Trialkyltin esters

In the solid state, the structures of most compoimds of most trialkyltin esters approximate die polymeric bridged structure where the oxygen atoms occupy the apical positions in a somewhat distorted trigonal bipyramid. For example, in trimethyltin acetate, the Sn-0 bond distances are 220 and 230 pm and the O-Sn-0 bond angle about 172°. For the corresponding triphenyltin acetate the values are 218.5 and 235 pm and 174°. 

These observations do not, however, mean that TBT carboxylates and TBTCl are ionic in nature. After detailed analysis of the physical evidence such as the low specific conductance and dipole moment of trialkyltin halides, Neumann has concluded that they have no "salt-like constitution" (6). Bonding in the trialkyltin carboxylates also is essentially similar to that in covalent alkyl esters, as evidenced by the low dipole moment of 2.2D for tributyltin acetate in benzene, as compared to 1.9D for alkyl acetates (7). 

The tin content of the tributylstannylcarboxymethyl cellulose ester is practically unchanged on boiling in water for one hour. When subjected to hydrolysis with 0.1 Af solutions of HCl and NaOH for 1 h at 20 C, the tributylstannyl residues of this ester are completely split off. Thus, polymeric acylates ctf trialkyltin, like low-molecular-weight or-ganotin compounds of this type, are unstable to the action of aqueous solutions of acids and alkalis. 

Several functionalized trialkyltin hydrides have been prepared and used in organic synthesis. For example, an optically active organotin hydride with binaphthyl as chiral center underwent hydrostannylation with methyl methacrylate leading to a -stannyl ester diastereoselectivity, however, was not sufficient [234]. Although a bowl-shaped organotin hydride with bulky aromatic substituents was prepared, the structurally novel tin hydride resulted in quite high chemoselectivity. When tris(2,6-diphenylbenzyl)tin hydride (TDTH) was used for competitive reduction of carbonyls under the influence of a Lewis acid it was observed that unsaturated carbonyl compounds such as benzaldehyde and a,/ -enones are highly resistant to TDTH reduction (Scheme 12.131) [235]. 

In the Suzuki coupling37 the trialkyltin functional group on the nucleophilic partner in the coupling reaction is replaced by a boronic acid 234. These stable compounds are easily made from simple organometallic compounds and boronic esters 233 followed by hydrolysis. 

Conversion of the acyliron group to an ester is accomplished by reaction with )V-bromo-succinimide (NBS) in alcoholic dichloromethane. This transformation is general and is compatible with the presence of the trialkyltin group. 

In efforts to prepare the ethynylphosphonic diesters 224 (R = H), reactions have been carried out with 223 (R = Me3Si, X = 1)" and with 223 (R = Et3Sn)" Thermolysis of the silicon-containing esters afforded the corresponding 224 (R = and the trialkyltin... 

Unfortunately, the radical chain reaction of simple S-phenyl thiol esters was not efficiently triggered by trialkyltin radicals [58]. The homolytic bond dissociation of S-2-naphthyl and other thiol esters by photolysis also seems to be a low quantum yield process ( =0.08-0.10) [59]. 

In 1980, Graf and co-workers reported that phenyl selenol esters underwent reduction to the corresponding aldehydes and alkanes (reduction and decar-bonylation) in the presence of trialkyltin hydrides and a free-radical initiator through generation of acyl radicals (Scheme 21) [97]. 

Unbranched and substituted alkyl carbamates, such as 146 or 147 [62,88] do not cause any problems in the deprotonation step. Deuteration (with CH3OD or dissolved CH3CO2D), methoxycarbonylation (gaseous CO2, followed by diazomethane after work-up or methyl chloroformate), alkylation with methyl iodide, substitution with trialkylsilyl chlorides, trialkyltin chlorides and even tri-methyllead bromide, addition onto aldehydes and ketones, and acylation with acid chlorides or esters, all proceed without difficulties. Although a ketone is formed in the latter reactions, which is at least 15 orders of magnitude thermodynamically more acidic than the alkyl carbamate we never observed enolate formation, racemization or epimerization - with one exception it occurred to some extent after formylation with formate esters [79]. 

An alternative procedure for reductive decarboxylation without the use of trialkyltin hydrides as hydrogen atom donors has been developed Alkane carboxylic acid esters derived from AT-hydroxypyridine-2-thione decomposed to alkyl radical, which can readily accept a hydrogen atom from t-BuSH (equation 74) to give alkanes. This reaction can be conveniently performed as a one-pot experiment wherein the acid chloride of an alkane carboxylic acid, the sodium salt of thiohydroxamic acid, t-BuSH and 4-dimethyl-aminopyridine (DMAP) in benzene solution are heated to reflux. This procedure works well for COOH groups attached to primary and secondary carbon atoms. Instead of AT-hydroxypyridine-2-thione, one can use other thiohydroxamic acids, viz. iV-hydroxy-AT-methylthiobenzamide, 3-hydroxy-4-methylthiazole-2(3if)-thione (equation 75) and l-iV-hydroxy-3-AT-methylbenzoylenethiourea for decarboxylation reactions. 

Although 2-[(trimethylsilyl)methyl]allyl esters and halides react with imines to give pyrrolidines (Scheme 22),the cycloaddition reaction with carbonyl compounds requires much more drastic conditions. On the other hand, 2-[(trialkyIstannyl)methyl]allyl acetate can react with aldehydes to give furanyl compound, where the presence of trialkyltin acetate, as a by-product, is crucial to compel the reaction successfully (Scheme 23). In fact, in the presence of the hialkyltin acetate, even the 2-[(trimethyl silyl)methyl]allyl ester undergoes [3 + 2] cycloaddition reactions with aldehydes (Scheme 24) and ketones successfully.Similarly, InCls is also an efficient additive for the cycloaddition reaction and both aldehydes and ketones are suitable as sub-strates. ... 

Allyl halides are probably the most common allylic reagents used. Palladium complexes with not only simple 77 -allylic moieties, but also cyclic derivatives, t/ -benzyls, 77 -allylic complexes derived from morphine alkaloids, and trialkyltin-subtituted T/ -allyls have been synthesized by oxidative addition of appropriate halides. Allyl carboxylates, especially acetates, are also common. When combined with other fimctionalities in the substrate, they lead to functionalized T/ -allylic Pd complexes. Examples of ester-substituted T/ -allyls obtained by reaction of the corresponding allylic trifluoroacetates 88 have been reported. Palladium complexes that bear an 7 -allyl with a pendant 2-pySiMc2-, where the py group coordinates to Pd, have been prepared from the corresponding allylic acetates 89. Allyl carbonates are also commonly used both in the stoichiometric preparation of -allylic complexes and in catalytic reactions that proceed through Pd -allylic intermediates. ... 

Hydroxy-esters.—a-Keto-esters can be reduced to a-hydroxy-esters using trialkyltin hydrides/ Unfortunately, reductions of ( - )-menthyl benzoylfor-mate with these reagents results in only 5—20% asymmetric induction. 

Trialkyltin alkoxides also react with ketene with formation of the normal insertion product antimony tris-alkoxides react with ketene to give a tris ester and triphenyllead acetate reacts with ketene in ethanol to give ethyl (triphenyllead)acetate . 

Finally to the list of carboxyl protecting groups must be added triethyl- and tri-n-butyl-tin esters [148, 149] which are prepared by heating the acid with the trialkyl tin oxide or hydroxide azeotropically in benzene. The ester group is readily cleaved by dilute acid or base and the tin can be recovered as the trialkyltin acetate. 

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