Chromium-catalyzed reactions

Bis-salicylidene (or bis-salen) derivatives of 1,2-diaminocyclo-hexane are prepared by treatment of the diamine (or its tartrate salt) with an o-hydroxybenzaldehyde derivative and are used for asymmetric manganese-, cobalt- or chromium-catalyzed reactions. The most important ligand of this type is A(A -bis(3,5-di-terf-butylsalicylidene)-diaminocyclohexane (12), the detailed synthesis of which has been published. Another important ligand is the atropoisomeric derivative (13) (eq 21). ... 

Hari A, Miller BL. Segregation of separate steps in chromium-catalyzed reactions for convenience and mechanistic analysis. Org. Lett. 2000 2 691-693. 

Katz and Sivavec, who used a Fischer tungsten carbene complex as a catalyst. After the report of a chromium-catalyzed reaction by Mori et al., they disclosed the utility of ruthenium-carbene complexes for the enyne metathesis (Scheme 24.44). The Ru-carbene-catalyzed reaction under the ethylene atmosphere brought about good results in the case of terminal alkyne substrates such as 164b. ... 

Copper—cadmium and zinc—chromium oxides seem to provide most selectivity (38—42). Copper chromite catalysts are not selective. Reduction of red oil-grade oleic acid has been accompHshed in 60—70% yield and with high selectivity with Cr—Zn—Cd, Cr—Zn—Cd—Al, or Zn—Cd—A1 oxides (43). The reduction may be a homogeneously catalyzed reaction as the result of the formation of copper or cadmium soaps (44). 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

To my knowledge, the first transition metal-catalyzed reaction utilizing S-S bond activation was reported by Holmquist el al. in 1960 [14]. The reaction of (PhS)2 with CO (950 atm) in the presence of chromium oxide on AI2O3 at 275°C furnished thioester 57 in 31% yield (Eq. 7.42). 

The multi-component procedure is also effective for the chromium-catalyzed addition of organic halides to aldehydes (the Nozaki-Hiyama-Kishi reaction) [73]. The active Cr(II) species is recycled by redox interaction with Mn powder as the stoichiometric co-reductant in the presence of MesSiCl (Scheme 34), which mainly liberates the chromium catalyst from the alkoxide adduct. The chemo- and diastereo-selective addition reaction is performed with a variety of organic halides and alkenyl triflates. In the case of crotyl bromide, the addition is highly stereoconvergent, i.e., the respective anti-... 

A wide range of nonacidic metal oxides have been examined as catalysts for aromatization and skeletal isomerization. From a mechanistic point of view, chromium oxide catalysts have been, by far, the most thoroughly studied. Reactions over chromium oxide have been carried out either over the pure oxide, or over a catalyst consisting of chromium oxide supported on a carrier, usually alumina. Depending on its history, the alumina can have an acidic function, so that the catalyst as a whole then has a duel function character. However, in this section, we propose only briefly to outline, for comparison with the metal catalyzed reactions described in previous sections, those reactions where the acidic catalyst function is negligible. 

C2-symmetric bis(oxazoline) and salen-metal complexes have recently been shown to be very effective ligands for iron(III),28 magnesium(II),29 copper(II),30 and chromium(in)31 complex-catalyzed reactions. 

Only a distillative work-up from an acidic oxidation reaction, for example, delivers 55, which then has to be isomerized by base to obtain 56 [113]. Similar precautions for the synthesis of 55 have been reported recently for a chromium-catalyzed oxidation [114] here again the key is to avoid a base and even a chromatographic workup, as already demonstrated for chromium-free Dess-Martin oxidations [110]. 

Exchange-inert complexes of Co(III) with nucleotides that have proven to be extremely useful as chirality probes because the different coordination isomers are stable and can be prepared and separated In addition, these nucleotides can be used as dead-end inhibitors of enzyme-catalyzed reactions and, since Co(III) is diamagnetic, a number of spectroscopic protocols can be utilized. See Exchange-Inert Complexes Chromium-Nucleotide Complexes Metal Ion-Nucleotide Interactions... 

The effect of methyl substituents on the (3-carbon on the rate of (3-elimination is complicated, different effects are observed for the acid dependent and acid independent process and for different central cations (Table V) (136). Methyl substituents on the (3-carbon enhance the specific rate of the acid independent path for both chromium(III) and copper(II) complexes, however no such effect was observed for the analogues (tspc)Co(III) complexes (136). The effect on the acid catalyzed reaction is even opposite for the chromium(III) and copper(II) complexes (136). 

As indicated in Scheme 27, indoles may be alkylated by their acid-catalyzed reaction with alcohols. Similarly, r-butylation of pyrroles has been effected by the acid-catalyzed reaction with t- butyl acetate (B-77MI30502), and the diarylmethylation of 1-methylpyrrole from the acid-catalyzed reaction with the chromium trichloride complex of the diarylcarbinol has been described (78JA4124). The alkylation of indoles by alcohols in the presence of the aluminum alkoxide and Raney nickel appears to be efficient for the synthesis of 3-substituted indoles, but is less successful in the alkylation of 2-methylindole (79JHC501). The corresponding isopropylation of pyrrole produces 2,5-diisopropylpyrrole and 1-isopropylpyrrolidine, as the major products (79JHC501). 

Tetrahydroisoquinolines can be produced by a nucleophilic attack on a chromium-activated aromatic ring (Equation 66) <1997TL7387>. The chromium is subsequently removed by oxidation. A highly diastereomeric ringclosing reaction occurs by the Pd-catalyzed reaction of the chiral iV-allylbenzylamine derivative 32 <1997LA1407> (Equation 67). 

Aminoindanol-derived Schiff bases were developed as tridentate ligands for the chromium-catalyzed hetero Diels-Alder reaction between weakly nucleophilic dienes and unactivated aldehydes.24 The generality of the utility of these Schiff bases, readily obtained by condensation of 1 with the corresponding aldehyde, was later demonstrated in the hetero Diels-Alder reaction between Danishefsky s diene and chiral aldehydes,25 in the inverse electron-demand hetero Diels-Alder reaction of a,P-unsaturated aldehydes with alkyl vinyl ethers,26 and in hetero-ene reactions.27... 

Fischer-type carbenes can also be modified via transition metal catalyzed reactions. Fischer chromium aminocarbene complexes can be used as nucleophiles in palladium-catalyzed allyUc substitution reactions with aUylic acetates and carbonates, alFording the corresponding allyl-substituted aminocarbenes. For example, reaction of the Uthiated carbene (15) gives (16) in good yield (Scheme 25). ... 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

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