Rhenium enolates

Narasaka and coworkers reported radical-polar crossover addition/cyclization reactions of phenacyl bromides 204 and electron-rich alkenes such as (silyl) enol ethers 205, catalyzed by the rhenium(I) complex 206 (Fig. 57) [302], The active catalyst 206A formed after thermal nitrogen elimination from 206 reduced 204 either directly or by oxidative addition/homolysis via rhenium enolate 204A to... 

The enolates (48) and (49) of the transition metals tungsten, rhenium and molybdenum can be successfully prepared by the nucleophilic displacement of a-chloro ketones and a-chloro esters with the appropriate transition metal anion (Scheme 6). They are isolated as C-bound enolate derivatives and, except for the rhenium enolate (49), do not undergo thermal aldol additions to benzaldehyde. However, Bergman and Heathcock et al. have found that an aldol reaction of complex (48) with benzaldehyde can occur on irradiation via the rearranged q -oxaallyl derivative (50), where the metal aldolate (51) can then be... 

Schiff base complexes to aromatic silyl enol ethers/ " olefins/ sugars,Mn ° Schiff base complexes, " and rhenium(III) complexes " has been reported. Similar reactions are observed with the Mn (N)porphyrin complexes. The reaction products are dependant on the nature of the Schiff base ligand, both the yield and the enantiomeric excess being affected. " Salen nitridomanganese(V) complexes have been incorporated into Zeolite... 

However, the more hindered, less basic lithium hexamethyldisilazamide reacts slowly with 1 at 0 °C to provide chemoselectively the desired enolate species 5. The a-protons of these rhenium-acyl complexes are believed to have a lower pKa than the cyclopentadienyl protons, but unless treated with hulky, selective hases the cyclopentadienyl protons exhibit greater kinetic acidity due to statistical factors and an earlier, reactant-like transition state since minimal rchybridiza-tion is required at the anionic center after cyclopentadienyl deprotonation. Equilibration of the cyclopentadienyl anion to the thermodynamically more stable enolate species cannot compete with the rapid acyl migration84. 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Another example of metathesis of olefins bearing functional groups at the a-position is the cross-metathesis of tetracyanoethene with ethyl a-cyanocinnamate to give l,l-dicyano-2-phenylethene and ethyl 2,3,3-tricyanopropenoate, eqn. (23), in the presence of certain rhenium(I)enolate complexes at 20°C (Hirano 1994). 

Rhenium A hydrocarboxylation with high selectivity for anti-Markovnikov addition and predominant (Z)-enol ester product is mediated by ReBr(CO)5 (1 mol%, 110°C, 15 h) [168]. The 7i-activation mechanism proposed by the authors does not fit to the observed anti-Markovnikov selectivity. Iridium The precursor complex [ IrCl(cod) 2] (1 mol%) combined with P(OMe)3 (4 mol%) and Na2C03 (2 mol%) produces a catalyst that adds carboxylic acids to terminal alkynes (toluene, 100°C, 15 h) to give a mixture isomers with variable selectivities, although the Markovnikov product is usually formed in excess (ca 5 1) [169]. The complex []IrCl(cod) 2] (1 mol%) in the presence of Na2C03 (0.6 equiv) is also a catalyst for the transvinylation of vinylacetate with diverse alcohols [170]. 

The C-bound metalla tautomers 2 are typical for the less electropositive metals [4e]. They have been postulated occasionally for zinc [20] and copper [21] but are a rule for mercury [10a]. Carbon-bound enolates of molybdenum, tungsten, manganese, rhenium, iron, rhodium, nickel, iridium, and palladium have been detected and characterized [22], but one has to be aware of the phenomenon that they exist in equilibrium with the O-bound metalla tautomers. The interconversion of the palladium enolates 14 and 15 (Scheme 1.6), whose activation barrier has been determined to amount to approximately 10 kcal mol , may serve... 

Scheme 3.11 0-bound rhenium(l) enolate 30 and iron(ll) enolates 31 and 32. 

A study, including computational results, of the role of copper enolate complexes in the a-arylation of enolates has shown that the reaction is likely to occur through oxidative addition of the iodoarene to form an aryl-copper(III) intermediate, (46), followed by reductive elimination to give the product. Copper(III) species are also likely to be involved in the meta-selective C-H arylation of acetanilides by diaryliodonium salts. However, radical species may be involved in the rhenium-catalysed electrophilic triflu-oromethylation of arenes using hypervalent iodine reagents. 

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