Stannylcuprates 1,4-addition

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

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

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

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... 

An expedient method for the preparation of isochromene carboxylates (203) has been developed, which relies on the regioselective 1,6-addition of various nucleophiles, such as Grignard reagents, alkoxides, and cyanide, to benzopyranylidenetungsten(O) complexes (201), followed by iodine oxidation of the addition intermediates (202)254 DFT calculations have been carried out to rationalize the unusual switch in stereoselectivity, where ynone and ynoate substrates exhibit opposite stereochemical preferences in the stannylcupration.255... 

These cuprates deliver the (CH3)3Sn group to organic substrates by conjugate addition or substitution of halo or triflate groups. Stannylcupration is also possible. 

The stannylcupration of alkynes has been widely studied. Reaction of alkynes with lithium bis(tributylstannyl) cuprate leads to r -2-(tri butyl stannyl) vinyl cuprates, which are synthetically equivalent to cis- 1,2-ethylene dianions. Addition of the tin-copper reagent across the triple bond occurs i>7/-stereospecifically, thus providing Z-vinylstannanes. Phenylacetylene reacts with the tin cuprate with a regiochemistry opposite to that of 1-decyne.294 The intermediate cuprates react well with the various electrophiles.295 For example, the reaction with ethylene oxide gives primary alcohols, and further treatment of their />-toluenesulfonates with butyllithium gives 1-substituted cyclobutenes (Equation (120)) 294... 

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