Factors affecting the electronic states of amidyls: evidence for Π—Σ mixing in simple amidyls

Abstract

N-Methyl-and N-acetyl-biphenyl-2-carboxamidyls, generated from the corresponding Nbromides in photochemically initiated alkyl radical or bromine atom chain reactions, cyclise to give Ar,-5 and Ar,-6 products, indicating the accessibility of a low-energy excited C-state for these species. N-Methoxybiphenyl-2-carboxamidyl radical, from photolysis of the N-chloro derivative, yields no products from Ar,-5 or Ar,-6 cyclisation; this indicates a thermodynamically stable ll-ground state and a high-energy and thermally inaccessible excited Sstate. The Nmethyl biphenyl -2-sulphonamidyl radical from photolysis of the N-iodo or N-bromo derivative affords only the Ar,-6 product; this represents chemical evidence for a large n-C energy separation. MNDO calculations predict the n-ground states of Nalkoxy-amidyls and -aminyls to be more nucleophilic than those of amidyls, imidyls, and sulphonamidyls, in keeping with the observed reactivities. The reactivities and regioselectivities observed, together with e.s.r. data and MNDO results, support mixing between the ll-ground and Xexcited states of amidyls, whereas alkoxyamidyls, cyclic amidyls, and sulphonamidyls have pure nground states. Mixing in amidyls is facilitated by their acyclic nature and by a small energy difference between their ll-ground and %excited states.Amidyl (l), imidyl (2), and sulphonamidyl (3) radicals belong to a class which, because of the non-bonding electrons, can have two distinct electronic states where the electron resides upon the singly bonded heteroatom. For amidyls these states are designated ll (4) and C (5). and theoretical (MO) 2*8-1 ' evidence points to a ll-ground state for amidyls (1). Calculations point to a n-ground state of imidyls (2).' '-13 Both e.s.r.6*1e16 and ab initio calculations l o point to a II-ground state for sulphonamid yls.The search for chemical evidence for the ground state structures of (1) and (2) has revealed interesting results. Skell and his co-workers l 7 have suggested that both Xand n-states of the succinimidyl radical can be generated selectively from Nbromosuccinimide in chain reactions involving, respectively, nucleophilic primary or secondary alkyl radicals and electrophilic halogen atoms in abstraction of the N-bromine atom. Although these routes have been supported by MNDO calculation^,'^ recent papers by Tanner1* and by Walling1g have questioned the intermediacy of different radical states in chain bromination. Ab initio calculations show that the Cstate is higher in energy than the ll-state by 84-105 kJ mol-'; MNDO provides a difference of 61 kJ mol-'.'The participation of the Z-state (5) of the biphenyl-2carboxamidyl radical (ll), formed by photolysis of the Niodamide (6), is indicated by the formation of Ar,-5 cyclised products (14)--(16).20-22 Although the intermediate radical (U) is thermodynamically less stable than (27), the Ar,-5 mode of addition is irreversible at temperatures below 333 K.23 This mode is peculiar to the X-state since in the transition state (28) there is optimum orbital overlap betwee...

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