Alkyne silanes, coupling reactions

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

The Houk group reported a theoretieal study of the Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane as reducting agent. They showed that the selectivity of the [5+2] cycloaddition is controlled by the anisotropic steric environment of the NHC ligands. 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

The use of enals in nickel-promoted intermolecular couplings was initially limited to stoichiometric [3 -1- 2]/[2 + 1] cycloadditions [26], This limitation was overcome with the development of a Ni(COD)2/PCy3/R3SiH/THF reaction system, whereby the reductive coupling of alkynes and enals was achieved to afford a highly chemo- and stereo-selective synthesis of Z-enol silanes (Scheme 8.6) [27]. 

The dearest empirical evidence for the productive involvement of an 1] , O-bound nickel enolate comes from the intermolecular reductive coupling of alkynes and enals (Scheme 8.9) [27]. The extremely high levels of Z-isomer stereoselectivity (>98 2) can best be rationalized via the metallacycle intermediate 5 which undergoes o-bond metathesis to afford nickel hydride 6, followed by reductive eUmina-tion to yield the Z-selective enol silane product 7. A mechanism consisting of a nickel Jt-allyl species would not be expected to lead to high selectivities of Z-enol silanes, and has been imphcated in reactions leading to the selective production of T-enol silanes [28],... 

A reaction of alkyn-l-yl(diorganyl)silanes with boraadamantane yielded a series of borahomoadamantanes in which the presence of the Si-H-B bridges has been confirmed inter alia by the reduced Jhsi coupling value in comparison with triorganosilanes (Fig. 1). 

The main advantage of using silyl ethers in cross<oupling reactions is the ability to incorporate them into molecules by a number of methods. Cyclic silyl ethers, as a class, nicely illustrate this attribute. The well-known hydrosilylation of alkynes to form vinylsilanes can easily be rendered intramolecular by attachment of the silane as, for example, a homopropargyl silyl ether to form an oxasilacyclopentane 108 (Scheme 7.28) [53]. In this stmcture, the double-bond geometry is defined by the stereochemical course of hydrosilylation and the ether tether defines the location of the silicon atom with respect to the alkene. Thus, the siHcon-oxygen bond in this molecule serves to direct the hydrosilylation, as well as to activate the siHcon for cross-coupling. 

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