Alkynyl-metal bonds

Cleavage of the alkynyl-metal bond in organometallic derivatives initiated by such agents as H+, Ag+, RCO+ and halogens may be considered formally as electrophilic substitutions. 

Cleavage of the alkynyl-metal bond by aqueous acids, as represented by equation (24), occurs under mild conditions when M is Ge, Sn or Pb. The kinetics... 

The aryl- and alkyl-transition metal bonds are thermodynamically less stable than the alkynyl-metal bond [223]. The number of intermolecular transmetala-tion reactions of alkynyl transition metal complexes, however, is much larger than those of alkyl and aryl complexes as shown above. The stable bridging coordination of alkynyl ligands of the intermediate dinuclear complexes with a 0,7t-coordination probably makes the transmetalation, which involves activation of the thermodynamically stable metal-alkynyl bond, kinetically favorable. 

There is wide diversity in the nature of organohalogen compounds but, of necessity, we have restricted this chapter to alkyl, cycloalkyl, alkenyl, alkynyl, and aryl halides. Some of the chemistry of the carbon-halogen bonds already will be familiar to you because it involves the addition, substitution, and elimination reactions discussed in previous chapters. To some extent, we will amplify these reactions and consider nucleophilic substitution by what are called the addition-elimination and elimination-addition mechanisms. Subsequently, we will discuss the formation of carbon-metal bonds from carbon-halogen bonds. The latter type of reaction is of special value because compounds that have carbon-metal bonds are potent reagents for the formation of carbon-carbon bonds, as we will show later in this chapter. 

In the majority of cases, organolithium compounds and Grignard reagents contain polarized but covalent carbon—metal bonds. Lithioalkanes, -alkenes, and -aromatics, on the one hand, and alkyl, alkenyl, and aryl magnesium halides, on the other hand, are therefore formulated with a hyphen between the metal and the neighboring C atom. Only lithiated alkynes and alkynyl Grignard reagents are considered to be ionic—that is, species with carbon, metal bonds similar to those in LiCN or Mg(CN)2. 

Work in our laboratory directed toward developing a class of transition-metal complexes that are structurally and electronically analogous to conjugated organic compounds and polymers led to the synthesis and spectroscopic study of alkynyl-substituted, quadruply metal-metal bonded compounds of the type M2(C=CR)4(PMe3)4 (6 M = Mo, W R =... 

Organo-silanes and -stannanes possessing sp caibon-metal bonds also add to carbonyls in the presence of Lewis acids. Alkynylation with silyl- and stannyl-alkynes is promoted by AlCb or Z11CI2 (equations 9 and lO). " Notably, the reaction of l,3-bis(trimethylsilyl)-l-prcq>yne with chloral affords ynic alcohol instead of allenylic alcohol, showing the preferential cleavage of sp C—Si bonds (equation 11). ... 

The chemistry of bis(alkynyl) metal complexes, including the titanium derivatives, focusing on the synthesis, chemical behavior, structure, and bonding of different type of mononuclear and heterometallic molecules has been discussed in a review.1280 A review focusing the preparation and the reactivity of compounds of the type Cp 2Ti-(ct-C=CR)2 (Cp = Cp, Gp ) has appeared which summarizes special aspects of C-C coupling and cleavage processes in organic reactions.1281... 

The bis(propynyl)beryllium compound [Be(C=CMe)2NMe3]2 is unusual in crystallizing with two types of dimeric molecule in the lattice, one of which has a diamond-shaped (Be-C)2 ring very similar to that of 31. The other, structure 32, has a nearly rectangular (Be-C)2 ring, explicable in terms of donation of charge from the alkynyl triple bond into the available metal orbital. 

Paul and coworkers have serutinised a series of differently substituted iron half-sandwich complexes Cp (dppe)Fe(—C=C—C6H4X-4) and their chemically oxidised cationic Fe(III) congeners bearing para substituents X of variable donor or acceptor capacity. They made a convincing case for n backdonation in acceptor-substituted derivatives based on IR, Raman, Mossbauer, and quantum-chemical studies.In the oxidised Fe(III) forms, on the other hand, alkynyl to metal donation dominates the Fe-alkynyl n bonding, especially when conjugated electron-rich substituents are present at the phenyl para position. These studies, in essence, demonstrate that the issue of the nature of the metal-alkynyl bond is not a trivial one. Every parameter, such as the metal ion, its... 

Vinyllead and alkynyllead tricarboxylates can only be prepared by metal-metal exchange, such as mercury-lead,ll6 ll7 tin-lead,ll6,ll7 boron-lead i or zinc-lead.l In the case of tin-lead exchange, cleavage of the methyl-tin bond is much slower than cleavage of the vinyl-tin or alkynyl-tin bond. Therefore (trimethyl)vinyltin derivatives are preferred to the corresponding (tributyl)vinyltin analogues for the synthesis of the unstable vinyllead and alkynyllead compounds. 

The reaction of halogeno transition metal complexes with [Cu(CsCR)] produces heterobimetallic complexes that have a bridging alkynyl ligand bonded to Cu in j7 -fashion and to the other transition metal in J7 -fashion (Scheme 5.34) [176-178]. The structure of the products suggests initial coordination of alkynyl ligand to transition metal to form the intermediate dinuclear complex with a or )7 )7 -coordinated alkynyl ligand. It is soon transformed into the product... 

This chapter focuses the attention on the reactions of nonstabilized carbanionic compounds such as alkyl, vinyl, aryl, alkynyl metals, etc., and the chemistry of dw stabilized system, i.e. allylic, propargylic or oxaallylic carbanions is presented in Volume 2 of diis series. Electrophiles with Om X bonds which are discussed include aldehydes, ketones, epoxides, aziridines, acetals, orthoesters and imines, all of which turn into highly reactive electrophiles in the presence of Lewis adds. 

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