Molybdenum complexes carbon

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). 

Asymmetric Synthesis Using a Chiral Molybdenum Catalyst In olefin metathesis, a double bond is cleaved and a double bond is formed. Thus, a chiral carbon center is not constructed in the reaction. To realize the asymmetric induction by ring-closing metathesis, there are two procedures a kinetic resolution and desym-metrization of symmetric prochiral triene. Various molybdenum complexes are synthesized in order to explore the viabihty of these approaches (Figure 6.2). 

An interesting allylic substitution reaction of ( )-cinnamyl methyl carbonate 143 has been examined by Pfaltz s group. The use of a molybdenum complex of ligand 144 resulted in 145 in 88% yield with an ee of 99% [for the (R) isomer] (Fig. 9.45). 

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... 

Molybdenum complexes of polysulfide ligands are reactive and, for example, readily make and break sulfur-carbon bonds, a property undoubtedly relevant to the involvement of Mo—S... 

Individual carbon atoms are very uncommon ligands for transition metals. The synthesis of a molybdenum complex containing a single C radical anion, uncomplexed to any other metals or functional... 

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... 

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

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. 

H) containing symmetrically bridging thioacyl and thioalkyl functions are obtained with molybdenum as the central metal (46). The dithiolactone S= C(CH2)2CH2S reacts similarly with the molybdenum complex, but in the product the a carbon is link to the nonadjacent sulfur atom by an alkyl chain (47). 

The alkylidyne ligands in the molybdenum complexes 179 were modified by deprotonation at the y -carbon with strong bases and subsequent addition of electrophiles [Eq. (146)]. Complexes 180 were desilylated by treatment with fluoride [Eq. (147)] (48). 

This is a general reaction for alkynes (acetylene, phenylacetylene, diphenylacetylene, cyclodecyne, etc.) that occurs at RT, producing 75% yields of products. The two cobalt atoms and two carbons form a tetrahedron. Analogous iron complexes are prepared in low yield by reaction of diphenylacetylene with Fe3(CO)i2 . Similar molybdenum complexes are prepared by addition of alkynes to molybdenum multiple bonds ... 

OC, Carbon monoxide (Continued) cobalt, iron, osmium, and ruthenium complexes, 21 58-65 iron complex, 21 66, 68 manganese complexes, 23 34 molybdenum complexes, 23 4-9 niobium complexes, 23 34 palladium complex, 21 49 rhodium complexes, 23 124 ruthenium complex, 21 30 OCH4, Methanol, iridium complexes, 23 127 rhodium complexes, 23 127, 129 OCjHs, Acetone, compd. with carbonyltri-p.-chloro-chlorotctrakis-(triphenylphosphine)diruthcnium (1 2), 21 30... 

---