With Doubly-bonded Electrophiles

The aminotin compounds show the same type of addition reactions which are discussed for the alkoxytin compounds in Section 14.1.3. 

The addition of dialkylaminostannanes to aldehydes and ketones often gives a mixture of tin-free products derived from addition and elimination reactions of the monomeric carbonyl compound and of its products of base-catalysed aldol condensation. With appropriate substituents, however, reasonable yields of the enamines can be isolated, for example 38 

In the reaction with chloral, Bu3SnCCl3 is eliminated, and diethylformamide is formed.39 

Distannazanes react with aldehydes and ketones to give the corresponding imines via addition to the carbonyl group, then elimination of bis(tributyltin) oxide (equation 16-49) 36 the intermediate can be identified by acetylation as shown in equation 16-46. 

Et2C=0 + (Bu3Sn)2NEt ---- Et2C -rp+ Et2C=NEt + (Bu3Sn)20 (16-49) 

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For propylene, the condensed Fukui function not only predicts that the electrophilic attack occurs on one of the doubly bonded carbon atoms, but it also predicts that there is a preference for the terminal carbon, in accordance with Markonikov s rule. Nucleophilic attack is predicted at the boron atom in BF3. 

The electron-deficient B of BH, as an electrophilic site, reacts with the n electrons of C==C, as the nucleophilic site. In typical fashion, the bond is formed with the C having the greater number of H s, in this case the terminal C. As this bond forms, one of the H s of BH, begins to break away from the B as it forms a bond to the other doubly bonded C atom giving a four-center transition state shown in the equation. The product from this step, CH,CH,CH,BH, (n-propyl borane), reacts stepwise in a similar fashion with two more molecules of propene, eventually to give (CH,CH2CH,),B. 

A plausible mechanism is shown in Figure 17.33. It is assumed that the enolate, not the ketone itself, reacts with the permanganate. The Mn( )4 anion presumably reacts with the enolate carbon electrophilically with one of its doubly bonded O atoms. The manganese(V) acid monoester C is formed and is oxidized by a permanganate ion to give the analogous man-ganese(VII) acid monoester F. The subsequent chemistry of F is difficult to predict. 

The trend in activation energies (Table I) shows the electrophilic nature of attack by the sulfur atom increasing number of alkyl substituents on the doubly bonded carbons decreases the value of E, while increasing number of halogen substituents on the doubly bonded carbons increases the value of . These variations are correlated with molecular properties such as ionization potentials, excitation energies, and bond... 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

The a-alkylation of carbonyl compounds by their conversion into nucleophilic enoiates or enolate equivalents and subsequent reaction with electrophilic alkylating agents provides one of the main avenues for regio- and stereo-selective formation of carbon-carbon a-bonds. " Classical approaches to a-alkylation typically involve the deprotonation of compounds containing doubly activated methylene or methine groups and having p/iTa values of 13 or below by sodium or potassium alkoxides in protic solvents. Since these conditions lead to monoenolates derived from deprotonation only at the a-site of the substrate, the question of the regioselectivity of C-alkylation does not arise (however, there is competition between C- and 0-alkylation in certain cases). In more recent years, dienolates of p-dicarbonyl compounds have been utilized in -alkylations with excellent success. 

Doubly lithiated nitroalkanes react rapidly with electrophiles providing an improved method for C-C bond formation from nitroalkanes.31... 

The deprotonated tris(ethylenediamine) complexes are, not unexpectedly, nucleophilic. The pale yellow doubly deprotonated complex [Rh(en — H)2(en)]I acts as a nucleophile toward Mel, MeBr, SOClj, SO2CI2, and triply deprotonated [Rh(en — 11)3] is electrophilically attacked by Mel (Scheme 16). The methylated species are air and HjO stable, but while air stable, the BF3 and SO2CI2 adducts hydrolyze rapidly. The low S—O stretching frequencies observed led to the suggested bonding configurations shown in Scheme 16, with the sulfur atoms bonded to two amine nit-... 

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