Epoxide deoxygenation

Formation of C-C Double Bonds by Epoxide Deoxygenation and Epoxide Dimerization... 

A limitation of epoxide deoxygenation with functionalized substrates became apparent in the second case where a mixture of olefins was isolated. Thus, the regioselectivity of epoxide opening and elimination of the titanium oxygen species was too low for practical use. 

Scheme 4 Epoxide deoxygenation via ET from metal complexes...

In a series of papers West and coworkers described the reactions of disilene 9 with several epoxides to give the disilyl enol ethers 125, the five-membered ring compounds 126 — the formal insertion products, as well as the products of epoxide deoxygenation, namely alkenes and the disilaoxirane 62 (equation 29)80,83,124 125. 

There are two especially relevant cases where Cp2TiCl-promoted epoxide deoxygenations have been demonstrated to conform to the requirements of selectivity, mildness, and wide functional group tolerance desirable in natural product synthesis the chemical correlation between cryptophycin-23 and cryptophycin-45 and the synthesis of anhydrovinblastine, the key intermediate in the synthesis of the anticancer drug Navelbine, from leurosine (Scheme 14) [91-93]. 

With sodium 0,0-dialkyl phosphorotelluronate, terminal epoxides are converted to alkenes very quickly, and cw-epoxides are reduced more easily than rraw5-epoxides (equation 31). The corresponding selenoate reagent has been applied to sugar epoxide deoxygenation (equation 32). 

Epoxide deoxygenation by or via phosphine oxides or selenides continues to attract attention. Further study of deoxygenation by triphenylphosphine selenide (30) has implicated episelenide intermediates, and n.m.r., chemical,... 

The conversion never reaches 100% whatever the reaction time, suggesting deactivation of the catalyst. Indeed, after addition of the epoxide, the intensity of signal B decreases with time. Similarly, the initial red-orange colour fades with time and vanishes at the end of substrate conversion. This colour may be attributed to the active complex. The catalytic site proposed above involves an oxophilic Ni centre. Deactivation might arise from oxidation of this active complex and simultaneous epoxide deoxygenation yielding the small amount of allylic tertiary alcohol la observed (Table 3). 

Lin and coworkers reported a DFT-level computational study of the hydrogen-driven epoxide deoxygenation and glycol DODH catalyzed by MeReOs (MTO) [29]. Very high enthalpic barriers were found for both the [2-1-3] and the [2-1-2]... 

Scheme 12 Sulfite-driven Re-catalyzed DODH and epoxide deoxygenation...

A graphical summary of the catalytic glycol deoxydehydration and epoxide deoxygenation reactions reported to date is given in Scheme 26. 

C-H Insertion. Insertion of carbenoid species generated from dimethyl diazomalonate in the presence of Rh into activated C-H bonds proceeds in moderate yields (eq 33). Sometimes undesired insertion into the C-H bond adjacent to the heteroatom complicates the utilization of Ganem epoxide deoxygenation procedure (eq 34). ... 

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