Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.

D i p a r t i m e n t o d i Chimica d e l P o l i t e c n i c o P i a z z a Leonard0 da Vinci 32, 20133 Milano -I t a l y A b s t r a c t -The moat r e c e n t m e c h a n i s t i c and s y n t h e t i c a s p e c t s of t h e s u b s t i t u t i o n o f p r o t o n a t e d h e t e r o a r o m a t i c b a s e s by nucleop h i l i c c a r b o n -c e n t e r e d r a d i c a l s a r e reviewed. From m e c h a n i s t i c p o i n t o f view t h e f o l l o w i n g a s p e c t s a r e d i s c u s s e d ; i ) S t r u c t u r er e a c t i v i t y r e l a t i o n s h i p ; i i ) R e a r o m a t i z a t i o n of t h e r a d i c a l add u c t s ; i i i ) S o l v e n t and i s o t o p e e f f e c t s ; i v ) Overlap a r e a w i t h i o n i c r e a c t i o n s . The s y n t h e t i c developments concern t h e F r e e -r a d i c a l r e a c t i o n s were c o n s i d e r e d f o r l o n g t i m e synonymous w i t h u n s e l e c t iv i t y . Apart from t h e u n i n t e r r u p t e d t h e o r e t i c a l i n t e r e s t and t h e importance i n t h e b a s i c c h e m i c a l i n d u s t r y ( v i n y l p o l y m e r i z a t i o n , o x i d a t i o n by m o l e c u l a r oxygen, c h l o r i n a t i o n o f methane e t c . , i n which t h e use of s t r u c t u r a l l y s i m p l e molecules makes l e s s d r a m a t i c t h e problems of s e l e c t z v i t y ) , t h e y were c o n s i d e r e d of poor i n t e r e s t by t h e o r g a n i c c h e m i s t s , w i t h f e w e x c e p t i o n s , f a r t h e s y n t h e s i s o f f i n e c h e m i c a l s a n d complex molecules, where a h i g h s e l e c t i v i t y i s an e s s e n t i a l cundzt i o n f o r t h e s y n t h e t i c s u c c e s s . Barton h a s been t h e most i m p o r t a n t among t h e s e e x c e p t i o n s : e v e r s i n c e 1960 he h a s 1 been a p r e c u r s o r showing by t h e "Barton r e a c t i o n ' ! t h e s y n t h e t i c p o t e n t i a l i t y of t h e f r e e -r a d i c a l r e a c t i o n s f o r t h e s e l e c t i v e s y n t h e s i s o f complex m o l e c u l e s . I n t h e l a s t 1 5 y e a r s f r e e -r a d i c a l r e a c t i o n s have, however, g a i n e d a remarkable p o s i t i o n among t h e s e l e c t i v e methods of s y n t h e s i s o f s o p h i s t i c a t e d molecules and 2 have been n o t i c e d a s t h e i m p o r t a n t f a c t o r i n b i o l o g i c a l p r o c e s s e s . I n this development t h e Barton r e s e a r c h h a s played a l e a d e r s h i p and a drawing r o l e . 3 I n 1968 we have showed, i n a p r e l i m i n a r y r e p o r t , t h a t a v a r i e t y of s e l e c t i v e r e a c t i o n s c o u l d be r e a l i z e d by t a k i n g advantage of t h e p o l a r e f f e c t s a r i s i n g from t h e n u c l e o p h i l i c c h a r a c t e r o f t h e c a r b o n -c e n t e r e d r a d i c a l s i n t h e r e a c t i o n s w i t h

Four different hexahelicenes, 5-aza-hexahelicene (1), hexahelicene (2), 2-methyl-hexahelicene (3), and 2-bromo-hexahelicene (4), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound 1; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound 1 is also examined from this point of view

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The substitution of protonated heteroaromatic bases by nucleophilic carbon‐centered radicals has been developed as one of the most general reactions in the heterocyclic series; its great interest results from the fact that it reproduces most of the numerous aspects of the Friedel‐Crafts aromatic substitution, but with opposite reactivity and selectivity. The most recent developments, concerning the generation of free radicals by iodine and hydrogen abstraction and decarboxylation of carboxylic acids are particularly discussed. Some processes of high synthetic value were designed on the basis of the kinetic and thermodynamic features of all the elementary steps involved in complex but selective chain processes. New general procedures for the monosubstitution in the homolytic alkylation and acylation are reported and discussed.

The oxidation of N-alkylamides by O(2), catalyzed by N-hydroxyphthalimide (NHPI) and Co(II) salt, leads under mild conditions to carbonyl derivatives (aldehydes, ketones, carboxylic acids, imides) whose distribution depends on the nature of the alkyl group and on the reaction conditions. Primary N-benzylamides lead to imides and aromatic aldehydes at room temperature without any appreciable amount of carboxylic acids, while under the same conditions nonbenzylic derivatives give carboxylic acids and imides with no trace of aldehydes, even at very low conversion. These results are explained through hydrogen abstraction by the phthalimide-N-oxyl (PINO) radical, whose reactivity with benzyl derivatives is governed by polar effects, so that benzylamides are much more reactive than the corresponding aldehydes. The enthalpic effect is, however, dominant with nonbenzylic amides, making the corresponding aldehydes much more reactive than the starting amides. The importance of the bond dissociation energy (BDE) of the O-H bond in NHPI is emphasized.

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