Hydrostannation alkynes

In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides (hydrostannolysis) (equation 1-6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn, 12 and in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1-7) followed a similar mechanism,13, 14 and these reactions now provide the basis of a number of important organic synthetic methods. 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Marshall, J.A. and Bourbeau, M.P. (2002a) Second-generation synthesis of the polyprojrionate subunit of callystatin A based on regioselective internal alkyne hydrostannation. Oig. Lett., 4, 3931-3934. 

The hydrostannation reaction can proceed either by a free-radical mechanism, or, with polar-substituted alkenes or alkynes, by a polar mechanism, respectively resulting in anti-Markownikoff or Markow-nikoff orientation. Both t3rpes of reaction are particularly suitable for preparing functionally substituted, organotin compounds. 

The tin hydrides find important applications as reducing agents. Many of their reactions (particularly the reduction of alkyl halides and the hydrostannation of simple alkenes and alkynes) arc known to proceed through RaSn- intermediates, and this aspect of their chemistry is referred to in Section II,G. 

In addition, Wu and Li recently have developed an efficient rhodium-catalyzed cascade hydrostannation/conjugate addition of terminal alkynes and unsaturated carbonyl compounds in water stereoselectively (Scheme 4.5).88... 

Hydrostannation refers to the addition of triorganotin hydride to different types of alkenes or alkynes, in the presence of catalysts, making possible the preparation of various types of stannylalkanes or stannylalkenes130. 

Hydrostannation, applied to alkynes with Pd as catalyst, produces vinylstannanes129,131 as mixtures of two regioisomers, a and b ... 

Using triethylborane, hydrostannation of alkynes can also be performed to achieve the addition of the triphenylstannyl group to the terminal acetylenic carbon. The reaction is regioselective, giving a mixture of E- and Z-l-triphenylstannyl-l-alkenes ... 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

Hydrostannation of alkenes and alkynes (Equation (14)) can involve nucleophilic (R3Sn H+), electrophilic (R3Sn+H ), or homolytic (R3Sm) tin, or the hydride as a metal complex (R3SnMH). 

Homolytic hydrostannation can also be initiated at room temperature by thiophenols, when a trace of oxygen may oxidize the thiol to the ArS radical, which abstracts hydrogen from the stannane to give the stannyl radical (Equation (15)).93 With azoisobutyronitrile (AIBN) initiation, alkynes undergo only monohydrostannation, but with the thiol, simple alkynes show bis(hydrostannation) or bis(stannation). 

Hydrostannation can be catalyzed by Lewis acids. If a mixture of an alkyne, tributyltin hydride, and Z1CI4 or HfCl4 is stirred in hexane, the product of ////-addition is formed (Equation (19)) 98 the hydride can be generated in situ from Bu3SnCl and EtjSiH, and B(C6F5)3 is an alternative Lewis acid.99 100 The mechanism of these reactions is still a matter of speculation. 

The hydrostannation of alkenes and alkynes can be catalyzed by a number of transition metals (Ni, Pd, Pt, Mo, etc.), but most studies have involved palladium.106 The mechanism that is most commonly accepted is shown for an alkyne in Scheme 1 the model for an alkene is similar. This mechanism accounts for the observations that the reactions normally involve cir-addition, that the R3Sn group enters so as to avoid steric hindrance by the largest substituent group on the alkene or alkyne, and that the regioselectivity depends on the polarization of the palladium-hydrogen bond in the sense Pd -H15-. 

If the acceptor molecule contains both an alkene and an alkyne group, hydrostannation occurs at the alkyne moiety. 1,6-Enynes undergo ring closure initiated by addition to the alkyne, as shown by the fate of the deuterium in Equation (26).108... 

The a-hydrostannation of activated alkynes RC=CCOR with Bu3SnH, catalysed by (Ph3PCuH)6, has been mentioned earlier.69... 

Since complete hydride abstraction by B(C6F5)3 from Bu3SnH has been observed,260 it is not surprising that the borane mediates hydrostannation reactions as well. Y. Yamamoto also reported the use of BfCfiFs) in the hydrostannation of alkynes the hydrostannane is generated in situ from silane in the presence of BtCgFsF.261 Furthermore, Maruoka et al. have reported B(C6F5)3 mediated hydrostannation of ketones, chemistry which is discussed in more detail in the next section. 

Hydrostannation of 1 -alkynes.1 Tributyltin hydride adds spontaneously to terminal alkynes to form a 1 1 mixture of ds- and f/ww-l-alkenylstannanes. However, use of this rhodium catalyst and a radical inhibitor (2, galvinoxyl) provides 2-alkenylstannanes as the major product. 

Hydrostannation of alkynes is one of the simplest and direct routes to vinylstannanes which have great versatility as building blocks in syntheses. However, a variety of regio and stereo isomers are usually formed.17-22 Until now, the selectivities are controlled by using different types of tin hydrides described above.26,92 For aryl-subsutituted alkynes, radical stannylation leads to the regioselective formation of the /3-adducts but with poor stereoselectivity. The addition of bulky tris[(phenyl dimethylsilyl)methyl]tin hydride indicates that these reactions take place with complete kinetic or thermodynamic stereoselectivity (Equation (30)).93... 

In the Bu3SnH-promoted radical reactions to aliphatic alkynes, using initiators such as AIBN, Et3B, and ultrasound104 furnishes /3-adducts as a mixture of (E)- and (Z)-isomers. Lewis acid catalysts give /3-(Z) isomers,96 whereas transition metal catalysts furnish the predominant formation of (3-(E) isomers.105 The a-stannylation of simple aliphatic alkynes, however, is particularly difficult because of the absence of anchor substituents such as ethers. In the general hydrostannations of aliphatic alkynes, a-adducts are obtained only as minor adducts in the Pd-catalyzed reaction (Equation (35)). 

Hydrostannation of bis-alkynes is most commonly used to prepare 1-stannacyclohexa-2,5-dienes (18) (59-63), though both 1-stannacyclo-penta-2,4-dienes (64) and l-stannacyclohepta-2,4,6-trienes (65) have been... 

The iododestannylation reaction is a commonly nsed reaction dne to the high reactivity of the formed vinyl iodides, which can be nsed for fnrther transformations. The stannylated precursors are easily prepared by hydrostannation of alkynes, and the iodide-tin exchange proceeds with an excellent regio- and stereocontrol. Fnrthermore, the reaction is compatible with the presence of several fnnctionalities snch as ethers ", alcohols , ketones , esters and amines. Fnnctionalized dienyltins were also nsed for the preparation of l-iodo-l,3-dienes, and the reaction was even extended to the preparation of fully stereo-determined iodo tetraenes. ... 

---