Chromium reaction with Lewis bases

The chromium-chromium quadruple bond is typically considered to be strong, and this is supported by the fact that reactions of [Cr2(p-02CR)4] with Lewis bases generate... 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

Similarly, chromium-complexed benzylic cations are also stabilized and organic reactions based on the benzylic cation species have been developed. For example, planar chiral o-substituted benzaldehyde dimethylacetal chromium complexes 4 were treated with 3-buten-l-ol in the presence of TiCl4 to give tet-rahydropyran derivatives with high diastereoselectivity (Eq. 5) [5]. The chromium-complexed benzylic oxonium ion 6 would be also generated and subsequent intramolecular cyclization afforded the cyclization product 7. Furthermore, the chromium-complexed benzyl alcohol derivative having electron-rich arene ring at the side chain produced tetrahydroisoquinoline skeleton by treatment with Lewis acid with stereochemical retention at the benzylic position (Eq. 6) [6]. 

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. 

It is apparent that if the Lewis base is charged rather than neutral, the substituted metal carbonyl will have the same charge as the Lewis base. In this manner certain anionic metal carbonyls can be synthesized by the displacement of carbonyl groups with anionic Lewis bases. Frequently used for this type of reaction are the halide ions and cyanide ion. Many of these reactions have been carried out on the relatively stable hexacarbonyls of chromium, molybdenum, and tungsten, their derivatives or other related... 

Observations on the reaction of ethylmalonatopentaamminecobalt(III) with Cr(H20)g + introduce a new element of interest. The chromium(III) products are the chelated malonate (67%) with a corresponding amount of free alcohol and the monodentate ester complex (33%). Since ester hydrolysis in the latter species is slow, we conclude that hydrogen results from the ester in the chelate form. Again, since ring closure of the monodentate product complex is slow, chelation must have occurred before Cr is oxidized to Cr . It is possible that formation of the chelate as primary product is complete, and that this product reacts in part to yield the monodentate product before hydrolysis occurs. Activation based on electron transfer to trap a function which is sensitive to a substitution-inert metal ion acting as a Lewis acid could presumably be extended to other more interesting situations. 

Notably, catalysts with redox properties, such as molybdenum-, chromium-, and vanadia-based catalysts, show high activity in various oxidative dehydrogenation reactions of hydrocarbons [45 8]. Factors influencing the reaction also include acid-base bifunctionality, which plays an important role in CO2-mediated dehydrogenation reactions [49]. Both basic sites and Lewis-acid vacant sites are important for hydrocarbons activation [50]. In fact, an enhanced basicity results in an improved performance because of the rapid desorption of the electron-rich alkenes, whereas Lewis acid sites enhance the dehydrogenation process [51]. In addition, in the presence of CO2 as feed, surface basicity favors the adsorption and reactivity of the acid CO2 molecules [52] (see also previous chapters). 

The by far strongest known Lewis acid is antimony pentafluoride, SbFj. Metal fluorides of aluminum, iron, chromium, etc., do not play a prominent role in this context, but instead the chlorides are considered as strong Lewis acids to be used, for example, in Friedel-Crafts reactions. In 1999, Christe et al. [59] introduced the pF scale that provided for the first time a quantification of Lewis acidity strength of metal fluorides and chlorides. Their approach is based on ab initio calculations of the free formation energies of the gas-phase reactions of molecular metal fluorides with a gaseous fluoride anion (Equation (6.11)) ... 

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