Organotin compounds enolates

Besides the silyl enolate-mediated aldol reactions, organotin(IY) enolates are also versatile nucleophiles toward various aldehydes in the absence or presence of Lewis acid.60 However, this reaction requires a stoichiometric amount of the toxic trialkyl tin compound, which may limit its application. Yanagisawa et al.61 found that in the presence of one equivalent of methanol, the aldol reaction of an aldehyde with a cyclohexenol trichloroacetate proceeds readily at 20°C, providing the aldol product with more than 70% yield. They thus carried out the asymmetric version of this reaction using a BINAP silver(I) complex as chiral catalyst (Scheme 3-34). As shown in Table 3-8, the Sn(IY)-mediated aldol reaction results in a good diastereoselectivity (,anti/syn ratio) and also high enantioselectivity for the major component. 

The stemona alkaloid stemonamide (49) was synthesized starting from a-stannyl acetate 47 and 2-stannyl pyrrolidine 48. The oxidative coupling of stannyl acetate 47 with acetylenic silyl enol ether affords the functionalized C-7 unit which corresponds to the side arm of the pyrrolidine ring. Then, introduction of the C-7 unit to the pyrrolidine ring is performed by the oxidative generation of acyliminium ion. The carbon skeleton of stemonamide was thus constructed efficiently as shown in Scheme 19 by employing organotin compounds. ... 

Baba and coworkers reported the unusual reactivity of highly coordinated tin compounds such as organotin(IV) enolates or organotin(IV) hydrides. NMR studies disclose the existence of five-coordinated tin enolate arising from the addition of certain ligands . In particular, the coordination to tin enolates by a bromide anion (from Bu4NBr) shows novel types of selective reactions. ... 

Darzens-type reactionThe Darzens reaction2 of a-halocarbonyl compounds with an aldehyde requires a base, which can promote side reactions. Darzens reactions can proceed under neutral conditions when effected with this neutral reagent (I cquiv.). Presumably, the reaction proceeds via an organotin(lV) enolate. When the reaction is conducted in THF, the rron.v-epoxidc is generally favored, but addition of HMPA can favor cis-sclectivity. ci.v-Selectivity is also favored in reactions of aliphatic aldehydes. The Darzens-type reaction can also be extended to or-halo esters, particularly in the presence of Bu4NF or LiBr. 

It is quite remarkable that in most cases CH-acids are added without either primary transformation into enolate forms or even the addition of a base capable of in situ deprotonation. The reaction with CH-acids without prior deprotonation was described for non-aqueous media for the palladium-catalyzed allylation with allyltin derivatives [69], though the mechanism proposed is quite specific and requires the presence of an organotin compound. By adjusting this mechanism to the reaction in water, the process shown in Scheme 5.5 may be considered. 

Tsuji J, Minami I, Shimizu I. Allylation of ketones via their enol acetates catalyzed by palladium-phosphine complexes and organotin compounds. Tetrahedron Lett. 1983 24 4713 714. 

The opposite polarity in the organotin reagent R3Sn5+X5 can be exploited in, for example, the reaction of a tin alkoxide with an vinyl ester,83 the addition of an alkoxy- or amino-tin compound to a ketene,84 the reaction of a silyl enolate with a tin chloride,85 or the addition of an alkyltin hydride under non-radical conditions to an a,P-unsaturated carbonyl compound.86... 

Organotin enolates (vinyloxystannanes) (14-18), containing an Sn-0 bond, are in metal-lotropic equilibrium with the corresponding Sn-C bonded a-stannylcarbonyl compounds (14-19), providing a further example of the equivalent behaviour of hydrogen (prototropy) and metals (metallotropy) (Section 3.1.1.7). 

The Mukaiyama aldol reaction is a favorable method for the synthesis of P-hydroxy carbonyl compounds in terms of environmental friendliness because the process uses less toxic silyl enolates compared to organotin enolates [52]. Yamagishi and CO workers have examined a BI NAPAg( I)-catalyzed asymmetric Mukaiyama aldol reaction with trimethylsilyl enolates and have found that the reaction is accelerated by BINAP-AgPFe in DMF containing a small amount of water to give the aldol adduct with high enantioselectivity (Scheme 18.14) [53]. 

Addition of tributylstannyl-lithium to crotonaldehyde and protection of the resulting alcohol with chloromethyl methyl ether gives the stannane (192), which reacts with both alkyl and aryl aldehydes RCHO to form specifically the t/rr o-hydroxy-enol ethers (193). These latter compounds have been used to prepare tra/i5-4,5-disubstituted butyrolactones by hydrolysis and subsequent oxidation. Palladium-catalysed carbonylation of RX in the presence of organotin species constitutes a useful synthesis of unsymmetrical ketones, and in the example reported this year RX is an arenediazonium salt. The reaction, which is basically an aromatic acylation, proceeds in good to excellent yield. Another Pd-catalysed reaction of aromatics, this time aryl bromides, is their reaction with acetonyltributyltin (194), prepared from methoxytributyltin and isopropenyl acetate, to give the arylacetones (195). ... 

There is recent spectroscopic evidence that compounds prepared via the enol -ester route (ll) or the addition of organotin hydride to Of, p-unsaturated ketones (l) exist as a mixture of both isomers. In search for a certain compound the reader should always consult Chapter 2.6.3 3 Table 112. 

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