Regioselectivity chromium carbonyl complexes

In chromium carbonyl complexes of indole, the metal is associated with the benzene ring, hence nucleophilic additions take place in that ring, usually at C-4 the example shows the relatively unusual attack at C-7 this regioselectivity can be induced to revert to the usual C-4 if an indole with a bulky A-protecting group is utilised. ... 

Cycloaddition of the carbene chromium complexes 97 with CO incorporation provides a versatile method for naphthol synthesis, in which the metallacy-clic intermediates 99 are involved [47]. An alternative entry to 101 is achieved by metal carbonyl-catalyzed rearrangement of the cyclopropenes 98 via the same metalla-cyclobutenes 99 and vinylketene complexes 100 [52], Mo(CO)6 shows a higher activity than Cr(CO)6 and W(CO)6. The vinylketene complex 103 is formed by the regioselective ring cleavage of 1,3,3-trimethylcyelopropene 102 with an excess of Fe2(CO)9 [53]. (Scheme 35 and 36)... 

Cathodic reduction of retinal leads to a regioselective coupling in the presence of malonic ester to produce the corresponding pinacol (Fig. 26) [129]. Chromium(III) ions facilitate the reduction and favor the regioselective coupling of conjugated dienones to pinacols. A Cr(III)-carbonyl compound complex is evidenced as the reason for the selectivity [130]. 

In a general illustration of the Dotz reaction a terminal or internal alkyne reacts with a carbene 123 and one carbonyl ligand at a [Cr(CO)3] template in a formal [3 + 2+1] cycloaddition reaction producing a chromium-complexed naphthol (124) under mild reaction conditions via the vinylketene intermediate 125 (see Scheme 57). Terminal alkynes (R1C = CR2 R HjR H) react with total regioselectivity, while the regiocontrol in the reaction course of internal alkynes... 

A number of metal complexes catalyze the hydrosilylation of various carbonyl compounds by triethylsilane. Stereoselectivity is observed in the hydrosilylation of ketones as in the reactions of 4-t-butylcyclohexanone and triethylsilane catalyzed by ruthenium, chromium, and rhodium metal complexes (eq 4). Triethylsilane and Chlorotris(triphenylphosphine)rho-dium(I) catalyst effect the regioselective 1,4-hydrosilylation of Q ,/3-unsaturated ketones and aldehydes. Reduction of mesityl oxide in this manner results in a 95% yield of product that consists of 1,4- and 1,2-hydrosilylation isomers in a 99 1 ratio (eq 5). This is an exact complement to the use of phenylsilane, where the ratio of respective isomers is reversed to 1 99. ... 

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