Hydrides stannyl

In the field of stannylium ions, n-BusSn B (C6F5)3H , synthesized by the oxidation of stannyl hydride with B(CgF5)3 (Section 2.1.2.2, Scheme 2.2, D), was characterized by Lambert et al. as a tricoordinate stannyl cation becanse of its low-field Sn NMR resonance of 360ppm. The same n-BusSn cation, having a TPFPB connterion, exhibited a less deshielded valne of +263 ppm (at room temperature), which, however, was corrected in snbseqnent stndies to +434ppm (at -Bn3Sn+ TFPB, ... 

The stannyl hydride 30, in contrast, showed nonequivalence of the geminal methyls for the neat sample up to 222 °C. Furthermore, no noticeable line broadening was observed in DMSO-dioxane at 160 °C. Thus, it may be concluded that the hydride 30 is configurationally stable while the chloride 29 is not. 

Upon exposure to 10% Pd/C in pentane, the nonracemic stannyl hydride 37 is converted to a mixture of meso and enantioenriched distannanes 4625. Dimerization of 37 can also be achieved by treatment with LiAlFLt or Me2Hg, but these reactions lead to mixtures of meso and racemic distannanes (equation 12). 

Polymer-supported tin hydrides are being increasingly used to avoid the separation problem,407 12 and the reagent Bu2SnHLi, described above, is useful for attaching a stannyl hydride group to a polymer such as poly(4-chlorobutyl-styrene).397 The use of a non-cross-linked, soluble polystyrene has also been recommended.288... 

Bulky stannyl anions which are not available from stannyl hydrides (see Section I.A.l) were prepared by using the reaction of alkali metals with tin halides (equations 24 - 26)31,40 41. 

Silyl and stannyl hydrides effect high yield reduction of aryl diazonium salts, and are compatible with a wider range of solvents than is HaP02. NaH prepared situ has been found to be much more active than the commercial product. Using the more active NaH, hydrogenolysis of benzylic halides is possible. Sodium borohydride has been reported to reduce nitriles to amines if Raney nickel is used as catalyst. 

Stannyl hydrides s. Tin hydrides, organo-Stark s. Dean Statistics, mathematical, optimization by - 13, 696 suppl. 27 Stereodiemistry... 

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. 

Radical homologation. This tin pinacolate is known to generate trimethyltin radicals at 60° and appears to be superior to tributyltin hydride as a source of stannyl radicals for addition of alkyl halides to O-benzylformaldoxime (equation I).1 Iodides, bromides, and selenides can be used as radical precursors. The same... 

Most of the many applications of the tin hydrides in organic synthesis proceed by radical chain reactions in which one step involves the reaction of a radical X- with the tin hydride to abstract hydrogen and generate a stannyl radical,... 

Stannyllithium compounds are important as sources of nucleophilic stannyl anions, and the dialkyltin lithium hydrides, R2SnLiH, have recently come to prominence as their reaction with electrophilic alkyl halides gives hydrides, R1R2SnH, with mixed alkyl groups (see Section 3.14.18.1).397... 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

The A -unsubstituted 5-dimethylamino-l,2,4,3-triazaphosphole is converted to its anion by sodium, potassium, or their hydrides (80ZN(B)1222>. Boryl (86PS(28)7I>, silyl (80ZN(B)1222>, stannyl, phosphino, and (thio)phosphinyl <80ZN(B)1222> derivatives have been prepared mostly by reaction of the triazaphosphole with the corresponding chloride the substituents become attached to N-2. 

The most general way to obtain chiral a-stannylated ethers today consists of the asymmetric reduction of acylstannanes34,35,36 using the 2,2 -dihydroxy-l,T-binaphthyl-modified lithium aluminum hydride (BINAL-H) reagent37 and etherification of the crude alcohol with chloro-methoxymethane. 

The mechanism of this transformation is outlined in Scheme 38 and each step has important features. In step 1, the tributyltin radical abstracts the radical precursor X. A possible side reaction, the addition of the tributyltin radical to the allylstannane, is much slower than comparable additions to activated alkenes. Even if this addition occurs, the stannyl radical is simply eliminated to regenerate the starting materials. Thus, for symmetric allylstannanes, this reaction is of no consequence. As a result, the range of precursors X that can be used in allylation is more extensive than in the tin hydride method. Even relatively unreactive precursors like chlorides and phenyl sulfides can be used if they are activated by adjacent radical-stabilizing groups. 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

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