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

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.

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compared to the literature value of -1.37 V. The reduction potential of NMN was measured as -1.65 V.Fluorescence Quenching of NMN. General Conditions. Fluorescence quenching experiments were carried out on 1 X 1 234 M solutions of NMN in spectral grade acetonitrile. Typically, five samples containing from 10'3 to 10~2 M quencher were prepared. The excitation wavelength was 300 nm unless competitive absorption from the quencher was significant, in which case a longer excitation wavelength was chosen. Fluorescence intensities were monitored at 371 nm. The data was interpreted by using the standard Stern-Volmer relationship.Fluorescence Quenching of NMN by 2,3-Butanedione. Estimation of the Singlet-State Lifetime of NMN. Efficient fluorescence quenching of NMN was observed in the presence of butanedione [(9.12 X 10"4)-(4.56 X 10~3) ], A Stern-Volmer plot of the data was linear with a slope of 101 ± 1. Since the singlet-state energy of butanedione is 65.3 kcal/mol and that of NMN is estimated to be 79 kcal/mol (vide supra) energy transfer should proceed at the diffusion-controlled rate, 2 X 1010 M/s in acetonitrile. The average feqr value for butanedione (and for 2,5-dimethyl-2,4-hexadiene) is 104 ± 3. The lifetime (r) of the singlet state of NMN is calculated as 5.2 ns.Solvent Isotope Studies. Identical samples containing 50 mg of NMP and 0.50 mL of 24 in 2 mL of the appropriate solvent (MeOH, MeOD, CH3CN, CD3CN) were irradiated through Pyrex in a merry-go-round apparatus. A minimum of three samples of each solvent was examined by HPLC vs. an internal standard. The 28/29 ratio was unchanged in MeOH/MeOD. The 28/29 ratio decreased by 16% in going from CH3CN to CD3CN.Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research (60740276) from the Ministry of Education, Science and Culture, Japan.

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