Transition metal stannyl

For example, the sensitive imidoyl chloride moiety at the C-3 position of the pyrazinone scaffold is known to vmdergo Stille reactions with a variety of tetraaryltin reagents, generating the corresponding 3-substituted pyrazi-nones (Scheme 10) [26]. Furthermore, the transition-metal-catalyzed stannyl-ation at the C-3 position is also documented in the hterature, in view of cross-couphng with a variety of alkyl and (hetero)aryl hahdes [26]. However, this strategy is completely restricted to the C-3 position, while the Cl atom of C-5 position was found to be inert under these conditions. 

Other methylphenyl(2-phenylpropyl)stannyl-transition metal complexes are oily compounds 18). Fractional recrystallization could therefore not be applied to separate those diastereomers. For the irondicarbonylcyelopentadienyl compound (77), the diastereomeric ratio (77)A/(77)B = 45/55 could be reached by the standard route (see Fig. 9) but could not be modified by column chromatography. 

Fig. 9. Synthesis of methylphenyl(2-phenyl)stannyl-transition metal complexes I8).

The optical stability of methylphenyl(2-phenylpropyl)stannyl-transition metal complexes decreases as follows ... 

In an interesting contrast to the analogous silicon and germanium systems, catalytic addition of the unactivated hexamethyldistannane to unsaturated substrate readily occurs with the common palladium catalysts Pd(PPh3)4 or Pd(dba)2. In fact, prior to 1991 this was the only distannane investigated for addition reactions. The first example of transition metal-catalyzed double stannylation of unsaturated molecules, palladium-catalyzed cis addition of hexamethyldistannane to 1-alkynes, was reported in 1983 by Mitchell and co-workers.66 More recently, the system has been extended to include other substrates and both the hexaethyl- and the hexabutyldistannanes. All examples reported involve the use of a Pd(0) complex as the catalytic species. 

Intermolecular free-radical additions of stannyl radicals to multiple bonds have emerged as important methods for the preparation of tetraorganostannanes which can be reacted further to afford new C—C bonds through transition metal mediated coupling processes (e.g. Stille coupling). There are numerous examples of this chemistry715-737, and this treatise will focus on a few selected examples. 

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)). 

Organosilyl substituents on a transition metal can be replaced by other silyl groups210 or, more readily, by germyl and stannyl groups35 160>,62> 169) because there is enhanced stability of El-M bonds in the order El = Sn > Ge > Si. Examples with some peculiarities are shown in Eqs. (51)-(53) ... 

The classical routes to 2//-pyran-2-ones (-pyrones) are illustrated in Scheme 124 . Additional examples include various transition metal-catalyzed cyclizations , e.g., the palladium-catalyzed reaction of -chloroacrylates with internal alkynes (Scheme 125) <2001NJC179> and the coupling of tributyl-stannyl allenes with (Z)—iodoacrylic acid (Scheme 126) <2005JOC6669>. 

Recent Developments in Theoretical Organometallic Chemistry. 15, I Redistribution Equilibria of Organometallic Compounds, 6, 171 Redistribution Reactions of Transition Metal Organometallic Complexes, 23, 9S Redistribution Reactions on Silicon Catalyzed by Transition Metal Complexes, 19, 213 Remarkable Features of (7] -Conjugated Diene) zirconocene and -hafnocene Complexes, 24, I Selectivity Control in Nickel-Catalyzed Olefln Oligomerization, 17, lOS Silyl. Germyl, and Stannyl Derivatives of Azenes. N H Part I. Derivatives of Diazene, N,H2, 23, 131... 

Carbostannylation sometimes proceeds with no activators, but the applicability of such reactions is severely limited to a few types of special substrates. The development of activation methods in the mid 1990s made carbostannylation a synthetically significant tool. Three major activators are now available. Thus, radical initiators, Lewis acids, and late transition metals were found to be effective as activators by Hosomi, Yamamoto, and Shirakawa-Hiyama, respectively. These activators are required only in a catalytic amount to promote the reaction. Scheme 5.7.2 summarizes the mechanism of the activation of carbon-tin bonds by these activators. The cleavage of carbon-tin bonds by a radical initiator gives a stannyl radical, which adds to an unsaturated bond. The resulting alkenyl or alkyl radical abstracts an R... 

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