Stannylcuprates

Scheme 3.21. Regioselecti - ity in the stannylcupration of 1-allcynes, -hydroxy-l-allcynes, enynes, and propargyl alcohols.

Another aspect of the previous procedure is the stannylcupration of N-protected propargylamines115. Thus RNHCH2C=CH (R = t-butoxycarbamoyl) reacts with Bu3SnCu(Bu)(CN)Li2 under very mild conditions and provides Bu3SnCH=CHCH2NHR in excellent yields. 

The use of stannylcuprate reagents is well established.112,113 The reactions occur under mild conditions. They are reversible, and the reactivity and regio- and stereo-selectivity are sensitive to the structure and reaction conditions, as illustrated by the reaction of allene with the lower-order cuprate Bu3SnCuCNLi and, more readily, with the higher order cuprate (Bu3Sn)2CuCNLi (Equation (29)).114... 

The reactions are faster in the presence of weak protic acids, which can quench the initial products under conditions where the stannylcuprate reagents are not affected. This is illustrated in Scheme 2. With no internal proton source, the thermodynamically more stable result of Irans-addition is favored, but, in the presence of a proton source, the initial adducts resulting from m-metallostannation are trapped.116,117,118... 

Scheme 3.2. Reactivity profile of stannylcuprates with carbon electrophiles [16b, d].

Scheme 3.9. Annulation and ring-formation strategies based on reactions between stannylcuprates and triflates of cyclic fS-keto esters [55a] and functionalized ynoates [55d],...

Scheme 3.11. Z Diastereoselectivities in conjugate additions of stannylcuprates to a,/ -unsaturated derivatives.

Scheme 3.12. Synthesis of chiral 4-stannylpyrrolin-2-ones by means of stannylcuprate additions to chiral 2-ynoates [61a] (Boc = t-butoxycarbonyl).

Scheme 3.13. Reagent regioselectivity in the stannylcupration of 2-ynoic acids [63].

Piers has exploited these 2-ynoate stannylcupration reactions in the preparation of donor and dipolar synthons (Scheme 3.14) [64]. Stereoselective stannylcupration followed by deconjugation provides a stereocontrolled route to vinyl 1,3-dipolar... 

Scheme 3.14. Selected synthons available through stannylcuprate additions to 2-ynoates [64].

Scheme 3.17. Synthesis of functionalized a-amino acids by silylcupration [72a] or stannylcupration [81c] of chiral propargyl amines (Boc = t-butoxycarbonyl TFA = trifluoroacetic acid).

The reaction has been incorporated into a synthetic approach to enediynes [77]. Structural and mechanistic studies by Oehlschlager established the reversibility of these stannylcupration reactions [25cj. Although the resultant vinylcopper reagents were thermodynamically favored, crossover experiments found facile ligand exchange processes. Efforts to control the regiochemistry of the addition were met... 

Scheme 3.19. Stannylcupration of acetylene and trapping of the vinylcuprate with electrophiles [79c]...

Scheme 3.20. Regioselective stannylcupration of 3-ynoic acids or esters [63aj.

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