Tributyltin stoichiometric reaction

Catalytic procedures (introduced by Kuivila and Menapace92) are easier to conduct and the tin hydride concentration is more easily controlled. A catalytic amount of tributyltin hydride or tributyltin chloride is mixed with the radical precursor, the alkene acceptor and a stoichiometric quantity of a coreductant such as sodium borohydride93 or sodium cyanoborohydride.29 Over the course of the reaction, the borohydride continuously converts the tin halide to tin hydride. The use of the catalytic procedure is probably restricted to halide precursors (tin products derived from other precursors may not be reduced to tin hydrides). This method has several advantages over the standard procedures (i) it is simple to conduct (ii) most functional groups are stable to the coreductants (especially sodium cyanoborohydride) (iii) the tin hydride concentration is known, is stationary (assuming that the tin halide is rapidly reduced to tin hydride), and can be varied by either changing the concentration of the reaction or the quantity of the tin reagent (10% is a typical value, but lower quantities can be used) and finally, (iv) the amount of tin hydride precursor that is added limits the amount of tin by-product that must be removed at the end of the reaction. 

Hoshino et al. [33] reported the first example of an enantioselective radical reaction employing a chiral Lewis add complex. The enantioselective reduction of a-methoxy-methyl-a-iodolactone 41 with tributyltin hydride (BusSnH) in the presence of stoichiometric amounts of the chiral complex of a chiral diamine 42 and Mgl2, gave the reduced product 43 in 88 % yield with 62 % ee (Sch. 17). Reaction using Mg(C104)2, TiCU, Znl2,... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

In 1989, a highly enantioselective aldol reaction of achiral silyl enol ethers of thiol esters with achiral aldehydes was developed by using a novel chiral promoter system consisting of chiral diamine-coordinated tin(II) triflate and tributyltin fluoride (or dibutyltin diacetate) [23]. When the silyl enol ether 16 of S-ethyl ethanethioate was treated with PhCHO in the presence of stoichiometric amounts of tin(II) triflate, (S)-l-methyl-2-[(piperidin-l-yl)-methyl]-pyrrolidine (18), and tributyltin fluoride, the aldol reaction proceeded at -78 °C to afford the corresponding adduct 17 in 78% yield with 82% ee (Scheme 4). 

Starting from fluoropyrazine, a regioselective synthesis of iodo- and Iributylstannyl substituted fluoropyrazines has been elaborated. Lithiation of fluoropyrazine with stoichiometric amounts of LTMP and iodine afforded the 2-fluoro-3-iodopyrazine 366 (E=I) as sole product otherwise a mixture of mono-, di-, and triiodo derivatives were formed (Scheme 62, Table 14) [147]. In a similar manner, use of tributyltin chloride as electrophile led to mono and di-stannylpyrazines [215]. Formation of compounds 369, 370 and 371 is a result of metalation at the position adjacent to the nitrogen atom without assistance of the fluorine atom as DMG. Such a metalation without a DMG has been previously reported during direct metalation of bare pyrazine by use of an excess of LTMP (4 equiv.) with very short reaction time (5 min) at low temperature -78 °C [216]. 

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