Asymmetric Catalysis by Vitamin B12: The Isomerization of Achiral Aziridines to Optically Active Allylic Amines

Achiral N-acylaziridines are isomerized to optically active N-acyl-allylamines in ee's of up to 95% by catalytic amounts of cob(I)alamin in MeOH.     

Asymmetric Catalysis by Vitamin B12. The isomerization of achiral epoxides to optically active allylic alcohols

Achiral epoxides are isomerized to optically active allylic alcohols under the influence of catalytical amounts of cob(I) alamin in protic polar solvents.     

Asymmetric Catalysis by Vitamin B12: The isomerization of achiral cyclopropanes to optically active olefins

Achiral spiroactivated cyclopropanes are isomerized to optically active (R)-(cycloalk-2-enyl)-Meldrum's acids ( = (R)-5-(cycloalk-2-enyl)-2,2-dimethyl-1,3-dioxane-4,6-diones) in high yield and ee's up to 86% by catalytic amounts of cob(I)alamin in polar protic solvents.     

Influence of microemulsions on enantioselective synthesis of (R)-cyclopent-2-enol catalyzed by vitamin B12

The influence of microemulsions on the vitamin B12-catalyzed enantioselective isomerization of 1,2-epoxycyclopentane (1) to form (R)-cyclopent-2-enol (2) has been examined. The reaction was initiated by a reduction of vitamin B12 to the Co(I) form by Zn/NH4Cl. The largest enantiomeric excess (e.e.) in the products was 52% for (R)-2 obtained in a bicontinuous sodium dodecyl sulfate (SDS) microemulsion. A water-in-oil SDS microemulsion gave a poorer percent e.e. probably because of limited catalyst utilization in the water droplets. The influence of the pH of the water phase, the amount of water, and the concentration of vitamin B12 on the enantioselectivity and yield of the reaction was also explored. Results suggest that factors such as higher water content and bicontinuous fluid structure facilitated efficient intermixing of catalyst with reactant to form a key organocobalt intermediate, thus improving enantioselectivity.     

the Mechanism of the Oxidation of Cob(I)alamins

Abstract

Spectroelectrochemical investigations of the reoxidation sequence of the reduced cob(I)alamin to the oxidized cob(III)alamin show that two different cob(II)alamin intermediates are formed during the processes which appear to correlate to base-on and base-off cob(II)-alamin species.     

Cob(I)alamin as Catalyst. 8th Communication. Cob(I)alamin and Heptamethyl Cob(I)yrinate During the Reduction of α,β-Unsaturated Carbonyl Derivatives

Hydrogen bonds as presented in Figure 2 cannot account for the enantioselective attack of cob(I)alamin ( 4 ( I )) or heptamethyl cob(I)yrinate ( 5 ( I )) on one of the two enantiotopic faces of the substrates. The attack of the strongly nucleophilic 3d orbital is preferentially directed to the re-side of the starting materials with (Z)-configuration and leads, after the highly stereoselective reductive cleavage of the Co, C bond, to saturated products with (S)-configuration in varying enantiomeric excesses (see Schemes 1, 3 and Table 1).     

Skeletal Migrations Observed during the Cob(I)alamin-Catalyzed Reduction of 4β-(tert-Butyl)-1β-(1-methylvinyl)cyclohexanecarbaldehyde

During the cob(I)alamin( 1(I) )-catalyzed reduction of 3 , intermediate formation of 2 and final generation of 4–10 was observed (see Scheme 1, cf. Tables 1 and 2). Identical products in similar ratios were generated starting from either 2 or 3 . Accepting the intermediate formation of six interconnected cobalt complexes, i.e. A–F (cf. Scheme 2), the generation of all the products observed can be explained.     

Stabilisation of Methylene Radicals by Cob(II)alamin in Coenzyme B12 Dependent Mutases

Coenzyme B₁₂ initiates radical chemistry in two types of enzymatic reactions, the irreversible eliminases (e.g., diol dehydratases) and the reversible mutases (e.g., methylmalonyl‐CoA mutase). Whereas eliminases that use radical generators other than coenzyme B₁₂ are known, no alternative coenzyme B₁₂ independent mutases have been detected for substrates in which a methyl group is reversibly converted to a methylene radical. We predict that such mutases do not exist. However, coenzyme B₁₂ independent pathways have been detected that circumvent the need for glutamate, β‐lysine or methylmalonyl‐CoA mutases by proceeding via different intermediates. In humans the methylcitrate cycle, which is ostensibly an alternative to the coenzyme B₁₂ dependent methylmalonyl‐CoA pathway for propionate oxidation, is not used because it would interfere with the Krebs cycle and thereby compromise the high‐energy requirement of the nervous system. In the diol dehydratases the 5′‐deoxyadenosyl radical generated by homolysis of the carbon–cobalt bond of coenzyme B₁₂ moves about 10 Å away from the cobalt atom in cob(II )alamin. The substrate and product radicals are generated at a similar distance from cob(II )alamin, which acts solely as spectator of the catalysis. In glutamate and methylmalonyl‐CoA mutases the 5′‐deoxyadenosyl radical remains within 3–4 Å of the cobalt atom, with the substrate and product radicals approximately 3 Å further away. It is suggested that cob(II )alamin acts as a conductor by stabilising both the 5′‐deoxyadenosyl radical and the product‐related methylene radicals.     

Synthesis of Optically Active Bifunctional Isoprenoid Building Blocks by Rhodium(I)-Catalyzed Asymmetric Allylamine to Enamine Isomerization

The application of the known asymmetric allylamine to enamine isomerization methodology to bifunctional C⁵-isoprenoid allylic amines of types IId and IIe (Scheme 1) is described. It is shown that a number of such substrates can be isomerized with enantioselectivities of > 90% ee. using cationie Rh¹ complexes containing (6. 6′-dimethylbiphenyl′2, 2′-diyl)bis(dipheny phosphine) (BIPHEMP; 9) as asymmetry-inducing ligand (Scheme 2, Tables 1 and 2). Synthetically most useful is the isomerization of the benzyloxy derivative 10a into the (E)-enamine 11a . This isomerization proceeds with very high enantioselectivity (98-99% ee) and affords, after enamine hydrolysis, the optically active 4-(benzyloxy)-3-methylbutanals ((R)- or (S)- 12 ) in chemical yields of ca. 90%. In conjunction, a short synthetic route to the starting material 10a has been developed which has a Pd-catalyzed amination of isoprene epoxide ( 30 ) as the key step. Thus, convenient and practical access to the optically active aldehydes (R)-and (S)- 12 is now at hand. These aldehydes are useful optically active bifunctional building blocks for isoprenoid homologation.     

Cob(I)alamin-Catalyzed Skeletal Migrations Observed during the Reduction of 4β-(tert-Butyl)-1α-(1-methylvinyl)cyclohexanecarbaldehyde

The cob(I) alamin (1(I)) -catalyzed² transformation of the aldehyde 2 has been studied (cf. Table 1). Kinetic examinations showed a rapid isomerization of 2 to 3 (cf. Fig. 1 and 2). From the transformations in glacial AcOH, the two cyclopropanols 5 and 7 were isolated as main products (cf. Tables 1–3 and Fig. 1 and 2). Using large amounts of 1(I) , the aldehyde 4 was very slowly transformed. Accepting the intermediate formation of 6 interconnected Co-complexes, i. e. A , B , C , D , E and F (cf. Scheme), the generation of all the products observed can be explained.     

Kinetics and mechanism of the reversible binding of nitric oxide to reduced cobalamin B(12r) (Cob(II)alamin)

The reduced form of aquacobalamin binds nitric oxide very effectively to yield a nitrosyl adduct, Cbl(II)-NO. UV-vis, (1)H-, (31)P-, and (15)N NMR data suggest that the reaction product under physiological conditions is a six-coordinate, "base-on" form of the vitamin with a weakly bound alpha-dimethylbenzimidazole base and a bent nitrosyl coordinated to cobalt at the beta-site of the corrin ring. The nitrosyl adduct can formally be described as Cbl(III)-NO-. The kinetics of the binding and dissociation reactions was investigated by laser flash photolysis and stopped-flow techniques, respectively. The activation parameters, DeltaH, DeltaS, and DeltaV, for the forward and reverse reactions were estimated from the effect of temperature and pressure on the kinetics of these reactions. For the "on" reaction of Cbl(II) with NO, the small positive DeltaS and DeltaV values suggest the operation of a dissociative interchange (I(d)) substitution mechanism at the Co(II) center. Detailed laser flash photolysis and (17)O NMR studies provide evidence for the formation of water-bound intermediates in the laser flash experiments and strongly support the proposed I(d) mechanism. The kinetics of the "off" reaction was studied using an NO-trapping technique. The respective activation parameters are also consistent with a dissociative interchange mechanism.     

Asymmetric synthesis of L-carnitine from (R)-3-chloro-1,2-propanediol

A practical chemical synthesis of L-carmtine(1) has been accomplished from(R)-3-chloro-1,2-propanediol((R)-4),which is a main by-product originated from(R,R)-Salen Co(Ⅲ) catalyzed hydrolytic kinetic resolution(HKR) of(±)-epichlorohydrin.(R)-4 was utilized as a chiral starting material to prepare the key intermediate cyclic sulfite((R)-S).The new synthetic approach demonstrated an efficient utilization of organic by-product for the asymmetric synthesis of bioactive compounds.     

Synthesis and Crystal Structure of （R）-4-Hydroxymethyl-2-thioxo Thiazolidine and Its Asymmetric Catalysis

(R)-4-Hydroxymethyl-2-thioxo thiazolidine as a new chiral catalyst in the asymmetric addition of diethyl-zinc to benzaldehyde was synthesized from (R)-4-hydroxymethyl-2-thioxo thiazolidine carboxylic acid and its crystal structure was determined by X-ray diffraction method. The compound was crystallized in the orthorhombic system, space group P212121 with unit cell dimensions a=0.67253(12) nm; b=0.89164(17) rim; c=1.06146(19) nm, volume 0.6365(2) nm3; Z=4, calculated denisity 1.557 Mg/m3; absorption coefficient 0.733 mm-1; F(000)=312. The X-ray crystal structure analysis reveals that the compound has a thione group.     

Asymmetric epoxidation of (Z)-enol esters catalyzed by titanium(salalen) complex with aqueous hydrogen peroxide

Titanium(salalen) complex 1 was an effective catalyst for asymmetric epoxidation of enol esters. Although (E)-enol esters were reluctant to proceed, (Z)-enol esters underwent asymmetric epoxidation to give the epoxides in high yields with high enantioselectivity ranging from 86 to >99% ee in the presence of aqueous hydrogen peroxide as the stoichiometric oxidant. Complete enantioselectivity was observed in the reaction of (Z)-3,3-dimethylbut-1-en-1-yl 4-methoxybenzoate. The obtained epoxide was readily transformed into the corresponding 1,2-diol by reduction with lithium borohydride without erosion of the high enantiomeric excess.     

(1R,2R)-(+)-(1,2)-DPEN-Bonded Sulfonic Acid Resin: A Trifunctional Heterogeneous Catalyst for Asymmetric Michael Additions of Acetone to Nitroolefins

Based on (1R,2R)-(+)-(1,2)-DPEN skeleton, a series of primary amine–sulfamide bifunctional catalysts were synthesized, which exhibited excellent catalytic performance in the Michael addition of acetone to β-nitrostyrene. Therefore, a trifunctional heterogeneous catalyst was designed and prepared by simple N-sulfonyl reaction of (1R,2R)-(+)-(1,2)-DPEN and sulfonyl chloride resin. It was employed for the aforementioned addition without any additive and satisfactory results (80.5% conversion; 84.3% ee) were obtained. Meanwhile, the structural and textural properties of the catalyst were characterized by infrared spectroscopy (FT-IR), elemental analysis, SEM, and N₂ adsorption and desorption experiments. Finally, the generality of the catalyst was investigated.     

Highly selective R,S-coordination of non racemic (1R,2R)-(1,2-dialkyl)-1,2-diamine cyclohexane derivatives to palladium dichloride

In non-racemic (1R,2R)-(1,2-dialkyl)-1,2-diaminocyclohexane palladium dichloride complexes the C2 symmetry of the diamine ligand is broken, resulting in selective R,S-coordination.     

Isomerization of Aldehydes Catalyzed by Rhodium(I) Olefin Complexes

A mixture of isomeric aldehydes is formed in a catalytic isomerization of alkyl aldehydes with [C(5)Me(5)Rh(olefin)(2)] complexes. This process offers an indirect method for altering the n:iso ratio of aldehydes formed in hydroformylation reactions. For example the reaction of n-butanal with [C(5)Me(5)Rh(CH(2)=CHMe)(2)] (1) results in the formation of a bis-alkyl carbonyl rhodium complex 2 which is the resting state during the isomerization of n-butanal to a mixture of n- and iso-butanals (see scheme). In the presence of other olefins transfer formylation is observed.