Hydrostannylation regioselective

Hydrostannylation. Regioselective conversion of 1-alkynes to 2-tributylstannyl-l-alkenes is accomplished with BuySnH in the presence of (r-BuNC)3Mo(CO)3/ By changing the catalyst to (p-02NC6H4NC)3W(C0)3 propargyl acetate furnishes a distannylated product. ... 

Hydrostannylation. Regioselective hydrostannylation of several types of alkynes (to yield mainly a-stannyl derivatives) is catalyzed by a molybdenum complex. 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

Overman et al. <1996JA9062> and Kibayashi et al. <2002JOC5517> have utilized the same alkynyl intermediate 222 as a substrate in their chemistry (Scheme 47). While the first group uses a nucleophilic attack of the triple bond on the iminium cation 223, the second group performs a regioselective hydrostannylation, followed by Pd-catalyzed carbonylation, to provide the precursor 224 to the cyclization step. 

Kazmaier and his associates introduced Mo(CO)3(CNBu-f)3(MoBl3) as a new and efficient catalyst for regioselective hydrostannylation chemistry (equation 116)844, while Smith and Lodise described the assembly of a subtarget of 13-deoxytedanolide using hydrostannylation as an important step (equation 117)845. Rizzacasa and his colleagues utilised palladium-catalysed hydrostannylation in the preparation of an important synthon... 

Palladium-catalyzed hydrostannylation is stereoselective, giving syn-adduct. Regioselectivity is not always perfect, however. Exclusive formation of (-B)-l-stannylalkene is obtained only when 1-alkyne with a bulky substituent is employed (Scheme 34) [147-150]. Nicolaou also reported a stereo- and regio-selective hydrostannylation (Scheme 35) [151]. Use of a combination of tri-butyltin chloride and poly(methylhydro)siloxane in the presence of TBAF is also an impressive method in this class [152]. 

They react with terminal alkynes by electrophilic addition of the empty p-orbital to the unsubstituted end of the triple bond 83. The intermediate would then be the more substituted vinyl cation 84. It is easier to draw this mechanism with R2BH than with the full structure for 9-BBN. The intermediate 84 is not fully formed before hydride transfer begins so that the reaction is semi-concerted and the transition state is something like 86. The result is a regioselective and stereospecific cis hydroboration of the triple bond to give the A-vinyl borane 85. The intermediate 84 is quite like the radical intermediate in hydrostannylation but the difference is that hydrogen transfer is intramolecular and stereospecific in hydroboration. 

The orientation and steric course of the hydrostannylation of alkynes to yield alkenylstannanes is determined by the nature of the reagent and the catalyst used. The trimethylstannylcopper reagent Me3SnCu Me2S adds regioselectively to terminal acetylenes 293 (n = 1,2,3,4 or 6, X = OH, Cl, OSiMe2Bu or 2-tetrahydropyranyl) to give, after treatment with methanol, the stannanes 294. ... 

Homoallylic alcohols are also hydrostannylated with excellent regioselectivity and good diastereoselectivity (eq 7). Here, the authors note again that the diastereoselectivity was higher in hexane compared to Et20, but that the stereochemical outcome was less dependent on the steric size of the a-substituent. 

Radical-Initiated and Transition Metal-Catalyzed Additions. Some radical and transition metal-catalyzed additions to TMSA are unique when compared with additions to other terminals alkynes, because they show remarkable regioselectivity and/or stereoselectivity. The regioselectivity of a metal-catalyzed addition may be complementary to that of a radical-initiated process (eq 12). For example, rhodium and molybdenum complex-catalyzed additions of trialkyltin or triaryltin hydrides to TMSA give mainly the 1,1-disubstituted ethylenes, whereas radical hydrostannylation through sonication or triethylhorane initiation gives the 1,2-adducts with the (E)-isomers predominating. Other terminal alkynes undergo radical or metal-catalyzed hydrostannylation with either poorer or reverse selectivity. 

Palladium-catalyzed cyclization was used by Grigg et al. [95] to construct complex heterocycles 47 (Scheme 32). Due to the excellent regioselectivity of the hydrostannylation of terminal alkynes, only one major isomer was formed in most cases. The strategy was used for preparation of combinatorial libraries. 

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