.PI. And .SIGMA. states of the N-formamido radical: an ab initio molecular orbital study

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...

show abstract

“…(b) N-Acetyl-N-bromobiphenyl-2-carboxamide (8).-Similar results were obtained upon photolysis of N-acetyl-N-bromobiphenyl-2-carboxamide (8). U.V.…”

supporting

confidence: 66%

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

Glover¹,

Goosen²,

McCleland³

et al. 1986

J. Chem. Soc., Perkin Trans. 2

View full text Add to dashboard Cite

show abstract

“…In the meantime, while arguments on the 2 and n amidyl radical configurations continued inconclusively, several works (14-18) on intramolecular reactivities, in particular cyclizations of alkenyl amidyl radicals, were published and interpreted in favor of 2 and (or) I I electronic configurations as the reactive state. Theoretical calculations (19,20) on the 2-n structure of amidyl radicals have not been able to give a definitive answer (19). Recently, Ingold's group (26)(27)(28)(29) has extended the esr observations reported earlier by Danen and Gellert (12), and proven convincingly that amidyl radicals (RCONR) possess n structure in N-alkyl or N-acyl chains and investigate competing intramolecular H-abstraction from the C(5)-hydrogens and additions to the double bonds.…”

Section: Resultsmentioning

confidence: 99%

“…The to publish this work now. major interest has been the electronic configuration from which amidyl radicals (RCONR) (12)(13)(14)(15)(16)(17)(18)(19)(20) react with substrates and its…”

mentioning

confidence: 99%

Chemistry of amidyl radicals: intramolecular reactivities of alkenyl amidyl radicals

Chow¹,

Perry²

1985

Can. J. Chem.

View full text Add to dashboard Cite

Amidyl radicals possessing Δ4,5, Δ5,6, and Δ6,7 double bonds were generated from the photodecomposition of nitrosamides and chloramides and the products were identified. Dichotomies of amidyl radical reactivities were discovered and compared with published kinetic rate constants. In complete reversal to intermolecular reactivities, intramolecularly the alkenyl amidyl radicals preferentially add to the double bonds rather than abstract a C-5 hydrogen even if it is allylic. In intramolecular competition, amidyl radicals add to an acyl side chain double bond more efficiently than to an alkyl one; this is just the opposite to intramolecular H-abstraction of amidyl radicals. Taken together with the published results, it is established that, in intramolecular attacks of double bonds, amidyl radicals selectively undergo the propia-addition to generate an exo-cyclic radical rather than the longa-addition to an endo-cyclic radical: this rule should replace the old one that amidyl radicals preferentially cyclize to form five-membered rings if choices are available.

show abstract

“…The difficulties in the calculations are that, to obtain a realistic prediction, the MO method which is used must be able to differentiate accurately ( a ) the relahive energies ofthe two hypes of double bonds compared to the single bonds, and (b) the energy of addition of an electron to a singly-occupied pa orbital of nitrogen compared to that for an s-p hybrid AO. Calculations for the NH, free radical (6) show that the STO-3G basis favours too strongly an s-p hybrid for the lone pair over a purely p orbital, whereas the 4-316 basis is better balanced. (Compared to the experimental advantage of ~3 1 kcal in favour of the ll state (1) the STO-3@ splitting is 'The optimum bond angle about the nitrogen is a good indicator of the delocalization of unpaired spin density.…”

Section: Resultsmentioning

confidence: 99%

Abinitio MO predictions for the geometries and energies of the Σ and II states of some conjugated free radicals

Baird¹,

Taylor²

1980

Can. J. Chem.

Self Cite

View full text Add to dashboard Cite

. Can. J. Chem. 58,733 (1980). Ab initio MO calculations are reported for the iowest C and II doublet states of the radicals 1-18. Configuration interaction is included, using the restricted open-shell MO's as a base. The important geometric variables are optimized in calculations using the §TO-3G basis set; in some cases energies and geometries are redetermined using the 4-31G basis. In most cases the energy separation between the C and FI doublet states is calculated to be small; the results are very dependent upon the basis set whenever the radical is localized upon a nitrogen atom. For radicals XYZ with different terminal groups, a valid prediction of the structure is obtained only if the basis set properly distinguishes between the n energies of X=Y and Y=Z bonds. Small basis sets appear to underestimate the n energy of carbonyl bonds, and thus are overbiased in favour of X=C-0' structures. Unsymmetrical structures are predicted for the C states of some radicals, even when the terminal groups are identical. On predit des structures dissymetriques pour les etats P: de quelques radicaux, mime quand les groupes terminaux sont identiques.[Traduit par le journal] Introduction(that perpendicular to the plane) whereas the other R/lolecules which have very low-lying excited primarily is a 2s, 2p hybrid of nitrogen. Since the states are expected whenever the hig.,est-occupied hybrid orbital is substantially more stable than is molecular orbital differs little in stability from one the 2~ 3 the energy splitting between the two doublet

show abstract

.PI. And .SIGMA. states of the N-formamido radical: an ab initio molecular orbital study

Cited by 16 publications

References 0 publications

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

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

Chemistry of amidyl radicals: intramolecular reactivities of alkenyl amidyl radicals

Abinitio MO predictions for the geometries and energies of the Σ and II states of some conjugated free radicals

Contact Info

Product

Resources

About