Allylic tributyltin reagents

Nakamura et al.100 found that in the presence of palladium catalysts imines undergo allylation readily, providing the corresponding homoallylamines with high yields. Thus, chiral palladium complex 157 has been synthesized and applied in the asymmetric allylation of imines using allyl tributyltin as the allylation reagent. In Scheme 3-54, moderate yield and up to 82% ee have been obtained with 157 as the chiral catalyst.101... 

MgBr2-mediated asymmetric nucleophilic addition of Grignard reagents and allyl-tributyltin to aldehydes bearing sugar-derived jS- or y-tetrahydropyranyloxy chiral auxiliaries designed to complex with MgBt2 has been achieved. ... 

Transmetalation of cinnamyl and crotyl tributyltin reagents by SnCl2 in acetonitrile has been proposed to explain the predominant formation of anti homoallylic alcohols from aldehydes (Table 23) [41]. In contrast, the syn adducts predominate when these reactions are conducted in CH2CI2. The exchange reaction in acetonitrile was confirmed by the quantitative formation of BusSnCl. Presumably the putative allylic SnCl species is stabilized by complexation with acetonitrile (Eq. 25). 

The silylated tin compound 199, obtained from tributyltin hydride and N-bis(trimethylsilyl)propargylamine (198) in the presence of a trace of AIBN (2,2/-azobisisobutyronitrile), is a versatile reagent for the preparation of allylic amines. Treatment with aryl bromides ArBr (Ar = Ph, 4-MeOCgH4, 4-O2NC6H4 etc.) under Pd(PPh3)4 catalysis yields the silylated amines 200, which are hydrolysed by acids to the free amines 201. 199 is converted into the lithium compound 202, which is transformed into 203 by aqueous ammonium chloride and into 204 by the action of alkyl halides RX (R = Me, Et or allyl) (equation 76)204. 

A further advantage is that the tin reagent is prepared in quantitative yield by coupling of tributyltin chloride and allyl chloride in the presence of Mg turnings (1 equiv.) in THF under ultrasound irradiation. 

Annelation.1 This reagent is used for conversion of 3-substituted aldehydes (or acetals) into methylenecyclohexanes. The conversion involves addition of the allylsilane group to the aldehyde or acetal to provide an adduct that undergoes radical cyclization via the allylic sulfide group. The phenylthio group is used to enhance 6-endo cyclization over the usual 5-exo cyclization. In addition it allows use of bis(tributyltin) as the initiator (14,173-174). Unfortunately the radical cyclization shows only slight stereoselectivity. 

Allylic oxidation of olefins is a reaction of considerable value in organic synthesis [18] and selenium dioxide itself or in combination with other cooxidants remains a highly predictable and reliable reagent to perform these reactions. Thus, selenium dioxide oxidation of (Z)-tributyltin 1-alkenylcarba-mates 46 constitutes the first successful example of such a conversion ever reported with an element other than hydrogen [19]. Namely, it was found that with the allylic stannanes 46a or 46b oxidation occurred smoothly within 15 min to deliver in good yields the expected corresponding allylic alcohols 47a and 47b, respectively (Eqs. 6 and 7). 

Tributyltin cuprate has been prepared and used as a milder alternative to tributyltin lithium for application in sugar chemistry. Thus tin derivative 39 was prepared from the corresponding allylic bromide without any allylic rearrangement. (See Section 4 below for the use of this reagent in the preparation of acyclic alkenes). 

Sodium borohydride adsorbed on alumina is a new reagent combination for the reduction of aldehydes and ketones to alcohols in aprotic solvents such as ether. Cyanoborohydride anion, [BHsCN] , supported on an anion-exchange resin containing quaternary ammonium groups has been found in preliminary results to be a successful and convenient reagent for, inter alia, the reduction of enones to allylic alcohols. Ease of work-up and retention of cyanide by the resin are obvious advantages. Carbonyl compounds can also be reduced to alcohols by tributyltin hydride in a cyclohexane-silica gel slurry aldehydes are selectively reduced in the presence of ketones and products are obtained by simple elution from the silica. Adsorption, and consequent polarization, of the carbonyl group by the silica seems to be important in the mechanism of this mild, non-basic, process. 

Tributyltin hydride-mediated hydrodebromination of bromoarenes is selectively carried out. However, removal of the tin residue should be carefully carried out. The 10% w/w anhydrous potassium carbonate-silica repeatedly reduces organotin impurity levels below 15 ppm in this system (Table 3-12). In addition, organotin reagents such as tributyltin hydride, allyl stannanes, vinyl stannanes, arylstannanes, and distannanes can be purified by the K2CO3 and silica column chromatography. On the contraiy, halostannanes and sulfonated stannanes are trapped through this process (Table 3-13). 

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