Cobalt enolates complexes

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

The cyclic cobalt-acyl complex 1 undergoes a-proton abstraction from the least-hindered face opposite the phosphane ligand upon treatment with lithium hexamethyldisilazide at 0 °C to generate the chiral enolate species 283. Treatment of 2 with primary iodoalkanes diastereoselec-tively produces the alkylated cobaltocycles 3 also via attack of the reagent on the face opposite the bulky phosphane. 

Scheme 7-22 shows the octahedral coordination of the cobalt (iii) center in two substituted enolate complexes [121]. 

Some very interesting structural features exist for neutral jS-keto-enolate complexes (18) of the type M(AA)nLm. Adducts with nitrogenous bases or water have yielded a 5-coordinate irregular Zn(AA)2 H20 complex and trans 6-coordinate cobalt(II) and nickel (II) complexes M(AA)2 2B, with B = H2O or pyridine. The vanadyl complex, VO(AA)2, contains a square pyramid of oxygens about the metal, while U02(AA)2-H2O shows a pentagonal bipyramidal structure. The anhydrous material is dimeric. 

FIGURE 19 First example of the Nicholas reaction [434, 173] [313] on a solid support. Various propa-rgylic alcohols were immobilized on a Merrifield resin. After forming the cobalt-alkyne complex, Nicholas reaction was performed as a diversifying cleavage step. Trimethylallylsilane, anisole, and silyl enol ethers were used as the carbon nucleophiles. 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Two convenient methods of synthesis of the nickel(II) and cobalt-(II) complexes are described here. Method A, based on reaction of the sodium enolate salt of l,l,l,5,5,5-hexafluoro-2,4-pentanedione with the transition-metal chlorides in dimethylformamide (dmf), gives almost quantitative yields of Ni(hfa)2(dmf)2 and Co(hfa)2(dmf)2. Method B gives lower yields (50-75%) of the same products but can be carried out rapidly in common laboratory equipment (Method A is best carried out by vacuum-line techniques7). 

Under the same conditions, ethylene oxide underwent primarily isomerization and polymerization, which could be prevented by use of a phosphine-modified Co catalyst (see below). By application of nonsymmetric epoxides, the regioisomeric ring-opening reaction has to be taken into consideration, which may lead to the terminal or branched aldehyde, or both. Especially, isomerization of the epoxide to the corresponding ketone or aldehyde is a serious side reaction, as illustrated in Scheme 6.104 by means of an unmodified cobalt catalyst [7]. In the absence of syngas, the cobalt-hydroxyalkyl complex can collapse under elimination of an enol, which rapidly undergoes tautomerization to give the stable ketone or aldehyde (with R = H). 

It has been noted that a positive activation volume (8.0 cm moP ) for the incorporation of Co " into N-methyltetraphenylporphyrin in DMF is consistent with an Id mechanism. The kinetics of the dissociation of the five-coordinate cobalt(II) complex of tris(2-dimethylaminoethyl)amine in solutions of chloroacetic acid in DMF and other solvents have been repor-ted, but no firm conclusions are drawn concerning the mechanism. The formation of the pentane-2,4-dione complex of Co(II) in aqueous solution has been studied/ and the values of kf for the reaction of the enol and keto tautomers (respectively, 136 and 0.84dm mor s 25 C, / = 0.5 mol dm ) found to be considerably lower than those predicted on the basis of the solvent exchange rate for this metal. 

Harrity exploited the carbocation stabilizing ability of cobalt-alkyne complexes to promote a novel O C rearrangement reaction. Exposure of cyclic enol ether 23 to diethylaluminum chloride promotes ionization of the C-O bond to yield the stabilized carbocation and an enolate. Bond rotation followed by C-C bond formation provides cyclohexanone product 24. ... 

Konno reported a detailed study of the reaction of fluorine-containing propargyl acetates under Nicholas conditions with a variety of nucleophiles. Allylstannanes and allylsilanes provided moderate yields of the desired products, while enamines, silyl enol ethers, and silyl ketene acetals furnished the target compounds with excellent efficiency. In one example, cobalt-alkyne complex 25 reacts with silyl ketene acetal 26 in the presence of trimethylsilyl triflate to yield, after cobalt decomplexation, ester 27. ... 

As part of his research into the synthesis of oxygen-bridged nine- and ten-membered cycloalkanes, Montana introduced a key carbon fragment via the Nicholas reaction. Boron trifluoride promoted reaction between silyl enol ether 28 and cobalt-alkyne complexed propargyl alcohol 29 provides substitution product 30 in excellent yield. ... 

Tyrrell demonstrated a three step tandem sequence involving an intermolecular Nicholas reaction, intramolecular Nicholas reaction, and a cationic cyclization. Treatment of silyl enol ether 55 with hexacarbonyl(propiolaldehyde diethyl acetal) dicobalt and boron trifluoride provides cobalt-alkyne complex 56. Exposure of this material to tetrafluoroboric acid promotes an intramolecular Nicholas reaction to form the second six-membered ring. Alkyne decomplexation with ceric ammonium nitrate enables the final cyclization step to yield the target tricycle 57." ... 

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