Dicobalt hexacarbonyls, acetylenic

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

An enantioselective intramolecular Pauson-Khand reaction based on chiral auxiliary-directed 7t-face discrimination in acetylenic 0-alkyl enol ether-dicobalt hexacarbonyl complexes, which proceeds with good yields and high facial diastereoselectivity, has recently been developed by M.A. Pericas, A. Moyano, A.E. Greene and their associates. The method has been applied to an enantioselective formal synthesis of hirsutene. Moreover, the process is stereodivergent and the chiral auxiliary -rran5-2-phenylcyclohexanol- is recovered in a yield as high as 92% [18]. 

The formation of the butenolactone complex, XII, by the action of carbon monoxide on acetylene dicobalt hexacarbonyl complexes, XI, (89) seems to be a closely related reaction. It probably involves the following steps ... 

Dicyclopentadienyldinickeldiphenylbutadiyne-dicobalt hexacarbonyl has been prepared from diphenyldiacetylene in which one triple bond acts as a bridging group between two nickel atoms and the other between two cobalt atoms (203). Reduction of the diphenylacetylene complex (R = R = Ph) with sodium and alcohol in liquid ammonia yields dibenzyl, showing that the diphenylacetylene grouping is bonded only to the nickel atoms. The corresponding complex of acetylene (R = R = H) has also been prepared from nickeloccne and acetylene (69) ... 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

When an alkyne reacts with [Co2(CO)g], the two C = C —C angles in the formed (n-acetylene)dicobalt hexacarbonyl complex are reduced to a value of =140° [26] therefore, dicobalt hexacarbonyl fragments have been used as protecting groups to allow geometrictilly disfavored cyclization reactions by bending an alkyne moiety [27]. Since the alkyne can be... 

The anti-proliferative effects of some dicobalt hexacarbonyl alkyne complexes derived from aspirin 21, from diphenyl acetylene 22, and from 2-propyn-l-ol have been studied on various cell lines, including... 

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

M3 thioheptacarbonyltricobalt), 46B, 789 Cl8HC05O15, (Methinyltricobalt enneacarbonyl)-acetylene-(dicobalt hexacarbonyl), 35B, 502... 

Ethynylcyclopropanes, like normal acetylenes, react with dicobalt octacarbonyl in ether to form stable dinuclear cluster-like hexacarbonyl complexes (equation 170)236. The complex with l-chIoro-2,2,3,3-tetramethylethynylcyclopropane reacts stereo- and regioselec-tively with norbomene in a typical Pauson-Khand reaction to give the exn-2-cyclopropyl substituted cyclopentenone (equation 171). Similarly, the reaction of 2-ethoxycyclo-propylacetylene with cyclopentene in the presence of Co2(CO)8 under CO gave 3-(2-ethoxycyclopropyl)-cw-bicyclo[3.3.0]oct-3-en-2-one (equation 172)242. 

The formation of ketonic products when acetylene hexacarbonyl dicobalt is treated with norbornadiene was a serendipitous discovery first reported in 1971 by Pauson and co-workers.1,2 The group were investigating the reaction between acetylene or phenylacetylene hexacarbonyl dicobalt and norbornadiene and found that varying the nature of the solvent changed the major product of the reaction (Scheme 1). 

Scheme 1 Original products identified from reaction between acetylene hexacarbonyl dicobalt (1) and norbornadiene.

In 1973, the group reported the preparation and characterisation of a number of ketonic products obtained from the reaction between variously substituted hexacarbonyl dicobalt complexes and norbornadiene.3 They were able to deduce that the products contained the original acetylene, one... 

The novel cobalt complex came about as a result of the intramolecular coordination of a double bond, present in one of the R groups on the acetylene, to one of the cobalt atoms - taking the place of a CO ligand (67 - 68). They found that the new pentacarbonyl complex could be readily formed in CDC13 at room temperature from the hexacarbonyl dicobalt precursor. Attempts to use the pentacarbonyl complex as a substrate in the PK reaction led to no formation of cyclopentenone product. It was proposed that this is due to the alkene occupying a pseudo-equatorial site -alkene insertion is thought to occur from the axial position (see section 2.4). 

Oxidation of these complexes releases the apical carbon atom and its substituent in the form of an organic derivative.2 Thus ceric ammonium nitrate oxidation of an RCCo3(CO)9 complex produces the carboxylic acid, RC02H, when the reaction is carried out in aqueous acetone. Thermolysis of RCCo3(CO)9 produces acetylenes or acetylene hexacarbonyl dicobalt complexes, depending on the reaction conditions. Useful applications of these cluster complexes in organic synthesis remain to be developed. 

Endo 3a1415,617,7a-Hexahydro-4l7-methano-2-lndene-i-one (2).1 A solution ol octacarbonyMcobalt (1.0 g, 3 mmol) and bicydo [2 2.11 hept-2-ene 1 (3.0 g. 32 mmol) in isooctane (100 ml) was stirred first with acetylene and then under a mixture ot 1.1 of acetylene and carbon monoxyde at 60-70°C until gas absorption ceased (total 1550 ml). The mixture was concentrated and the residue chromatographed on neutral alumina. Petroleum ether PhH (1 1) eluted acetylene hexacarbonyl dicobalt 70 mg, PhH CHCI3 (1.1) eluted a yellow oil which alter rfisfiltation afforded 3 54 g of 2 (74%), bp 101-102°C (15 mm) Colorless crystals (rom pentane, mp32°C. 

Based on this strategy, Garcia et al. have used oxazaborolidine, (R)-2, and effectively reduced several a,p-acetylenic ketones. The e.e. value of the products was in the range 90 to 97%. To improve the results further, the monobranched ketones were complexed with hexacarbonyl dicobalt complexes and snbjected to reduction. Unfortunately, the reactions were sluggish and under forced conditions or modifications of oxazaborolidine resnlled only in low yields and enantioselectives (Table 21.3). 

The PKR was first reported as a stoichiometric reaction between norbomadiene and a complex of acetylene bound to hexacarbonyl dicobalt (Equation 17.72). Pauson and coworkers also reported the first catalytic intermolecular formation of a cyclopentene from an alkyne, an olefin, and CO (Equation 17.73). The first intramolecular version of the PKR was reported almost 10 years later by Shore (Equation 17.74). At this point, the intramolecular PKR has been studied in more detail than the intermolecular PKR. 

Moreover, the decomposition ofC H3C=Cr(CO)4Br in the presence of dicobalt octacarbonyl resulted in interception of CH3 C to give CH3CC03-(CO)9 (Fischer and Daweritz, 1975). In this context, it is then not surprising that the decomposition of RCC03 ( 0)9 compounds gives either acetylenes or acetylenedicobalt hexacarbonyl complexes ... 

---