Chiral pyridine N-oxides

High enantiomeric excesses for the addition of chloride to meso-epoxides were also obtained with use of a planar-chiral pyridine N-oxide 17 developed by Fu... 

Tao B, Lo MMC, Fu GC (2001) Planar-chiral pyridine N-oxides, a new family of asymmetric catalysts exploiting an rj -CsArs ligand to achieve high enantioselectivity. J Am Chem Soc 123 353-354... 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

Denmark and Fan (06TA687) reported that the oxidative dimerization of chiral pyridine N-oxides was highly diastereoselective for the formation of the P-configuration, (P)-(R,R)-260, of the chiral axis. 

Takenaka N, Sarangthem RS, Captain B (2008) Helical chiral pyridine N-oxides a new family of asymmetric catalysts. Angew Chem Int Ed 47 9708... 

Kobayashi and co-workers reported similar enantioselectivity switch in the bi-nol-yterrbium(III) triflate complex-catalyzed cycloaddition reactions [69] between N-benzylidenebenzylamine N-oxide and 3-crotonoyl-2-oxazolidinone [70]. The reaction in the presence of MS 4 A showed an exclusively high enantioselectivity of 96% ee, while that in the absence of MS 4 A (-50% ee) or in the presence of pyridine N-oxide (-83% ee) had the opposite enantioselectivity (Scheme 7.24). This chirality switch happens generally for the combination of a wide variety of nitrones and dipolarophiles. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

PhlO, cat chiral Mn salen complex, pyridine N-oxide (enantioselective)... 

Some examples of asymmetric epoxidations of alkenes using chiral ruthenium porphyrins have been reported for example, the previously reported D4-sym-metrical chiral ruthenium porphyrin complex Run(D4-Por )(CO)(MeOH) [58], which produced (R)-styrene oxide in 57% ee with Cl2PyNO as a donor, was readily converted into the dichloro derivative A [59] (Fig. 11). This di-chlororuthenium porphyrin gave (R)-styrene oxide in 69% ee using Cl2PyNO and was highly active (875 TON in 1.5 h). The use of unsubstituted pyridine N-oxide or NMO as oxidants resulted in low substrate conversions as well as... 

R = Me) hydrolysis of the intermediate affords the 4-hydroxy compound (127 R = OH) in low yield (Equation (14)) <85CJC33I3>. Reaction of the chiral oxathiolanone oxide (128) with TFAA and an arene gives some asymmetric induction in the formation of (129) (Equation (15)) <90TA143>. With the unsubstituted compound (39), Pummerer reaction allows the introduction of a variety of nucleophiles at the 4-position to give (130) and oxidation of the intermediate with pyridine N-oxide affords access to the little known 4,5-dione (35) (Equation (16)) <93JHC663>. 

A similar (salen)manganese(III) catalyst was used by Katsuki for asymmetric sulfide oxidations [35]. Chiral complex 20 bears additional asymmetric carbons in the salicylidene part of the salen. In this system, hydrogen peroxide, which was the preferred oxidant in the Jacobsen procedure, turned out to be inefficient. Instead, iodosylbenzene was chosen, and in the presence of only 1 mol % of catalyst several aryl alkyl sulfides were oxidized in acceptable yields having enantiomeric excesses in the range of 8% to 90%. As in the Jacobsen-KatsuJd-epoxida-tion, the presence of additives such as pyridine N-oxide has a beneficial effect on chemical and optical yields. In addition, such co-ligands suppress the overoxidation of sulfoxides to the corresponding sulfones so that a sulfoxide sulfone ratio of 47 1 can be achieved. Consequentely, as shown for the case of thioanisole. 

Komatsu synthesized the chiral salen complex 185 and demonstrated that aziridinations of various styrene derivatives proceeded enantioselectively in the presence of pyridine N-oxide (Equation 31) [145], Thus, treatment of... 

Kureshy s group [83] immobilized the Mn(Salen) catalyst axially in the nanopores of MCM-41 via pyridine N-oxide (Scheme 10.15). These immobilized catalysts showed higher enantioselectivity (69% ee) than their homogeneous counterparts (51% ee) for the asymmetric epoxidation of styrene and were also effective for the asymmetric epoxidation of bulkier substrates such as indene and 2,2-dimethylchromene (conversion 82-98% ee 69-92%). The catalysts could be recycled for at least four times without loss in performance. The increase in ee values was attributed to the unique spatial environment constituted by the chiral Salen ligand and the surface of the support. 

Scheme 10.15 Asymmetric epoxidation on chiral Mn(salen) catalyst axially immobilized in the nanopores of MCM-41 via pyridine N-oxide. Reprinted with permission from Ref. [83]. Copyright 2005 Elsevier.

Methyl trichlorosilyl ketene acetal reacts with aromatic and aliphatic ketones (the former enantioselectively), using chiral pyridine bis-N-oxide catalysts.134 Computations and an X-ray crystal structure of a catalyst-SiCU complex have helped to elucidate the mechanism. 

Pyridine-type N-oxides (Fig. 7.2) represent another, no less-successful class of catalysts for the allylation reaction. Thus, Nakajima first demonstrated that the axially chiral biquinoline N,N -bisoxide 17 can indeed catalyze the allylation... 

Several ruthenium complexes bearing chiral Schiff s base ligands have been published. RuL(PPh3)(H20)2], complex C (Fig. 11), with PhIO produced (S)-styrene oxide in 80% ee [61]. Chiral Schiff s base complex D was examined using molecular oxygen with aldehyde, with or without 2,6-dichloropyridine N-oxide as an axial ligand. Styrene oxide was produced in up to 24% ee[62]. A chiral bis(oxazolinyl)pyridine ruthenium complex E with iodosylbenzene diacetate PhI(OAc)2 produced (lS,2S)-fra s-stilbene oxide in 74% ee [63]. Similarly, chiral ruthenium bis(bipyridine) sulfoxide complex F [64] was effective in combination with PhI(OAc)2 as an oxidant and resulted in in 33% ee for (R,R) trans-stilbene oxide and 94% ee for (R,R) trans-/i-Me-styrene (after 75 h at 25 °C). 

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