Molybdenum complexes stereochemistry

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. 

Molybdate, pentachlorooxy-stereochemistry, 50 Molybdate, tetrakis(dioxygen)-stereochemistry, 94 Molybdenum complexes history, 21... 

The chemistry of dienes coordinated to the cationic CpMo(CO)j fragment has been exploited many times for complex molecule synthesis. Originally, Faller showed that the cationic molybdenum complex in Equation 11.44 undergoes nucleophilic attack by hydride, deuteride, methyl lithium, and enamines to produce the Ti -allyl complex. As expected, attack of the nucleophiles occurs at a terminal position and exclusively from the face opposite the metal. Trityl cation abstracts a hydride from this allyl product from the face opposite the metal to regenerate a diene complex. Pearson has used this sequence of nucleophilic attack and hydride abstraction to synthesize substituted cyclohexenes with control of stereochemistry as shown in Scheme 11.6. ... 

There is a marked effect on the resulting stereochemistry of the products of metathesis reactions depending on the transition metal. By careful analysis of the cisitrans ratios at very low conversion of 2-butenes, obtained by metathesis of c/5-2-pentene, different behaviour is observed for tungsten and molybdenum complexes [40,41]. 

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. 

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). 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

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