 

So far only a few examples of the uses of organosilicon compounds in cross-couplings have been published. Noteworthy is the smooth reaction with a sterically hindered substrate (87).295 The synthesis of the alkaloid nitidine included a cross-coupling step using an alkenylsilane (88),296 while the syntheses of some antitumor agents involved the alkenylation of unprotected iodouracyls using alkenyl-silicon species.297... 

As is also true for silicon and tin compounds, the high stability of boronates, particularly cyclic esters, allows them to be incorporated into and carried through as substituents in a range of reaction types, such as the synthesis of pyrazole boronates for cross couplings (see 4.2.7.4). 

Summary Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of rathenium (e.g., Grabbs catalyst). 

The similarity of reaction rates and yields conld be explained by either interconversion of these species to one another, or conversion of each of them to a more advanced, common reactive intermediate. H-NMR analysis of a mixture of TBAF with either siletane 13, silanol 32, disiloxane 202, or fluorosilane 203 shows only two species that are formed almost immediately. One is identified as the disiloxane of the corresponding silanol, and the other species an unknown compound 204 (or 205) containing both silicon and fluorine as determined by Si- and F-NMR (Scheme 7.51). Moreover, the ratio of204 (205) to disiloxane increased with TBAF stoichiometry under typical conditions for cross-coupling the ratio is heavily in favor (>10 1) of204 (205). 

Heat-curing of silicone rubbers usually involve free-radical initiators such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and f-butyl per-benzoate, used in quantities of 0.5—3%. These materials are stable in the compounds at room temperature for several months but will start to cure at about 70°C. The curing (cross-linking) is believed to take place by the sequence of reactions shown in Figure 4.37. The process involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the thermal decomposition of the peroxide and subsequent cross-linking by coupling of the polymer radicals. 

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