Molybdenum allyl complexes chiral

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

Progress has also been made in the use of other metals. For example, iridium-catalyzed processes have been developed for AAA reactions. The regioselectivity of this reaction is analogous to molybdenum. Allylation of soft carbon nucleophiles has been reported using chiral iridium complexes, with the products isolated in high enantiomeric purity (> 91%... 

Scheme 6.29. Diastereoselective cuprate addition to chiral molybdenum Ti-allyl enone complex 147.

Enantioselective epoxidation of allylic alcohols using hydrogen peroxide and chiral catalysts was first reported for molybdenum 7B) and vanadium 79) complexe. In 1980, Sharpless 80) reported a titanium system. Using a tartaric acid derivative as chiral auxiliary it achieves almost total stereoselection in this reaction. 

The chiral ligand (44) was prepared starting from the cyclic a-amino acid (S)-proline80). Recently, similar chiral catalysts and related molybdenum complexes involving optically active N-alkyl-P-aminoalcohols as stable chiral ligands and acetylacetone as a replaceable bidentate ligand, were designed for the epoxidation of allylic alcohols with alkyl hydroperoxides which could be catalyzed by such metal complexes 8,). 

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. V-alkylation of ephedrine is a convenient method for obtaining tertiary amines which are useful as catalysts, e.g., for enantioselective addition of zinc alkyls to carbonyl compounds (Section D. 1.3.1.4.), and as molybdenum complexes for enantioselective epoxidation of allylic alcohols (Section D.4.5.2.2.). As the lithium salts, they are used as chiral bases, and in the free form for the enantioselective protonation of enolates (Section D.2.I.). As auxiliaries, such tertiary amines were used for electrophilic amination (Section D.7.I.), and carbanionic reactions, e.g., Michael additions (Sections D. 1.5.2.1. and D.1.5.2.4.). For the introduction of simple jV-substituents (CH3, F.t, I-Pr, Pretc.), reductive amination of the corresponding carbonyl compounds with Raney nickel is the method of choice13. For /V-substituents containing further functional groups, e.g., 6 and 7, direct alkylations of ephedrine and pseudoephedrine have both been applied14,15. 

Cyclopentadienyl complexes of molybdenum have also been used in asymmetric synthesis, mainly for the addition of allyl groups to carbonyl compounds (Section D. 1.3.3.3.). The chiral ligand was prepared from (—)-menthone via menthyl tosylate (see Section 3.5.1. for the alcohol)49 and introduced in an allylmolybdenum complex to give 1950. For an early review on catalytic applications of such compounds, see ref 51. 

Another feature that is crucial in considering rearrangements in monosubstituted allyls is the effect on the chirality 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 5%. Thus if one considers complexes like (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 chirality in the allyl moiety. 

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. 

Many diastereoselective allylations form a new stereocenter at one of the allylic carbons and at the nucleophilic carbon. For example, an iridium complex containing a phosphite ligand catalyzes enantioselective and diastereoselective formation of products containing two stereocenters, one at the original nucleophile and one at the original allyl electrophile (Equation 20.58). In another example shown in Equation 20.59, Trost s palladium catalyst leads to the reaction of allylic esters with chiral azlactone pronucleophiles with high diastereomeric and enantiomeric excess, as does the related molybdenum catalyst. In these cases, the metal appears to control the new stereocenter at the allyl group, as well as the relationship between this stereocenter and the new stereocenter formed at the nucleophile. 

Using the same concept, Hoveyda and co-workers recently employed molybdenum derived chiral complexes to develop a net catalytic enantioselective, cross metathesis process based on either the kinetic resolution of racemic allylic alcohols or the asym-... 

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