J. Org. Chem. 1981,46,3831-3835
3831
Kinetics and Mechanisms of Nucleophilic Displacements with Heterocycles as Leaving Groups. 3.' N-( Substituted benzyl)-2,4,6-triphenylpyridiniums: Effects of Benzyl Substitution on First- and Second-Order Rates2 Alan R. Katritzky,* Giuseppe Musumarra, and Kumars Sakizadeh The School of Chemical Sciences, University of East Anglia, Norwich, England, the Department of Chemistry, University of Florida, Gainesville, Florida 32611, and Instituto Dipartimentale di Chimica e Chimica Industriale dell'Universit6 di Catania, Catania, Italy Received March 3, 1981 1-@-Methoxybenzy1)- and 1-(2-furfuryl)-2,4,6-triphenylppidiniums react with piperidine in chlorobenzene by sN1 and sN2 mechanisms, the proportion of sN1 increasing with temperature. Other 1-(substituted benzyl) derivatives show only sN2 reaction. Activation parameters support this interpretation. Substituent effects on sN1 and sN2 rates are discussed.
We have shown that nucleophilic displacement reactions of l-benzyl-2,4,6-triphenylpyridiniumcan be studied conveniently in nonpolar solvents and that steric3 and electronic4 effects in the leaving group can profoundly influence the rate and mechanism of such reactions. Preliminary experiments which indicated a very large rate enhancement for the displacement of l-(p-methoxybenzyl) groups from pyridiniums led to the present investigation of the effect of benzyl substituion on rates. Extensive previous work on the nucleophilic substitutions of benzyl halides has encompassed studies of substituent effects in substrate6 and n u ~ l e o p h i l eand ~ ~ , of ~ solvent sensitivity.' The solvolysis of benzyl chlorides has been explored with regard to the effects,of substituents, of added nucleophiles,8 and of ~olvent.~ Preparation of Compounds. 2,4,6-"Iliphenylpyrylium perchlorate was reacted with the appropriate benzylamine to give the 2,4,6-triphenyl-l-(substituted benzy1)pyridinium perchlorates (lc-h, Table I, Chart I; exceptionally, compound lb was prepared as the tetrafluoroborate). The fury1 derivative 2 was prepared similarly from furfurylamine. The p-methoxy derivative (lh) reacted preparatively with pyridine to give the expected product (3)in high yield. Kinetic Determinations. Reactions :u1chlorobenzene solution, with piperidine, morpholine, or pyridine as nucleophile, were followed spectrophotornetrically under pseudo-first-order conditions as previoiisly described.l0 Results are given in Tables 11-IV. In dl cases, the observed rate was a linear function of the piperidine concentration. We have shown previously3s0that kohlfor the reaction with piperidine is of l-benzyl-2,4,6-triphenylpyridinium independent of the substrate concentration. In the present work, reproducibility was good except for the p-methoxybenzyl compound, where difficulty was encountered at the higher temperatures. In most cases the rate was directly dependent on [piperidine] (Figure I.). However, for the N-p-methoxybenzyl and N-2-furfuryl derivatives a positive intercept was obtained by extrapolating the nucleophilic concentration to zero (Figures 2 and 3). For the p-methoxybenzyl compound, when morpholine and pyridine were used as nucleophiles in place of piperidine, different slopes but the same intercepts were found (Figure 2). This behavior, with positive or zero intercepts depending on the N substituent, recalls that of N-benzylpyridiniums *Towhom correspondence should be addressed at the University of Florida.
chart I
m I
CY
cio;
2
R
=
a
b
H
p-nie
c
m-m
d
p-F
e
f
p-ci
rp-ci
g
p-30~
h
p - ~ ~ e
with different leaving group^.^ The observed rate variations can therefore be interpreted in terms of the sN2 mechanism, together with (for the p-methoxy and furfuryl derivatives) a contribution from the s N 1 mechanism; the s N 2 rate, but not the s N 1 , rate depends on the nature of the nucleophile. Calculated s N 2 and s N 1 rate constants are given in Table V. Simultaneous SN2 and SN1Reaction Mechanisms. The mechanism of nucleophilic substitution has been controversial. Sneen proposed unification of the chemical duality of the s N 1 and s N 2 mechanisms in which d nucleophilic substitution involved the formation of ion pairs." (1) Part 2 A. R. Katritzky, A. M. El-Mowafy, G. Musumarra,K. Sakizadeh, C. Sana-Ullah,S. M. El-Shatie,and S. S. "bind J. Org. Chem., preceding paper in this issue. (2) For a preliminary communication of part of this work see A. R. Katritzky, G. Musumarra, K. Sakizadeh, S. M.M. El-SWie, and B. Jovanovic, Tetrahedron Lett., 21, 2697 (1980). (3) See ref 1. (4) A. R.Katritzky, J. Adamson, E. M. Elieeeou, G. M u s m a , R. C. Patel, K. Sakizadeh, and W. K. Yeung, unpublished work. (5) (a) J. W.Baker, J. Chem. SOC.,2631 (1932); (b) F.P. Ballistreri, E. Maccarone, and A. Mamo, J. Org. Chem., 41, 3364 (1976). (6) (a) D. H.Peacock, J. Chem. SOC.,1975 (1924); (b) P. S. Radhakrishna Murti and G. P. Panigrahi, Zsr. J. Chem., 6,137 (1988); (c) J.Indian Chem. SOC., 45,323 (1968); (d) Bull. Chem. SOC.Jpn., 43,Sl (1970); (e) S.P.sakeena and A. N. Bose, Zndiun J. Chem., 13,421 (1975). (7) F. P.Balliitreri,E. Maccarone, G. Musumarra, and G. A. Tomaselli, J. Org. Chem., 42, 1415 (1977). (8) B. Bensley and G. Kohtam, J. Chem. SOC., 4747 (1967); 0 Kohnstan, A. Queen, and B. Shillaker, h o c . Chem. SOC.,London, 157, (1959);A. Queen, Can. J. Chem., 57,2646 (1979). (9) (a) E. Tommila, Acto Chem. Scond., 20,923 (1966); (b) E. Tommila and I. P. P i t h e n , ibid, 20, 937 (1966); (c) E. Tommila and M. Savolainen, ibid., 20, 946 (1966). (10) A. R. Katritzky, G. Musumarra, K. Sakizadeh, and M. MisicVukovic, J. Org. Chem., accompanying paper in this issue.
0022-3263/81/1946-3831$01.25/00 1981 American Chemical Society
3832 J. Org. Chem., Vol. 46, No. 19, 1981
Katritzky, Masumarra, and Sakizadeh
Table I. Preparation of 1-Substituted 2,4,6-TriphenylpyridiniumPerchloratesa compd lbb IC Id le
1-substituent p-MeC,H,CH, m-MeC,H,CH, p-FC,H,CH, p-ClC,H,CH,
If lg lh
m-ClC,H,CH, p-O,NC,H,CH, p-MeOC,H,CH,
2
2-f~ryl-CH~
% cryst yield solvent mp, "C 81 EtOH 175 71 EtOH 127 65 EtOH 124 93 Me,CO- 143 Et,O 82 EtOH 177 69 EtOH 125 73 Me,CO- 144-145 Et,O 67 Me,CO- 122 Et,O
% found
lit. mp, "C
cryst form 174-176ad white needles whiteneedles white needles 143'~~ white plates
% required
C
H
N
74.2 72.3 69.6 67.4
4.9 5.0 4.5 4.5
2.6 2.7 2.5 2.5
formula C,,H,,BF,N C,,H,,ClNO, C,,H,,ClFNO, C,,H,,Cl,NO,
148e3f
white needles 69.2 4.3 2.6 C,,H,,Cl,NO, white needles 65.9 4.2 5.1 C,,H,,ClN,O, whiteplates 70.2 4.8 2.6 C,,H,,ClNO,
122c*gdec
cream plates
C
H
N
74.6 72.7 69.8 67.7
5.2 5.1 4.5 4.3
2.8 2.7 2.7 2.6
67.7 4.3 2.6 66.3 4.2 5.2 70.5 5.0 2.7
2.9 C,,H,,ClNO,
2.7
Satisfactory analyses were obtained for all compounds. This compound is a tetrafluoroborate. Perchlorate. A. R. Katritzky, J. B. Bapat, R. J. Blade, B. P. Leddy, P.-L. Nie, C. A. Ramsden, and S. S. Thind, J. Chem. SOC.,Perkin Trans. 1 , 418 (1979). e Tetrafluoroborate. A. R. Katritzky, U. Gruntz, A. A. Ikizler, D. H. Kenny, and B. P. Leddy, J. Chem. SOC., Perkin Trans. 1 , 436 (1979). g A. R. Katritzky, M. F. Abdel-Megeed, G. Lhommet, and C. A. Ramsden, J. Chem. SOC., Perkin Trans. 1 , 426 (1979). Table XI. Pseudo-First-Order Rate Constants ( k o b d ) for the Reaction of 1-RCH,-Substituted 2,4,6-Triphenylpyridinium Cations with Piperidine in Chlorobenzene at 100 "C [piperidine], mol L-'
p-MeC,Hqa (. l b .)
0.032 0.08 0.16 0.24
28.8
m-MeC,H, (.I C ).
p-FC,H, (. I d .)
p-ClC,H, (. l e .)
29.2 71.4 127 194
24.3 57.9 107 152
23.3 50.0 97.2 149
138
m-ClC,H, (. I f, )
p-NO,C,H, (. l a- .) 10.8 26.1 47.3 73.2
16.0 34.6 69.0 91.8
a Concentration of Dvridinium eaual to 3.2 X lo-' mol L-'. Concentration of Rvridinium eaual to 1.6 X 10- mol L-', Additional values f i r - l b ( R = p-MeC,H,) ([piperidine] in mol L-', lO5hobd in s-Ij:- 0.0642, 56.1; 0.00321, 3.20; 0.0016,
1.68; 0.00064, 1.63.
Table 111. Pseudo-First-Order Rate Constants ( k o b e ) for the Reactions of l-@-Methoxybenzyl)-2,4,6-triphenylpyridinium Cations ( l h ) with Nucleophiles in Chlorobenzene 1OSk0bd,s-' morpiperidine pholine, pyridine, [nucleophile], mol L-' 35'C" 40 "C 45 O c a 50 'Ca 55'Ca 60"CGb 40°C 40 "C 0.32 0.24 0.16 0.08 0.032 a
8.9 6.8 4.5
16.7a 12.4a 7.gc 5.33c
25.8 21.3 15.8
38.3 29.7 23.9
109.3 98.2 80.1 68.8 64.4
73.7 61.5 49.6
9.85
4.66 4.78 4.20 4.35 3.91
7.27 5.83 4.62
mol L-I. Additional value: [piperidine] Concentration of pyridinium equal to 3.2 x mol L-I. Concentration of pyridinium equal to 1.6 X
= 0.016
mol L-'; 1 0 5 k 0 b d
= 53.0 s - ' .
Table IV. Pseudo-First-Order Rate Constants (k.134) for the Reactions of 1-(2-Furyl)-2,4,6-triphenylpyridinium Cation (2)with Piperidine in Chlorobenzenea 105kObsd,s-1
[piperidine], mol L-I 0.032 0.08 0.16
c
45
50
55
60
69
"C
"C
"C
"C
"C
4.60 7.00
9.61 13.4 19.7
15.7 21.8 29.8
28.3 38.1 54.3
48.8 68.2 92.9
130 174 230
X 10.'
mol
,40
10.5
Concentration of pyridinium equal t o 3.2 L-1. a
This approach was criticized particularly by McLennan, who favored the traditional duality.12 The evidence for and against each viewpoint has been reviewed.I3 (11)R.A. Sneen and J. W. Larsen, J. Am. Chem. SOC.,91,6031(1969); R. A. Sneen, Acc. Chem. Res., 6 , 46 (1973). (12)D.J. McLennan, Acc. Chem. Res.,9,281 (1976). (13) D.J. Raber, J. M. Harris, and P. v. R. Schleyer, "Ions and Ion Pairs in Organic Reactions", Vol. 2, M. Szwarc, Ed., Wiley, New York, 1979;T. W. Bentley and P. v. R. Schleyer, Ado. Phys. Org. Chem., 14, l(1977).
We believe that the present results show a clear duality of mechanism for the reactions of 1-(pmethoxybenzy1)and 1-(2-furfuryl)-2,4,6-triphenylpyridiniumwith nucleophiles in chlorobenzene. The interpretation is further supported by the similar results already reported for N benzylpyridiniums with different leaving group^.^ These results do not support the Sneen unified theory of nucleophilic substitution. In our work, t h e use of neutral nucleophilic and nonnucleophilic solvents exclude complications in the interpretation arising from salt effects and solvolysis reactions. Under pseudo-first-order conditions (excess of nucleophile), the unified mechanism for our reactions (eq 1) would require a kinetic dependence according to eq 2. RX+
k
k-i
kobd
R+-X
kdN1
RN+
+X
= k~k,[Nl(k-I + k,[NI)
+ k[N1
(1) ( 2)
(3) In place of kinetic dependence according to eq 2 we clearly find the kinetic dependence expected for the sikbsd
=
k0
J. Org. Chem., Vol. 46, No. 19, 1981 3833
N-(Substituted benzyl)-2,4,6-triphenylpyridiniums
Table V. First-Order (k ') and Second-Order (k,) Rate Constants for the Reactions of 2,4,6-Triphenyl-l-(substituted benzy1)pyridinium Cations and Heterocyclic Analogues with Nucleophiles in Chlorobenzene slope intercept compd 1-substituent PhCH, lag p-MeC,H,CH, lb
IC
Id le If 1g lh
2
m-MeC,H CH, p-FC,H,CfI, p-ClC,H,CH, m-ClC,H,CH, p-0,NC,H4CH, p-MeOC,H,CH,
2-fu~l-CH,
teFP, nucleophile C Nn piperidine 100 6 piperidine 40 1 60 1 80 1 100 6 piperidine 100 4 100 4 piperidine piperidine 100 4 piperidine 100 4 piperidine 100 4 piperidine 35 3
morpholine pyridine piperidine
40 45 50 55 60 100 40 40 40 45 50 55 60 64 100
rb 0.9996
1O3k,/ 105k1,c$d error, (k, t re1 S% 10k,)e k,f
