Chromium-catalysed 1,2-addition

JACOBSEN S SALEN AS A CHIRAL LIGAND FOR THE CHROMIUM-CATALYSED ADDITION OF 3-CHLORO-PROPENYL PTVALATE TO ALDEHYDES A CATALYTIC ASYMMETRIC ENTRY TO SYN-ALK- 1-ENE-3.4-DIOLS... 

The catalytic asymmetric allylation of aldehydes is another reaction that has received a great deal of attention. Both allylstannes and the less reactive allylsilanes undergo addition to aldehydes with high ee in the presence of enantiomerically pure Lewis acids and Lewis bases and asymmetric versions of the chromium-catalysed Kishi-Nozaki-Hiyama reaction utilising allyl halides have recently been developed. 

Addition of acetic anhydride to a solution of chromium trioxide in water caused violent boiling [1], due to the acid-catalysed exothermic hydrolysis of the anhydride [2],... 

Sodium perborate oxidation of alcohols by is aided by Aliquat, but also requires the addition of chromium oxide [17]. However, the long reaction times at 60-80°C and the variable yields do not make the procedure particularly attractive. In contrast, direct epoxidation of a,p-unsaturatcd ketones has been conducted with moderate success using sodium perborate catalysed by tetra-n-hexylammonium hydrogen sulphate [18, 19]. 

Macroporous glycidyl methacrylate-ethylene glycol dimethacrylate (GMA-EGDM) copolymer beads were synthesised and characterised for pore volume and surface area. These reactive copolymers were derivatised with 2-picolyl amine and coordinated with chromium and vanadium ions. The peroxocomplexes of these supported metal complexes were generated by the addition of hydrogen peroxide / tertiary butyl hydroperoxide(tert. -BHP) and shown to catalyse a variety of oxidation reactions. 

Catalysed aerobic epoxidation of the styrene-cobalt complex, followed by the addition of trimethylsilyl isothiocyanate, yields 4-phenyloxazolidine-2-thione 123 <96TL7315>. The 4-substituted 2-oxazolidinone 124 undergoes a "chromium Reformatsky" reaction with aldehydes RCHO (R = i-Pr or Ph) in the presence of chromium(ll) chloride and lithium iodide to give mainly the unusual anti-aldol products 125 in excellent dia,stereomeric excess and yield <97TL4,387>. 

Chromium. The mechanism of hydrogenation at this rarely studied centre has been probed by product analyses and deuterium tracer experiments. The catalysts used were Cr(arene)(CO)3, which selectively catalyse 1,4-addition to diolefins. Key intermediates are Cr(H)2(CO)3 and Cr(H)2(CO)3-(diene). ... 

Other Reagents. Cobalt(i) complexes have been proposed as intermediates in the chromium(ii) reduction of macrocyclic cobalt(m) complexes. The reaction [CoLd(NH8)2] + + Cr + [Ld=(22)] is autocatalytic, and it is also strongly catalysed by the addition of [CoLt(HaO)2] [Lt=ligand (23)]. In the proposed mechanism (44)... 

Interest continues in the effects of micellar agents on rates of inorganic reactions. Aquation of oxalato-cobalt(m) and -chromium(iii) complexes is mentioned in a review of reversed micellar systems predicted and actual rates have been compared for a variety of reactions, including the mercury(n)-catalysed aquation of [CoX(NH3)g] + cations. Micellar effects on uncatalysed aquation of cis- and of tra j -[CoCl2(en)2]+ are small (as expected) but real. Other reactions for which the kinetic consequences of micelle or polyelectrolyte addition have been d cribed include complex formation from nickel(ii), the conversion of ammonium cyanate into urea, and the hydrolysis of pyrophosphate. ... 

Several reports of the use of complexes 14,15 and 16 or similar sutetrates as chiral catalysts or auxiliaries have been published. In one example, 5 mol% of the complex 16 catalysed the addition of diethylzinc to benzaldehyde to afford (5)-l-phenylpropan-l-ol in 97% yield and 96% ee (Scheme 6.19). Using complexes 14 and 15, samples of the same enantiomer were produced with an enantioexcess of 93% ee (the isolated yields were 83% and 87%, respectively). By comparison, the corresponding chromium-free derivative 18 resulted in only 24% ee to give the (S)-alcohol. 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

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