Chromium carbene reagents

Recently, reaction of this lithium reagent with Cr(CO)6 followed by the treatment with 1,2-diphenylacetylene afforded a highly substituted fluorophenol via a chromium carbene intermediate [127] (Scheme 48). 

Barbier reaction Samarium(II) iodide, 270 Benzoannelation Chromium carbene complexes, 82 Dicarbonylcyclopentadienylcobalt, 96 Ethyl (Z)-3-bromoacryIate, 130 Grignard reagents, 138 Methyl acrylate, 183 Methyllithium, 188 Ruthenium(III) chloride, 268 Benzoin condensation Benzyltriethylammonium chloride, 239 3-EthyIbenzothiazolium bromide, 130 Benzoylation (see also Acylation) Cadmium, 60 Dibutyltin oxide, 95 Birch reduction Birch reduction, 32... 

Nitropropane, 200 Simmons-Smith reagent, 275 Four-membered rings Chromium carbene complexes, 82 Menthol, 172... 

Chromium carbene complexes, 82 Ethyl diazo(lithio)acetate, 230 Reformatsky reagent, 348 Titanium(IV) chloride, 304 By other reactions Di- p.-carbonylhexacarbonyldicobalt, 99... 

Chromium-based reagent systems, for pinacol coupling, 11, 63 Chromium carbenes, in ene-yne metathesis, 11, 272 Chromium carbonyl compounds with bridging hydrides, 5, 206 computational studies and spectroscopy, 5, 203 experimentally determined structures, 5, 204 nitro and nitroso compounds, 5, 205 silatropic migrations, 5, 249 with very weakly bonded ligands, 5, 205 Chromium carbonyl hydrides, preparation and characteristics,... 

Beyond the use of the Fischer-type chromium carbenes as stoichiometric reagents and the Grubbs-type ruthenium carbenes as versatile catalysts for the preparation of organic compounds, Schrock s molybdenum and tungsten complexes of the general composition M(CHR)(NAr)(OR )2 (and derivatives thereof) and... 

The chiral Fischer-type chromium carbene complex of furan 28, shown in Scheme 14, participated in nucleophilic 1,4-addition with organolithium reagents followed hy alkylation in a regioselective and diastereoselective manner, creating a quaternary C-3 stereocenter in the 2,3-trisubstituted 2,3-dihydrofuran products after oxidative decom-plexation and reductive cleavage of the chiral auxiliary <2003CEJ5725>. 

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

Organochromium reagents have found varied use in organic synthesis. Aryl-chromium complexes influence significantly the reactivity of the aromatic ring and have been used widely. Other unsaturated chromium complexes, alkyl-chromium species and chromium carbenes promote useful transformations and continue to attract attention. 

Straightforward access to pyrazole carbene chromium complex LXI results from the 1,3-dipolar addition of diazo compounds to acetylenic chromium carbene LX the reagent thus obtained is a useful synthon for the preparation of novel heterocyclic systems such as pyrazole [1, 5-a]pyridine quinone LXIII derivatives [100] in low to moderate yield (10-51 o) according to the substituents (Figure 5). 

Kinetic studies of the reaction of oxo(salen)chromium(V) triflate with alkynes suggest that a metallaoxetane or a chromium carbene acts as an intermediate in the reaction.326 The bridging methylene complexes, (THF)nClxMo(0) 2( X-CH2)2. are prepared by treatment of MoCly(THF)2 (x = 1, y = 3, z = 2 X = 2, y = 4, z s= 0) with two equivalents of MeLi,322 both complexes act as chemoselective reagents for alkene addition to a carbonyl compound. 328... 

Organometallic reagents were used for the synthesis of bicyclic aromatic compounds via Moore-type cyclization. Rahm and Wulff described the new synthesis of 5-hydroxyindolines with the use of a chromium carbene complex bearing alkynyl substituent 22." The amino-tethered bis-alkynyl carbene complex 22 was transformed into indoline 23 by thermolysis in the presence of a hydrogen source. The low yield of product 23 was improved when the reaction was carried out in the presence of the electrophile, added to protect the phenol function. This process involves the insertion of one carbon monoxide group from the chromium complex into the skeleton of an eneyne compound 24. The resulting enyne-ketene 25 undergoes a cycloaromatization reaction to afford the 1,4-diradical intermediate 26. Subsequent demetalation yields product 23."... 

Several investigators have attenuated the potent nucleophilic-ity of vinyllithium intermediates by transmetalation or trapping. Vinylstannanes, chromium-carbene complexes, and particularly vinylsilanes have been prepared. Alkenyllithiums react faithfully to give 1,2-addition products with a, -unsaturated ketones, and mixed cuprate reagents have also been prepared with some success by trapping vinyllithiums with phenylthiocopper for 1,4-addition (eq... 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

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