Addition of stannanes

Palladium-catalyzed procedures have also been developed for addition of stannanes to alkynes,143 and these reactions usually occur by syn addition. 

Support-bound stannanes have been prepared from phenyllithium bound to macro-porous polystyrene and chlorostannanes [14,41], by treatment of support-bound alkyl chlorides with lithiated stannanes [21,41], and by radical or palladium-mediated addition of stannanes to alkenes and alkynes (Figure 4.7 [42-47]). The chloride of poly-styrene-bound chlorostannanes can be displaced by treatment with arylzinc reagents, thereby yielding resin-bound arylstannanes [46]. Polystyrene-bound stannanes have also been prepared by copolymerization of 4-[2-(dibutylchlorostannyl)ethyl]styrene with styrene and divinylstyrene [48],... 

Azodiisobutyronitrile Addition of stannanes to unsatd. carbon-carbon bonds... 

Without additional reagents Addition of stannanes to nitrogen-nitrogen double bonds... 

Oxidative addition of stannanes HSnR3 to [OsClCp(PPr 3)2] generates the hydrido-stannyl-Os(lv) derivatives [Os(SnR3)HClCp(PPr 3)] (R=Bu, Ph). In the presence of an excess of HSnBu"3, the complex [Os(SnBu"3)HClCp(PPr 3)] is converted to the dihydride derivative [Os(SnBu 3)H2Cp(PPr 3)]. The half-open ruthenocenes [Ru(7] -CH2CMeCHCMeX)Cp ] (X = CH2, O) also undergo oxidative addition of SnCl to afford the... 

Cobalt naphthenate di-tert-butyl peroxide, or palladium (10% on charcoal) or hexa chloroplatinic acid are effective in catalysing the addition of stannane to olefins to form the corresponding tetra-alkyltins. 

Tin/lithium exchange on the a-alkoxy stannanes and subsequent addition of carbon dioxide led to optically active (7-protected a-hydroxy acids 18 with retention of configuration and without any loss of stereochemical information11. 

In Lewis acid catalyzed carbonyl additions of allylsilanes and -stannanes, syn diastereoselectiv-ity predominates, irrespective of the double-bond configuration, indicating that open-chain transition states are involved. 

Tributyl[( )-l-(methoxymethoxy)-2-butenyl]stannane (4) is available by the addition of tri-butyltin lithium to 2-butenal followed by treatment with chloromethoxymenthane 03. The resulting diastereomers were separated by chromatography to give stereochemically homogeneous a-alkoxystannanes 5 and 6104. 

Stannylation of lithiated allyl ethers gives (Z)-3-alkoxyallylstannanes (1)115,116, whereas mixtures of (Z)- and ( )-tributyl(3-methoxy-2-propenyl)stannanes (2) were obtained from free-radical addition of tributyltin hydride to l-methoxy-l,2-propadienel16. 

Table 7. 4-Hydroxy-l-alkcnyl Diisopropylcarbamates from 2-Alkenyl Diisopropylcarbamates by Ho-moaldol Addition of Carbonyl Compounds to Trichlorotitanates (via Stannanes)...

Stannane 6a underwent facile transmetalation in tetrahydrofuran at — 78 °C on treatment with butyllithium to afford 6b. Addition of the lithium reagent 6b to a solution of 1.1 equivalents of copper(I) bromide-dimethyl sulfide in 1 1 diisopropyl sulfide/tetrahydrofuran at — 78 °C gave the copper reagent 6c, which reacted with methyl vinyl ketone at —78 "C in the presence of boron trifluoride-diethyl ether65, producing 7 in 55% yield65. 

In contrast, a-alkoxycuprate 10, prepared from the corresponding stannane, gave the conjugate addition product with only 0-20% ee upon treatment with 2-cyclohexenone in the presence ofchlorotrimethylsilane67. However, the conjugate addition of 10 to ethyl propynoatc in the presence of chlorotrimethylsilane provided 11 in 85% yield with nearly 96% ee67. 

As with the silanes, the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C-Sn bond and it is a weaker bond.156 The most useful reactions in terms of syntheses involve the Lewis acid-catalyzed addition of allylic stannanes to aldehydes.157 The reaction occurs with allylic transposition. 

With chiral aldehydes, reagent approach is generally consistent with a Felkin model.163 This preference can be reinforced or opposed by the effect of other stereocenters. For example, the addition of allyl stannane to l,4-dimethyl-3-(4-methoxybenzyloxy)pentanal is strongly in accord with the Felkin model for the anti stereoisomer but is anti-Felkin for the syn isomer. 

Reactions proceeding through open TS In this group, exemplified by BF3-catalyzed additions of allylic silanes and stannanes, the degree of stereochemical control is variable and often moderate. The stereoselectivity depends on steric factors in the open TS and can differ significantly for the E- and Z-isomers of the allylic reactant. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

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