Asymmetric epoxidation vanadium

Vanadium-catalyzed asymmetric epoxidation has recently been re-examined with a newly designed chiral hydroxamic acid (3).43-45 The hydroxamic acid (3) forms a 1 1 complex with vanadium ions and induces high enantioselectivity (Scheme 6). 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

SCHEME 60. Optically active hydroxamic acid ligands for the vanadium-catalyzed asymmetric epoxidation of allylic alcohols,... 

TABLE 18. Enantiomeric excesses obtained in the vanadium-catalyzed asymmetric epoxidation of aUyhc alcohols using enantiomeric pure ligand 140a and racemic as well as enantiomericaUy pure hydroperoxide 16a... 

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

The asymmetric epoxidation of homoallylic alcohols has continued to be a problematic area. A potential solution has recently been published <07JA286 07T6075>. The use of bis-hydroxamic acid 1 as a chiral ligand for a vanadium catalyst has provided both excellent yields and enantioselectivity. This method works well with both cis- and trans-alkenes. 

Homoallylic alcohols can be asymmetrically epoxidized using a chiral vanadium catalyst equipped with the hydroxaraic acid ligand 45, as exemplified in Yamamoto s concise synthesis... 

The hrst of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with f-butyl hydroperoxide in Chapter 33,... 

R. C. Michaelson, R. E. Palermo, K. B. Sharpless, Chiral hydroxamic acids as ligands in the vanadium catalyzed asymmetric epoxidation of allylic alcohols by tert-butyl hydroperoxide, /. Am. Chem. Soc. 99 (1977) 1990. 

Katsuki-Sharpless asymmetric epoxidation. Since its introduction in 1980 [10], the Katsuki-Sharpless asymmetric epoxidation (AE) reaction of allylic alcohols has been one of the most popular methods in asymmetric synthesis ([11-14]). In this work, the metal-catalyzed epoxidation of allylic alcohols described in the previous section was rendered asymmetric by switching from vanadium catalysts to titanium ones and by the addition of various tartrate esters as chiral ligands. Although subject to some technical improvements (most notably the addition of molecular sieves, which allowed the use of catalytic amounts of the titanium-tartrate complex), this recipe has persisted to this writing. 

Epoxidations. Grafting tantalum onto silica to form a useful catalyst for the Sharpless asymmetric epoxidation of allyl alcohols is contrary to the ineffective titanium species on a similar support. Vanadium-complexed chiral hydroxamic... 

When the previously cited transition structure 193 is applied to the asymmetric epoxidation of allylic alcohols, it must be modified to include binding of the peroxide, the allylic alcohol, and also the chiral tartrate. The metal in the new model is titanium rather than vanadium, and tetraisopropoxy titanium was found to react with 2 equivalents of diethyl tartrate to form a species such as 212, where OR = 0/-Pr and CO2R = CO2Et.3i7.3i8 The tartrate can bind to titanium from either the bottom or the top face.3i8 The nature of the... 

Other metal-based epoxidation catalysts have been explored to overcome some of the hmitations of the Sharpless procedure. One drawback with the Sharpless asymmetric epoxidation is the slightly lower ees often obtained when using cis-olefin substrates. The group of Yamamoto have achieved highly enantioselective epoxidations of ds-alkenes using vanadium(V) oxytriisopropoxide in the presence of C2-symmetric bishydroxamic acid ligands such as (4.23). In contrast to the Sharpless procedure this process is not hampered by the presence of air or... 

SCHEME 34.15. Asymmetric epoxidation of allyl alcohols 54 leading to enantioenriched epoxides 56 mediated by vanadium(V) catalysts with a chiral hydroxamic acid ligand 55 and t-butyl hydroperoxide as a terminal oxidant. 

Hoshino Y, Yamamoto H. Novel a-amino acid-based hydroxamic acid hgands for vanadium-catalyzed asymmetric epoxidation of aUylic alcohols. J. Am. Chem. Soc. 2000 122 10452-10453. 

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