No abstract

Diazoalkanes and their conjugate acids, diazonium ions, are important reactive intermediates which have been implicated in the carcinogenicity and mutagenicity of N-alkyl-N-nitroso compounds. 2 Alkanediazoates have been studied extensively and have been shown to decompose via diazonium ion intermediates. 3 Nitrous acid deaminations of aliphatic amines are known to involve the same diazonium ion intermediates. 4,5 It has been postulated that N-alkyl-N-nitroso compounds may, under certain conditions, lead to the formation of diazoalkanes in biological systems, 2,6 and their studies serve as models from which inferences about the biological implications have been made. Despite the considerable interest surrounding these reactive intermediates, there is little quantitative information on their reactivities. The only known bimolecular rate constant for protonation of an unstabilized diazoalkane by acidic species, such as hydronium ions, is that for protonation of diazomethane by hydronium ion at 25°C, estimated as k H + ≈ 4 × 10 8 M -1 s -1 . 7 The laser flash photolysis (LFP, 308 nm, with either UV or IR detection of transients) of alkoxy substituted ∆ 3 -1,3,4oxadiazoline precursors (1) has allowed us to determine rate constants for protonation of diazoalkanes in both aqueous and nonaqueous solutions.The mechanism for the photochemical decomposition of alkoxy substituted ∆ 3 -1,3,4-oxadiazolines 1 involves initial R-scission of the excited state to form biradicals, 2, which subsequently undergo efficient -scission selectively, to give primarily diazoalkanes, 3, and an ester or carbonate (if R 1 ) alkoxy). 8,9 The formation of the two products upon 308 nm LFP of 1a in acetonitrile at 25°C, is readily confirmed with use of timeresolved infrared (TRIR) detection. 10 Absorptions assigned to the diazo band of 2-diazopropane, centered at 2036 ( 3 cm -1 , and to the carbonyl band of methyl acetate, centered at 1744 cm -1 (not shown), were formed instantaneously (within the response time of the instrument) from 1a, Figure 1. Both absorptions were persistent under the experimental conditions. Under identical conditions, 308 nm LFP (UV-vis detection) of 1a resulted in an instantaneous bleaching of its absorption at 322 nm and was accompanied by the instantaneous appearance of a strong persistent band, centered at 250 nm, assigned to 3a. 11 Both the UV and the IR absorptions assigned to 3a decayed with first-order kinetics (τ ) 30µs) when trifluoroacetic acid (TFA, 3.0 mM) was present ( Figure 1).Rate constants for protonation of 3a by carboxylic acids with pK A 's ranging from 10 to 23 in acetonitrile, 12 at 25°C, µ ) 0 M, are listed in Table 1. They were obtained as the slopes of linear plots of observed rate constants (determined from LFP TRIR measurements) Vs acid concentration. 14 The linear leastsquares analysis of log k HA Vs log K A of the acids (data adjusted for statistics) by the Brønsted procedure gave R ≈ 0.25. 15 The reaction between 3a and hydronium ion was also studied by monitoring decay of the UV absor...

The 9-[N,N-dimethyl(thioformamidyl)]-9-fluorenyl cation was generated under stable ion conditions and characterized by UV/visible and NMR spectroscopy and methanol trapping reactions. The same cation was generated by laser excitation of the appropriate chloride precursor in 2,2,2-trifluoroethanol, and rate constants for nucleophilic quenching by alcohols and several anions were measured. The quenching data for this and other 9-fluorenyl cations demonstrate that the reactivity decreases for 9-substituents in the order H > carbomethoxy > N,N-dimethyl(thioformamidyl), demonstrating that the thioamidyl group imparts substantial kinetic stabilization to an adjacent cationic center. Both steric and electronic factors are suggested to be important in determining this reactivity order.