Dienes hydrostannation

Pd-catalyzed hydrostannation of dienes with HSnBu3 affords the (Z)-2-alke-nylstannane 89 with high regio- and stereoselectivities[84], Dimerization-dou-... 

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

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. ... 

Hydrostannation of 1,3-dienes produces aUyltins. Specific modifications of certain fimctionally substituted alkyltin compounds have also been used (equation 34 and Scheme 7). 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

This access to a , y-bis(tricyclohexyltin)alkanes proves to be very convenient when the corresponding organic dihalides are available. When this is not the case, and when a , y-dienes are more easily accessible, a double hydrostannation of these unsaturated compounds can be used. As indicated above, this radical addition has to be conducted in less mild conditions than for the addition of tri-n-butyltin hydride, for instance, because of the bulkiness of the organotin center and also of the more nucleophilic character of the tricyclohexyltin radical. Nevertheless, the treatment of 4,4 -di(butenyl)biphenyl or 4,4 -bis(but-3-enyloxymethyl)biphenyl with tricyclohexyltin hydride at 130 °C over seven days affords the corresponding adducts 72 and 75 in good yields (Scheme 3.7.11). Due to its rapid decomposition at this temperature, the AIBN initiator has to be added in small portions for the duration of the reaction. 

It has also been established that consecutive esterification and cross-coupling affords the same macrolide segment (Scheme 12.111) [213]. This tandem cross-coupling procedure is an efficient means of assembly of a C2-C5 diene in (-[-macro-lactone A (Scheme 12.112) [214]. In this reaction a bromoethyne was used as the starting component otherwise a mixture of regioisomers (7 3 P/a) was obtained on hydrostannation. 

Acetylenes and allenes can also be smoothly hydrostannated under these conditions 444 for example, tributyltin hydride and acetylene give tributyl-(vinyl)tin in very good yield.445 Conjugated dienes afford mainly the 1,4-adducts.446... 

RXN17 Hydroboration, Hydrogermylation, Hydrosilylation and Hydrostannation of Alkynes, Allenes, Dienes and Enynes... 

In contrast with the Pd-catalyzed hydrosilation of conjugated dienes discussed in Sect. B, the corresponding hydrostannation has not been extensively studied. In a pair of reports published in 1992, Miyake and Yamamura, described the results obtained with several representative conjugated dienes. By using BusSnD, the position of hydride or deuteride incorporation has also been estabUshed (Scheme 32). The selective formation of (Z)-l-stannyl-2-alkenes closely parallels the regio- and stereochemical profiles observed in the Pd-catalyzed hydrosilation (Sect. B). Further delineation of the scope, limitations, and other aspects of the reaction is very desirable. 

Functionally-substituted allylstannanes result from the corresponding allyl-sulphone and Bu sSnH, which forms adducts with conjugated dienes, and reduces the keto functional group in a-keto esters. Hydrostannation of 1-alkynols... 

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