Camphor catalytic reactions

Further evidence for the formation of intermediate compounds in catalytic reactions is afforded by the observation (a) that optically active camphor is formed from optically inactive (racemic) camphor carboxylic acid in the presence of the d- or /-forms of quinine, quinidine or nicotine and (6) that optically active bases, e.g., quinidine, catalyze the synthesis of optically active mandelonitrile from benzaldehyde and hydrocyanic acid.10 These results hardly admit of any other interpretation than the intermittent production of a catalyst-reactant compound. 

The original details of the influence of optically active additives on catalytic reactions were reported by Isoda et al. Hydrogenation of C=0 and C=N bonds in diethyl 2-oxoglutarate and diethyl 2-oximinoglutarate were carried out in the presence of modifier compounds (camphor, bomeol, amino acids) in situ at elevated hydrogen pressures and temperatures. 

The reaction of diethylzinc or dimethylzinc with prochiral ketones, in the presence of a stoichiometric amount of Ti(OPr )4 and a catalytic amount (20%) of camphor-sulfonamide derivative 136, leads to the formation of the corresponding tertiary alcohols with enantiomeric ratios of up to 94.5 5.5. 

The amino alcohol-catalyzed enantioselective addition of dialkylzincs to aldehydes, detailed in Chapter 5 (27), is accomplished with polymer catalysts containing DAIB, a camphor-derived auxiliary, and other chiral amino alcohols (28). Reactions that involve matrix isolation of the catalyst not only result in operational simplicity but also greatly facilitate understanding of the reaction mechanism. In solution, the catalytic chiral alkylzinc alkoxide derived from a dialkylzinc and DAIB exists primarily as dimer (27) however, when immobilized, its monomeric structure can be maintained. 

Although cobalt catalysts have been rarely used in cyclopropanation reactions, Nakamura and coworkers2 1 have developed the camphor-based complex (35) as a useful asymmetric catalyst, as shown in a typical example in equation (16). High yields were obtained with dienes and styrenes but cyclopropanation did not occur with simple alkenes. Studies with cu-ife-styrene showed that, unlike other catalytic systems, the reaction was not stereospecific with respect to alkene geometry. 

Both imprinted polymers showed an enhancement in the catalytic activity that was about 50-fold higher than the control polymer (P0) and turnover of the catalytic cavities was also demonstrated. However, when comparison was made with a polymer containing Co(II) but which was not imprinted with the template (PI), the rate acceleration dropped to about fourfold. In addition, the control of the enantioselectivity of the reaction was very low. In fact, the polymer, imprinted with the diketone derived from the / -camphor, was able to catalyse the reaction, between the 5-camphor and benzaldehyde, with an acceleration rate almost identical to that obtained with the polymer imprinted with the opposite enantiomer. The rate enhancement between the two polymers was in fact equal to 1.04. 

In works made under direction of N.M. Emanuel they showed that catalytic system [Cu2+... A. .. 02] where A" - the anion form of substrate (anion form of substrate was formed at the expense of its deprotonation under the action of bases introduced into the system) is extremely effective in reactions of oxidation of fluorinated alcohols with general formula H(CF2CF2)nCH2OH (where n=l-6) [2], camphor [3] and diacetone-L-sorbose [4] with... 

The asymmetric conjugate addition of diethylzinc with chalcone was also catalyzed by nickel and cobalt complex (Eq. (12.31)) [71]. A catalytic process was achieved by using a combination of 17 mol% of an aminoalcohol 34 and nickel acetylacetonate in the reaction of diethylzinc and chalcone to provide the product in 90% ee [72, 73]. Proline-derived chiral diamine 35 was also effective, giving 82% ee [74]. Camphor-derived tridentate aminoalcohol 36 also catalyzes the conjugate addition reaction of diethylzinc in the presence of nickel acetylacetonate to afford the product in 83% ee [75]. Similarly, the ligand 37-cobalt acetylacetonate complex catalyzes the reaction to afford the product in 83% ee [76]. 

The understanding of the degradation of natural products such as camphor has been greatly enhanced by understanding the catalytic cycle of the cytochrome P-450 enzyme P-450cam in structural detail.3,4 These enzymes catalyze the addition of 02 to nonactivated hydrocarbons at room temperatures and pressures - a reaction that requires high temperature to proceed in the absence of a catalyst. O-Methyltransferases are central to the secondary metabolic pathway of phenylpropanoid biosynthesis. The structural basis of the diverse substrate specificities of such enzymes has been studied by solving the crystal structures of chalcone O-methyltransferase and isoflavone O-methyltransferase complexed with the reaction products.5 Structures of these and other enzymes are obviously important for the development of biomimetic and thus environmentally more friendly approaches to natural product synthesis. 

Optically pure sultams have been used by Oppolzer as chiral auxiliaries in various asymmetric transformations, including Diels-Alder reaction, aldoliza-tion, conjugate addition, his-hydroxylation, and catalytic hydrogenation [42,43]. In the literature, the most commonly used chiral sultam is derived from camphor (Oppolzer s sultam). The ready access to 80 and other chiral sultams from the Diels-Alder cycloadducts could further expand the scope of their use as chiral auxiliaries in asymmetric synthesis. 

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