Experiments on the Chaperon Effect in the Nitration of Aromatics

Abstract

A nitro group may be effectively delivered to the ortho position of alkylbenzenes, provided that a suitable chaperon function is located in α-position and a dilute solution of HNO3 in CH2Cl2 is used. The carbonyl function of an aldehyde or ketone is the best choice, but a carboxyl, alkoxycarbonyl, and amide groups all work well. The ether function showed a less pronounced ortho orientation effect, whereas the hydroxyl group was too prone to oxidation. Side reactions were minimal under the conditions employed. A para chaperon effect was seemingly at work in the CH2Cl2 nitration of benzenepropanenitrile. All the results were compared with the corresponding classical nitration in H2SO4.

Oxidation of Benzylic Alcohols and Ethers to Carbonyl Derivatives by Nitric Acid in Dichloromethane

“…In the course of our studies concerning the synthetic applications of the HNO 3 /CH 2 Cl 2 system, previously found to be very effective in ortho-oriented nitrations of some benzylic substrates (chaperon effect), [1] as well as a useful heterolytic (nitrolytic) reagent suitable for the deprotection of N-Boc derivatives, [2] tert-butyl and 1-adamantyl carboxylates, [3] we set out to explore the synthetic potential implicit in our initial observation [1] that methyl phenylmethyl ether (1a) underwent a rapid and quantitative transformation into benzaldehyde (2a), thus preventing any occurrence of ring nitration [4] and without undergoing further oxidation to benzoic acid (3, Scheme 1). [5] Such behaviour appeared particularly noteworthy in view of the fact that compound 1a was reported to meet a completely different fate under classical nitration conditions.…”

Directing effects in nitration of 1-adamantyl bearing aromatic ketones

“…This effect was attributed to nitration mechanism including an active nitrating agent complexed to carbonyl oxygen to form six-member ring transition state of lowest energy leading to formation of ortho isomer preferably. 16,17 On the contrary, the opposite ortho/para ratio of value of 0.5 was obtained when nitroguanidine was employed in conc. sulfuric acid at -15 °C (Table 1, entry 14).…”

“…specific association between carbonyl and nitrating species; b) ortho nitration is not unusual, but sometimes may be reduced by steric hindrance or c) the para position suffers special deactivation by the -M effect. The complexation of active nitrating agent seems to be responsible for high ortho/para ratio observed in the case of β-ketones, 16,17 but it should be noted that even inverse than statistical ortho/para ratio was obtained in strong acid medium (Table 1, entry 14). The trends in regioisomer ratio observed under comparable conditions are very similar for both groups of ketones.…”

“…Nitration has been one of the most studied and best understood organic reactions from mechanistic as well as preparative point of view. 15 Beside conventional nitrating agents as nitric acid 16,17 or nitric-sulfuric acid (mixed acid), many others agents were presented e.g. cerium(III) nitrate, 18 N 2 O 5 (ref.…”

Electrochemical Synthesis of Nitroaromatic Ketones

“…1-(4-Nitrophenyl)acetone (6): [9] 3-(4-Nitrophenyl)-2-butanone (7a): [10] [11] [12] [13] [9] 2,4-Dinitrophenylacetone (10): [5] [7] 2,4-Dinitrophenylacetophenone (12): [5] 2,4,6-Trinitrophenylacetone (13): [14] 1-(2,4,6-Trinitrophenyl)-2-butanone (14): [15] 2,4-Dinitronaphthylacetone (15): [16] …”