Molybdenum complexes isolation

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). 

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

As described in many reviews, Trost and his co-workers have carried out a pioneering work on the molybdenum-and tungsten-catalyzed allylic alkylation of allylic esters regioselectivity of the reaction is often complementary to the palladium-catalyzed allylic alkylation. The first asymmetric version was disclosed by Pfaltz and Lloyd-Jones in 1995 (Equation (63)). They used a catalytic amount of a novel tungsten complex, prepared from [W(CO)3(MeCN)3] or [W(cycloheptatriene) (COIs] and optically active (diphenylphosphino)phenyloxazolines 57, for the allylic alkylation of 3-aryl-2-propenyl phosphate with dimethyl sodiomalonate to isolate the corresponding branched alkylated compounds as a major isomer with an excellent enantioselectivity (96% ee). Unexpectedly, 3-aryl-2-propenyl carbonates are shown to be unreactive. It is worth noting that an isostructural molybdenum complex does not promote the catalytic alkylation under the same reaction conditions. In contrast, Lloyd-Jones and Lehmann reported the stereocontrolled... 

The reactions of dinuclear thioaldehyde and -ketone complexes with PR3 take a different course. When the dimeric complexes 112 are treated with PMe3 a Ti-S bond in 112 is cleaved and the monomeric complexes 113 are reversibly formed (see Scheme 26), 69 The reaction of 112 with benzene-thiol led to a Ti-C cleavage and formation of 1,3-propanedithiolato-(phenylthiolato) complex.69 The reaction of binuclear p-191 rf-thioketone molybdenum complexes related to 115 (M = Mo, E = S, R = Aryl) with P(OEt)3 also gave mononuclear complexes [Mo(CO)2(Et) i72-S = C(Aryl)2 (i75-C5H5)] by a rather complicated reaction sequence. When other phosphites were employed additionally binuclear complexes derived from addition of the phosphite to one molybdenum atom were isolated.225... 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

Electrocyclization of E is expected to give cyclohexadienone complex F a related molybdenum complex 6 (Scheme 4), in which two carbonyl ligands have been replaced by alkyne ligands, has been isolated from the reaction of a vinyl molybdenum carbene complex with 3-... 

Several mixed bisalkyne molybdenum complexes have been prepared [Eq. (32)] (87). Alkyne exchange is known to occur in the Mo(CO)-(RC=CR)(S2CNMe2)2 reagents (90), and this complicates isolation of pure mixed bisalkyne products. 

Similarly, orange-red needles (mp 53 °C) of 1-fluoroborepin molybdenum carbonyl complex 18 were obtained by conversion of 1-methoxyborepin 17 to the corresponding Mo(CO)3 complex followed by immediate treatment of the crude complex with boron trifluoride in pentane, as in Scheme 1. The intermediate molybdenum complex of 17 could be isolated as air- and moisture-sensitive crystals (mp 110-115 °C) but prolonged manipulation led to contamination by the corresponding 1-hydroxyborepin complex <19970M1884>. 

From the reaction of vinyl methyl ketone with (CH3CN)3M(CO)3, (M = Cr or Mo), King (348) has isolated the complexes, (vinyl methyl ketone)sM, as yellow volatile solids. Although the molybdenum complex... 

The positions of the photoluminescence (i.e., phosphorescence) and its corresponding excitation bands are nearly identical to those of the M0O4 complex in CaMo04. The energetic positions of the peak maxima of the excitation and photoluminescence spectra of five- and six-coordinated oxo-molybdenum complexes are known to exist at lower energy levels, i.e., at approximately 400-333 nm (25,000-30,000 cm ) for the excitation and at 714-588 nm (14,000-17,000 cm" ) for the photoluminescence, respectively 130). Hence, the photoluminescence spectrum shown in Fig. 26 can be attributed to the presence of an isolated four-coordinate molybdenum oxide species. 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

In addition to the compound classes mentioned so far, cyclopropenylidene complexes have also been reported. The first mononuclear carbene complex not stabilized by a heteroatom in the (X-position was the chromium complex 27, which proved to be unexpectedly stable. This complex was obtained in 20% yield by treatment of 3,3-dichloro-l, 2-diphenylcyclopropene with disodium pentacarbonylchromate at — 20 °C. However, isolation of the complex is easier when it is prepared from 3-ethoxy-l,2-diphenylcyclopropenium tetrafluoroborate. The corresponding molybdenum complex can also be obtained in this way. 

---