Nucleophilic attack, propargylic centers

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

However, not only heterocyclic but also carbocyclic systems can be made via nucleophilic attack at propargylic centers. This is usually done intramolecularly as shown in Eq. (7), when a malonic ester anion displaces a propargylic chloride [16]. 

Cyanide addition to the lactamic carbonyl group has been described in a reaction in which the cyanide ion acts as a catalyst (Fig. 14).The intermediate acyl cyanide can be attacked by an added nucleophile (allylic, propargylic, benzylic alcohols, aniline, benzylmercaptan). Comparative experiments were carried out using more classical procedures, such as under catalysis by potassium cyanide with stirring at room temperature, and with sodium alkoxides at -78 C. This last method provides the highest yields, up to 95% in most of the cases tested, but the sonochemical method proceeds under less basic conditions. Both methods preserve the integrity of the asymmetric center. 

The propargylic cations [Co2(/i,i/2,Tj3-RC2CR2)(CO)6]+ react as electrophiles with a variety of heteroatom- and carbon-centered nucleophiles to provide, following demetalation, propargylated products with complete regioselectivity. Complexation of the triple bond circumvents isomerization to allenic products. Reaction with asymmetrical ketones results in attack by the cation exclusively (>95%) at the more substituted a-carbon.72,74 (See Scheme 11.)... 

Doubly (and even triply) charged anions appeared on the horizon of organic chemistry relatively recently, They were immediately recognized as extremely useful intermediates due to the synthetic opportunities offered by their unique pattern of reactivity. Besides the dianion 174, the dianions of carboxylic acids 178 (Scheme 2.82), propargylic dianion 179, and the dianion 180 derived from propargyl alcohol are now widely used as nucleophiles. Selectivity in electrophilic attack for these species is governed in much the same way as was described for 174 the least-stabilized anionic center is the preferential site of attack by the electrophile (marked with an asterisk in Scheme 2.82). 

The jt-allyl ligand bonded to cationic Pd(II) center tends to undergo nucleophilic substitution by organic nucleophiles from the opposite side of the metal center (Chapter 8). Attack of Pd(0) or Pt(0) complex in this fashion causes inversion of stereochemistry of the complex repetition of the reactions results in epimerization. The allyl ligand transfer between Pd centers takes place much faster than that from Pd to Pt. Isomerization between the Pt(II) complexes with a propargyl ligand and that with an allenyl ligand is catalyzed by Pd(0) complex (Eq. 5.58) [200,201]. 

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