Cobalt, complexes with alkynes

Many new carbonylic complexes have been obtained from the interaction of alkyne carbonylic complexes of cobalt and iron. Lactonyl complexes form at 70°C under CO pressure of 200 x 10 Nm" (152 x 10 Torr) from hexacarbonylcobalt complexes with alkynes ... 

The high selectivity of the reaction of cobalt-carbene complexes with alkynes for furan products was taken advantage of in the synthesis of bovolide, a natural flavor constituent of butter. The carbene complex (230) was prepared in two steps from n-pentanal and was treated with 3 equiv. of 2-butyne. The crude reaction mixture, which presumably contained the furan (231), was treated directly with 3 equiv. of trimethylsilyl iodide to give bovolide in - 50% yield from carbene complex (230). 

Wulff, W.D., Gilbertson, S.R. and Springer, J.P. (1986) Reactions of cobalt carbene complexes with alkynes - ri -vinylketene complex intermediates and a... 

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]. 

In organometallic systems in particular, clusters with bridging alkylthiolates are well known, as exemplified by Co2(Cp)2(/i-SCH3)2, which undergoes reactions with alkyne-cobalt complexes to yield trinuclear clusters.170... 

Benzocyclobutenedione 57 is transformed to the phthaloylmetal complex 58 by treatment with Fe(CO)5, RhCl(PPh3)3, and CoCl(PPh3)3. The phthaloyliron complex 58 (M=Fe) reacts with alkynes, and subsequent acidification under air then gives the naphthoquinone 59. The cyclization of the phthaloyl-cobalt 58 (M=Co) with alkynes requires AgBF4-activation [32]. (Scheme 22)... 

Although disubstituted alkynes are used successfully as two-carbon components in chromium-mediated and -catalyzed [6 + 2]-reactions, the use of terminal alkynes produces a [6 + 2 + 2]-reaction (Section 10.13.3.7). Buono and co-workers have discovered that when a cobalt catalyst is employed, several monosubstituted alkynes can be used in [6 + 2]-cycloadditions with cycloheptatriene (Scheme 35). The use of a chiral BINOL-phosphoramidite cobalt complex affords an enantioselective [6 + 2]-cycloaddition reaction (Equation (18)).121... 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

Pauson-Khand cyclopentenone synthesisThe cycloaddition of an alkene with an alkyne complexed with Co2(CO) usually furnishes a mixture of two cy-clopentenones when the alkene is unsymmetrical. The regioselectivity can be improved markedly if the alkene bears a heteroatom that can coordinate with the cobalt complex. Both sulfur and nitrogen ligands can improve the yield and regio-control of this reaction. 

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts332. Lautens and coworkers327 extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

All the reactions described so far are carried out using stoichiometric amounts of Co2(CO)8. However, a coordinatively unsaturated 02(00)5 species would be liberated when the rearrangement proceeds, and if this cobalt species could form an alkyne-Co2(CO)6 complex with another molecule of 1-(1-alkynyl) cyclopropanol, the reaction should proceed with only a catalytic amount of 02(00)8 [13, 18]. In practice, when 1-phenylethynylcyclopropanol (11b) is treated with a 10 or 20 mol% amount of 02(00)8, the rearranged product 13b is obtained in 43 and 59 % yield, respectively. Thus, the reaction in fact proceeds with a catalytic amount of 02(00)8 however, the efficiency is low and a complex mixture of by-products is also obtained. As this low efficiency could be ascribed to instability of the unsaturated cobalt species, the catalytic reaction could be made more efficient by the addition of a stabilizing additive. 

In spite of its simplicity and conceptual clearances, the original protocol has suffered from many intrinsic problems in a practical sense. For example, the reaction with the alkyne-cobalt complexes provided low chemical yields and required harsh reaction conditions. In addition, it was also difficult to extract the obtained product from the sticky metallic residue. Those problems can be accounted for based on the widely accepted mechanism given in Scheme 1, which, as proposed by Magnus," is supported by many theoretical studies. ... 

The complex reacts readily with ligands such as tertiary phosphines, phosphites, or arsines to give substitution of a CO ligand on the cobalt atom. With CO, however, the complex reacts to give 2-phenylindazolone and 3-phenyl-2,4 lif, 3f/)-quinazolinedione,15 whereas the reaction with CO and hexafluoro-2-butyne affords an anilinoquinoline, probably via an intermediate complex in which the alkyne and CO have inserted into the Co—C bond.16... 

The cobalt(O) complex shown in Entry 2 (Table 3.49) could be prepared either by heating a mixture of an alkyne cobalt carbonyl complex with polystyrene-bound tri-phenylphosphine, or by pretreating resin-bound triphenylphosphine with dicobalt octacarbonyl and then treating the resulting support with the alkyne. 

Reaction with cyclobutenediones. This cobalt complex inserts into di-methylcyclobutenedione (1) to give the cobalt complex 2 in high yield. The complex 2 reacts with alkynes slowly at 120° to give complexes of benzoquinones such as 4. The reaction is facilitated by replacement of the CO ligand of 2 by S(C2H5)2,... 

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