Hexacarbonyl complex

Table 8-6. Bond lengths for neutral hexacarbonyl complexes of Cr, Mo, and W in Oh symmetry [A],...

The molybdenum hexacarbonyl complex was recently introduced as a condensed source of carbon monoxide for Heck carbonylations [29]. This easily handled and inexpensive solid delivers a fixed amount of carbon monoxide when heated to approxi-... 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

An enantioselective intramolecular Pauson-Khand reaction based on chiral auxiliary-directed 7t-face discrimination in acetylenic 0-alkyl enol ether-dicobalt hexacarbonyl complexes, which proceeds with good yields and high facial diastereoselectivity, has recently been developed by M.A. Pericas, A. Moyano, A.E. Greene and their associates. The method has been applied to an enantioselective formal synthesis of hirsutene. Moreover, the process is stereodivergent and the chiral auxiliary -rran5-2-phenylcyclohexanol- is recovered in a yield as high as 92% [18]. 

The dppm-subsituted complexes of the asymmetric diynes RC=CC=CSiMe3 (R = Ph, tol) are readily obtained from thermal reactions of the hexacarbonyl complexes with dppm or by direct reactions of the diynes with Co2(M-dppm)(CO)6. In these cases there is some evidence for regioselectivity due to steric interaction... 

Fig. 3 Surface-reactivity of the molybdenum hexacarbonyl complex as a function of the area density of - OH groups on an alumina surface...

Tandem reaction generally attracts much interest because it allows us to effect multiple transformations, all in one pot. Two types are known at present. The most popular approach is a combination of several reactions with PKR. Properly functionalized 1,6- or 1,7-enynes are readily obtained from the propargyl alcohol-dicobalt hexacarbonyl complexes and a properly nucleophilic allylic moiety in the presence of a Lewis acid. The resultant enynes are subjected to the promoter-assisted PKR without purification to afford the desired PKR products (Scheme 9).82,82a-82e... 

The formation of the butenolactone complex, XII, by the action of carbon monoxide on acetylene dicobalt hexacarbonyl complexes, XI, (89) seems to be a closely related reaction. It probably involves the following steps ... 

It is, however, for the transition metals themselves that DFT has proven to be a tremendous improvement over HF and post-HF methods, particularly for cases where tlie metal atom is coordinatively unsaturated. The narrow separation between filled and empty d-block orbitals typically leads to enormous non-dynamical correlation problems with an HF treatment, and DFT is much less prone to analogous problems. Even in cases of a saturated coordination sphere, DFT methods typically significantly ouqierform HF or MP2. Jonas and Thiel (1995) used the BP86 functional to compute geometries for the neutral hexacarbonyl complexes of Cr, Mo, and W, the pentacarbonyl complexes of Fe, Ru, and Os, and the tetracarbonyl... 

Ethynylcyclopropanes, like normal acetylenes, react with dicobalt octacarbonyl in ether to form stable dinuclear cluster-like hexacarbonyl complexes (equation 170)236. The complex with l-chIoro-2,2,3,3-tetramethylethynylcyclopropane reacts stereo- and regioselec-tively with norbomene in a typical Pauson-Khand reaction to give the exn-2-cyclopropyl substituted cyclopentenone (equation 171). Similarly, the reaction of 2-ethoxycyclo-propylacetylene with cyclopentene in the presence of Co2(CO)8 under CO gave 3-(2-ethoxycyclopropyl)-cw-bicyclo[3.3.0]oct-3-en-2-one (equation 172)242. 

The reaction of trimethylsilylacetylenedicobalt hexacarbonyl complex and divinyl sulfone in refluxing toluene afforded a dihydrothiepine 1,1-dioxide 218 in 30% yield (Scheme 29) <1999JA8237>. Subsequent bromination-dehydrobromination gave 4-trimethylsilylthiepine 1,1-dioxide 219 in 70% yield. Metalation using [(MeCN )3Cr(CO)3] provided the complex 220 in 85% yield, which was used for [6n+2n] cycloaddition and subsequent Ramberg-Backlund rearrangement (Sections 13.03.5.2 and 13.03.6.2). 

Heterostannenes, preparation and characteristics, 3, 870 Heterosubstituted arenes, metallation, 9, 17 Hexaaryldiplumbanes, preparation, 3, 887 Hexabutyldistannane, preparation, 3, 856 Hexacarbenes, in cobalt(III) complexes, 7, 19 Hexacarbonyl complexes, with molybdenum kinetics and reactivity, 5, 395 photochemistry, 5, 393 solid-support studies, 5, 394 spectroscopic and theoretical studies, 5, 392 Hexadentate iV-substituted triazacyclononane, synthetic applications, 1, 70... 

The expected hexacarbonyl dicobalt complex was formed when the al-kyne and dicobalt octacarbonyl were stirred in CH2C12 (73 —> 74). If the same reaction was performed in the presence of ten equivalents of NMO, the novel pentacarbonyl complex could be isolated (73->75). On dissociation of the CO ligand, accelerated by the addition of NMO, the sulfur atom is able to coordinate to the vacant site creating a stable complex. Treatment of the complex with CO resulted in regeneration of the hexacarbonyl complex and, unlike the isolated alkene-pentacarbonyldicobalt-alkyne species 68, heating of the pentacarbonyl complex 75 led to the formation of the desired cyclopentenone. A similar sulfur stabilised intermediate was isolated by Pericas and co-workers while investigating tethered chiral auxiliaries in the PK reaction.87,88... 

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