Cobalt catalysts conjugation reactions

The amount of cobalt complex in this step influences the reaction rate, but not the yields. Indeed, with only 0.3 equivalent of cobalt catalyst, the arylzinc compound is consumed after 24 h instead of 10 h when 1 equivalent was used. An excess of the activated olefin is required to optimize the yield of the conjugate addition. Under these conditions, this process has been studied with various aryl halides (X = Br, Cl) and activated olefins. Yields range from 40 to 80%. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Acylmetal complexes generally react with conjugated dienes to produce useful organic compounds. For example, acylcobalt carbonyls add to butadiene, producing 1 -acylmethyl-7r-allylcobalt derivatives, which then undergo elimination of the elements of HCo(CO)3 on treatment with base to give 1-acyl-diene u6). This reaction can be made catalytic with respect to the cobalt catalyst under proper reaction conditions. 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

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... 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

A variety of substrates have been catalytically hydrogenated at room temperature and 1 -atm. hydrogen pressure by pentacyanocobaltate(ll) anion. Conjugation is required for the reduction of C=C bonds The effects of detailed molecular structure on reducibility and of cyanide-cobalt ratio on mode of reduction have been noted Poisoning and reactivation of the catalyst as well as the effect of alkali are described, and mechanisms are tentatively proposed for these phenomena It is concluded that the aging reaction of pentacyanocobaltate(ll) is reversible A dimerization of acrylic acids at elevated temperatures was found ... 

The addition of hydrogen cyanide (HCN) to carbon-carbon double bonds activated by electron-withdrawing groups in the presence of a base as a catalyst (a variation of the Michael Reaction) has been known for a long time. Nitriles were also obtained by hydrocyanation of branched olefins, such as isobutylene and trimethylethylene, in vapor phase reactions in particular the reactions over alumina (3) and cobalt-on-alumina (4) were reported in the late 1940s and early 1950s. Addition of HCN to conjugated dienes in the presence of cuprous salts (vapor and liquid phase) was reported as early as 1947 (5). 

Dioximato-cobalt(II) catalysts are unusual in their ability to catalyze cyclopropanation reactions that occur with conjugated olefins (e.g., styrene, 1,3-butadiene, and 1-phenyl-1,3-butadiene) and, also, certain a, 3-unsaturated esters (e.g., methyl a-phenylacrylate, Eq. 5.13), but not with simple olefins and vinyl ethers. In this regard they do not behave like metal carbenes formed with Cu or Rh catalysts that are characteristically electrophilic in their reactions towards alkenes (vinyl ethers > dienes > simple olefins a,p-unsaturated esters) [7], and this divergence has not been adequately explained. However, despite their ability to attain high enantioselectivities in cyclopropanation reactions with ethyl diazoacetate and other diazo esters, no additional details concerning these Co(II) catalysts have been published since the initial reports by Nakamura and Otsuka. 

An unusual synthesis of acyldienes from conjugated dienes, carbon monoxide, and alkyl or acyl halides using cobalt carbonylate anion as a catalyst should be mentioned here (57). The reaction apparently involves the addition of an acylcobalt carbonyl to a conjugated diene to produce a l-acylmethyl-7r-allylcobalt tricarbonyl, followed by elimination of cobalt hydrocarbonyl in the presence of base. The reaction can thus be made catalytic. Since the reaction was discussed in detail in the recent review by Heck (59), it will not be pursued further here. 

The cobalt(II) cyanide complex Co(CN)5 " in aqueous solution acts as a homogeneous catalyst for the selective hydrogenation of conjugated dienes to monoenes (81). The initial step in this catalysis is the reaction... 

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