The Electrophilic Substitution of Heteroaromatic Compounds. Part 53. The Experimental Orientation in the Electrophilic Substitution of 1-Phenylpyridinium Cations, and a Theretical Treatment of Electrophilic Substitution Orientations in N-Phenylheterocycles

matractm it ration, svlfonarion and bromination of the title compound occur in the mara position of the 1-phenyl group.This meta substitution, and the para substitution previously observed for other N-phenylheterocycles, cannot he explained by static reactivity indices." o r e v e r , a HNOO analysis, raking account of the near degeneracy of the two top HOMO,^, and using experimentally probable interannular anglee, allows rationalization of the orientations observed.vorlaender has that the 1-phenylpyridinium cation (11 undergoes nitration at the et.position of the phenyl substituent.he reported orientation is sanewhat surprising, as K phenyl groups attached to five-membered rings fend to be nitrated in the position, even when reaction takes place an a cation (cf.pyrazoles 1,3 3-pyra~olonss.~5-pyra~olones.~isidazoles 661.m t h e r m o r e , the indo~enium cation (11 undergoes nitration exclusively para to the positive pole to yield i.7 mreover, while the nitration of 1-phenyl-4-pyridone lcf. 9 1 is reported to occur in the para p o ~i t i o n .~ 1-phenyl-5-methyl-2-pyridone (cf.101 1% nitrated in the a posit ion.8 OH / Table I. 13cNXF? Chemical Shifts of 1-Phenylpyridinium Tetrafluoroborate Derivatives.I pyridinim ring phenyl ring compd.C-2 C-3 C-4 C-1 C-2 C-3 C-4 C-5 C-6 4 144.8 128.2 146.6 -141.8 124.7 124.1 127.8 130.2 131.3 d Solutions in Dnso-& except 2 in OMS&& + TPA.b Peaks overlap.There appears to be no previovs record of other electrophilic substitution reaotions of the 1phenylpyridinium cation 1 1 1 .Because Of these facts we have reinveetigatsd the nitration and studied some further typical electrophilic substitutions.we find that compound 1 1 1 indeed undergoes nitration (fuming HN03, d. 1.5.85 OCI, sulfonafion (30 \ aleun.118 OCI, and bromination (Br2, 150 OC), to give in each case the corresponding msfa svbetifvfsd darivafive 12).131, and I s in 60. 46, and 45% isolated yield, respectively.he canpounds were isolated as the tetrafluoroborate salts.Examination of the crvde reaction products by TLC and by 13c m gave no evidence for the formation ~f any -or para-substituted derivatives.he orientation of the products was proven by 13c Nnn (Table 1 1 which ahore clearly the asymmetrical nature of the substitution in the phenyl ring.Calculations: calcvlatione were carried out on compavnde 1, 2, and 7-10 using the published M W W formalimo and parameterizationl1 uith standard program package^.^^^^.The geometry of compound 5 r a a completely optimized.I" other cases the optimization was limited to the interannular distances and angles with the remaining geonetrical variablee fixed at the NNW values for the correeponding mononuclear species.me coefficients of the highest ~ccupied molecular orbital (~01401 and the prtial atomic charges at tha m s t a and pera positions are summarized in able 2. m t h aete of simple reaotivity indicsa suggest'2"3 that electrophilic eubstitution should occvr predominantly a t the meta position in a11 compounds.This, however, is in contradiction uith experiment for all but the l-phenylpyridinium cation (I).Unfortunately, there are at lsaer two difficulties associated with this approach.he HNW calculations lead to energy minima which, a p r t from the rigid indolanium ion 121.correspond to interannurar angles (alpha) near 9 0 ' .~t seems highly likely that this is an artifact of the w e n knovn tendency of the method to overestimate non-bcnded rep~leione.11.'~