Stannanes electrophilic substitution reactions

The heteroaromatic stannanes undergo the normal electrophilic substitution reactions of their protic precursors, and often to an enhanced degree. They are often prepared with the aim of a subsequent Stille cross-coupling reaction, and oligothiophenes with potentially useful optical and electron properties have been prepared by coupling between stannyl- and bromo-thiophenes, for example, Equation (63).204... 

Alkenes undergo electrophilic substitution reactions by the same addition-fragmentation mechanism as do arenes. Alkenylsilanes and -stannanes are especially good substrates for electrophilic substitution reactions because the carbo-... 

Aside from substitution reactions on silicon, interesting reactivity on SCB-substituted methylstannane has been described <2005CC3047>. Upon treatment of the mixed SCB/stannane substrate with -BuLi, transmetallation predominates over ring-opening and ambiphilic SCB-methyllithium can be generated (product formed after quenching with electrophile was isolated in 50-60% yield, Scheme 64). 

The utility of this chenustry in synthesis is considerably enhanced by the versatility of the aiylsilane and stannane functionalities in further transformations such as electrophilic substitution and migration reactions (Scheme 31). 

The P-effect similarly enhances the reactivity of alkynyl- (Section 8.2.2), alkenyl-(Section 8.1.2), allyl- (Section 9.1.3.2), and aryl- (Section 7.1) stannanes in their reactions with electrophiles (equations 3-33-3-36), and its effect can be recognised in other contexts such as the ene reactions of allylstannanes (Section 9.1.3.4), and the charge-transfer reactions and ring-substitution reactions of benzylstannanes.22... 

Tin, zinc, boron, and mercury metallopyrimidines in alkylation and arylation reactions Acylation by electrophilic substitution Acylation by palladium-catalyzed reactions with stannanes Tin metallopyrimidines in acylation reactions Coupling reactions in quinazolines and perimidines Alkylation and arylation by organometallic adduct formation... 

Alkenyl silanes and stannanes have the potential for nucleophilic delivery of vinyl groups to a variety of electrophiles. Demetallation also occurs in these reactions, so the net effect is substitution for the silyl or the stannyl group. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

The electrophile for the cyclization reaction of an a-amino-organolithium compound is not restricted to a terminal (or phenylthio-substituted) ahcene and examples have been reported using carboxylic amides, alkynes and allyhc ethers." " For example, Fautens and Kumanovic reported that treatment of the bicyclic stannane shown in Scheme 24... 

The lithiation and silylation of the amide 71 is enantioselective for a very different reason. When the intermediate organolithium is made from the racemic stannane 72, it still gives the product 74 in good enantioselectivity provided the electrophile reacts in the presence of (-)-sparteine.72 The reaction must therefore be an enantioselective substitution. Furthermore, reaction of the deuterated analogue 75 gives a result which is not consistent with asymmetric deprotonation yield, deuterium incorporation and product ee are all high. 

An interesting carbocyclization process was observed when alkenyl stannanes were treated with electrophilic selenenylating reagents containing a non-nucleo-philic counterion. Thus, Nicolaou showed that compound 213 reacted with AT-PSP 11 to form the intermediate 214 which then afforded the cyclopropane derivative 215 (Scheme 32) [109]. Further examples were reported by Herndon [110]. As indicated in Scheme 32, in the presence of tin tetrachloride, the stannane 216 was converted into the cyclopentane derivative 217. This cyclization reaction proved to be quite general with respect to a variety of substitution patterns but it appears to be restricted to the formation of three- and five-membered ring. 

Three-membered rings have not been metaUated directly in the absence of anion-stabilising substituents but simple lithio derivatives of aziridines can be prepared by exchange from the corresponding stannane. iV-f-Butylsulfonyl-aziridines can be substituted, via non-stabiUsed lithio intermediates, by reaction with LiTMP, usually in the presence of the electrophile. ... 

ACSA(B)62]. The stannanes are available from enol ethers by a-lithiation and quenching with trialkylstannyl chloride. The coupling reactions have been run on derivatives that had either a chlorine atom in an activated position or a bromine atom in the benzenoid position. Mild acid hydrolysis of the a-pyrimidinylalkenyl ethers yields ketones, the acyl-substituted pyrimidines. In the 4,5-dichloro derivative (130), the masked acyl group is introduced into the electrophilic 4-position (131). In the 2,5-disubstituted pyrimidine (133), having a methyl group in the 5-position and a chlorine atom in the 2-position results in the addition of a masked acyl group in the electrophilic 2-position (134). When the 5-substituent in the latter example is a bromine atom, the chemoselectivity leads to masked acylation in 5-position (135). This reaction sequence constitutes a convenient... 

A halogen atom in an electrophilic position can be substituted using a metal stannate. Reactions of the 4-iodo derivative (191) with stannyllith-ium, -sodium, or -copper reagents are run at - 78°C to form the 4-stannane (192) (89T993). 

---