Dicobalt hexacarbonyl complex

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]. 

Tandem reaction generally attracts much interest because it allows us to effect multiple transformations, all in one pot. Two types are known at present. The most popular approach is a combination of several reactions with PKR. Properly functionalized 1,6- or 1,7-enynes are readily obtained from the propargyl alcohol-dicobalt hexacarbonyl complexes and a properly nucleophilic allylic moiety in the presence of a Lewis acid. The resultant enynes are subjected to the promoter-assisted PKR without purification to afford the desired PKR products (Scheme 9).82,82a-82e... 

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 ... 

Later on, Schreiber used consecutively these two reactions in the key step for the synthesis of diterpene (-i-)-epoxydictimene (73), starting from natural (R)-pulegone [116,117]. This approach was built on their preliminary studies on Lewis acid mediated intermolecular Nicholas reactions [118]. They prepared functionalized enyne 70 bearing a mixed acetal. This compound was transformed into its dicobalt-hexacarbonyl complex and, in the presence of a carefully selected Lewis acid, it formed a stabilized carbocation by release of the more accessible ethyl moiety. This cation reacted intramolecularly with the allylsilane giving the central eight membered ring of the natural product... 

A diastereoisomeric mixture of dicobalt hexacarbonyl complexes 33 reacted with trifluoroborane etherate at -20 °C to give the reduced product 3.3 (minus a benzyl ether) as a single diastereoisomer after decomplexation of the metal with cerium(IV) ammonium nitrate. Suggest a mechanism for the formation of 33 which accounts for the stereochemistry of the product. 

The total syntheses of (+)-secosyrins 1 and 2 was achieved and their relative and absolute stereochemistry was unambiguously established by C. Mukai and co-workers. To construct the spiro skeleton of these natural products, the intramolecular Nicholas reaction was utilized. The alkyne substrate was first converted to the dicobalt hexacarbonyl complex by treatment with Co2(CO)8 in ether. Exposure of the resulting complex to boron trifluoride etherate at room temperature brought about the ring closure with inversion of configuration at C5 to afford the expected tetrahydrofuran derivative. The minor product was the C5 epimer which was formed only in 15% yield. 

Balsells, J., Vazquez, J., Moyano, A., Pericas, M. A., Riera, A. Low-Energy Pathway for Pauson-Khand Reactions Synthesis and Reactivity of Dicobalt Hexacarbonyl Complexes of Chiral Ynamines. J. Org. Chem. 2000, 65, 7291-7302. 

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. 

The reactions of boronates 2.68 (Re = Rz = H, R = i-Pr) are somewhat less selective [698], However, by using the arenechromium tricarbonyl complex of benzaldehyde or dicobalt hexacarbonyl complexes of a-alkynylaldehydes, homoal-lyl alcohols are obtained with a high selectivity after decomplexation [722, 1203] (Figure 6.44). These selectivities are interpreted by distorted chair transition states (Figure 6.44). In the reactions of allylboranes, the approach of the aldehyde minimizes both the steric interactions with the boron substituents and the eclipsing 1,3-interactions of the aldehyde C-R bond with the B-C bond. In the case of boronates 2.68, repulsive interactions between the oxygen lone purs are also avoided [698,1204] (Figure 6.44). 

Decomplexation. Dicobalt hexacarbonyl complexes of alkynes are efficiently decomposed at room temperature with NaSMe in DMF. 

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... 

Alkyne-dicobalt hexacarbonyl complexes, formed in situ from alkynes RC=CH (R = octyl or Ph), cobalt(II) bromide, zinc dust and carbon monoxide in THF, give mixtures of 489 and 490 by the action of norbornene. ... 

According to these results, practical rate increase in PK reaction can be achieved through acceleration of the initial CO dissociation step. The loss of a CO ligand normally proceeds thermally or upon treatment with a mild oxidant such as an amine N-oxide [132, 133]. Pericas et al. have studied the different possibilities of dissociative loss of CO from the dicobalt hexacarbonyl complexes of... 

The asymmetric allylboration of 2-decynal (18) provides addition products 19-21 in 72%, 72% and 58% ee with reagents 1-3, respectively. In contrast, the allylborations of 2-dec5mal-dicobalt hexacarbonyl complex 22 with 1-3 followed by decomplexation with Fe(N03)3 in EtOH at room temperature provides adducts 19-21 with improved%ee s of 92%, 96% and 86%, respectively, in 85-95% yield. 

Scheme 3.68 Dicobalt hexacarbonyl complex of propynals in organic synthesis erythro-selective aldol reaction with silyl enol ethers.

In one case, electrophilic substitution at the coordinated triple bond has been demonstrated. The dicobalt-hexacarbonyl complexes below [Eqs. (45) and (46)] undergo acylation, albeit rather inefficiently, in a process which is undoubtedly facilitated by the susceptibility of the C—Sn bond to electrophilic attack (Seyferth and White, 1971). 

---