Perruthenate, derived catalyst

Limitations of the reaction due to the substitution pattern of the allylic alcohols were overcome by the use of tetrapropylammonium perruthenate (TRAP) as a catalyst and monosubstituted, disubstituted and trisubstituted allyl alcohols were converted into the corresponding saturated aldehydes and ketones [5]. Intermediacy of the ruthenium alkoxide in this reaction was evidenced from the complete lack of reactivity of the trimethylsilyl ether derived from the allylic alcohol. 

Tetrapropylammonium perruthenate Pr4N+Ru04 and N-methylmorpholine-N-oxide (NMO) as catalytic oxidants of primary, secondary, allylic and benzylic alcohols to carbonyl derivatives. The same catalyst polymer supported perruthenate (PSP) used as efficient oxidant (see 1st edition). 

As we were not only interested in the development of a racemization method but also wanted to evaluate an asymmetric synthesis articulated around the imine intermediate 7 vide infra), we initially investigated its controlled preparation by oxidation of the unwanted (7 )-TH[3C 4 obtained from the mother liquors. Among the various methods initially tested, good results (approximately 75 to 80% in situ yield of imine 7) were obtained with NaOCl in methanol/THF at 0 to 5°C for 3 h. The major by-product is the overoxidized (3-carboline derivative (5 to 10%), although in some experiments, low levels of the unstable N-chloroamine intermediate were also detected. Later on, approximately 68% in situ yield was obtained with tetra-n-propylammonium perruthenate (0.05 equiv) as catalyst with iV-methylmorpholine oxide (1.5 equiv) as cooxidant in acetonitrile at room temperature. However, in this latter method, up to 16% of totally oxidized (3-carboline was also formed. The imine 7 was then directly reduced with sodium borohydride to produce the racemic material in approximately 50% isolated overall yield. Although the aromatic (3-carboline by-product was easily removed upon salt formation, the above approach suffered from several major drawbacks difficulty to control the overoxidation of the desired dihydro-(3-carboline to the (3-carboline on... 

As a further extension, the Kim group very recently developed the aerobic oxidation and [l,5]-hydride transfer/cyclization sequence starting from readily available ortho tertiary amine substituted cinnamyl alcohols 24 (Scheme 4.12). The tetrapropylammonium perruthenate (TPAP) was identified as the competent catalyst for the initial aerobic oxidation of the allylic alcohols. The synthetically useful tetrahydroquinoline derivatives 25 were prepared in moderate yields and high level of enantioselectivity. 

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