Stannyl substituents

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

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. 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

Stannylated ferrocenes can be prepared by the reaction of a lithiated ferrocene with an organotin chloride, or of a lithiated stannylcyclopentadiene with FeCl2, for example, Scheme 10.305 The products can be described by the shorthand FcSn.. . where x and y indicate the number of stannyl substituents in each of the two rings. 

Most of the allenes with silyl or stannyl substituents are prepared by the routes discussed in Chapter 9. 

In all these structures the Sn-C-C=C angle is close to optimum for g-tc interaction between the C(allyl)-Sn g bond and the n system. In the two crystallographically independent molecules of cyclopenten-2-yl-triphenylstannane 197 and cyclohepten-2-yltriphenylstannane 199, the stannyl substituent takes up a pseudoaxial orientation. Although cyclohex-2-enyltriphenylstannane 198 exists in the solid state with the stannyl substituent in a pseudoequatorial orientation (Sn-C-C=C 120°), it exists in solution predominantly in a pseudoaxial conformation. Analysis of the structural parameters in 198 and 199 provides some tentative structural evidence for the presence of the o-Jt interaction in these compounds the Sn-C(allyl) bond distances, which are 2.189(5) and 2.182(5) A, respectively, are slightly longer than the normal Sn-C(aliphatic) distance, which is typically of the order of 2.13 A, consistent with the expected structural effects of electron donation from the Gc Sn orbital into the n orbital. 

FIGURE 70. Calculated transition states at MP2 of the 1,2-migration of silyl, germyl and stannyl substituents of radicals H3E-CH2X to H3EX-CH2 (X = CH2, NH, O E = Si, Ge, Sn). Bond distances are in A, angles in deg. The calculated activation energies (kcal mol 1) at QCISD//MP2 refer to the forward reaction ( A E1 i ) and the reverse reaction (A E ), respectively. Reproduced by permission of The Royal Society of Chemistry from Reference 190... 

Table 24 Synthesis of phospholes with silyl and stannyl substituents...

The reaction of bis-silyl- or stannylcarbodiimides 41 with carboxylic acid halides or anhydrides,affords carbonylcarbodiimides 42 having one silyl or stannyl substituent. 

The synthetic power of these silylcuprate and stannylcuprate reactions lies in the synthetic utility of the product silanes and stannanes for carbon-carbon bond formation and also in the utilization of the silyl [38] or stannyl substituents as agents for stereocontrol and regiocontrol. Additionally use of appropriate silylcuprates permits conversion of the produced C Si bond into a C OH bond (Tab. 3.1) [39]. This C Si to C OH conversion is a particularly difficult transformation for an allyl silane and the development of lithium diphenyl(2-methyl-2-butenyl)silylcuprate for this purpose illustrates the characteristic transformations of silylcuprates (Scheme 3.1) [14e]. Several silyl substituents convertible into hydrosy groups are not amenable to the cuprate methodologj vinyl silanes - generated by... 

The isomerization of 3 into 15 implies the migration of the two substituents from the carbon atom to the two nitrogen atoms, and also a rearrangement of the CNN skeleton. Since under these experimental conditions diazo derivatives usually loose dinitrogen, this complicated process is very surprising One can easily anticipate that the migrating aptitude of the stannyl substituents is a key point of the process. 

Pyrroles. The general synthesis of substituted pyrroles by the [3+2]cycloaddition pathway involving TOSMIC and conjugated carbonyl compounds has been extended to 3-arenesulfonylacrylic esters. C-stannylation prior to the cycloaddition enables the preparation of pyrroles containing a stannyl substituent at C-2 ... 

Hyperconjugation by a C-Sn o bond (and indeed by most carbon-metal a bonds) is much more effective than C-H hyperconjugation, and it is an important factor in determining the structure and stability of not only radicals and cations, but also of compounds with filled n systems such as allyl-, benzyl-, and cyclopentadienyl-stannanes. The importance of vinyl-, allyl-, and aryl-stannanes in organic synthesis owes much to the stabilisation of radical and cation intermediates by a stannyl substituent, and under suitable conditions this can accelerate a reaction by a factor of more than 1014. 

In the 1,4-substituted bicyclooctyl system (3-24), the stannyl substituent shows a substantial 5-effect,42 but no evidence could be found for a -effect in (3-25)43... 

Table 3-1 Estimated values of electronic stannyl substituent constants.

In 4-trimethylstannylbenzoic acid, with an observed pKa of 5.98 (estimated with the above data, 6.12) compared with benzoic acid, pKa 4.20, the stannyl substituent is electron releasing. On the other hand, the ESR spectra of the trimethylstannylcyclopen-tadienyl radical (see Section 20.2), and of the trimethylstannylbenzene radical anion (see Section 20.4) indicate that, under these conditions, the stannyl group is weakly electron attracting. 

Table 3-2 Estimated values of steric stannyl substituent constants.

The reactions of diarylcarbenium ions, Ar2CH+, with allylstannanes are kinetically first order in each reagent and involve irreversible addition to give the cation which is stabilised by the P-stannyl substituent, followed by loss of R3Sn+ (or sometimes attack of a nucleophile at C2).40... 

---