Octacarbonyldicobalt complexes

A complexation-initiated reaction was realized for the first time as depicted below. Thus, the octacarbonyldicobalt complex of furan is subjected to silica gel and gives rise to the adduct with a seven-membered ring owing to bending of the triple bond to a structure with an angle of around 140° when the alkyne is allowed to react with Co2(CO)8 at room temperature . 

A hitherto unknown type of rearrangement of l-(l-alkynyl)cyclopropanols (321) to cyclopent-2-en-l-ones (322) mediated by octacarbonyldicobalt" and hexacarbonyldicobalt" complexes has been described. A possible pathway for the transformation is outlined in Scheme 101. A -proton transfer accompanied by a metal-mediated Stevens rearrangement, which converts a coordinated dimethylsulfane... 

Only severely sterically hindered alkynes, such as [(OC)9Co3CC=CC Co3(CO)9],6 fail to react with octacarbonyldicobalt. This steric effect is also illustrated by the reactions of [Cp(OC)2FePPh2C=CR]+ (R = H, Me, Ph, p-tolyl, t-Bu) with octacarbonyldicobalt.7 All of the cationic alkyne complexes form hexacarbonylalkyne-dicobalt derivatives except when the substituent is the bulky t-Bu group. 

The heterobinuclear complexes [MCo(/u,-FiCC2CF3)2(CO)3Cp] have been prepared by addition of octacarbonyldicobalt to the mononuclear alkyne complexes [M(F3CC2CF3)2(Cl)Cp] (M = Mo or W)24 demonstrating that cobalt-carbonyl fragments can coordinate to an alkyne already bound to another metal-ligand fragment see Eq. (5). 

This review has already indicated numerous applications of dicobalt-alkyne complexes in organic synthesis. Like the Nicholas reaction (see Section II,D), the Pauson-Khand reaction has seen widespread use.175 This reaction is a three-component cycloaddition of alkynes with alkenes and carbon monoxide which occurs in the presence of octacarbonyldicobalt to afford cyclopentenones, as shown in Eq. (16). 

The formation of alkyne-hexacarbonyldicobalt complexes from the direct reaction of alkynes with octacarbonyldicobalt is represented by Eq. (1). Alkynes that undergo reaction according to the equation include... 

In the synthesis of octacarbonyldicobalt(O), an important by-product is the hydride CoH(CO)4, which is promptly decomposed to Co2(CO)s by adventitious oxygen or spontaneously (equation 18) see Hydride Complexes of the Transition Metals). 

The alkyne-Co2(CO)6 complexes 1 are the binuclear cluster complexes of the acetylenic derivatives with the hexacarbonyldicobalt moiety. These complexes can be readily prepared by treatment of alkynes with commercially available octacarbonyldicobalt [Co2(CO)g] and can regenerate the parent triple bond functionality under some mild oxidation conditions. Two synthetically very useful reactions have so far been developed by taking advantage of the characteristic properties of the alkyne-Co2(CO)6 complexes 1 one is so-called Nicholas reaction" and the other is so-called Pauson-Khand reaction (Scheme 1). The alkyne-Co2(CO)6 complexes 1 possessing a hydroxyl group or its equivalent at carbon p- to alkyne moiety (propargyl alcohol derivatives) could easily... 

Treatment of the yellow-brown octacarbonyldicobalt, Co2(CO)g, with an acetylene in ethereal or hydrocarbon solvent at room temperature under an inert atmosphere generates over several hours time the red-violet hexacarbonyldicobalt-alkyne complex, Co2(CO)s al-kyne. These species may be isolated and purified under inert atmosphere conditions at room temperature and readily characterized by the usual spectroscopic techniques [89]. Reaction of this complex with an olefin occurs at temperatures ranging from about 60 to 120 °C over a period of hours to days Eq. (36) depicts one of the best examples [1, 90]. 

Alkynes and Allenes.—Ethynyl steroids (239) readily form a stable hexacarbonyl-dicobalt complex (240) on treatment with octacarbonyldicobalt. The complex serves to protect the triple bond during reactions elsewhere (c.g. olefin reduction with di-imide, or hydroboronation). The ethynyl group is regenerated by treating the complex with iron(iii) nitrate in ethanol. ... 

Octacarbonyldicobalt and the alkylidyne-molybdenum or alkylidyne-tungsten Tp complexes react to afford a family of species Tp MCo2(/i3-CR)(CO)8 (M = W,... 

Co(EtOOCCH = CHCOOEt)2 (NCMe)2] as shown by X-ray analysis has a trigonal-pyramidal structure with olefin molecules occupying positions at the pyramid base. Arylfulvenes react with octacarbonyldicobalt to give dinuclear complexes of the formula... 

The alkynyl ligands CFj-CiC MPhj (M = P or As) react readily with octacarbonyldicobalt to form complexes COi(CO)io(PhsM-C C CF3)j (94), an A -ray crystal structure determination showing (for M = P) that both the alkyne and the phosphorus are co-ordinated to the metal. ... 

Preparation of Complexes with Transition Metals. 3-Trimethylsilyl-l-prop3me reacts with octacarbonyldicobalt to give the complex shown in eq 27. This complex can react with alkenes to lead to 2-trimethylsilylmethylcyclopentenones (28-30%). 

A short route to known prostaglandin synthons involves heating the octacar-bonyldicobalt complex of the appropriate acetylene and ethylene in an autoclave at 160 C (Scheme 22). Octacarbonyldicobalt is also a useful catalyst for the carbonylation of arylmethyltriethylammonium salts, leading to a new arylacetic... 

To a solution of propynyl alcohol 48 (0.80 g, 2.7 mmol) in anhydrous dichloromethane (12 mL), under an atmosphere of nitrogen, was added octacarbonyldicobalt (1.02 g, 3.0 mmol) and the reaction was stirred at ambient temperature. The progress of the reaction was monitored by observing the evolution of carbon monoxide from the reaction mixture. TLC analysis, after fifteen minutes, showed the presence of a faster moving compound ( fO.45, 2 1 hexane diethyl ether). The reaction mixture was then cooled to -10 °C whereupon tetrafluoroboric acid diethyl ether complex (0.52 mL, 3.0 mmol, 85% by volume) was added and the mixture left to stir. TLC analysis, after five minutes, showed the presence of a new compound (R( 0.65, 2 1 hexane diethyl ether). To the reaction mixture, maintained at -10 °C, was added dropwise methanolic ceric ammonium nitrate (CAN, 6.67 g, 12.20 mmol, 30 mL) until the evolution of carbon dioxide ceased and the yellow color of CAN persisted (about fifteen minutes). TLC analysis of the reaction mixture revealed the presence of a new compound (/ f 0.40, 3 1 hexane diethyl ether). Residual methanol was removed in vacuo and the residue was partitioned between diethyl ether (25 mL) and water (25 mL). The aqueous phase was extracted with diethyl ether ( 3 x 20 mL) and the combined organic extracts were dried over anhydrous magnesium sulfate, filtered, and the solvent removed in vacuo to afford an oil. Purification was effected by column chromatography on silica (3 1 hexane diethyl ether) to afford the desired compound 49 (0.53 g, 66%) as a yellow oil. 

Tetrafluoroethylene reacts with octacarbonyldicobalt to give initially the cr-complex (125), which loses CO when heated in vacuo to give the fx,-... 

Bis(pentafluorophenyl)disulphide and octacarbonyldicobalt yield the disulphide-bridged complex COj(CO)8[(C F8)aS2], assigned structure (79>. ... 

For metallization, hb-PPh was admixed with octacarbonyldicobalt to form a cobalt complex. Refractive index values were measured for the neat hb-PPh and for the hb-PPh cobalt complex, indicating refractive index values of 1.704-1.681 together with low optical dispersion for the hb-PPh-cobalt complex, as verified by the high value of the Abbe number. The authors concluded that these materials could act as precursors to magnetic ceramics with high magnetizabilities. 

In an analogy to the well-known octacarbonyldicobalt-mediated Pauson-Khand reaction, the formal [2+2+1] cycloaddition of alkyne, alkene, and carbon monoxide can be promoted by pentacarbonyliron. However, the iron-mediated [2+2+1] cycloaddition of two alkynes with carbon monoxide has attracted much more attention. When heated in glyme at 140 °C in the presence of stoichiometric amounts of penta-carbonyliron, substituted trimethylsilylacetylenes afford tricarbonyliron complexed cyclopentadienones in moderate to good yields (Scheme 4-3). ... 

Octacarbonyldicobalt shares with many other metal carbonyls and other organometallics the ability to catalyze the cyclotrimer-ization of alkynes and related condensations, e.g. of two alkyne and one nitrile molecule to give pyridines. Derived compounds including Hg[Co(CO)4]2, Co(CO)3NO (obtained from Co2(CO)g with NO or other nitrosating agents) and the alkyne complexes (RC2R )Co2(CO)6 (see below) all share this activity, but none of these compare in efficiency with the preferred CpCoL2 catalysts. 

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