A practical synthesis of (4-1-stannylalk-1 -enes containing a range of oxygen and nitrogen functionality is highlighted, involving hydrostannylation followed by palladiumcatalysed carbon-bromine bond cleavage reactions of 1-bromoal kynes.Vinylstannanes are pivotal intermediates in a wide range of carbon+arbon bond forming reactions involving electrophiles,' frequently under palladium catalysis.2 As a result of their central importance, considerable effort has been expended towards the regio-and stereo-controlled synthesis of geometrically defined (@-and (a-, 1-and 2-vinylstannanes. The most popular and commonly employed synthetic routes to vinylstannanes involve hydrostannylations 3-12 or stannylmetallations of monosubstituted alkynes. Whilst there have been several developments in controlling both the regio-and stereo-chemistry of hydrostannylations of alkynes, unfortunately mixtures of isomers are commonly observed. Recently Hodgson et al." described an approach to (a-1-vinylstannanes which uses a modified Takai reaction between an aldehyde and Bu3SnCHBr2 in the presence of CrCI,.In contemporaneous investigations of the scope of the intramolecular Stille coupling reaction between vinylstannanes and vinyl halides in the elaboration of polyene macrolides, e.g. macrolactin A,= and polyene macrolactams, e.g. leinamycin 23 and virginiamycins," we have evaluated a range of methods for the stereocontrolled synthesis of several (@-1 -vinylstannanes, including those methods highlighted above. In many instances either poor yields or poor stereocontrol, or both, resulted. In 1990, Zhang et al. lo revealed that 1 -bromoalkynes can be converted into (@-1 -vinylstannanes in a regioselective manner using two equivalents of tributylstannane in the presence of PdCI2( PPh3)2. The corresponding chloroalkynes did not react in an analogous manner, and the reactions were thought to occur via an alkylidene carbenoid intermediate, viz 2, and involve palladium-catalysed carbon-bromine bond cleavage (Scheme 1). Surprisingly, this method has not been developed or 1 Br 2 H 3 Scheme 1 Reugenrs: i, Bu,SnH, Pdo exploited in synthesis by other researchers.To highlight the scope for this method, we now report the concise synthesis of a range of sensitive heteroatom-functionalised (E)-1 -vinylstannanes from hydrostannylations of appropriate 1 -bromoalkynes under palladium catalysis.1-Bromoalkynes can be simply prepared from the corresponding alkyne by reaction with N-bromosuccinimide in the presence of a catalytic amount of silver nitrate.25 They can also be elaborated from aldehydes via 1, I-dibromoalkene intermediates, using Grandjean's modification 26 of the Corey-Fuchs procedure. In this manner a variety of oxygen-and nitrogenTable 1 Hydrostannylations of bromoalkynes 1 to give 3 1 R Isolated yield of 3" (%) a B z O C O e 78 p-MeC&42H,O-77 C 80 Me' d EtoY O -74 Me f 82 72 86 70 @ E : 2 > 95: 5 in all cases.substituted I -bromoalkynes were synthesised (Table 1). When solutions of the 1-bromoalkynes in THF were treated with 2.2 equiv. of trib...
