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O,O-Diprotonated nitro olefins undergo three alternative electrophilic reactions which yield α-phenylacetones, 4H-1,2-benzoxazines and biphenylacetone oximes depending on the reaction conditions (temperature and time) and aromatic substrates. Although these reactions are seemingly divergent, a common intermediate of a phenylated protonated aci-nitro species, derived from the dication, is postulated to be involved in the reactions. Furthermore, the formation of benzoxazines and biphenylacetone oximes can be interpreted in terms of participation of novel chemical species with phenylethylene dication character derived from the common intermediate.
Nitrostyrene (2a) yields a strongly colored stable species formed by protonation in trifluoromethanesulfonic acid (TFSA). On the basis of analysis of the protonation sites by NMR spectroscopy, it was concluded that the species formed in TFSA is the O,O-diprotonated β-nitrostyrene.
Initially, we thought that the formation of the dication (1a) depended on the substituent effect of the phenyl group, stabilizing the cation center conjugation. However, a similar dication (1b, R1=R2=CH3) is formed from 1-nitro-2-methyl-1-propene (2b) in TFSA at -5°C, and is stable enough to be observed by NMR spectroscopy.
A recent study revealed that the dication 1e (R1=H, R3=CH3), formed from 2-nitropropene, reacts with benzene at low temperature (-40°C) to give a phenylated acetone in high yield after quenching with methanol and water (Scheme II).1
Acid-Catalyzed Reactions of 2-Nitropropene at a Low Temperature. Formation of 1-Phenylacetones.
The dication 1e of 2-nitropropene reacts with benzene at -40°C in the presence of TFSA (10 equiv. with respect to the nitro olefin) to give a-phenylacetone 4e in 85% yield after methanol-water work-up, i. e., pouring of the reaction mixture after 1 min into a large excess amount of dry methanol (100 ml) cooled at -78°C with vigorous stirring, subsequent addition of water, and extraction with methylene chloride (Scheme II and Table I).1
Treatment of the intermediate solution with methanol, followed by aqueous work-up, quantitatively converts the stable protonated aci-nitro compound to the corresponding carbonyl compound by a mechanism similar to that of the Nef reaction2 (5 → 6 → 7), involving attack of more nucleophilic methanol on the cation 5e through a ketal intermediate which could be isolated by careful work-up (Scheme III). This work-up procedure is important to get a high yield of the α-phenylacetone: straightforward aqueous quenching of the reaction mixture (5e obtained after 1 min at -40°C) gave the ketone 4e in a lower yield (55%), together with 1-phenyl-2,2-dinitropropane (13%) and α-phenylacetone oxime (17%).3 The stepwise work-up eliminates the complex accompanying reactions.
The reactions of the dication 1e with substituted benzenes (anisole, toluene or chlorobenzene) give ortho- (8A,B,C) and para-substituted arylacetones (9A,B,C) in high yields under similar reaction conditions (Scheme IV).1 In the case of anisole, the ortho isomer 8A is favored over the para isomer while in the cases of toluene and chlorobenzene, the para isomers (9B,C) are favored over the ortho isomers.
2-Nitropropene (2e) was prepared by elimination of water from 2-nitro-1-propanol4 with phthalic anhydride (1.35 equiv.) by heating to 140°C under reduced pressure (78 mmHg).5 The distilled yellow oil was dried over CaCl2, and the filtrate was redistilled to give pure 2e as yellow low-melting-point crystals (59°C/80 mmHg, lachrymatory). 1H NMR: 6.42 (1H, br s), 5.60 (1H, br s), 2.26 (3H, s).
2-Nitro-1-propanol was prepared by the condensation reaction of formalin (37% in water) and nitroethane in the presence of NaOH at ambient temperature for 3 hr (bp. 57°C/1.5 mmHg).
- R1R2C=N+(OH)O- → R1R2C=O + H+ + HNO + H2O
- R1R2C=N+(OH)O- + HNO → R1R2C=N-OH + HNO2
- R1R2C=N+(O-)O- + NO+ → R1R2C(NO)(NO2) → R1R2C(NO2)(NO2)