Synthesis electrophilic/nucleophilic metal compounds

Three-component reactions between organic electrophile (halide, ester, etc.), carbon monooxide, and organic nucleophile (organometallic compound) (Equation (1)) catalyzed by transition metal complexes afford a powerful method for the synthesis of various ketones. The pioneering works in this area appeared in the early 1980s. 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

Applications of the stannylalkali metal compounds in synthesis are based on their provision of nucleophilic R3Sn for reaction with electrophiles. Kinetic and mechanistic studies have been carried out only where the electrophile is an alkyl or aryl halide, when the behaviour of R3S11M has been referred to as that of a supemucleophile, and a variety of detailed mechanisms have been identified by which overall substitution can be achieved (see Section 4.2). 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

The ROP of lactide in the presence of metal compounds of tin, aluminum, zinc, titanium or zirconium as catalysts proceeds via the coordination-insertion mechanism. The initiators in this type of reaction are usually metal alkoxides. In the first step, temporary coordination of lactide through the carbonyl group with the metal in the initiator leads to increased nucleophilicity of the alkoxide and electrophilicity of the carbonyl group, thereby facilitating the insertion of the monomer into the metal O bond. This is the most investigated and applied method for the synthesis of PLA due to the mild reaction conditions, since the reaction proceeds via covalent species. High molecular weights of 200000 g mol are easily achievable with minimum side reactions and racemization. 

A major application of ion exchange in non-aqueous solvents is for the synthesis of organometallic compounds. The common methods fall into two distinct classes. In one, the reagents are electrophilic carbon compounds, such as alkyl halides, and nucleophilic metal complexes, such as carbonylmetallates. In the other, the relative polarities of the reagents are reversed and nucleophilic carbon compounds, e.g. alkyllithium reagents, and electrophilic metal complexes, such as metal halides, are employed. Both classes of reaction are ion exchange processes and typically involve the formation of a salt as the low-solubility by-product. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

A greatly improved experimental procedure for the synthesis of thieno[2,3-d]-1,2,3-thiadiazole caiboxylates 68 was reported by Stanetty et al. and involved diazotisation of aminothiophene derivatives 67 <99JHC761>. In these systems, substituents could be introduced into the 5-position by nucleophilic displacement of a chlorine atom or by metallation of the unsubstituted compound (68 R = H) and subsequent electrophilic quenching <99JPR391>. 

Carbon disulfide is the dithio derivative of C02. It is only a weak electrophile. Actually, it is so unreactive that in many reactions it can be used as a solvent. Consequently, only good nucleophiles can add to the C—S double bond of carbon disulfide. For example, alkali metal alkoxides add to carbon disulfide forming alkali metal xan-thates A (Figure 7.4). If one were to protonate this compound this would provide compound B, which is a derivative of free dithiocarbonic acid. It is unstable in the condensed phase in pure form, just as free carbonic acid and the unsubstituted carbamic acid (Formula B in Figure 7.3) are unstable. Compound B would therefore decompose spontaneously into ROH and CS2. Stable derivatives of alkali metal xanthates A are their esters C. They are referred to as xanthic add esters or xanthates. They are obtained by an alkylation (almost always by a methylation) of the alkali metal xanthates A. You have already learned about synthesis applications of xanthic acid esters in Figures 1.32, 4.13, and 4.14. 

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