C-Stannylation

Few new methods for direct functionalization of pyrazoles were reported. Notable is the N-hydroxylation of pyrazoles (c/., 17) with benzoyl peroxide [95LA1563]. These N-hydroxypyrazoles, after O-protection, undergo a-metallation and electrophile trapping processes to afford functionalized N-hydroxypyrazoles (19) [95JOC4998], The O-stannylpyrazole (20), on metallation, undergoes an 0 >C stannyl migration to stannylpyrazole (21). Direct sulfinylation of the 5-amino-3-cyanopyrazole (26) occured at C-4 [EUP668269]. 

Although 2 was inert to tributyltin chloride 215), reaction of the readily available 1-vinylcyclopropanols 69 a, b (vide supra, Sect. 4.1) with trimethyl- or tributylstannyl chloride in ether in the presence of triethylamine and DMSO, resulted in ring opening and 3-C-stannylation to provide the enone 411 Eq. (99)217). Under the same conditions, Me3SiCl led to O-silylation exclusively 180) (vide supra. Sect. 4.5.2.1, Scheme 8). Alkylation and acylation of these intermediates involving trans-metalation2161 have been investigated. 

Application of triorganostannyl enolates to organic synthesis was investigated on O-stannyl and C-stannyl enolates. " Sn NMR spectra showed that tribntylstannyl enolate is a mixture of both enolate kinds at lower temperatnre. The O- to C-stannyl enolate ratio is temperature-dependent and it was confirmed to be reversible. 

Trienes A formal 1,4-dehydration of propargylic alcohols to afford cumulenes is realized on C-stannylation and subsequent treatment with MsCl-EtjN. 

Pyrroles. The general synthesis of substituted pyrroles by the [3+2]cycloaddition pathway involving TOSMIC and conjugated carbonyl compounds has been extended to 3-arenesulfonylacrylic esters. C-stannylation prior to the cycloaddition enables the preparation of pyrroles containing a stannyl substituent at C-2 ... 

See also ret 149 see ret 333 for solution- and solid-state Sn NMR data of C-stannylated carbohydrate derivatives. See also ret 181. 

With the substrate (83), the 0-stannyl ketene acetal (84) can be made by 1,4-hydrostannation. Normally, addition of tin hydride to alkenic esters gives the C-stannylated compounds. An example is the reaction of (85) with tri-n-butyltin hydride. ... 

Alkynes are converted to ( )-alkenes in a microwave-assisted hydrosilylation-Stille coupling process, and both reactions are catalyzed by (Ph3P)4Pd. Interestingly, arylation of tributylstannylacetylene is achieved via a Stille coupling and then C-stannylation. ... 

C-Stannyl derivatives of sugars [e.g. (437) and (438)] have been prepared by either the displacement of primary tosylates or the reaction of suitably protected epoxides with triphenyltin lithium in anhydrous... 

The synthesis of tributylstannylglycals by treating glycals with /-butyllithium then tributyltin chloride is successful when the 0-3 is unsubstituted otherwise furans are formed. However, Scheme 1 shows how glycal phenyl sulfones with 0-3 substituents can be converted to the C-stannyl analogues, useful for preparing 1-C-arylglycais. ... 

Scheme 1-175. Divergent site affinities of electrophiles make benzoxazol-2-ylIithium (228a) to opt for C-stannylation and through its ring-opened isomer 228b for O-silylation.

Tin enolates are in metalotropic equilibrium between the O- and C-stannylated forms, so that the enolate form can be considered as the oxygenated analog of an allyl tin. Thus, the 8, 2 reaction can be extended to tin enolates, used as electron-rich scavengers for carbon-centerd radicals [311]. A synthetically useful extension of this reaction proposed the carbostannylation of alkenes with tin enolates [312], which can be associated to cascade radical process (Scheme 6.37). 

Sulfonation of the aromatic ring of l, 2, 2 -trifluorostyrene below 0 C does not give satisfactory yields with chlorosulfonic acid or a sulfur trioxide-dioxane complex Tar forms on heating In contrast, under similar conditions ipso substitution IS facile at the position of a trialkylsilyl or -stannyl group Thus, 4-trimethyl-silyl-T 2, 2 D-trifluorostyrene affords the corresponding trimethylsilyl sulfonate [20] (equation 9)... 

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

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. 

In terms of relative Co—C bond energies, those in the trihalomethyl complexes Co(OEP)CX3 are observed to be qualitatively weaker than in Co-alkyl porphyrin complexes. The high thermal stability of the cobalt porphyrin stannyl complexes was interpreted as an indication that, surprisingly, the Co—Sn bond is stronger than the Co—C bond in Co(Por)(alkyl) complexes. " ... 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

---