Complexes, alkyne-metal cyclotrimerization

Grothahn, D.B. (1995) Transition metal alkyne complexes transition metal catalyzed cyclotrimerization, in Comprehensive Organometallic Chemistry II, vol. 12 (eds. E.W. Abel, F.G.A. Stone, G. Wilkinson, and L.S. Hegedus), Pergamon, Oxford, pp. 741-770. 

Among transition-metal complex catalyzed reactions of alkynes with carbon-heteroatom unsaturated compounds the most studied is co-cyclotrimer-ization of alkynes with nitriles to pyridines. For this process the same complexes can be used as for the cyclotrimerization of alkynes. The first report of a cyclopentadienylcobalt complex catalyzed co-cyclotrimerization of alkynes with nitriles appeared in 1973 [92] and was soon followed by other papers [93]. Co-cyclotrimerization of alkynes and nitriles with all its aspects has been recently reviewed [94] and because of that we will focus only on recent developments in this area. In this regard, advances have been made in simple co-cy-clotrimerization of ethyne with various nitriles [95], combinatorial synthesis of substituted pyridines [96], and co-cyclotrimerization of hydroxyalkynes with nitriles in aqueous media catalyzed by cobalt complex with hydrophobic chain attached to the cyclopentadienyl ring [97]. 

Among the most exciting frontiers in boratabenzene chemistry is the development of transition metal-boratabenzene complexes as catalysts. As early as 1984, it had been demonstrated that these adducts can accelerate useful reactions— specifically, Bonnemann established that (C5H5B-Ph)Co(cod) serves as a catalyst for pyridine-forming cyclotrimerization reactions of alkynes and nitriles.39... 

The cobalt catalyzed cocyclization of alkynes with heterofunctional substrates is not limited to nitriles. cpCo-core complexes are capable of co-oligomerizing alkynes with a number of C,C, C,N or C,0 double bonds in a Diels-Alder-type reaction. Chen, in our laboratories, has observed that these cycloadditions are best performed with the help of stabilizers such as ketones or acetic esters that are weakly coordinated to the cobalt and prevent the alkynes from being cyclotrimerized at the metal center... 

Metal-catalyzed [4 + 2 + 2] cyclotrimerizations of either heteroatom-containing enyne 62 with 1,3-butadiene (Eq. 17) [42] or heteroatom-containing dienyne 64 with an alkyne (Eq. 18) [43] are effected by cationic rhodium complexes generated in situ from a chlo-rorhodium complex modified with silver salts. These processes afford eight-membered ring products 63 and 65, respectively. In both processes, the nature and amount of the silver salt profoundly affect the outcomes. 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

Alkynes can be selectively dimerized, cyclotrimerized, or polymerized with a large variety of transition metal and lanthanide catalysts nickel also catalyzes the cyclote-tramerization of HC=CH to cyclooctatetraene. Very electrophilic complexes such as Cp 2LnR and Group 4 compounds,137 as well as 18-electron species such as Cp RuH3(L) and Ru(Tp)Cl(PPh3)2, catalyze the linear dimerization of terminal alkynes 138... 

The involvement of metalocyclopentadiene intermediates in the cyclotrimerization of alkynes (Scheme 6) has been established for some metal systems 162, 198, 201-204). However, there is no evidence to indicate participation of cobaltocyclopentadiene species in cyclotrimerization reactions involving carbonylcobalt complexes. It is worth noting, however, that 1,4-addition of an alkyne to such an intermediate would lead to the formation of 1,2,4- and 1,3,5-substituted benzenes. Moreover, the 1,2,4-derivative would be favored statistically. This line of reasoning has led to the incorporation of a cobaltocyclopentadiene intermediate in the mechanism proposed 120) (Scheme 7) for the Co4(CO)ia-catalyzed cyclotrimerization of PhC=CH. 

Arene complexes are usually prepared by the following methods (i) metal vapor synthesis (see Metal Vapor Synthesis of Transition Metal Compounds).(ii) AEAIX3 reduction of a metal halide in the presence of the arene and (hi) alkyne cyclotrimerization (see Cyclodimerization -tri-merization Reactions). Synthetic procedmes to obtain r] -arene derivatives have been reviewed by Pampaloni and Calderazzo. Over the past ten years, studies in this... 

The cyclotrimerization of alkynes to give benzene derivatives is perhaps the most general reaction of these compounds in the presence of transition metal complexes. Practically any mono- or di-substituted alkyne, in addition to acetylene itself, may be cyclotrimerized. In addition, cocycloadditions involving more than one different alkyne are possible with some degree of selectivity, and intramolecular versions of the reaction have seen sophisticated development. 

A wide variety of homogeneous and heterogeneous catalysts are available for alkyne cyclotrimerization. As a result, numerous mechanistic pathways have been established for the different versions of this process, each characteristic of the metals involved in the system. The most common involves the intermediacy of metallacyclopentadienes, derived as already shown from any number of metal fragments and two alkynes. Upon opening a vacant coordination site, these systems may readily complex a third alkyne, which may insert to give a transient metallacycloheptatriene from which the benzene product is ultimately released via reductive elimination of the metal (Scheme 24). ... 

As is clear from the introductory discussion, most, if not all, of the d-block transition metals are expected to participate in reactions that are related to those discussed here. In addition to the Co-based methodology mentioned earlier, some related reactions of Pd and are known. Also related are the cyclization reactions of metal-carbene complexes containing Cr, Mo, W and other transition metals with alkynes and alkenes and a recently reported Nb- or Ta-promoted diyne-alkyne cyclization reaction, which appears to be closely related to a number of previously developed alkyne cyclotrimerization reactions, such as those catalyzed by Co. Investigations of reactions involving other transition metals may prove to be important especially from the viewpoint of developing asymmetric and catalytic procedures. 

The cyclotrimerization of alkynes with transition metal complexes proceeds via metallacyclopropenes and -cyclopentadienes. It is surprising that a similar reaction can be carried out by introducing a P=S function in place of an alkyne. Its alkyne-like behavior is to be traced back to the... 

The cyclotrimerization of alkynes by transition metal complexes proceeds via metallacyclopropenes and -cyclopentadienes depending on the alkyne, the products are metallacycloheptatrienes (103) or -bicycloheptadienes (102) (Scheme 36).The first mode is a Diels-Alder addition of a... 

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