The chemistry of nitroso-compounds. Part III. The nitrosation of substituted benzenes in concentrated acids

Abstract

Rates of nitrosation are reported for benzene, toluene, diphenyl ether, anisole, and phenol in 1 0 . 4 ~-H C J 0 , a t 52.9 "C, for benzene in 1 ~. ~M -H , S O , , and for [aH6]benzene in 1 2M-D2S0,. The effective reagent under these conditions is NO+ and substitution occurs predominantly a t the para-position of monosubstituted substrates. Benzene reacts about 50 times more rapidly in HzS04 than in HCIO, of the same Ho acidity, and about 8.5 times more rapidly than [2H6]benzene in D2S04, which shows that H+-loss from the Wheland intermediate is ratelimiting. Rates of nitrosation are moderately increased by electron-donating substituents and log (partial rate factors) for the para-position correlate with Q+ parameters to give p = -6.9. This relatively low p value is consistent with the rate-limiting step suggested. The reactivity of NO+ is discussed and it is shown to be at least 1 014 times less reactive than NOz+. The significance of the findings to recent explanations of aromatic reactivity in nitrations is also discussed.London S.W.7 THE reaction of nitrous acid with aromatic substrates is usually difficult in the absence of powerfully activating substituents such as OH, OR, and NR,. Often the nitroso-product formed initially undergoes further transformation and nitrous acid is unstable in moderately acidic solutions at high temperature^.^ This combination of adverse factors has discouraged detailed investigation of these reactions and relatively little is

Synthesis of Benzimidazoles by Phosphine‐Mediated Reductive Cyclisation of ortho‐Nitro‐anilides

“…The formation of the phosphine imide 11 and, particularly, the contrast between the formation of the benzimidazole 8 from 3, but of the benzimidazolone 9 from 4, were surprising. We had expected the reaction mechanism for the cyclisation of the nitroanilides 1 -4 to be similar to the one suggested for the cyclisation of 4-(acylamino)-5-nitrosopyrimidines [1], i.e., reduction of the NO 2 to the NO group, followed by addition of the phosphine to the NO group, and formation of an aza-Wittig reagent 5 ) via a nitrene or nitrenoid intermediate. Surprisingly, however, no intermediate phosphine imide was observed during the formation, under milder reaction conditions, of an 8-(tert-butyl)guanine from a 4-(pivaloylamino)-5-nitrosopyrimidine [1].…”

“…Oxaziridine intermediates were considered before as intermediates in related reactions [42]. The NÀO bond of 19 may open either to generate the N-oxide 20 that will be deoxygenated by Ph 3 P, leading to the dimethylbenzimidazole 8, or to the orthodiazaquinoid intermediate 21 that will generate the rearranged product 9 either by a [1,5] sigmatropic rearrangement [43], or by a 1,2 pinacolÀpinacolone type migration of the i-Pr group. It is, however, not clear why there should be such a dichotomy in the opening of the oxaziridine substituted by either a Me, or an i-Pr group, so that one oxaziridine will react while maintaining the aromatic ring intact, and the other one lead to dearomatization.…”

Nitrosation kinetics of phenolic components of foods and beverages

“…The nitrosonium ion NO ϩ being some 10 14 times less reactive than the nitronium ion NO 2 ϩ [1,2] much less attention has been paid to the C-nitrosation of aromatics than to their nitration, which is now well un-cursors of genotoxic substances, it is of some concern that phenol, catechol, vanillin, and other phenolics have been detected in smoked fish and meats [14]. It has also been reported that the presence of phenolic compounds can block the N-nitrosation reactions of other substrates [15,16], and although this has been questioned on the grounds that N-nitrosation is catalyzed by monohydroxy phenols [17] (but not by all polyhydroxyphenols [15]), it should be borne in mind that catalytic activity is only observed when the concentration of nitrosating agent significantly exceeds the concentration of phenol, which rarely occurs in vivo or in environmentally significant situations.…”

Nitrosation kinetics of phenolic components of foods and beverages

“…This mechanism implies the rate equation (4) The slow step in Scheme I is hypothesized, following Challis [1], to involve protonation of the nitroso oxygen in the transition state (Scheme II). Since this implies susceptibility to basic catalysis by organic bases, Schemes I and II are supported by the finding that, for phenol, k 2corr depends linearly on buffer concentration, …”