CO, Carbon monoxide chromium complexes

CF3H, Methane, trifluoro-cadmium complex, 24 55 mercury complex, 24 52 CF3NOS, Imidosulfurous difluoride, (fluorocarbonyl)-, 24 10 CH2, Methylene ruthenium complex, 25 182 CH2CI4P2, Phosphine, methylenebis-(dichloro)-, 25 121 CH3, Methyl cobalt complexes, 23 170 mercury complexes, 24 143-145 platinum complex, 25 104, lOS CNO, Cyanato silicon complex, 24 99 CN2OS2, l,3k, 2,4-Dithiadiazol-5-one, 25 53 CO, Carbon monoxide chromium complexes, 21 1, 2 23 87 cobalt complex, 25 177 cobalt, iron, osmium, and ruthenium complexes, 21 58-65 cobalt-osmium complexes 25 195-197 cobalt-ruthenium cluster complexes, 25 164... 

CH Methyl, cobalt complexes, 23 170 CO, Carbon monoxide, chromium complexes, 21 1, 2 23 87... 

Catalysts which have been heated for one hour at 250°C. with carbon monoxide at 100 mm., cooled to 35°C., and pumped to remove physically adsorbed gas have far greater activities than untreated catalysts at all chromium concentrations (7). At low pressures, CO treatment increases the activity by at least an order of magnitude. As described in a previous publication, on the basis of infrared evidence, it is believed that the CO-treated catalyst is characterized by the presence of a carbon monoxide-chromium complex. 

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

Chelating polyolefins displace carbon monoxide from chromium hexa-carhonyl to form stable olefin complexes. Thus cyclo-octa-1,5-diene gives the yellow complex [Cr(CO)4(C8H12)] for which the cfs-structure (VI M = Cr) is proposed (79). 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

Exposure of a tetrahydrofuran solution of W(CO)6 and the 1,2-B9C2H,, 2 ion to ultraviolet radiation produced immediate carbon monoxide evolution. Ultimately the air-sensitive (1,2-B9C2H,, )W(CO)32 ion was obtained as the tetramethylammonium salt (18). The corresponding chromium and molybdenum complexes have been obtained in the same manner (17, 18). These dianions undergo nucleophilic reactions characteristic of the analogous 77-C5H5Mo(CO)3 ion... 

Functionalized chromium carbene complexes can be prepared by the reaction of polyfunctional diorganozincs with photochemically generated Cr(CO)5.THF. The resulting intermediate ate complexes furnish under 1 atm of carbon monoxide an acyl complex which can be treated with Meerwein salt (Me30+ BF4" in dry CH2C12) at -30°C to give a chromium carbene complex (Scheme 9.3). 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

The chemistry of (l-alkynyl)carbene iron complexes is different from that of chromium and tungsten compounds. [4+2] cycloaddition of cyclopentadiene to a (l-alkynyl)carbene iron complex 11,m (R = SiMe3, r-Bu, c-QHn, n-Pr, Ph) affords (l-alkenyl)carbene Fe(CO)4 complexes 55, but these are readily isomerized at 50°C to Fe(CO)2 complexes 57 by insertion of carbon monoxide into the Fe = C bond of an intermediate Fe(CO)3... 

A systematic investigation of ligand dissociation from chromium carbonyl complexes provided evidence regarding electronic effects. The complexes Cr(CO)sT, trans-Cr(CO)4T2, and trans-Cx CO)AlAJ have been investigated for their reaction with carbon monoxide. The order of Cr-T bond stabilities in Cr(CO)sT is ... 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

From these properties it is concluded that low-valent metals do not favor water in the ligand sphere. Not that Cr(CO)6 is a stable compound because of the outstanding jr-backbonding of carbon monoxide, while Cr(FI20)6 does not exist -quite contrary to the common [Cr(H20)6]3+. On the other hand, trivalent chromium does not form the (hypothetical) cationic carbonylchromium complex Cr(CO)6]3+. This is, in short, one major reason why so little is known about typical organometallic water complexes. 

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