Chromium mediated addition

The chromium-mediated addition of propargyl halides to carbonyl compounds was studied by Gore and coworkers. ° Unlike the crotylchromium reagents already described (vide supra), which react ex-... 

Allylic organometallics modified at the metal center by chiral adjuvants add to aldehydes and ketones to provide optically active homoallylic alcohols. This process has been described for reagents containing boron, tin and chromium metal centers. Gore and coworkers have shown that a chromium-mediated addition reaction of allylic bromides to simple aldehydes that uses a complex of lithium N-methyl-norephedrine and chromium(Il) chloride occurs with modest (6-16% ee) enantioselectivity (equation 61, Table 8). 

The synthesis of another Ci-Ce ketone 210 used an asymmetric Mukaiyama aldol reaction catalyzed by oxazaborolidine 135 (Scheme 9-58) [78]. This concise synthesis is reminiscent of an earlier approach to the bryostatins (see Scheme 9-42) and can also be compared with the chromium-mediated addition of a-bromoimide 211 to simple aldehyde 212 to give 213, which proceeded with excellent stereo-control (>99 1) [76]. 

Chromium(II) Chloride Mediated Addition of Allyl Bromides to Aldehydes General Procedure12 ... 

Method B Alternatively, a suspension of 3.32 g (27 mmol) of anhyd chromium(II) chloride in THF is used. Chromium(If ) Mediated Addition ... 

In addition to the aforementioned total syntheses. Shannon et al. observed the formation of an N-C3-linked dimer during the transformation of a 3-bromocarbazole to a 3-cyanocarbazole by reaction with copper(I) cyanide in DMF under reflux (668). Harrity et al. reported the synthesis of non-natural (+ )-N,N -dimethylbismurrayafoline A via a chromium-mediated benzannulation, followed by a palladium-catalyzed oxidative coupling reaction (669). 

The chromium(II)-mediated addition (Hiyama reaction) of chiral allylic bromide 835 to lactaldehyde 831 proceeds with high Felkin—Anh selectivity to furnish exclusively adduct 836 [230]. In addition to the Felkin model, the high stereoselectivity is also explained by the effect of matched pairing of the two reaction partners. If the corresponding R-enantiomer of THP-lactaldehyde 831 is employed ( mismatched pair ), a mixture of three diastereomers (3 1 1) is produced. The THP group of 836 can be removed in the presence of the TBPS protecting group by treatment with PPTS in methanol (54% yield). 

In 1983, Nozaki, Takai, Hiyama, and their coworkers disclosed that vinyl and aryl iodides or bromides are reduced with chromium(n) chloride, and that the resulting organochromium(in) species react smoothly with a host of aldehydes to give allylic or benzylic alcohols in excellent yields.6 As shown in Scheme 1, the chromium(n) chloride-mediated carbonyl addition can be conducted efficiently at... 

Scheme 1. The chromium(n) chloride-mediated carbonyl addition process of Nozaki, Takai, and Hiyama.

Ethyl (Z)-2-bromomethyl-2-heptenoate and aldehydes condense on reaction with chromium(II) chloride to furnish cw-3,4-disubstituted dihydro-3-methylene-2(3 //)-( uranones exclusively16, indicating that a (Z)-allylchromium complex might serve as reactive intermediate in the. mv-selec-tive addition step due to the bulky 2-substitucnt. Alternatively, an acyclic transition state for the reaction of the ( )-diastereomer, mediated by the Lewis acid dichloroaluminum hydride, has been discussed16. 

The catalytic process is also achieved in the Pd(0)/Cr(II)-mediated coupling of organic halides with aldehydes (Scheme 33) [74], Oxidative addition of a vinyl or aryl halide to a Pd(0) species, followed by transmetallation with a chromium salt and subsequent addition of the resulting organo chromate to an aldehyde, leads to the alcohol 54. The presence of an oxophile [Li(I) salts or MesSiCl] allows the cleavage of the Cr(III) - 0 bond to liberate Cr(III), which is reduced to active Cr(II) on the electrode surface. 

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