Molybdenum enantiomerically pure

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

Molybdenum catalysts such as 1 can also lead to the isomerization of alkenes [810-812]. Care is due in particular if enantiomerically pure olefins with the stereogenic center near the C-C double bond are to be metathesized, or when strained rings are to be formed [811]. 

Chiral molybdenum complexes of llil-pyran.1 Enantiomerically pure Mo-com-plexes, (S)- and (R)-l, of 2//-pyran have been prepared by known methods (13, 194-195) from d- and L-arabinose, respectively. They react with a wide range of nucleophiles at an allylic position with 96% ee. The resulting complex can react with a second nucleophile at the other allylic position to form c/y-disubstituted complexes, also with high enantioselectivity. The sequence can be used to obtain chiral cis-2,6-disubstituted tetrahydropyrans such as 2, a component of the scent gland of the civer cat. 

The cycloadducts are readily converted to the corresponding enantiomerically pure amino acids by reductive cleavage of the N —O bond with molybdenum hexacarbonyl in acetonitrile/water 93 7, followed by hydrolytic removal of the chiral auxiliary, performed with 6 M hydrochloric acid 11. 

Hansson, S, Miller, J F, Liebskind, L S, Synthesis and reactions of enantiomerically pure molybdenum rr-complexes of 2H-pyran. A general approach to the enantiospecihc synthesis of cA-2,5-disubstituted-5,6-dihydro-2H-pyrans and cA-2,6-disubstituted tetrahydropyrans, J. Am. Chem. Soc., 112, 9660-9661, 1990. 

Finally, the use of stoichiometric amounts of transition metal complexes can play an important role in the synthesis of functionalized piperidines. <01H14.39> Liebeskind and coworkers have developed a chiral transition metal complex and have used it in the synthesis of (-)-indolizidine 209B <01JA12477>. A lipase mediated allylic alcohol resolution provides access to both antipodes of enantiomerically pure allyl acetates (115) which can be used to form an ri -allyl molybdenum complex (116), Hydride abstraction followed by methoxide quench yields a reactive species 117 which may be further functionalized through reactions with Grignard reagents. The eventual products 119 arc 2,3,6-trisubstituted piperidines in enantiomerically pure form. 

Enantiomerically pure manganese complexes using ligands other than the salen structure have been reported, but so far with lower enantioselectiv-ities. Better results have been achieved using molybdenum complexes bearing hydroxamic acid ligands and TBHP or cumylhydroperoxide as oxidant. This system has been used to effect the epoxidation of a range of olefins with up to 96% ee. 

For the second building block for verrucarin A (380), a derivative of verrucarinic acid (465) was synthesized in enantiomerically pure form from diester 461. Cleavage with pig liver esterase led to monoester 462, which was reduced to the alcohol with borane dimethylsulfide complex and protected with TBSCl to obtain the molecule 463. a-Hydroxylation with molybdenum oxide generated alcohol 464, and final protection and saponification afforded compound 465 (Scheme 8.16). 

Inexpensive and commercially available d- and L-arabinose are useful starting materials for large-scale preparation of the enantiomerically pure chiral molybdenum diene of 2H-pyrans 118 (15) and 119 (1/ ), respectively. Being potent electrophiles, these air-stable cations react with various nucleophiles, for example, H", LiR(R=alkyl, enolate, alkynyl, vinyl) and RjMgBr (R = vinyl, aryl), at the coordinated enolic diene... 

Liebeskind s second-generation approach to the stereocontrolled synthesis of 2,3,6-trisubstituted piperidines was based on the use of a range of molybdenum-containing scaffolds that were not only general and efficient precursors for the regjo- and stereodivergent preparation of the targets, but could also be obtained in enantiomerically pure form. This impressive... 

The widely used asymmetrical epoxidation of allylic alcohols developed by Katsuki and Sharpless employs tert-batyl hydroperoxide (TBHP), enantiomerically pure tartaric acid esters, and isopropyl titanium(TV) (6). Similar results were obtained using TBHP and vanadyl acetyl acetonate VO(acac>2 and hexacarbonyl molybdenum [7,8]. The drawback of these reactions are their restriction to allylic alcohols, which is required as anchor group for the formation of the intermediate complex. 

Kinetic resolution of chiral dienes has been realized by RCM with enantiomeri-cally pure molybdenum carbene complexes [888-890]. High enantiomeric excesses of recovered diene (> 99% ee) and cycloalkene (93% ee) could be obtained. 

O DeU, R. McConviUe, D. H. Hofmeister, G. E. Schrock, R. R. Polymerization of enantiomeric-aUy pure 2,3-dicarboalkoxynorbornadienes and 5,6-disubstituted norbornenes by well-characterized molybdenum ring-opening metathesis polymerization initiators. Direct determination of tacticity in cis, highly tactic and trans, highly tactic polymers. /. Am. Chem. Soc. 1994, 116, 3414-3423. 

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