Equilibrium acidities and homolytic bond dissociation enthalpies of the acidic C-H bonds in as-substituted triphenylarsonium and related cations

Article abstract of DOI:10.1021/jo981129i

Equilibrium acidities (P.KHAS) of As-fluorenyltriphenylarsonium, As-phenacyltriphenylarsonium, six As-(panz-substituted benzyDtriphenylarsonium [p-GC6H4CH²⁺AsPh3] (G = H, Me, CF3, CO2Me, CN, and N02), and six P-(para-substituted benzyl)tri(ra-butyl)phosphonium [p-GC6H4CH²⁺P(w-Bu)3] (G = H, Me, CF3, CO2Me, CN, and NO₂) bromide salts, together with the oxidation potentials [SOX(A-)] of their conjugate bases (ylides) have been determined in dimethyl sulfoxide (DMSO) solution. Introduction of an a-triphenylarsonium (a-PhsAs+) group was found to increase the adjacent C-H bond acidities by 13-20 pK units (18-27 kcal/mol). The equilibrium acidities for the two series p-GC6H4CH²⁺AsPh3 andp-GC6H4CH²⁺P(n-Bu)3 cations were found to be nicely correlated with the Hammett er constants of the correspondingpara-substituents (G) (Figures 1 and 2). The homolytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds determined by using eq 1 reveal that an a-PhsAs"1" group increases the BDE value of the adjacent acidic C-H bond by 2-5 kcal/mol, whereas the substituent effects for an a-Ph3P+ or a-(w-Bu)3Pf group was found to be dependent on the nature of the substituents attached to the a-carbon atom. Good linear correlations were obtained for the equilibrium acidities of As-(para-substituted benzyDtriphenylarsonium and P-(parasubstituted benzyl)tri(?-butyl)phosphonium cations with the oxidation potentials of their conjugate bases (ylides) as shown in Figures 3 and 4, respectively.

Full text of DOI:10.1021/jo981129i

7072
J . Org. Chem. 1998, 63, 7072-7077
Equ ilibr iu m Acid ities a n d Hom olytic Bon d Dissocia tion
En th a lp ies of th e Acid ic C-H Bon d s in As-Su bstitu ted
Tr ip h en yla r son iu m a n d Rela ted Ca tion s1
J in-Pei Cheng,* Bo Liu, Yongyu Zhao, and Xian-Man Zhang*
Department of Chemistry, Nankai University, Tianjin, 300071, People’s Republic of China and
Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459
Received J une 12, 1998
Equilibrium acidities (pKHAs) of As-fluorenyltriphenylarsonium, As-phenacyltriphenylarsonium, six
As-(para-substituted benzyl)triphenylarsonium [p-GC6H4CH2+AsPh3] (G ) H, Me, CF3, CO2Me, CN,
and NO2), and six P-(para-substituted benzyl)tri(n-butyl)phosphonium [p-GC6H4CH2+P(n-Bu)3] (G
) H, Me, CF3, CO2Me, CN, and NO2) bromide salts, together with the oxidation potentials [Eox(A-)]
of their conjugate bases (ylides) have been determined in dimethyl sulfoxide (DMSO) solution.
Introduction of an R-triphenylarsonium (R-Ph3As+) group was found to increase the adjacent C-H
bond acidities by 13-20 pK units (18-27 kcal/mol). The equilibrium acidities for the two series
p-GC6H4CH2+AsPh3 and p-GC6H4CH2+P(n-Bu)3 cations were found to be nicely correlated with the
Hammett σ- constants of the corresponding para-substituents (G) (Figures 1 and 2). The homolytic
bond dissociation enthalpies (BDEs) of the acidic C-H bonds determined by using eq 1 reveal that
an R-Ph3As+ group increases the BDE value of the adjacent acidic C-H bond by 2-5 kcal/mol,
whereas the substituent effects for an R-Ph3P+ or R-(n-Bu)3P+ group was found to be dependent on
the nature of the substituents attached to the R-carbon atom. Good linear correlations were obtained
for the equilibrium acidities of As-(para-substituted benzyl)triphenylarsonium and P-(para-
substituted benzyl)tri(n-butyl)phosphonium cations with the oxidation potentials of their conjugate
bases (ylides) as shown in Figures 3 and 4, respectively.
Sch em e 1
In tr od u ction
Although there are oceans of research papers about the
phosphonium ylides (Wittig reagents) primarily used as
reactive intermediates (synthons) to convert the carbonyl
double bonds (>CdO) of aldehydes and ketones into the
corresponding olefinic carbon-carbon double bonds (>Cd
C<),2 relatively little attention has been paid to the
arsonium ylides presumably due to the psychological
barrier in that the word arsenic has been associated with
poisoning for many centuries.3,4
Introduction of an R-triphenylarsonium group is known
to significantly increase the equilibrium acidity of the
adjacent C-H bond. For example, the pKHA value of As-
phenacyltriphenylarsonium cation was found to be 8.25
in 95% ethanol solvent,5 and the pKHA value of As-
fluorenyltriphenylarsonium cation was reported to be 7.8
in 31.7% water-dioxane.6 The large acidifying effects
of the R-triphenylarsonium groups were believed to be
caused by a combination of the Coulombic effects of the
positive charge on the arsenic atom and the resonance
delocalization of the negative charge into the available
4d vacant orbitals of arsenic as shown in Scheme 1.4 The
postulated resonance delocalization (covalent structure)
(II) seems to be supported by the near planar carbanion
structure of the stabilized arsonium ylides as obtained
from X-ray diffractions.7 The As-C bond length of the
arsonium ylides was found to be shorter than the sum of
the singly covalent bonded radii of carbon and arsenic.4,7
Recently, we8 have found that the acidifying effects of
an R-triphenylphosphonium group are about 9∼20 pK
units stronger than those of an R-trimethylammonium
group on the related adjacent C-H bonds. The larger
acidifying effects of the R-triphenylphosphonium than
those of the R-trimethylammonium group were attributed
to the larger polarizability effects rather than the reso-
nance delocalization of the negative charge into the
vacant 3d orbitals of phosphorus.8a,b The latter is very
unlikely because the R-triphenylphosphonium groups
(3) (a) Huang, Y.-Z. Acc. Chem. Res. 1992, 25, 182-187. (b) Huang,
Y.-Z.; Shen, Y.-C. Adv. Organomet. Chem. 1982, 20, 115. (c) Huang,
Y.-Z.; Xu, Y.; Li, Z. Org. Prep. Proceed. Int. 1982, 14, 373. (d) Huang,
Y.-Z.; Shi, L.-L.; Yang, J .-H.; Xiao, W.-J .; Li, S.-W.; Wang, W.-B. In
Heteroatom Chemistry; Block, E., Ed.; VCH Publishers: New York,
1990; pp 189-206.
(4) (a) Lloyd, D.; Gosney, I.; Ormiston, R. Chem. Soc. Rev. 1987,
16, 45-74. (b) Lloyd, D.; Gosney, I. The Chemistry of Organic Arsenic,
Antimony and Bismuth Compounds; Patai, S., Ed.; Wiley: Chichester,
1994.
(5) J ohnson, A. W.; Amel, R. T. Can. J . Chem. 1968, 46, 461.
(6) J ohnson, A. W.; LaCount, R. B. Tetrahedron 1960, 9, 130.
(7) (a) Fronza, G.; Bravo, P.; Ticozzi, C. J . Organomet. Chem. 1978,
157, 299. (b) Shao, M.-C.; J in, X.-L.; Tang, Y.-G.; Huang, Q.-C.; Huang,
Y.-Z. Tetrahedron Lett. 1982, 5343. (c) Fan, Z.-C.; Shen, Y.-C. Acta
Chim. Sin. 1984, 42, 759. (d) Ferguson, G.; Rendle, D. F.; Lloyd, D.;
Singer, M. I. C. J . Chem. Soc. Chem., Commun. 1971, 1647. (e)
Ferguson, G.; Rendle, D. F. J . Chem. Soc., Dalton Trans. 1976, 171.
(1) J .P.C. and X.M.Z. would like to dedicate this paper to Professor
F. G. Bordwell for his pioneering work to determine the BDEs of the
acidic H-A bonds from the acidity and oxidation potential measure-
ments.
(2) (a) March, J . Advanced Organic Chemistry, 3rd ed.; Wiley: New
York, 1985; p 845. (b) J ohnson, A. W.; Kaska, W. C.; Starzewski, K. A.
O.; Dixon, D. A. Ylides and Imines of Phosphorus; Wiley: New York,
1993. (c) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863-927.
(d) Vedejs, E.; Peterson, M. J . Top. Stereochem. 1994, 21, 1 and
references therein.
S0022-3263(98)01129-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/09/1998

Acidic C-H Bonds in As-Substituted Triphenylarsonium
J . Org. Chem., Vol. 63, No. 20, 1998 7073
Ta ble 2. Equ ilibr iu m Acid ities a n d Hom olytic Bon d
Dissocia tion En th a lp ies of th e Ben zylic C-H Bon d s in
As-(P a r a -su bstitu ted ben zyl)tr ip h en yla r son iu m Ca tion s
were found to have no stabilizing effects on the adjacent
radicals.8 In the present paper, we extend our studies
to examine the effects of R-triphenylarsonium groups on
the equilibrium acidities and the BDEs of the adjacent
acidic C-H bonds. Effects of R-tri(n-butyl)phosphonium
and R-triphenylphosphonium groups have also been
compared on the equilibrium acidities and BDEs.
a
c
acids
PhCH2+AsPh3
pKHA
Eox(A-)b
BDEHA
22.7
23.8
18.4
18.2
17.6
16.1
-0.558
-0.630
-0.290
-0.276
-0.221
-0.062
91.5
91.4
91.8
91.9
92.3
93.9
4-MeC6H4CH2+AsPh3
4-CF3C6H4CH2+AsPh3
4-CH3OCOC6H4CH2+AsPh3
4-NCC6H4CH2+AsPh3
4-O2NC6H4CH2+AsPh3
Resu lts a n d Discu ssion
a
Equilibrium acidities (in pK units) measured in DMSO by the
Equ ilibr iu m Acid ities. As-Fluorenyltriphenylarso-
nium, As-phenacyltriphenylarsonium, six As-(para-sub-
stituted benzyl)triphenylarsonium (p-GC6H4CH2+AsPh3)
(G ) H, Me, CF3, CO2Me, CN, and NO2), and six P-(para-
substituted benzyl)tri(n-butyl)phosphonium [p-GC6H4-
CH2+P(n-Bu)3] (G ) H, Me, CF3, CO2Me, CN, and NO2)
bromide salts were synthesized according to the litera-
ture or modified literature procedures.8 The equilibrium
acidities of these salts were measured in dimethyl
sulfoxide (DMSO) solution by the overlapping indicator
titration method.9 All of the conjugated bases (ylides)
derived from these salts were found to be stable enough
to allow three-point titrations to be made under the
experimental conditions of measurement. The counterion
(Br-) was shown to have no effects on the equilibrium
acidity measurement.8a The related equilibrium acidi-
ties, together with those for the related parent com-
pounds are summarized in Tables 1∼3.
overlapping indicator titrations.9 Irreversible oxidation poten-
b
tials (in volts) of the triphenylarsonium ylides measured in DMSO
solution by conventional cyclic voltammetry. c Homolytic bond
dissociation enthalpies (in kcal/mol) of the benzylic C-H bonds
determined by using eq 1.
Ta ble 1. Com p a r ison of Equ ilibr iu m Acid ities a n d
Hom olytic Bon d Dissocia tion En th a lp ies of th e Acid ic
C-H Bon d s in Differ en t On iu m Ca tion s
a
h
acids
PhCH3
pKHA
Eox(A-)e
BDEHA
(43)b
88-89i
90.5
89.3
91.5
79.5
84.6
81.3
82.3
93.0
96.5
96.2
95.5
PhCH2+NMe3
PhCH2+PPh3
PhCH2+AsPh3
Fluorene (FlH2)
9-Me3N+FlH
31.9c
17.4d
22.7
22.6b
17.8c
6.6d
-1.143c
-0.381d,f
-0.558
-1.060b
-0.563c
-0.044d,g
-0.191
-0.609b
0.141c
9-Ph3P+FlH
9-Ph3As+FlH
PhCOCH3
9.8
24.7b
14.6c
6.0d
PhCOCH2+NMe3
PhCOCH2+PPh3
PhCOCH2+AsPh3
F igu r e 1. Correlation of the equilibrium acidities of P-(p-
substituted benzyl)triphenylarsonium cations measured in
DMSO solution against the Hammett σp- constants of the para
substituents (Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991,
91, 165-169).
0.639d
0.450
8.6
a
Equilibrium acidities (in pK units) measured in DMSO by the
overlapping indicator titrations.9 This study unless otherwise
indicated. Reference 16. c Reference 23. Reference 8a. e Irre-
versible oxidation potentials (in volts) of the onium ylides mea-
sured in DMSO by conventional cyclic voltammetry. This study
unless otherwise indicated. f The reversible oxidation potential is
-0.339 V, which was measured by fast-scan CV.8b g The reversible
oxidation potential is -0.021V, which was measured by fast-scan
b
d
units weaker than those of the corresponding P-(para-
substituted benzyl)triphenylphosphonium cations.8b The
weaker acidifying effects of R-triphenylarsonium than
those of the R-triphenylphosphonium groups are con-
sistent with the relatively higher reactivities of the tri-
phenylarsonium ylides than those of the analog-
ous triphenylphosphonium ylides in the related Wittig
reactions.3c,4-6,10
CV.8b Homolytic bond dissociation enthalpies (in kcal/mol) of the
h
i
acidic C-H bond determined by using eq 1. Reference 22.
Examination of Table 1 shows that the acidifying
effects of the R-triphenylarsonium group are about 6∼9
pK units stronger than those of the R-trimethylammo-
nium group, but about 2∼5 pK units weaker than those
of the R-triphenylphosphonium group. The equilibrium
acidities of As-(para-substituted benzyl)triphenylarso-
nium cations (Table 2) were also found to be about 5 pK
In an earlier paper,8b we have reported that the
equilibrium acidities of P-(para-substituted benzyl)triph-
enylphosphonium cations were well correlated with the
Hammett σ- constants of the para-substituents. Not
surprisingly, the equilibrium acidities of the As-(para-
substituted benzyl)triphenylarsonium and P-(para-sub-
stituted benzyl)tri(n-butyl)phosphonium cations can also
be well correlated with the Hammett σ- constants of the
para-substituents (Figures 1 and 2). Similar linear
correlations of the equilibrium acidities with the Ham-
mett σ- constants have been observed for many other
families of weak acids such as phenols,11 thiophenols,12
(8) (a) Zhang, X.-M.; Bordwell, F. G. J . Am. Chem. Soc. 1994, 116,
968-972. (b) Zhang, X.-M.; Fry, A. J .; Bordwell, F. G. J . Org. Chem.
1996, 61, 4101. (c) Liu, B.; Huan, Z.-W.; Cheng, J .-P. Acta Chim. Sin.
1997, 55, 123. (d) Cheng, J .-P.; Liu, B.; Lu, Y.; Mi, J .-L.; Huan, Z.-W.
Chem. J . Chin. Univ. 1997, 18, 1080. (e) Liu, B. Ph.D. Dissertation,
Nankai University, Tianjin, China, 1995. (f) Zhao, Y. Ph.D. Disserta-
tion, Nankai University, Tianjin, China, 1994.

7074 J . Org. Chem., Vol. 63, No. 20, 1998
Cheng et al.
Ta ble 3. Equ ilibr iu m Acid ities a n d Hom olytic Bon d
Dissocia tion En th a lp ies of th e Ben zylic C-H Bon d s in
P -(P a r a -su bstitu ted ben zyl)tr i(n -bu tyl)p h osp h on iu m
Ca tion s
a
c
acids
pKHA
Eox(A-)b
BDEHA
PhCH2+P(t-Bu)3
21.9
23.1
18.9
17.9
17.5
15.0
-0.620
-0.694
-0.370
-0.358
-0.292
-0.127
89.0
88.9
90.6
89.6
90.5
90.9
4-MeC6H4CH2+P(t-Bu)3
4-CF3C6H4CH2+P(t-Bu)3
4-CH3OCOC6H4CH2+P(t-Bu)3
4-NCC6H4CH2+P(t-Bu)3
4-O2NC6H4CH2+P(t-Bu)3
a
Equilibrium acidities (in pK units) measured in DMSO by the
overlapping indicator titration.9 b Irreversible oxidation potentials
(in volts) of the tri(n-butyl)phosphonium ylides measured in DMSO
solution by conventional cyclic voltammetry. c Homolytic bond
dissociation enthalpies (in kcal/mol) of the benzylic C-H bonds
determined by using eq 1.
to eq 1.16 The calculated C-H BDEs together with those
for the parent compounds and few N-substituted tri-
methylammonium and para-substituted triphenylphos-
phonium cations are also summarized in Tables 1∼3 for
comparison.
BDEHA ) 1.37pKHA + 23.1Eox(A-) + 73.3 (1)
F igu r e 2. Correlation of the equilibrium acidities of P-(p-
substituted benzyl)tri(n-butyl)phosphonium cations measured
in DMSO solution against the Hammett σp constants of the
para substituents (Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev.
1991, 91, 165-169).
Deprotonation from the substituted onium cations and
neutral weak acids is different; the former changes from
positive to neutral species (ylide), and the latter changes
from neutral to negative species. But the deviation on
the equilibrium acidity measurement due to the different
activity coefficients of the charged species has been shown
to be smaller than the experimental error ((0.1 pK units)
of the overlapping indicator titration.8b,9 The oxidation
potentials of the conjugate bases (ylides) [Eox(A-)] are
readily obtained by means of conventional cyclic volta-
mmetry (scan rate: 100 mV/s).16 Although most oxida-
tion potentials measured by the conventional cyclic
voltammetry are irreversible, the irreversible oxidation
potentials for the conjugate anions were found to agree
within potentials as obtained by fast-scan cyclic voltam-
metry8b,17 or second harmonic alternating current volta-
mmetry (SHACV).18 Nevertheless, the BDE values ob-
tained with the irreversible oxidation potentials accord-
ing to eq 1 have been shown to agree within ( 2 kcal/
mol with the best literature BDE values.16 Furthermore,
the BDEs for the same family of weak acids are expected
to be accurate with each other even less than 0.5∼1 kcal/
mol since the oxidation potential measurement is repro-
ducible to (0.020 V (∼0.5 kcal/mol)16b and the acidity
measurement is accurate to 0.15 kcal/mol.9
-
anilines,13 10-substituted-9-methylanthracenes,14 and tolu-
enes.15 But the Hammett correlation slopes for the As-
(para-substituted benzyl)triphenylarsonium, P-(para-
substituted benzyl)tri(n-butyl)phosphonium, and P-(para-
substituted benzyl)triphenylphosphonium cations8b were
found to be only about 5 pK units, which are much
smaller than those for the para-substituted toluenes (12
pKHA units)15 and 10-substituted-9-methylanthracenes
(10 pKHA units).14 The diminution of the Hammett
correlation slopes for the para-substituted benzylic onium
cations is clearly associated with the strong stabilization
of the onium groups on the adjacent negative charge
(Scheme 1), which in turn decreases the influence of the
para substituent G.8b
Hom olytic Bon d Dissocia tion En th a lp ies of th e
Acid ic C-H Bon d s. The homolytic bond dissociation
enthalpies of the acidic C-H BDEs in eight As-substi-
tuted triphenylarsonium cations and six P-(para-substi-
tuted benzyl)tri(n-butyl)phosphonium cations have been
determined from the equilibrium acidities and the oxida-
tion potentials of their conjugate bases (ylides) according
Examination of the column 4 of Table 2 shows that the
benzylic C-H BDEs of the six As-(para-substituted
benzyl)triphenylarsonium cations average 92.7 ( 1.2
kcal/mol. Examination of Table 3 shows that the average
benzylic C-H BDE of the six P-(para-substituted benzyl)-
tri(n-butyl)phosphonium cations is 89.0 ( 0.9 kcal/mol,
which is essentially same as the average benzylic C-H
(9) (a) Bordwell, F. G.; Algrim, D.; Vanier, N. R. J . Org. Chem. 1977,
42, 1817. (b) Sim, B. A.; Milne, P. H.; Griller, D.; Wayner, D. D. M. J .
Am. Chem. Soc. 1990, 112, 6635.
(10) (a) Matthews, W. S.; Bares, J . E.; Bartmess, J . E.; Bordwell, F.
G.; Cornforth, F. D.; Drucker, G. E.; Margolin, Z.; McCollum, G. J .;
Vanier, N. R. J . Am. Chem. Soc. 1975, 97, 7006-7014. (b) Bordwell,
F. G. Acc. Chem. Res. 1988, 21, 456 and the references therein.
(11) (a) Nesmeyanov, N. A.; Mikulshin, V. V.; Revtov, O. A. J .
Organomet. Chem. 1968, 13, 263. (b) Akanes, G.; Songstad, J . Acta
Chem. Scand. 1964, 18, 655. (c) Froyen, P. Acta Chem. Scand. 1971,
25, 2541. (c) Lloyd, D.; Singer, M. I. C. Tetrahedron 1972, 28, 353.
(12) Bordwell, F. G.; McCallum, R.; Olmstead, W. N. J . Org. Chem.
1984, 49, 1424.
BDE (89.6 ( 0.3 kcal/mol) of the P-(para-substituted
+
benzyl)triphenylphosphonium cations [p-GC6H4CH2
-
PPh3] (G ) H, Br, CN, and NO2) even though the
equilibrium acidities of the former are about 5 pK unit
(13) Bordwell, F. G.; Hughes, D. L. J . Org. Chem. 1982, 47, 3224.
(14) Bordwell, F. G., Zhang, X.-M.; Cheng, J .-P. J . Org. Chem. 1993,
58, 6410-6416.
(15) Zhang, X.-M.; Bordwell, F. G.; Bares, J . E.; Cheng, J .-P.; Petrie,
B. C. J . Org. Chem. 1993, 58, 3051-3059.
(16) (a) Bordwell, F. G.; Cheng, J .-P.; Harrelson, J . A., J r. J . Am.
Chem. Soc. 1988, 110, 1229. (b) Bordwell, F. G.; Cheng, J .-P.; J i, G.-
Z.; Satish, A. V.; Zhang, X.-M. J . Am. Chem. Soc. 1991, 113, 9790. (c)
Bordwell, F. G.; Zhang, X.-M. Acc. Chem. Res. 1993, 26, 510.
(17) Bausch, M. J .; Gostowski, R. J . Org. Chem. 1991, 56, 6260.

Acidic C-H Bonds in As-Substituted Triphenylarsonium
J . Org. Chem., Vol. 63, No. 20, 1998 7075
F igu r e 4. Plot of the oxidation potentials of the P-(p-
substituted benzyl)tri(n-butyl)phosphonium ylides versus the
equilibrium acidities of their conjugate acids in DMSO solu-
tion.
F igu r e 3. Plot of the oxidation potentials of the As-(p-
substituted benzyl)triphenylarsonium ylides versus the equi-
librium acidities of their conjugate acids in DMSO solution.
weaker than the latter.8a,b The weaker acidifying effects
of R-tri(n-butyl)phosphonium relative to R-triphenylphos-
phonium groups can be attributed to the relatively
smaller polarizability effects of the n-butyl than of the
phenyl groups. The polarizability effects of a phenyl
group are known to be larger than those of an alkyl
group. For example, replacement of the alkyl group in
alkylthioacetonitriles RSCH2CN (R ) Me, Et, Pr, and Bu)
by a phenyl group was found to increase the equilibrium
acidities by 2∼4 pK units, but it results in negligible
effects on the acidic C-H BDEs.19 Equilibrium acidity
increase accompanied with a near constant BDE has been
considered as the characteristics for the polarizability
effects.19a
Lin ea r Cor r ela tion of Oxid a tion P oten tia ls of As-
(P a r a -su b st it u t ed b en zyl)t r ip h en yla r son iu m a n d
P -(P a r a -su bstitu ted ben zyl)tr i(n -bu tyl)p h osp h on i-
u m Ylid es ver su s Equ ilibr iu m Acid ities of Th eir
Con ju ga te Acid s. The near constant benzylic C-H
BDEs of As-(para-substituted benzyl)triphenylarsonium
(Table 2) and P-(para-substituted benzyl)tri(n-butyl)-
phosphonium cations (Table 3) require that the oxidation
potentials of the onium ylides be linearly correlated with
the equilibrium acidities of the corresponding conjugate
acids. Indeed, the oxidation potentials of As-(para-
substituted benzyl)triphenylarsonium and P-(para-sub-
stituted benzyl)tri(n-butyl)phosphonium ylides were found
to be well correlated with the equilibrium acidities of
their conjugate acids as shown in Figures 3 and 4,
respectively. The slopes for the linear correlations are
near unity when both axes are expressed in kcal/mol.
Similar linear correlations of the oxidation potentials of
the conjugate bases versus their equilibrium acidities
have been observed in few other families of weak
acids.8a,b,14,20
CH2+P(n-Bu)3 cations (G ) CN and NO2) are about 1∼2
kcal/mol higher than those of the rest cations. The
slightly higher benzylic C-H BDEs for those containing
strong electron acceptors (CN and NO2) are presumably
due to the increase of the ground-state effects caused by
the polar interaction of the strong electron acceptors with
the weak polar C-H bonds.21
No Reson a n ce Deloca liza tion of th e Un p a ir ed
Electr on in to th e 4d Or bita ls of Ar sen ic. Examina-
tion of Table 2 shows that the benzylic C-H BDEs of
the As-(para-substituted benzyl)triphenylarsonium cat-
ions are in the range of 91.4-93.9 kcal/mol. This means
that introduction of an R-triphenylarsonium group
strengthens the benzylic C-H bond by about 2∼5 kcal/
mol since the benzylic C-H BDE of toluene is well-known
to be about 88∼89 kcal/mol.22 In addition, the acidic
C-H BDEs of As-fluorenyltriphenylarsonium and As-
phenacyltriphenylarsonium cations (Table 1) were also
found to be about 2∼3 kcal/mol higher than those of their
parent compounds.
The bond-strengthening (radical destabilizing) effects
of the R-triphenylarsonium group were found to be
remarkably similar to those of the R-trimethylammonium
(R-Me3N+) group.23 The bond-strengthening effects of
R-trimethylammonium groups were believed to be caused
by the field/inductive effects of the positive charge on the
nitrogen atom.23 Theoretical calculations also show that
an R-ammonium (R-H3N+) group decreases the adjacent
methyl radical stability by about 4 kcal/mol.24 Therefore,
the bond-strengthening effects of the R-triphenylarso-
nium groups can also be attributed to the field/inductive
radical destabilizing effects of the positive charge on the
arsenic atom. In other words, the resonance delocaliza-
tion of the unpaired electron into the related vacant 4d
orbitals of arsenic is negligible as shown in Scheme 2.
No resonance delocalization of the unpaired electron
into the 4d orbitals of arsenic implies that the resonance
delocalization of the negative charge in the corresponding
ylides into the vacant 4d orbitals of arsenic should also
Examination of Tables 2 and 3 shows that the benzylic
C-H BDEs of the p-GC6H4CH2+AsPh3 and p-GC6H4-
(18) (a) Arnett, E. M.; Harvey, N. G.; Amarnath, K.; Venimadhaven,
S. J . Am. Chem. Soc. 1990, 112, 7346. (b) Bordwell, F. G.; Harrelson,
J . A., J r.; Lynch, T.-Y. J . Org. Chem. 1990, 55, 3337.
(19) (a) Bordwell, F. G.; Zhang, X.-M. J . Am. Chem. Soc. 1994, 116,
973. (b) Zhang, S.; Zhang, X.-M.; Bordwell, F. G. J . Am. Chem. Soc.
1995, 117, 602.
(20) (a) Bordwell, F. G.; Bausch, M. J . J . Am. Chem. Soc. 1986, 108,
1979. (b) Bordwell, F. G.; Cheng, J .-P.; Bausch, M. J .; Bares, J . E. J .
Phys. Org. Chem. 1988, 1, 209. (c) Bordwell, F. G.; Cheng, J .-P.; Satish,
A. V.; Twyman, C. L. J . Org. Chem. 1992, 57, 6542. (d) Bordwell, F.
G.; Harrelson, J . A.; Zhang, X.-M. J . Org. Chem. 1991, 56, 4448.
(21) Bordwell, F. G.; Zhang, X.-M.; Satish, A. V.; Cheng, J .-P. J . Am.
Chem. Soc. 1994, 116, 6605.
(22) (a) Berkowitz, J .; Ellison, G. B.; Gutman, D. J . Phys. Chem.
1994, 98, 2744. (b) Lide, D. R. CRC Handbook of Chemistry and
Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995-1996; p 9-64.
(23) (a) Bordwell, F. G.; Zhang, X.-M. J . Org. Chem. 1990, 55, 6978.
(b) Zhang, X.-M.; Bordwell, F. G.; Van Der Puy, M.; Fried, H. E. J .
Org. Chem. 1993, 58, 3060.

7076 J . Org. Chem., Vol. 63, No. 20, 1998
Cheng et al.
Sch em e 2
Lack of the resonance delocalization of the unpaired
electron into the 4d orbitals of arsenic for the radical
cation Ph3As+CH•Ph is probably due to the greater size
and diffuseness of the arsenic 4d-orbitals compared with
the phosphorus 3d-orbitals, which will prevent the ef-
fectiveness of the pπ - dπ orbital overlap.
be negligible.8 Thus, the large acidifying effects of the
R-triphenylarsonium groups should be caused by a
combination of the polarizability effects and the field/
inductive effects of the positive charge on the arsenic
atom. The short ylidic As-C bond length6,7 can be
attributed to the electrostatic attraction of the opposite
charge on the arsenic atom and the carbanion center.25
Is Th er e Reson a n ce Deloca liza tion of th e Un -
p a ir ed Electr on in to th e 3d Or bita ls of P h osp h o-
r u s? Introduction of an R-triphenylphosphonium group
into the acidic sites of CH3CN, CH3COOEt, and CH3COR
was found to strengthen the adjacent acidic C-H bonds
by 2∼3 kcal/mol, which is similar to the effects for the
introduction of an R-trimethylammonium group.8a In
other words, there is no resonance delocalization of the
unpaired electron for the radical cations Ph3P+CH•CN,
Ph3P+CH•COOEt, and Ph3P+CH•COR, formed by removal
of one hydrogen atom from the corresponding precursor
cations. This conclusion is in accord with the ESR
studies, which showed that the majority of the unpaired
electron in the radical cation (Ph3P+CH•COOH) is local-
ized on the carbon atom rather than delocalized into the
3d orbitals of phosphorus.26
Su m m a r y a n d Con clu sion s
The acidifying effects of R-triphenylarsonium groups
(R-Ph3As+) are about 13 ∼ 20 pK units (18 ∼ 27 kcal/
mol) and are attributed to a combination of the Coulombic
and polarizable interaction on the ylide anions. Deter-
mination of the homolytic bond dissociation enthalpies
reveals that introduction of an R-triphenylarsonium
group decreases the adjacent radical stabilities by 2∼5
kcal/mol, which is similar to that of an R-Me3N+ group,
indicating that there is no (pπ-dπ) resonance delocal-
ization of the unpaired electron into the 4d orbitals of
arsenic. But the radical stabilization effects of the
R-phosphonium groups in the radical cations R3P+CH•G
were found to be dependent on the nature of the sub-
stituent G. The 4∼5 pK unit weaker acidifying effects
of an R-tri(n-butyl)phosphonium than of an R-triph-
enylphosphonium group are attributed to the much larger
polarizability effects of phenyl than of n-butyl groups.
Exp er im en ta l Section
Substituent is known to have dual effects on the
adjacent radical stabilities: i.e., the field/inductive effect
decreases the radical stability, but the resonance delo-
calization will increase the radical stabiliy.23,27 When an
electron acceptor and electron donor are both attached
to the same carbon radical center, the net radical
stabilizing effect is usually greater than the sum of the
individual substituent effects.28 But when two electron
acceptors were both introduced to the same carbon
radical center, the net radical stabilizing effect will be
much smaller than the sum of the individual substituent
effects due to the strong polar interactions of the two
electron acceptors.20d,21 For example, introduction of a
PhSO2 or NO2 group into methane decreases the C-H
BDE by 6 or 7 kcal/mol respectively,29 but no additional
radical stabilizing effect was found when they were
introduced into the acidic site of CH3SO2Ph.20d On the
other hand, introduction of an electron donor such as a
phenyl20d or a thiophenyl group30 into the acidic site of
CH3SO2Ph was found to decrease the acidic C-H BDEs
by an additional 8∼10 kcal/mol.
The 1H NMR spectra were recorded on a J EOL FX-90Q
NMR spectrometer with tetramethylsilane as internal stan-
dard. Melting points were measured on a Yanaco apparatus
and were uncorrected. The electrochemical measurements
were performed on a BAS-100B electroanalytical instrument.
Ma ter ia ls. The synthesis of P-fluorenyltriphenylphospho-
nium and P-phenacyltriphenylphosphonium bromides has
been described previously.8a As-Fluorenyltriphenylarsonium,31
As-phenacyltriphenylarsonium,32 and P-(para-substituted ben-
zyl)tri(n-butyl)phosphonium bromides33 were synthesized ac-
cording to the related literature procedures. As-(para-
substituted benzyl)triphenylarsonium bromides were synthe-
sized by a modified literature procedure.34 A solution of
triphenylarsine (Ph3As) (3.06 g, 10 mmol) in 20 mL of ni-
tromethane was added dropwise into a solution of para-
substituted benzyl bromide (10 mmol) in 20 mL of ni-
tromethane. The mixture was allowed to reflux for 4∼6 h and
gave a white precipitate. The crude products were purified
by recrystallization from acetone/ethanol. The melting points,
1H NMR chemical shifts, and elemental analyses (for new
compounds) are summarized in Table 4.
p KHA Mea su r em en t. The equilibrium acidities of the eight
As-substituted triphenylarsonium and six para-substituted tri-
(n-butyl)phosphonium bromides were measured in DMSO
solution at the room temperature by the overlapping indicator
titration method as described previously.9 The results, to-
gether with the indicator used, are summarized in Table 5.
Comparison of Tables 2 and 3 shows that the benzylic
C-H BDEs of the p-GC6H4CH2+P(n-Bu)3 cations are also
about 2 ∼ 3 kcal/mol less than those of the corresponding
p-GC6H4CH2+AsPh3 cations. More significantly, the acidic
C-H BDE (88 kcal/mol) of Ph3P+CH2SPh cation was
found to be even about 5 kcal/mol lower than that (93
kcal/mol) of CH3SPh.8a The less radical destabilizing or
some radical stabilizing effects of the R-phosphonium
groups are caused presumably by the radical stabilization
by some resonance delocalization of the unpaired electron
into the 3d orbitals of phosphorus. This is not surprising
since phenyl (Ph) and thiophenyl (SPh) groups are known
to be electron donating, but cyano (CN), ethoxycarbonyl
(COOEt) and carbonyl (COR) are electron accepting.29
(25) Whangbo, M.-H.; Wolfe, S.; Bernardi, F.Can. J . Chem. 1975,
53, 3040.
(26) (a) Hudson, R. F. Pure Appl. Chem. 1964, 9, 371. (b) Lucken,
E. A. C.; Mazeline, C. J . Chem. Soc. A. 1966, 1074.
(27) Zhang, X.-M. J . Org. Chem. 1998, 63, 3590.
(28) Viehe, H. G.; J anousek, Z.; Merenyi, R.; Stella, L. Acc. Chem.
Res. 1985, 18, 148.
(29) Bordwell, F. G.; Zhang, X.-M.; Alnajjar, M. S. J . Am. Chem.
Soc. 1992, 114, 7623.
(30) Zhang, X.-M. Unpublished results.
(31) Wittig, G.; Felletschin, G. Ann. 1944, 555, 133.
(32) J ohnson, A. W. J . Org. Chem. 1960, 25, 183.
(33) Qi, Z.; Shao, J .; Lin, C. Synth. Commun. 1991, 21, 869.
(34) Zhong, Q.; Liu, C.-Q.; Shao, J .-G. Chin. J . Org. Chem. 1991,
11, 58.
(24) Pasto, D. J .; Krasnansky, R.; Zercher, C. J . Org. Chem. 1987,
52, 3062.

Acidic C-H Bonds in As-Substituted Triphenylarsonium
J . Org. Chem., Vol. 63, No. 20, 1998 7077
Ta ble 4. Meltin g P oin ts, 1H NMR Ch em ica l Sh ifts, a n d Elem en ta l An a lysis of th e On iu m Br om id e Sa lts
compound
mp (°C)a (lit. mp)
1H NMRe δ (CDCl3)
elemental analysis (calcd) f
9-Ph3As+FlH Br-
180-181 (178-179)b
184-185 (186)c
172-172.5
7.01-8.12 (m, 23H), 8.62 (s, 1H)
6.5 (s, 2H), 7.3-8.2 (m, 20H)
5.60(s, 2H), 7.11-7.52(m, 5H),
7.82(m, 15H)
2.43(s, 3H), 5.55 (s, 2H), 6.93-7.51
(m, 4H), 7.85 (s, 15H)
3.94 (s, 3H), 5.81 (s, 2H),
7.32-8.10 (m, 19H)
6.12 (s, 2H), 7.12-8.03 (m, 19H)
PhCOCH2+AsPh3 Br-
C6H5CH2+AsPh3 Br-
C: 62.85 (62.91)
H: 4.71 (4.69)
C: 63.53 (63.56)
H: 5.02 (4.92)
C: 60.41 (60.58)
H: 4.52 (4.52)
C: 62.01 (62.18)
H: 4.17 (4.21)
N: 2.67 (2.98)
C: 57.09 (57.27)
H: 3.88 (3.88)
C: 57.29 (57.49)
H: 4.13 (4.05)
N: 2.57 (2.68)
p-MeC6H4CH2+AsPh3 Br-
p-MeO2CC6H4CH2+AsPh3 Br-
p-NCC6H4CH2+AsPh3 Br-
161-162
160-161
188-189
p-CF3C6H4CH2+AsPh3 Br-
p-NO2C6H4CH2+AsPh3 Br-
162-163
163-164
6.02 (s, 2H), 7.11-8.12 (m, 19H)
6.23 (s, 2H), 7.53-8.12 (m, 19H)
C6H5CH2+P(n-Bu)3 Br-
148.5-150 (144-148)d
108-110
0.80-1.21 (m, 9H), 1.33-180 (m, 12H),
2.20-2.81 (m, 6H), 4.35 (d, 2H),
7.33-7.81 (m, 5H)
p-MeC6H4CH2+P(n-Bu)3 Br-
p-MeO2CC6H4CH2+P(n-Bu)3 Br-
p-NCC6H4CH2+P(n-Bu)3 Br-
p-CF3C6H4CH2+P(n-Bu)3 Br-
p-NO2C6H4CH2+P(n-Bu)3 Br-
0.80-1.12 (m, 4H); 1.22-1.81 (m, 12H),
2.34 (s, 3H); 2.45-2.85 (m, 6H),
4.25 (d, 2H), 7.13-7.64 (m, 4H)
0.81-1.22 (m, 9H), 1.23-1.82 (m, 12H),
2.33-3.01 (m, 6H); 4.10 (s, 3H),
4.51-5.02 (d, 2H), 7.51-8.25 (m, 4H)
0.79-1.09 (m, 9H); 1.15-1.70 (m, 12H);
2.13-2.80 (m, 6H); 5.02(d, 2H),
7.42-8.13 (m, 4H)
0.81-1.10 (m, 9H); 1.20-1.81 (m, 12 H);
2.21-2.79 (m, 6H); 4.57 (d, 2H),
7.45-8.05 (m, 4H)
0.87-1.22 (m, 9H); 1.23-1.82 (m, 12H),
2.33-2.81 (m, 6H), 4.55-5.15(d, 2H),
7.84-8.45 (m, 4H)
C: 61.92 (62.02)
H: 9.25 (9.35)
88.-89.5
C: 58.36 (58.47)
H: 8.52 (8.41)
160-161.5
157-158
C: 59.91 (60.30)
H: 8.30 (8.35)
N: 3.28 (3.52)
C: 54.11 (54.43)
H: 7.90 (7.53)
169-170
C: 54.50 (54.55)
H: 7.86 (7.95)
N: 3.19 (3.35)
a
b
Melting point; in °C; uncorrected. J ohnson, A. W. J . Org. Chem. 1960, 25, 183. c Aksnes, G.; Songstad, J . Acta Chem. Scand. 1964,
18, 655. Ramirez, E.; Madaan, C. P.; Smith, G. P. Tetrahedron, 1966, 22, 567. e Chemical shifts in ppm. f Elemental analysis for the
d
new compounds. The numbers in the parenthesis are calculated from the related molecular formula.
Ta ble 5. Equ ilibr iu m Acid ity Mea su r em en ts for th e On iu m Br om id es by th e Over la p p in g In d ica tor Titr a tion
pKHA
(assigned)
cations
pKHA
indicator
pKIN
runsj
a
i
9-Ph3As+FlH Br-
9.77 ( 0.11
8.56 ( 0.02
22.68 ( 0.07
23.83 ( 0.06
18.17 ( 0.06
17.67 ( 0.15
18.40 ( 0.02
16.11 ( 0.12
21.92 ( 0.12
23.07 ( 0.02
17.86 ( 0.04
17.49 ( 0.08
18.85 ( 0.21
15.59 ( 0.25
COMFHb
CNFHc
FHd
10.35
8.3
9.8
8.6
3
2
2
2
2
2
2
2
2
2
2
2
2
2
PhCOCH2+AsPh3 Br-
PhCH2+AsPh3 Br-
22.6
22.6
17.9
17.9
17.9
15.0
20.66
22.6
18.9
17.9
18.9
15.0
22.7
23.8
18.2
17.6
18.4
16.1
21.9
23.1
17.9
17.5
18.8
15.6
p-MeC6H4CH2+AsPh3 Br-
p-MeO2CC6H4CH2+AsPh3 Br-
p-NCC6H4CH2+AsPh3 Br-
p-CF3C6H4CH2+AsPh3 Br-
p-NO2C6H4CH2+AsPh3 Br-
C6H5CH2+P(n-Bu)3 Br-
FH
PFHe
PFH
PFH
J 1Y300f
2NPANHg
FH
p-MeC6H4CH2+P(n-Bu)3 Br-
p-MeO2CC6H4CH2+P(n-Bu)3 Br-
p-NCC6H4CH2+P(n-Bu)3 Br-
p-CF3C6H4CH2+P(n-Bu)3 Br-
p-NO2C6H4CH2+P(n-Bu)3 Br-
CNAHh
PFH
CNAH
J 1Y300
a
b
d
Equilibrium acidities measured in DMSO. 9-(Methoxycarbonyl)fluorene. c 9-Cyanofluorene. Fluorene. e 9-Phenylfluorene. f (Phe-
g
h
i
j
nylsulfonyl)acetonitrile. 2-Naphthylacetonitrile. 4-Chloro-2-nitroaniline. Equilibrium acidity of the indicator used. Numbers of the
experimental measurements.
Electr och em ica l Mea su r em en t. All electrochemical ex-
periments were carried out in DMSO solution under an argon
atmosphere as described previously.16,35 n-Bu4NPF6 (0.1 M)
was employed as the supporting electrolyte. A standard three-
electrode cell consists of platinum disk (ø ) 1 mm) as working
electrode and platinum wire as counter electrode. The scan
rate was 100 mV/s. AgNO3/Ag (0.1 M) couple was used as the
reference electrode. The reported oxidation potentials were
all referenced to the ferrocenium/ferrocene couple, which was
used as an internal standard. In the electrochemical cell
(BAS), the conjugate bases (ylides) were generated by addition
of a stock potassium dimsylate solution to a 5∼8 mL solution
of the onium bromide salt to be deprotonated. The concentra-
tion of the onium ylides generated was about 2 mM.
Ack n ow led gm en t. This research was supported by
the National Science Foundation of People’s Republic
of China (NSFC Contract Nos. 29392206 and 29472053).
(35) (a)Cheng, J .-P.; Zhao, Y. Tetrahedron 1993, 49, 5267. (b) Cheng,
J .-P.; Zhao, Y.; Huan, Z.-W. Sci. China B 1995, 38, 1417.
J O981129I