Reactions of Acetylene Complexes

The acetylene molecule, particularly in the case of weak alkyne complexes, may undergo dissociation reaction. This process occurs quite readily in copper group compounds. In pentacoordinate complexes, many dissociation reactions are known only in solution. 

Thermal decompositions may lead to release of the alkyne or, most frequently, to 

Acetylenes undergo exchange reactions or they may be displaced by other ligands which are present in excess in the reaction mixture or exhibit considerably stronger complex formation properties. For cationic coordination compounds of the type [PtMe(RCCR)L2] the ease of alkyne substitution by other ligands decreases as follows  

Alkyne substitution reactions by Lewis bases also take place for many other complexes. In such reactions, the following ligands are most commonly utilized phosphines, arsines, phosphites, P(OR)3, CO, etc. 

In neutral and anionic platinum(II) complexes the alkyne may be displaced by thiourea, KI, KSCN, KNO2, and Na2S203. Acetylenes exhibit a relatively strong trans effect. In [PtCl3(RCCR)] , the chlorine atom which is trans to acetylene is weakly bonded. The v(Pt-Cl . J frequency is lower than that of v(Pt —Cl ) by 20cm Consequently, ligands that are trans to alkyne readily undergo substitution. 

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Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

Instances in which cyclobutadiene complexes are the major products from the reactions of acetylene complexes with additional alkyne are uncommon. These generally have been found to be significant products with sterically hindered alkynes and with palladium and platinum metals. For example, phenyl tert-butyl acetylene was converted to the corresponding cyclobutadiene complex (one isomer) upon treatment with (PhCN)2 PdCl2 (Hos-okawa and Moritani, 1969) [Eq. (71)]. With sterically less demanding tolane,... 

Yamamoto et al. have developed a catalytic enantioselective carbo-Diels-Alder reaction of acetylenic aldehydes 7 with dienes catalyzed by chiral boron complexes (Fig. 8.10) [23]. This carbo-Diels-Alder reaction proceeds with up to 95% ee and high yield of 8 using the BLA catalyst. The reaction was also investigated from a theoretical point of view using ab-initio calculations at a RHF/6-31G basis set. 

The dinuclear rhenium disulfido complex with n,rj -S2 ligands (ReCp )2(/U-S2)2 (Cp =7j -C5Me4Et) (type IIa2 complex in Fig. 2) undergoes an insertion reaction of acetylene and ethylene into the S-S bond according to Scheme 57. 

While Cp2Zr(CO)(PPh3) was found to be more reactive toward acetylenes than Cp2Zr(CO)2 (2), no monocarbonyl-Tj2-acetylene complexes of zirconocene were observed in contrast to the reaction of acetylenes with Cp2Ti(CO)(PPh3) (42) (50). Instead the reaction of Cp2Zr(CO)(PPh3) with RC=CR (R = Et, Ph) led directly to the respective zirconacy-clopentadienes (58). 

The reactions of polysulfido complexes with activated acetylenes have been studied in some detail (4, 5). 1,4- (MeCp2Ti)2(S2)2 reacts with dimethylacetylene dicarboxylate (DMAD) to yield MeCp2TiS2C2R2... 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

The reaction of acetylenes with the iron carbonyls gives a wide variety of stable complexes, the composition of which depends on the alkyne, the carbonyl, the reaction temperature, and the solvent used in the preparation (Table III) 116, 117, 118, 120, 182, 187). Iron dodecacarbonyl yields as many as seven different kinds of complexes, depending on the acetylene studied. The complexes are generally yellow, red, or black, soluble in organic solvents, and are invariably diamagnetic they show infrared absorption... 

High temperature, pressure, and catalyst are required to achieve addition of ammonia to alkynes. Acetylene and ammonia yield a complex mixture of heterocyclic nitrogen bases.311,312 Ethylideneimines, thought to form through the intermediate enamines, are the products of the reaction of acetylene with primary alkylamines in the presence of catalysts.313... 

In 2001, Van den Hoven and Alper reported the unexpected 2(Z)-6(ft)-47/-[l,4]-thiazepin-5-one 215, as the major product, from the reaction of acetylenic thiazole 214 with carbon monoxide and hydrogen in presence of a zwitterionic rhodium complex and triphenyl phosphite. After optimization of the reaction condition, the pressure, and the temperature, up to 90% yield is achieved with good selectivity for thiazepine 215 over thiazole side products 216-218 (Scheme 38) <2001 JA1017>. 

Another unusual three-component coupling reaction involving an imine as intermediate has been developed by Ishii who has shown that a C-H bond adjacent to the nitrogen atom of an imine can be activated by an iridium complex. Carbo-metallation reactions of acetylenic compounds may then be achieved, which lead to unsaturated imines 155 (Scheme 8.67) [122]. 

The field of acetylene complex chemistry continues to develop rapidly and to yield novel discoveries. A number of recent reviews 1-10) covers various facets including preparation, structure, nature of bonding, stoichiometric and catalytic reactions, and specific aspects with particular metals. The first part of this account is confined to those facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts will be made to give a consistent interpretation of the reactivities of coordinated acetylene in terms of a qualitative molecular orbital picture. 

A further reaction of metalocyclopentadiene complexes with acetylenes leads to metalocycloheptatriene complexes by metalocyclic enlargement (3, 10, 98) or to benzene derivatives by reductive elimination (57, 70, 73, 77, 82, 98) ... 

Metalocycles are also formed by the reaction of acetylenes with metal carbonyls or with isonitrile complexes (3, 73, 99-102). Their formation may involve monohaptoacetylene intermediates. 

Studies on elementary reactions of acetylenes with metal complexes are now beginning to shed some light on the nature of activation caused by complexation. This activation is not a simple process. Many low-valent d8-d10 metal complexes and also some early transition metal compounds with higher oxidation state (d -d2 complexes) are capable of activating acetylenes. As already described, in the former complexes, interaction (c ) would lead to activation of an r)2-acctylcne ligand to an -acetylene having some radical as well as some anionic character ... 

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds 103, 116). The reaction patterns are generally similar to the cocyclization wdth carbon monoxide which is already known 103, 117). Low-valent nickel, palladium, or cobalt complexes are active in the following reactions 102, 103) for which intervention of acetylene complexes has been suggested ... 

Yamazaki et al. (91, 119) and Bonnemann el at. (120) have recently reported catalytic syntheses of substituted pyridines from acetylenes and nitriles. Various cobalt complexes serve as active catalysts, in particular, CpCo(PPh3)2 (91) or Co(C8Hi2) (C8Hi3) (120). Similar reactions of acetylenes with CS2 or RNCS also give new heterocycles (91) ... 

The linear coupling reaction of acetylene and acrylonitrile afforded 2,4,6-hep-tatrienenitrile by dimerization of acetylene and insertion of one molecule of acrylonitrile [88] (Eq. 66). The reaction involves the formation of a ruthena-cyclopentadiene complex, which also catalyzed the reaction. 

Carbonylation of the parent acetylene via stoichiometric or catalytic reactions involving transition-metal carbonyl complexes has been extensively studied. Various types of carbonylation reactions of acetylene were discovered. In 1968, Pino et al. [30] reported on the synthesis of hydroquinone via a Ru3(CO)12-cat-alyzed carbonylation of acetylene with H2 or H20. The product formally consisted of two molecules of acetylene and CO, and one molecule of H2 (Eq. 14). To achieve a good yield of hydroquinone, the H2 pressure must be kept under... 

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