Molybdenum, stereochemistry

Table 23.2 Oxidation states and stereochemistries of compounds of chromium, molybdenum and tungsten...

Molybdenum, tetrakis(l-adamantoxo)(dimethy]amine)-stereochemistry, 1, 44... 

Molybdenum, tetrakis(diethyldithiocarbamato)-stereochemistry, 1, 94 Molybdenum, tetrakis(dithiobenzoato)-stereochemistry, 1, 94... 

Molybdenum, tris(dimethyldithiocarbamato)(phenylazo)-stereochemistry, 1, 82 Molybdenum, tris(dithioglyoxal)-structure, 1,63... 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

This reaction probably proceeds via the neutral hydride that undergoes reductive elimination of germane. The stereochemistry observed (retention of configuration) is in agreement with a reductive elimination and with the assumption that the formation of the molybdenum-germanium bond proceeds with retention of configuration (cf. Sect. 3.1.2). 

Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 

The molybdenum-hydroperoxide complex (Step 3) reacts with the olefin in the rate-determining step to give the epoxide, alcohol, and molybdenum catalyst. This mechanism explains the first-order kinetic dependence on olefin, hydroperoxide, and catalyst, the enhanced reaction rate with increasing substitution of electron-donating groups around the double bond, and the stereochemistry of the reaction. 

Hughes 76 studied the stereochemistry of the homogeneous disproportionation of 2-pentene using soluble molybdenum catalyst system. His results reveal that the molybdenum catalyst exhibits a high degree of stereoselectivity c/s-2-pentene disproportionated preferentially to c/s-2-butene and cis-3-hexe-ne, and trans-2-pentene reacted to yield preferentially fraras-2-butene and... 

Table 18-C-i Oxidation States and Stereochemistry of Molybdenum and Tungsten... 

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. 

The anti addition of amines to the double bond of cationic (alkene)(cyclopentadienyl)di-carbonyliron complexes and to the analogous molybdenum and tungsten complexes has been reported31 33. The adducts underwent carbonyl insertion-cyclization to give chelate complexes, which were then oxidized to /8-lactams. For example, from the Fp complexes of ( )- and (Z)-2-butene the corresponding /8-lactams were obtained diastereoselectively in 10-15% yield by the direct oxidation of the benzylamine adducts with chlorine at low temperature33. The stereochemistry was determined by H-NMR spectroscopy. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

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