Chromium-catalysed reactions

A key step in the synthesis of hydroxyanthecotulide has been reported to involve a stere-ocontrolled chromium-catalysed reaction of 3-(bromomethyl)furan-2(5//)-one with an enynal and a mild gold-catalysed Meyer-Schuster rearrangement (Scheme 98). ... 

The role of metallacycles, as intermediates in chromium-catalysed ethylene trim-erisation, is highlighted in this article. It is also shown that when 37 is treated with ethene in a protonolysis reaction, hexane and butane are liberated (1 3 ratio hexane butane). This observation gives strength to the argument that metallacycles are intermediates in chromium-catalysed ethylene trimerisation. Scheme 13 provides a simplified representation for chromium-based ethylene trimerisation. 

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

Pyrroles 531 are formed from the chromium complex 529 and alkynes 530 (R = H, Me or Ph = Me, Ph or NEt2). The dicobaltoctacarbonyl-catalysed reaction of cyanotrimethylsilane with a variety of acetylenes R C=CR (R R = alkyl or Ph) furnishes pyrroles 532, in which the bulkier of the two substituents of unsymmetrical internal acetylenes appears at the position marked with an asterisk . An indole synthesis from o-iodo-aniline and alkynes R C=CR (R R =alkyl or Ph) in the presence of palladium(II) acetate, triphenylphosphine, lithium chloride and potassium carbonate has been described (equation 56). In the case of unsymmetrical alkynes, the bulkier substituent tends to be in position 2 of the indole. ... 

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>. 

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. 

The use of base-catalysed reactions for the template synthesis of co-ordinated, often macrocyclic ligands was discussed in the introduction to this chapter. " Chromium(m) Complexes.—Studies of the base hydrolysis of chromium(ra) complexes at high pH are relatively rare, probably because of the ease with which polymeric hydroxy-complexes can be precipitated. Studies of aqua-chromium(m) complexes even at low pH invariably show that conjugate-base formation is important owing to the acidity of the co-ordinated water molecules. Conjugate-base formation is apparent when the observed pseudo-first-order rate constant, k, varies with acidity according to the equation A =A o+ -x/[H+]. Recent examples include studies of the [Cr(HaO)6(NHs) + and [Cr(ox)2(N3)(H20)]2- ions." ... 

Another feature of chromium(m) substitutions which distinguishes them from analogous cobalt(m) reactions is the dominance of stereoretentive pathways, trans-Complexes of cobalt(m), in particular, often undergo stereochemical change, but this is rarely the case for chromium(in). Despite these characteristics which distinguish chromium(m) and cobalt(m) substitutions, there are still many features which make the chemistries of these two metal ions comparable. For example, the relative labilities of analogous complexes is similar, and acid- and base-catalysed reactions oftra occur for related compounds. These similarities will be apparent in the following sections. 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Haloarene chromium tricarbonyl complexes are activated to nucleophilic attack by thiolate anions [58, 59]. High yields of the thioethers are obtained under liquiddiquid two-phase conditions, but optimum yields are achieved under soliddiquid conditions. In many cases the thioether is produced directly but, where the reaction mixture contains thioether and its chromium complex, the thioether can be isolated by degradation of the complex with iodine or an excess of the thiol. Both 1,2- and 1,4-dichlorobenzenes yield only monothioethers, even when an excess of thiolate anion is used. In contrast, 1,3-dichlorobenzenes produce a mixture of the mono- and dithioethers [59]. Aryl allyl thioethers have been produced under catalysed Heck reaction conditions from S-allyl thiocarbamates and iodobenzene [60]. 

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]. 

The kinetics of chromium(l 11 )-catalyscd oxidation of fonnic acid by Ce(TV) in aqueous H2SO4 can be rationalized in terms of initial formation of an outer-sphere complex involving oxidant, catalyst, and substrate (S), Ce(TV)(S)Cr(III), followed by an inner-sphere complex Ce(III)(S)Cr(IV). It is proposed that electron transfer occurs within this complex from substrate to Cr(TV) (with elimination of H+) followed by fast reaction to give CO2 (again with elimination of H+).54 In contrast, there was no kinetic evidence for the accumulation of a corresponding inner-sphere intermediate in the osmium(VIII)-catalysed Ce(TV) oxidation of DMSO to dimethyl sulfone here, the observed rate law was rationalized in terms of rate-determining bimolecular electron transfer from DMSO to Os(VHI) in an outer-sphere step.55 The kinetics of oxidation of 2-hydroxy-l-naphthalidene anil by cerium(IV) in aqueous sulfuric acid have been... 

Enyne intramolecular metathesis reactions, of the type shown in equation 61, can be very useful in organic synthesis. A number of such reactions, catalysed by tungsten or chromium carbene complexes, have been reported634,635,737 - 740. The ruthenium carbene catalysts 18-20 (Table 2) are likely to be increasingly used for this purpose because of their stability, ease of handling and good yields, as in the synthesis of various 5-, 6- and 7-membered heterocycles, e.g. equation 67741. 

What effect does co-ordination of the amino acid derivative 3.1 have upon the rate of hydrolysis The rates of the hydrolysis reactions depicted in Fig. 3-8 are only slightly more rapid than those of the free amino acid esters, and, in general, the rates of reactions involving monodentate TV-bonded ligands very closely resemble those for acid-catalysed hydration. This monodentate bonding mode is only exhibited with non-labile ions such as cobalt(m) or chromium(m) and is relatively rare even then. 

Substituted bicyclo[ . 1.0]alkanes may also be obtained by condensation of secondary amines with 2-haloketones. A variety of nucleophilic reactions can be carried out on the intermediate cyclopropaniminium salt 116251 (Scheme 108). Competing alkene scission and cyclopropanation occurs on reaction of enamines with pentacarbonyl-chromium carbene complexes252 (Scheme 109). N-Silylated allylamines and their derived N-silylated enamines undergo rhodium or copper catalysed cyclopropanation by methyl diazoacetate253 (Scheme 110). 

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