Cobalt allenes

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

Here we might note that cobalt(II) hydroxide, but not the oxide, also forms cements (Allen et al., 1984 Mansion Gleed, 1985 Prosser et al., 1986). It also is used in controlled-release devices for supplying trace elements to cattle and sheep. Nothing is known of its structure. 

Copper(II) oxide and cobalt(II) hydroxide form cements with solutions of many multifunctional organic acids propanetricarboxylic acid, tartaric acid, malic acid, pyruvic acid, mellitic acid, gallic acid, tannic acid and phytic acid (Allen et al., 1984 Prosser et al., 1986). These have been used mainly in cement devices for the sustained release of copper and cobalt (Manston et al., 1985 Mansion Gleed, 1985). Little is known about... 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Allen (106) also studied cobalt hydroformylation with a polymer-bound catalyst. The polymer was formed from diphenyl-p-styrylphosphine cross-linked with divinylbenzene. 2-Hexene was the substrate, and reaction conditions were 175°C and 1500-3000 psi of 1/1 H2/CO. The product aldehyde was 55% linear, and the effluent product solution contained 20-50 ppm cobalt. 

Allenes, while arguably underused in synthesis as a whole, have become popular functionalities in cycloisomerization chemistry and provide access to a wide variety of products. Ruthenium, cobalt, platinum, palladium, rhodium, and iridium catalysts are efficient in the transition metal-catalyzed Alder-ene reactions of allenes. 

The asymmetric synthesis of allenes via enantioselective hydrogenation of ketones with ruthenium(II) catalyst was reported by Malacria and co-workers (Scheme 4.11) [15, 16]. The ketone 46 was hydrogenated in the presence of iPrOH, KOH and 5 mol% of a chiral ruthenium catalyst, prepared from [(p-cymene) RuC12]2 and (S,S)-TsDPEN (2 equiv./Ru), to afford 47 in 75% yield with 95% ee. The alcohol 47 was converted into the corresponding chiral allene 48 (>95% ee) by the reaction of the corresponding mesylate with MeCu(CN)MgBr. A phosphine oxide derivative of the allenediyne 48 was proved to be a substrate for a cobalt-mediated [2 + 2+ 2] cycloaddition. 

In 1996, Malacria et al. [139] reported on cobalt-mediated reactions of the related allenynes. Heating the allenyne 222 in the presence of cpCo(CO)2 accompanied by a photochemical activation of this organometallic compound delivered the cross-conjugated trienes 223 (Scheme 15.71). The second triple bond present in the substrate did not participate in the reaction, underlining the higer reactivity of the allene unit. 

An allene is a very promising unsaturated partner in cobalt-mediated [2 + 2 + 2]-cycloaddition reactions. Exposure of an allenediyne to a stoichiometric amount of CpCo(CO)2 in boiling xylenes under irradiation for 5 h furnishes red-brown complexes in 42% isolated yield (Scheme 16.76) [84—87]. Treatment of a 7 3 mixture of the two diastereomers thus obtained with silica gel provides an oxygen-sensitive cobalt-free tricydic compound. 

Scheme 16.77 Cobalt-mediated formal Alder ene reaction of an allene.

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

T. L. Blair, J. R. Allen, S. Daunert, and L. G. Bachas, Potentiometric and fiber optic sensors for pH based on an electtopolymerized cobalt porphyrin, Anal. Chem. 65, 2155-2158 (1993). 

The intramolecular 2 - - 2 - - 1-cycloadditions of allene, alkyne (106), and carbon monoxide yield a -methylene-(107) or 4-alkylidene-cyclopentenones (108) depending on the allene structure or the reaction conditions (Scheme 4i).i59.i6o The cobalt-catalysed 4 - - 2 - - 2-cycloaddition of norbornadienes (109) with buta-1,3-dienes readily produces cycloadducts (110) when a bimetal system is used (Scheme A2) ... 

Substrates possessing an allene that participate in the Alder-ene reaction are less common, but a few examples are known. Malacria [11] and Livinghouse [12] have independently used cobalt to effect intramolecular allenic Alder-ene reactions but the scope of these reactions was not investigated. Sato has performed an allenic Alder-ene reaction to form five-membered rings, using stoichiometric amounts of titanium [13], and Trost has shown that 1,3-dienes can be prepared via an intermolecular Alder-ene reaction between allenes and enones using a ruthenium(II) catalyst [14]. 

Khulbe and Mann [155] have obtained infrared spectra of allene adsorbed on silica-supported cobalt, nickel, palladium, platinum and rhodium. The spectra were similar for all the metals, although variations in band intensity from metal to metal were observed. Addition of hydrogen to the allene-precovered surface resulted in similar spectra to those found for chemisorbed and hydrogenated propene in which the surface species was thought to be an adsorbed prop-1-yl group. The authors concluded that the initial allene spectrum was consistent with the adsorbed species being a 1 2-di-o-bonded allene (structure K)... 

An asymmetric preparation of alkenylcyclopropanes has also been realized by the use of palladium(O) complexes carrying chiral ferrocenylphosphine ligands (equation 22)38. The requisite rt-allyl palladium intermediates can also be generated from allene and meth-ylenecyclopropane derivatives, 1839 and 1940, in the presence of palladium(O) complex and alkenyl or aryl halide (equations 23 and 24). The cobalt complexes, 20, similarly afford the corresponding alkenylcyclopropanes upon exposure to LDA (equation 25)41. 

Among unique transformations on the metallacyclobutene framework, allene complexes of cobalt can be prepared by fluoride-induced desilylation of cobaltacyclobutene complex 39 three isomeric complexes bearing the same disubsituted allene are obtained (Equation 41) <1998JA1100>. 

Allen MJ, Myer BJ, Millett PJ, et al. 1997. The effects of particulate cobalt, chromium and cobalt-chromium alloy on human osteoblast-like cells in vitro. J Bone Jt Surg Am 79-B(3) 475-482. 

The structure of 19 was unambiguously confirmed by an X-ray diffraction study. A mechanistic rationale is depicted in Scheme 11. After Co2(CO)6-complexed alkyne A is obtained, a Sn2 attack of the Co2(CO)6 fragment opens the epoxide moiety to afford intermediate B, which subsequently incorporates CO to give C. The latter rearranges into cobalt-stabilized cyclic allene species D. The net result is a [5 + 1] cyclization that creates the lactone group. Coordination of the tethered olefin leads to oxidative cyclization to give E. Finally, insertion of CO followed by reductive elimination affords the desired product 19. 

Several authors have demonstrated that a Pauson-Khand type formation of methylenecyclopente-nones from enynes, allenes and carbon monoxide occurs with stoichiometric ammounts of [Fe2(CO)y] or [MoCCO) ], and catalytically with [Cp2Ti(CO>2] [19, 29]. Gazes et al. recently reported that cobalt-mediated inter- and intramo-... 

Intramolecular allene alkyne cyclizations using cobalt complexes are nonspecific and both double bonds of the allene participate in the reaction affording [4.3.0] and [3.3.0]ring systems (Scheme 260). Both inter- and intramolecnlar Pauson - Khand-type reactions have been reported using... 

In coordination polymerization it is generally accepted that the monomer forms a 7r-complex with the transition metal prior to insertion into the growing chain. In general these complexes are insufficiently stable to be isolated although complexes of allene [69] and butadiene [70] have been reported. With allene the complex was formed prior to polymerization with soluble nickel catalysts, and cis coordinated butadiene forms part of the cobalt complex, CoCj 2H19, which is a dimerization cateilyst. 

No other groups of compound have clearly defined chain transfer activity. Ethyl trichloroacetate reduces the molecular weight of ethylene/propene copolymers using VOCl3/Al2Et3Cl3 [108] but as the rate of polymerization is also increased the effect may be caused by an increase in the number of active centres. a-Olefins and allenes reduce the molecular weights of cis polybutadiene obtained with soluble cobalt catalysts [139], but in this case it is not clear whether transfer or termination processes are involved. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

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