Molybdenum complexes isomerization

The formation of 2H-pyrroles (21) and a pyrrole derivative (22) from the reaction of 3-phenyl-2//-azirines and acetylenic esters in the presence of molybdenum hexacarbonyl is intriguing mechanistically (Schemes 24, 25).53 Carbon-nitrogen bond cleavage must occur perhaps via a molybdenum complex (cf. 23 in Scheme 26) but intermediate organometallic species have not yet been isolated.53 Despite the relatively poor yields of 2H-pyrrole products, the process is synthetically valuable since the equivalent uncatalyzed photochemical process produces isomeric 2H-pyrroles from a primary reaction of azirine C—C cleavage54 (Scheme 24). 

Molybdenum complexes A (Figure 3.46) react efficiently with terminal and internal alkenes in toluene (e.g. 500 eq. Z-2-pentene are metathesized in 2 min at 25 °C 20 eq. of styrene in 2 h at 25 °C). These catalysts also oligomerize 2,4-hexadiene [808] and 1,5-hexadiene [809] and promote RCM of enol ethers. Isomerization of alkenes by catalysts A is a potential catalytic side-reaction [810-812]. 

Alkylidene carbonyl iridium complexes, reactions, 7, 275 Alkylidene compounds, NLO properties, 12, 121 Alkylidene-containing complexes, in molybdenum complexes, Schrock-type complexes, 5, 524 a-Alkylidene cyclic carbonyl compounds, isomerization,... 

Another feature that is crucial in considering rearrangements in monosubstituted allyls is the effect on the chirahty and stereochemistry. In crotyl complexes, formation of a a-bond at the unsubstituted terminus provides a path for racemization for the stereogenic center at the substituted terminus (equation 21). Formation of the a-bond at the monosubstituted terminus, however, results in conversion to a different isomer (equation 22). The most stable isomer is the syn isomer (72) and, in the absence of a substituent on the central carbon, the anti isomer (74) will only occur to the extent of f 5Vo. Thus if one considers complexes hke (acac)Pd(allyl), some racemize, whereas others only isomerize because there is no path for racemization (equation 23). These concepts have been used effectively by Bosnich in the design of systems for asymmetric allylic alkylation. These concepts also allow the rationalization of why certain substrates give low enantiomeric yields. It should be noted here that the planar rotation found in some of the molybdenum complexes retains the chirahty in the allyl moiety. 

Norbornadiene (NBD) in (NBD)M(CO)4 (353) is readily displaced by CO (274) or (2-allylphenyl)(diphenyl)phosphine (50, 312). Although the latter reaction gives the compound of expected composition, (C2iHi9P)M(CO)4, both the chemical and spectral data indicate that it has the structure (26) in which the C21H19P ligand is the isomeric (2-propenyl)(diphenyl)phosphine. For the molybdenum complex this structure has been confirmed by X-ray diffraction (379). 

Relative susceptibility of isocyanide and carbonyl ligands to insertion depends on reaction systems. The greater thermodynamic stability of the iminoacyl complexes than the acyl complexes was demonstrated by thermal isomerization of acyl(isocyanide)molybdenum to iminoacyl(carbonyl)molybdenum complexes (Eq. 7.13) [94]. 

Using this model, the authors determined kinetically the temperature in the bubbles, 5200 K, and 1900 K in the limit layer. 3 the presence of ligands (phosphines, olefins), new complexes are formed. An olefin, e.g., 1-pentene, sonicated with catalytic amounts of iron pentacarbonyl is isomerized to a mixture of ( )- and (Z)-2-pentene. A similar reaction was observed with manganese, chromium, tungsten, and molybdenum complexes. ... 

For example, the ferf-amyl hydroperoxide-molybdenum complex is presumed to attack a-pinene selectively from the least hindered side to give only one isomeric epoxide [388], equation (240). 

In subsequent chapters, only hydroformylations with Co, Rh, Ru, Pd, Pt, Ir, and Fe will be discussed in detail. Occasionally also molybdenum complexes (e.g., wer-Mo(CO)3(/ -C5H N-CN)3) [18] or osmium complexes (e.g., HOs(r -02CR)(PPhg)2) have been investigated [19]. Only recently, HOs(CO)(PPh3)3Br was evaluated for the hydroformylation of several olefins [20]. A main concern was the high isomerization tendency (up to 39%) noted. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

The second example refers to oxomolybdenum dithiolate complex [Md O(qdt)2], where qdt stands for quinoxaline-2,3-dithiolate. This complex also manifests thermal valence isomerism. Helton et al. (2001) also explain it with the change of the molybdenum oxidation state ... 

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