Tin-to-lithium exchange

The progress has been much slower with acyclic systems. However, the trans-metallation of N-benzyl-N-Cbz- [350], N-Boc-N-methyl- [351], and N-Boc-N-tertbu-tylthiomethyl- [352] protected aminotins provided the corresponding amino-lithium derivatives that may be trapped with electrophiles with complete retention of configuration. This chemistry was applied to the preparation of /3-aminoalco-hols, N-methyl-/3-aminoalcohols and N-methyl-/3-amino acids. It can be noted that the preparation of enantio-enriched a- or /3-amino carbanions can be achieved starting from linear racemic precursors when the tin-to-lithium exchange is done in the presence of (-)-sparteine [353]. 

Finally, an aza-Wittig rearrangement of acyclic enantio-enriched N,N-diallyllic a-amino alkylithium prepared via a tin-to-lithium exchange was reported, the process proceeding predominantly with inversion of configuration at the lithiumbearing carbon terminus [361]. 

The tin-to-lithium exchange in alkenyltins is characterized by the preservation of the alkene stereochemistry and is compatible with several functionalities on the alkene moiety such as amino, hydroxyl and ester groups. The transmetallation of functionalized vinyltins has found an application in the synthesis of complex molecules such as unsaturated fatty acids [362], prostaglandin side-chain [363], various antibiotics ]364] and a biotoxin [365]. 

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

Tin/lithium exchange on the a-alkoxy stannanes and subsequent addition of carbon dioxide led to optically active (7-protected a-hydroxy acids 18 with retention of configuration and without any loss of stereochemical information11. 

Two unique type Had syntheses of pyrroles that were reported both involved cyclopropane fragmentations. The first allowed for a synthesis of 2-arylpyrroles <06SL2339>. In the event, treatment of stannylcyclopropane 25 with -BuLi followed by benzonitrile produced 2-phenylpyrrole 26 via tin-lithium exchange, addition to the nitrile, ring fragmentation of ketimine intermediate, intramolecular condensation, and loss of dibenzylamine. 

Tetrahydrofurans.s The (tributylstannyl)methyl ethers (1) of homoallylic alcohols (9, 475) on treatment with butyllithium undergo tin-lithium exchange to a-... 

The tin-lithium exchange reactions are thought to proceed with retention of stereochemistry. However, as the stannanes employed in this study were racemic, there is no evidence in support of this pathway. 

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyUithiums. These results are in sharp contrast to the earlier tin-lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (ii)-vinyllithiums. ... 

Tin and selenium derivatives (runs 11 and 12) undergo lithium-tellurium exchange preferentially to lithium-tin and lithium-selenium exchange. - ... 

Adding electrophiles externally (after complete lithiation) failed, and in order to make potential ligands based upon the arenechromium skeleton, it was necessary to useBusSnCl in an internal quench procedure. Reduction of 430 (X = Sn) yielded a phosphine without decomplexation, and tin-lithium exchange can lead to a variety of products 431 in about 70% ee. 

The tin-lithium exchange reaction does not play an important role for the generation of 1-halo-l-lithioalkenes. Whereas in a-bromoalkylstannanes tin-lithium exchange occurs upon treatment with n-butyllithium , the bromine rather than the tin is replaced against lithium, when a-bromovinylstannanes are allowed to react with w-butyllithium . 

The tin-lithium exchange is of particular interest for the generation of configurationally stable, non-racemic a-lithiated alkyl ethers 19 (equation 10). The metal-metal exchange has been found to occur under retention of the configuration. Examples of this method... 

The tin-lithium exchange is also suitable for the generation of a-lithiated oxiranes 53245-247 jjyg jQ jjjg enhanced acidity of a carbon atom incorporated into a three-membered ring, the metalation of epoxides by treatment with various alkyllithium reagents of lithium amide bases also permits one to obtain carbenoids 53 in situ (equation 35) °. 

In our group, the tin-lithium exchange has been used to synthesize the doubly lithiated dimethyl sulphane 99 by the reaction of bis(tributylstannomethyl)sulphane (98) with two equivalents of n-butyllithium in diethyl ether". Two equivalents of tetrabutylstannane are cleaved off in this reaction (Scheme 34). 

Also by tin-lithium exchange, Ashe and coworkers were able to generate 2,5-dilithio-l,5-hexadiene (104), a divinyllithinm componnd . The reaction, starting from 2,5-bis(trimethylstanno)-l,5-hexadiene (103), is beheved to proceed via the dimeric pen-tacoordinated lithium stannate 105 (Scheme 36). 

De Meijere, StaUce and coworkers were able to generate dilithiated 111, where the tricyclic non-metalated form can be considered as a subunit of the smallest possible fullerene By a two-fold tin-lithium exchange of the bis(trimethylstanno) derivative 110 with methyllithium, the dilithium compound 111 was cleanly obtained (Scheme 39) its solid-state structure could be clarified by means of X-ray crystallography. 

In contrast to tin-lithium exchange, doubly hthiated bis(hthiomethyl)silanes 117a and 101 with methyl or phenyl substituents at the silicon centre are accessible from bis(teUurio-methyl)silanes 116a,b by a two-fold teUurium-lithium exchange (Scheme 42) . 

In 1991, Kessar and coworkers demonstrated that the kinetic barrier could be lowered by complexing the tertiary amine with BF3, snch that i-BuLi is able to deprotonate the ammoninm compound, which can be added to aldehydes and ketones as shown by the example in Scheme 4a. Note the selectivity of deprotonation over vinyl and allyl sites. A limitation of this methodology is that the ylide intermediate does not react well with alkyl hahde electrophiles. To get aronnd this, a seqnence that begins with the stannylation and decomplexation shown in Scheme 4b was developed. The stannane can be isolated in 94% yield (Scheme 4b) and snbseqnently snbjected to tin-lithium exchange to afford an unstabilized lithiomethylpiperidine that is a very good nucleophile. However, isolation of the stannane is not necessary and a procedure was devised in which the amine is activated with BF3, deprotonated, stannylated, decomplexed from BF3 with CsF, transmetalated back to lithium and alkylated, all in one pot (Scheme 4c). ... 

Generation of an a-amino-organolithium by tin-lithium exchange was first introduced by Peterson in 1970, who showed that these compounds add well to aldehydes, but... 

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