Making photochemically generated phenyl cations visible by addition to aromatics: Production of phenylcyclohexadienyl cations and their reactions with bases/nucleophiles

Article abstract of DOI:10.1021/ja980712d

The benzenediazonium cation and its 4-fluoro, 4-chloro, 4-bromo, 4- methyl, and 4-methoxy derivatives were photolyzed in 1,1,1,3,3,3- hexafluoroisopropyl alcohol (HFIP) with 20-ns pulses of 308-nm light from a XeCl excimer laser. This leads to a pronounced and permanent depletion of the parent compounds (quantum yields 0.89-0.96), but no signal from a transient is seen. However, on addition of aromatics such as, e.g., mesitylene, strong signals due to species with λ(max) at 250-260 and 350-390 nm are detected. In the absence of bases/nucleophiles other than the aromatics, these species, which do not react with oxygen, have a lifetime in the microsecond to millisecond range. On the basis of their absorption spectra and their reactivity with typical bases/nucleophiles such as halides, alcohols, and ethers, the transients are identified as cyclohexadienyl cations formed from the photoproduced 'invisible' phenyl cations by addition to the ring of the added aromatics. On the basis of competition data for reaction with HFIP and aromatic, the substituents Me, MeO, and Cl lead to a weak increase in selectivity of the phenyl cation, whereas F and Br do not influence the selectivity.

Full text of DOI:10.1021/ja980712d

J. Am. Chem. Soc. 1998, 120, 11925-11931
11925
Making Photochemically Generated Phenyl Cations Visible by
Addition to Aromatics: Production of Phenylcyclohexadienyl Cations
and Their Reactions with Bases/Nucleophiles
S. Steenken,*,1 M. Ashokkumar,1,2 P. Maruthamuthu,1,2 and R. A. McClelland*,3
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, D-45413 Mu¨lheim, Germany, and
Department of Chemistry, UniVersity of Toronto, Canada M5S 1A1
ReceiVed March 3, 1998
Abstract: The benzenediazonium cation and its 4-fluoro, 4-chloro, 4-bromo, 4-methyl, and 4-methoxy derivatives
were photolyzed in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) with 20-ns pulses of 308-nm light from a
XeCl excimer laser. This leads to a pronounced and permanent depletion of the parent compounds (quantum
yields 0.89-0.96), but no signal from a transient is seen. However, on addition of aromatics such as, e.g.,
mesitylene, strong signals due to species with λmax at 250-260 and 350-390 nm are detected. In the absence
of bases/nucleophiles other than the aromatics, these species, which do not react with oxygen, have a lifetime
in the microsecond to millisecond range. On the basis of their absorption spectra and their reactivity with
typical bases/nucleophiles such as halides, alcohols, and ethers, the transients are identified as cyclohexadienyl
cations formed from the photoproduced “invisible“ phenyl cations by addition to the ring of the added aromatics.
On the basis of competition data for reaction with HFIP and aromatic, the substituents Me, MeO, and Cl lead
to a weak increase in selectivity of the phenyl cation, whereas F and Br do not influence the selectivity.
Introduction
probably e0.5 ns. In the case of p-amino-substituted phenyl
cations, which exist in the triplet state, their lifetime in liquid
solution has recently been measured to be <15 ps.11 Phenyl
cations stabilized by seVeral methoxy groups appear to have
long lifetimes in LiCl matrices at 77 K.12
The reactions of arenediazonium ions have attracted consider-
able interest, both from a synthetic and from a mechanistic point
of view.4 The diazonium ions are able to undergo homolytic
decomposition, as in the Gomberg-Bachmann reaction,5 or
heterolysis, as in, e.g., solvolysis reactions. A particularly
fascinating aspect is the gradual changeover from heterolytic
to homolytic dediazoniations, which leads to the question of a
common intermediate for these two types of reactions.6
A point of interest for the heterolytic reactions has been
whether aryl cations are real intermediates. These cations are
expected to be highly reactive due to the large degree of
localization of charge at CR in an orbital of significant s
character.7,8 This question has been positively answered on the
basis of carefully performed product analysis and kinetic studies
We have recently found that the solvent 1,1,1,3,3,3-hexafluoro-
2-propanol (HFIP)13 is sufficiently weakly nucleophilic to allow
the direct, time-resolved detection of highly reactive carboca-
tions such as the (antiaromatic) 9-fluorenyl cation14 or benzyl
cations.15 The solvent HFIP is also moderately polar (ꢀ )
16.6),16 so it should support ionic processes. Diazonium salts
are obviously good precursors for carbocations because N2 is
such an excellent leaving group. Finally, photochemical excita-
tion should speed up bond breakage as found in many analogous
cases.17,18 Therefore, the combination of benzenediazonium
cations as precursors, HFIP as a solvent, and photolysis to
accelerate bond rupture should provide conditions suited for the
+
on the thermal decomposition of PhN2 in H2O and in 2,2,2-
trifluoroethanol.9 It was, however, concluded that the phenyl
cation is extremely short-lived. This result has been qualita-
tively supported by 337-nm laser flash photolysis studies of
monosubstituted arenediazonium cations in aqueous solutions,10
where it was concluded that the lifetime of the aryl cations is
(11) Gasper, S. M.; Devadoss, C.; Schuster, G. B. J. Am. Chem. Soc.
1995, 117, 5206.
(12) Ambroz, H. B.; Przybytniak, G. K.; Stradowski, C.; Wolszczak,
M. J. Photochem. Photobiol. A 1990, 52, 369.
(13) For quantitative data on the solvolyzing power and the nucleophi-
licity of HFIP, see: Schadt, F. L.; Schleyer, P. v. R.; Bentley, T. W.
Tetrahedron Lett. 1974, 2335.
(14) (a) McClelland, R. A.; Mathivanan, N.; Steenken, S. J. Am. Chem.
Soc. 1990, 112, 4857. (b) Cozens, F. L.; Mathivanan, N.; McClelland, R.
A.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1992, 2083. (c) Cozens, F.
L.; Li, J.; McClelland, R. A.; Steenken, S. Angew. Chem., Int. Ed. Engl.
1992, 31, 743. (d) McClelland, R. A.; Cozens, F. L.; Li, J.; Steenken, S. J.
Chem. Soc., Perkin Trans. 2 1996, 1531.
(15) McClelland, R. A.; Chan, C.; Cozens, F.; Modro, A.; Steenken, S.
Angew. Chem. 1991, 103, 1389; Angew. Chem., Int. Ed. Engl. 1991, 30,
1337.
(16) Murto, J.; Kivinen, A.; Lindell, E. Suomen Kemistilehti B 1970,
43, 28. These authors also determined the viscosity of HFIP to be 1.579 cP
at 25 °C.
(1) Max-Planck-Institut.
(2) Department of Energy, University of Madras, India.
(3) University of Toronto.
(4) For reviews, see e.g.: Zollinger, H. Acc. Chem. Res. 1973, 6, 335.
Zollinger, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 141.
(5) See: Ru¨chardt, C.; Freudenberg, B.; Merz, E. Chem. Soc., Spec. Publ.
1965, 19, 154.
(6) Broxton, T. J.; Bunnett, J. F.; Paik, C. H. J. Chem. Soc., Chem.
Commun. 1970, 1363.
(7) Gleiter, R.; Hoffmann, R.; Stohrer, W.-D. Chem Ber. 1972, 105, 8.
(8) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1977,
99, 1.
(9) (a) Swain, C. G.; Sheats, J. E.; Harbison, K. G. J. Am. Chem. Soc.
1975, 97, 783. (b) Bergstrom, R. G.; Landells, R. G. M.; Wahl, G. H.;
Zollinger, H. J. Am. Chem. Soc. 1976, 98, 3301.
(10) Scaiano, J. C.; Kim-Thuan, N. J. Photochem. 1983, 23, 269.
(17) Bartl, J.; Steenken, S.; Mayr, H.; McClelland, R. A. J. Am. Chem.
Soc. 1990, 112, 6918.
10.1021/ja980712d CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/06/1998

11926 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Steenken et al.
study of even extremely reactive carbocations such as the σ-type
phenyl cations.
Deoxygenated ≈0.1-0.2 mM solutions of the diazonium ions
+
4-R-C6H4N2 (R ) H, Br, Cl, Me, MeO) were irradiated in
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at 20 ( 1 °C with 20-
ns pulses of 308-nm light. Permanent negative changes of
optical density (OD) were seen at the wavelengths of the two
main absorption bands of the parents (between 260 and 320
nm and between e200 and 235 nm for R ) H to MeO). The
amount of depletion of the diazonium ions was strictly
proportional to laser dose, showing that the process is mono-
photonic. Negative signals arise when the precusor is more
absorbing at a particular wavelength than the product (or
intermediate) that is formed on photolysis. With our system,
the light-induced “negative“ spectra were within experimental
error ((10%) exact mirror images of the absorption spectra of
the starting materials. This shows that the products of the
photolysis have much weaker absorptions than the diazonium
cations. The (negative) signals were the same with respect to
shape and amplitude when the solutions were saturated with
oxygen, indicating that the photolysis probably does not lead
to radicals since these would be scavenged by oxygen leading
to peroxyl radicals which absorb in the 230-250-nm region.
Absence of observable transients was also reported for aqueous
solutions of benzenediazonium salts on photolysis with 337-
nm light.10
Experimental Section
Except 4-ClC6H4N2PF6 and 4-BrC6H4N2BF4 (from Aldrich), the
+
(substituted) benzenediazonium compounds Ar′N2 were synthesized
-
(as BF4 salts) using literature procedures.19 The aromatic derivatives
(from Aldrich, purity g 97%) used as phenyl cation scavengers were
passed over basic Al2O3 in order to remove any polar impurities.
1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP; from Hoechst) was purified
by fractional distillation over Na2SO4 (to remove water), then over
NaHCO3 (to remove trace acid impurities), and finally by zone refining
at -15 °C (to remove UV-absorbing impurities) to a purity of >99.9%
(GC). The diazonium salts were dissolved in HFIP to give concentra-
tions of 0.1-0.2 mM. Shortly after preparing the solutions (which
contained in addition 0-0.2 M benzene derivative and were deoxy-
genated by bubbling with argon), they were pumped through the 2 ×
4 mm Suprasil quartz cell (flow rates ca. 0.2-0.5 mL/min) and
photolyzed with 20-ns pulses of 308-nm light (ca. 5-40 mJ/pulse)
from a Lambda-Physik excimer laser. Prolonged standing of the
solutions leads to trapping of moisture with a concomitant reduction
of signal amplitude and lifetime. There is also a slow thermal
decomposition (half-lives several hours). The light-induced optical
transmission changes were digitized by Tektronix 7612 and 7912
transient recorders interfaced with a DEC LSI11/73+ computer which
also process-controlled the apparatus and on-line preanalyzed the data.
Final data analysis was performed on a Microvax I connected to the
LSI.
The quantum yields for depletion of the arenediazonium
cations were determined by measuring the pulse-induced
(negative) ∆OD changes (in a range of laser dose varying by a
factor of 5) at or close to the λmax of the diazonium absorption
bands and comparing them with the (positive) ∆OD obtained
at 320 nm on photolysis of a solution in acetonitrile of
acetophenone of the same absorbance at 308 nm as that of the
diazonium salt. With acetophenone, the transient absorbing at
320 nm is the triplet whose extinction coefficient is 12 600 M-1
cm-1 and which is formed with a quantum yield of unity.20 On
this basis, the quantum yields for destruction of the diazonium
cations R-C6H4N2+ were found to be 0.91 (R ) H), 0.92 (R )
Me), 0.96 (R ) MeO), 0.89 (R ) F), 0.93 (R ) Cl), and 0.91
(R ) Br), which are essentially twice the values measured for
(some of) the cations in water-methanol mixtures.21
The error in the quantum yields and extinction coefficients is
estimated as (10%.
For identification of photochemical products, a ≈1 mM solution of
the diazonium salt in HFIP was photolyzed in the presence of 0.05-
0.5 M benzene, toluene, or anisole with excitation at 300 nm in a
Rayonet reactor. The irradiated solutions were analyzed by GC and
GC-MS. Authentic samples of biphenyl, 2-methylbiphenyl, 3-meth-
ylbiphenyl, 4-methylbiphenyl, 4-methoxybiphenyl, fluorobenzene, 4-flu-
oroanisole (all from Aldrich), and phenyl 1,1,1,3,3,3-hexafluoroiso-
propyl ether and 3,5-dimethoxyphenyl 1,1,1,3,3,3-hexafluoroisopropyl
ether (isolated from scaled up reactions) served as standards to verify
and quantify the products. 4-Methoxyphenyl 1,1,1,3,3,3-hexafluoro-
isopropyl ether was identified only by GC-MS. For quantitative
studies, the GC response of the particular product was determined by
injecting known quantities of the standard material. Reaction of the
benzenediazonium ion with the isomer ratio determined at several
concentrations of toluene in the range 0.1-0.5 M was independent of
concentration. The measurement of kAr/kHFIP from the reaction of the
benzenediazonium ion was carried out with six concentrations of
benzene in the range 0.05-0.5 M in HFIP. The ratio [biphenyl]:
[PhOCH(CF3)2] was obtained by GC and was linear in benzene
concentration; the rate constant ratio kAr/kHFIP is obtained as the slope
of this plot. In all cases the reactions were carried out to approximately
complete conversion, and the total yield of products (HFIP ether,
fluorobenzene, and biaryls) was essentially quantitative.
To the solutions containing the diazonium ions were then
added alkylated benzenes, in particular, symmetric 1,3,5-
trisubstituted ones. As an example, in Figure 1 is shown the
+
result obtained on 308-nm photolysis of 4-MeO-C6H4N2 in
the presence of 1,3,5-triisopropylbenzene (TPB). In addition
to the pronounced depletion of the parent diazonium ion (at
≈310 nm), one now observes positiVe ∆OD at three wave-
lengths: approximately equally strong bands at 365 and 260
nm and a band (by a factor ≈ 10 stronger) at ≈200 nm (shown
in reduced size in Figure 1). These band positions and relative
intensities are very similar to those of the cyclohexadienyl-type
cations formed by protonation of these trialkylbenzenes (see
Table 1). The latter cations can be formed by proton addition
to the ground-state aromatic in superacids, where NMR studies
establish that they are 2,4,6-trialkylbenzenonium ions, i.e.,
arising from protonation at a nonalkylated position.22,23 These
Results and Discussion
1. Formation of R-C6H4+ Adducts to 1,3,5-Trialkyl- and
Hexamethylbenzenes and Their Optical Absorption Spectra.
(18) (a) McClelland, R. A.; Banait, N.; Steenken, S. J. Am. Chem. Soc.
1986, 108, 7023. (b) 1989, 111, 2935. (c) McClelland, R. A.; Kanagasa-
bapathy, V. M.; Steenken, S. Ibid. 1988, 110, 6913. (d) McClelland, R. A.;
Kanagasabapathy, V. M.; Banait, N.; Steenken, S. Ibid. 1989, 111, 3966.
(e) Steenken, S.; McClelland, R. A. Ibid. 1989, 111, 4967. (f) Schnabel,
W.; Naito, I.; Kitamura, T.; Kobayashi, S.; Taniguchi, H. Tetrahedron, 1980,
36, 3229. (g) Kobayashi, S.; Kitamura, T.; Taniguchi, H.; Schnabel, W.
Chem. Lett. 1983, 1117. (h) Van Ginkel, F. I. M.; Visser, R. J.; Varma, C.
A. G. O.; Lodder, G. J. Photochem. 1985, 30, 453. (i) Kobayashi, S.; Zhu,
Q. Q.; Schnabel, W. Z. Naturforsch. 1988, 43b, 825. (k) Minto, R. E.; Das,
P. K. J. Am. Chem. Soc. 1989, 111, 8858. (l) Johnston, L. J.; Lobaugh, J.;
Wintgens, V. J. Phys. Chem. 1989, 93, 7370. (m) Alonso, E. O.; Johnston,
L. J.; Scaiano, J. C.; Toscano, V. G. J. Am. Chem. Soc. 1990, 112, 1270.
(19) Vogel, A. J. Textbook of Practical Organic Chemistry, 4th ed.;
Longmans Green: London, 1978; pp 705-707.
(20) Lutz, H.; Breheret, E.; Lindqvist, L. J. Phys. Chem. 1973, 77, 1758.
(21) Becker, H. G. O.; Ebisch, R.; Israel, G.; Kroha, G.; Kroha, W.;
Brede, O.; Mehnert, R. J. Prakt. Chem. 1977, 319, 98. Becker, H. G. O.;
Fangha¨nel, E.; Schiller, K. Wiss. Zeitschr. Techn. Hochsch. Leuna-
Merseburg 1974, 16, 322. Schulte-Frohlinde, D.; Blume, H. Z. Phys. Chem.
(Frankfurt/M.) 1968, 59, 282, 299.
(22) (a) Dallinga, G.; Mackor, E. L.; Stuart, A. A. V. Mol. Phys. 1958,
1, 123. (b) Colpa, J. P.; MacLean, C.; Mackor, E. L. Tetrahedron 1963, 19
(Suppl. 2), 65. (c) Bakoss, H. J.; Ranson, R. J.; Roberts, R. M. G.; Sadri,
A. R. Tetrahedron 1982, 38, 623. (d) Smith, G. P.; Dworkin, A. S.; Pagni,
R. M.; Zingg, S. P. J. Am. Chem. Soc. 1989, 111, 525.

Making Photochemically Generated Phenyl Cations Visible
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11927
a
+
Table 1. Absorption Maxima λ1 and λ2 (nm) of the Cyclohexadienyl (CHD) Cations Formed by Addition of p-R-C6H4 to Alkyl-Substituted
+
Benzenes (Ar) in HFIP and Partitioning of R-C6H4 between Ar and HFIP
+
p-R-C6H4
R ) H
R ) Me
R ) OMe
R ) F
R ) Cl
R ) Br
λ1
H+ adduct
Arb
λ1
λ2
λ1
λ2
λ1
λ2
λ1
λ2
λ1
λ2
λ2
λ1
λ2
1,3,5-TMB
1,3,5-TEB
1,3,5-TPB
1,3,5-TBB
HMB
350
362
363
371
386
260
264
264
268
273
354
363
363
374
394
260
264
264
266
270
356
362
366
374
390
264
260
260
276
272
353
357
362
367
394
258
258
260
263
266
353
362
364
373
388
258
262
260
267
270
352
357
362
370
388
257
262
262
268
272
350c
360d
365d
370d
390c
260c
260
260
270
275c
e
kAr/kHFIP
R ) H
R ) Me
R ) OMe
R ) F
R ) Cl
R ) Br
1,3,5-TEB
140
72
150
100
190
100
73
150
120
80
220
120
84
110
80
60
1,3,5-TPB
1,3,5-TBB
HMB
34
42
60
30
90
40
benzene
7f
Apparent Quantum Yield for Formation of CHD Cationg
R ) H
R ) Me
R ) OMe
R ) F
R ) Cl
R ) Br
H+ adduct
1,3,5-TMB
HMB
0.27
0.18
0.31
0.23
0.32
0.26
0.28
0.20
0.51
0.47
0.35
0.26
0.08c
0.04c
a The λmax values are accurate to (3 nm. In all cases there is a third absorption band at λ ≈ 200 nm (see text and Figure 1). b 1,3,5-TMB )
1,3,5-trimethylbenzene; 1,3,5-TEB ) 1,3,5-triethylbenzene, 1,3,5-TPB ) 1,3,5,-triisopropylbenzene; 1,3,5-TBB ) 1,3,5-tri-tert-butylbenzene; HMB
) hexamethylbenzene. c From ref 24. See also: Cozens, F. Ph.D. Thesis, University of Toronto, 1991. d From Cozens, F. Ph.D. Thesis, University
of Toronto, 1991. e Value obtained from the competition analysis of the dependence on [Ar] (in the range 0 to ≈0.1 M) of the yield of CHD+, see
eq 6. Error limits (15%. Unitless, both kAr and kHFIP are expressed as second-order rate constants. f From the dependence on [benzene] of the
g
product ratio [biphenyl]:[C6H5OCH(CF3)2] determined by GC. Measured using 0.2 M Ar. The quantum yields are based on the assumption that
the ꢀ values at λ1 are the same as that of the H+ adduct of 1,3,5-TMB (ꢀ at 358 nm ) 9200 M-1 cm-1, ref 22b) or of HMB (ꢀ at 392 nm ) 10 400
M-1 cm-1, ref 22b), and using the acetophenone triplet for calibration (see text).
trialkylbenzenes, however, have no absorption at 308 nm, and
an experiment carried out under the same conditions as that of
Figure 1, but with no diazonium salt present, produced no
transient upon 308-nm excitation. This rules out the possibility
that the cyclohexadienyl-type spectrum obtained with the
diazonium ion arises by a photoprotonation mechanism.
The upper part (above the dashed line) shows positiVe bands
at 365, 260, and e200 nm (the 200-nm band is shown
compressed in size by the factor 10) and a strong negative band
at ≈310 nm. The part below the dashed line (OD scale is on
the left-hand side) is an expansion (by the factor 16) of the
positive ∆OD of the upper part. In the insets a and b are
presented the decays of the CHD cation at 260 and 365 nm.
Inset c show the dependence on [MeOH] of the rate of decay
of the cation measured at 365 nm.
Figure 1. Changes in OD (normalized to 100 mJ/laser pulse) observed
The absorptions were found to decay by first-order kinetics
at 250 ns after the pulse on 308-nm photolysis of an air-saturated 0.1
with the same rates in the regions 250-270 and 350-390 nm,
mM solution of 4-MeO-C6H4N2BF4 in HFIP at 20 ( 1 °C in the
consistent with a single transient. The rates were not influenced
presence of 50 mM 1,3,5-triisopropylbenzene. The upper part (above
by oxygen nor was the shape of the absorption spectrum altered,
evidence that there are no detectable radicals present in the
system. However, addition of small concentrations of nucleo-
philes/bases such as halides, alcohols, or ethers led to a drastic
reduction of the lifetime of the transient. This kinetic behavior
is characteristic for carbocations,17,18 including cyclohexadienyl
cations generated by photoprotonation in HFIP.24
the dashed line) shows positiVe bands at 365, 260, and e200 nm (the
200-nm band is shown compressed in size by the factor 10) and a strong
negative band at ≈310 nm. The part below the dashed line (OD scale
is on the left-hand side) is an expansion (by the factor 16) of the positive
∆OD of the upper part. In the insets a and b are presented the decays
of the CHD cation at 260 and 365 nm. Inset c shows the dependence
on [MeOH] of the rate of decay of the cation measured at 365 nm.
These observations, especially the absorption spectra that
make it clear22-24 that the cation is of the cyclohexadienyl
(CHD) type, suggest a mechanism involving photoheterolysis
of the diazonium ion (eq 1) followed by attachment of the aryl
cation to the trialkylbenzene to form a 1-aryl-2,4,6-trialkyl-
benzenonium ion (e.g., eq 2).
same cations with identical absorption spectra are produced also
upon 248-nm-light-induced protonation of the first excited
singlet state of the trialkylbenzene in the solvent HFIP.24,25 The
(23) For a review, see: Koptyuk, V. A. Arenium IonssStructure and
Reactivity. In Topics in Current Chemistry; Boschke, F. L., Ed.; Springer:
Berlin, 1984; Vol. 122, p 96.
(24) Steenken, S.; McClelland, R. A. J. Am. Chem. Soc. 1990, 112, 9648.
(25) In a manner similar to that observed with TMB,24 the H+ adducts
of TEB, TPB, and TBB have been produced by 248-nm photoprotonation
in HFIP (Cozens, F. Dissertation, University of Toronto, 1992).
To further test whether photohomolysis is a side reaction in
the dediazoniation, ≈0.5 mM solutions of 4-MeO-C6H4N2BF4
in HFIP (with 2% cyclohexane added as an H donor) were

11928 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Steenken et al.
(2)
photolyzed with a medium-pressure Hg lamp using a cutoff filter
at 300 nm, and after 10-50% conversion, the solutions were
analyzed by GC for anisole, the product expected from the
4-methoxyphenyl radical. No traces of anisole were found,
which is strong evidence that in HFIP decomposition of the
electronically excited 4-methoxybenzenediazonium cation does
not proceed by homolysis. Analogously, in the case of the 3,5-
dimethoxybenzenediazonium ion, photolysis in HFIP was found
to lead to the 3,5-dimethoxyphenyl ether of HFIP as the product
(characterized by both GC/MS and NMR) and no 3,5-
dimethoxybenzene, which again shows that photodediazoniation
occurs by heterolysis (analogous to eq 1).
Figure 2. Dependence on [TEB] of the yield (OD) of CHD cation
+
(measured at 360 nm) from the reaction of 4-ClC6H4 (from 308-nm
photolysis of 0.1 mM of the PF6- salt) with TEB in air-saturated HFIP,
squares. The filled circles contain the same data, however analyzed
+
according to competition (see text) for 4-ClC6H4 between HFIP and
TEB.
The first-order decay of the CHD cation is proposed to be
due to reaction with solvent (analogous to eq 5), the latter acting
as a Brønsted base. The CHD cation (the Brønsted acid)
deprotonates to give the corresponding phenyl-substituted
aromatic.26 This is concluded from product analysis data on the
case the transient had a band in the region 350-390 nm, one at
≈260-270 nm, and one at ≈200 nm.29 Consistent with the
presence of only one species, the rate of decrease of OD
(measured at 250-260 and 340-400 nm) was found to be in
all cases independent of wavelength. As shown in Table 1,
the spectroscopic data are characteristic for CHD cations.22-24
Examination of the effect of substituents on the position of
the bands reveals that the substituent R (on the phenyl cation)
has no influence, whereas substituents on the aromatic shift λmax
to higher wavelengths with increasing number or electron-
donating power (e.g., for the first band, λmax increases from 350
to 390 nm in going from TMB to HMB).
+
reaction when C6H5 is formed photochemically in HFIP
containing 0.1-0.5 M toluene. In addition to the ether PhOCH-
(CF3)2, the product from reaction with the solvent (eq 3), the
three methyl biphenyls are formed as products with an o:m:p
ratio of 55:22:23 (eqs 4 and 5). This isomer ratio indicates
that the phenyl cation is electrophilic but has only a low regio-
selectivity with toluene.27 When toluene was replaced by
benzene, biphenyl was a product. A similar experiment with
4-MeOC6H4+ in the presence of benzene yielded 4-MeOC6H4-
OCH(CF3)2 and 4-methoxybiphenyl (and 4-fluoroanisole, see
later).
2. Reactivity of 4-RC6H4+. As expected on the basis of
eqs 2 and 3, the yield of CHD cations was found to increase
with the concentration of the aromatic scavenger. Figure 2
(squares) shows a typical dependence of CHD+ optical density
on the concentration of the aromatic. The plot is not linear; at
higher concentrations of the scavenger the yield tends toward a
plateau. Such a dependence can be quantitatively explained in
terms of competition for the aryl cation Ar′+ between the
aromatic Ar (eq 4) and the solvent (eq 3). This leads to eq 6,
1/OD(CHD+) ) const + {[(const)kHFIP[HFIP]]/(kAr[Ar])}
(6)
where kHFIP and kAr are second-order rate constants for reaction
of the solvent and aromatic and const ) 1/(ꢀ(CHD+)d[Ar′+]).
In the latter term, ꢀ(CHD+) is the extinction coefficient of the
cyclohexadienyl cation, d is the path length of the cell (4 mm),
and [Ar′+] is the constant concentration of Ar′+ produced in
the pulse at fixed [Ar′N2+] and laser dose. Equation 6 predicts
that a plot of 1/OD(CHD+) vs 1/[Ar] should be a straight line
whose intercept if divided by its slope is equal to kAr/
(kHFIP[HFIP]).
Spectra similar to that shown in Figure 1 were observed on
photolysis of 4-MeO-C6H4N2+ in the presence of other benzene
derivatives such as mesitylene (1,3,5-trimethylbenzene (TMB)),
1,3,5-triethylbenzene (TEB), 1,3,5-triisopropylbenzene (TPB),
1,3,5-tri-tert-butylbenzene (TBB), and hexamethylbenzene (HMB)
(28) (a) Other alkylated benzenes such as toluene, the xylenes, and
additional polymethylated benzenes also give absorption spectra charac-
teristic of CHD cations. In some cases, there is evidence for the formation
of more than one isomer (Ashokkumar, M.; Steenken, S. Unpublished
results). (b) In the case of 4-FC6H4N2+, the strong absorption of the
diazonium ion at ≈260-280 nm makes it impossible to measure the
expected λ2 band of the CHD cation.
(29) Experimentally, the band at ≈200 nm is not easy to detect, due to
the very low levels of analyzing light intensity typically available at this
wavelength. The exact position of this band is therefore not known with
sufficient precision and thus not given in Table 1.
+
or on photolysis of the diazonium cations 4-R-C6H4N2 (R )
H, Me, Br, Cl) in the presence of these aromatics.28 In every
(26) In an analogous system, the rate of deprotonation from the cation
was found to be larger than that for addition of the (highly nucleophilic)
solvent water to the cation (see: Boyd, D. A.; McMordie, R. A. S.; Sharma,
N. D.; More O’Ferrall, R. A.; Kelly, S. C. J. Am. Chem. Soc. 1990, 112,
7822).
(27) For comparison, a considerably larger regioselectivity is shown by
the 9-fluorenyl cation in the reaction with toluene (o:m:p ratio ) 5:5:90).33

Making Photochemically Generated Phenyl Cations Visible
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11929
Figure 2 depicts such a competition plot for the system
4-ClC6H4+ plus TEB. From the slope of the straight line (full
circles) and the intercept and taking [HFIP] ) 9.5 M, the ratio
kAr/kHFIP is calculated to be 220. Values ranging between 30
and 290 thus determined for a series of phenyl cations and
aromatics are presented in the second section of Table 1.
The question arises as to whether the aryl cation that is
reacting here is in the ground state or in some vibrationally or
electronically excited state. If the assumption is made that the
aryl cation reacts with the aromatic with a rate constant ∼ 5 ×
109 M-1 s-1 (see later), the lifetimes of the aryl cations in this
system are on the order of a few nanoseconds. Barring some
unusual behavior of excited aryl cations, such lifetimes are
significantly long to ensure that the intermediate has reached
its ground state before capture by nucleophiles.
These data show that the parent phenyl cation (R ) H) is
the least selective (most reactive) ion and that Me, MeO, and
Cl increase the preference for reaction with the aromatic relative
to solvent. The effect is, however, not very large (factor e 2).
Since the substituent is remote from the reaction site, the
observed effect cannot be of steric but must be of electronic
nature. It is interesting that not only MeO but also Cl appear
to act as electron-donating groups with respect to the positive
charge in the σ-orbital of the phenyl cation. This has been
predicted on theoretical grounds7 on the basis of through-bond
interaction of the lone pairs of the substituent with the σ-orbital
of the cationic site.30 In contrast to Cl is the effect of F and of
Br: These atoms do not increase the selectivity of the phenyl
cation with respect to Ar and HFIP. It is notable that a strongly
destabilizing effect predicted8 for F does not show up in the
reactivity of the cation.
The preference of the phenyl cations to react with the aromatic
relative to HFIP decreases in going from 1,3,5-trimethyl- to the
corresponding isopropyl and tert-butyl compounds. This is
likely caused by the increasing steric hindrance in this series
which dominates over the increasing electron-donating effect
of the alkyl substituents. The fully methylated compound HMB
is the most electron-rich (on the basis of the ionization
potentials31) or most basic32 compound of the series and is
therefore expected to react most rapidly if the electron density
or basicity were decisive. The aryl cation however has to add
to an ipso position, i.e., an already occupied position. The steric
hindrance exerted by the methyl group at the reaction site slows
the addition reaction down to the extent that the resulting rate
is a factor ≈4 lower than in the case of the less electron-rich or
less basic but sterically more accessible TMB.
As a further check of the competition data, the quantum yields
for formation of CHD cation were determined by measuring
the OD changes at the CHD+ λmax values, using the acetophe-
none triplet for actinometry (see section 1) and assuming that
the extinction coefficients of the phenyl-CHD+’s are the same
as those22,24 of the corresponding proton adducts. The values
found are listed in the last section of Table 1. They are between
0.2 and 0.5, i.e., a factor of 2-3 lower than expected on the
basis of the kAr/kHFIP ratios and the scavenging capacity of Ar
at 0.2 M. There is one simple explanation possible for this
imperfection: The extinction coefficients of the phenyl-CHD
cations are lower than those22,24 of the corresponding H+
adducts. An additional possibility is that there is ion pairing
Figure 3. Dependence of the quantum yield for depletion of
+
4-MeOC6H4N2 (squares) and the yield of CHD cation (measured by
+
the ∆OD at 365 nm) from the reaction of 4-MeOC6H4 with 0.2 M
TPB in HFIP-n-BuCl mixtures (full circles). Inset a shows the kinetics
+
of depletion of 4-MeOC6H4N2 and inset b that for formation of the
CHD+ on the same time scale, both in in pure HFIP. Inset c shows the
slow additional buildup (2 µs/div) of the CHD cation from 4-MeOC6H4+
and 0.2 M TPB in 80% n-BuCl/20% HFIP, see footnote 38.
+
-
between Ar′N2 and BF4 (or PF6-) and that some of the
photochemically produced phenyl cations are scavenged by the
counteranion before they react with the aromatic to give the
CHD cation. In support of this is the observation in yields of
20-25% of the product C6H5F from the parent or 4-MeOC6H4F
from the 4-methoxybenzenediazonium salt. (The possibility also
exists that these fluorinated products are derived from fluoride
extraction from the solvent.)
An attempt was made to measure the rate constants kAr
directly by monitoring the rate of buildup of CHD cations at
low concentrations of Ar. Down to concentrations of ≈10 mM,
the rates were found to be ≈5 × 107 s-1, essentially independent
of [Ar]. The value of 5 × 107 s-1 corresponds to the reciprocal
of the laser pulse width (20 ns) and thus reflects the effective
time resolution of the apparatus (see Figure 3, insets a and b,
for an example). At concentrations below 10 mM the signal
amplitudes (see, e.g., Figure 2) were not sufficiently strong for
reliable analysis. Taking kobsd for the formation of CHD+ at
10 mM Ar to be g5 × 107 s-1, kAr results as g5 × 109 M-1
s-1. This number is reasonable considering the value of 1.9 ×
109 M-1 s-1 directly measured14c for reaction of the more
stabilized and sterically more demanding 9-fluorenyl cation with
TMB. The aryl cations are likely reacting at the diffusion limit
(or at least very near it), and thus, a value of 5 × 109 is not
unexpected. Taking 5 × 109 for kAr, 100-200 for kAr/kHFIP
,
and [HFIP] in HFIP ) 9.5 M, the lifetimes of the aryl cations
are 2-4 ns,33 shorter than the laser pulse with our instrument.
It is therefore not surprising that transients11 are not seen on
photolysis of the diazonium salts in the absence of the aromatic
scavengers.34
3. Solvent Effects. The substituted benzenediazonium ions
were also photolyzed in other solvents, such as 2,2,2-trifluo-
(33) (a) With lifetimes of this order of magnitude, most of the reaction
of the cation with the added nucleophile is occurring through the free ion,
and preassociation of the nucleophile with the precursor does not account
for much of the trapping. For example, the 4-chloro-N-acetylnitrenium ion
is estimated to have a lifetime (in water) of 0.6 ns; around 75% of its reaction
with added azide ion is still occurring through the free ion.33b (b) Novak,
M.; Kahley, M. J.; Lin, J.; Kennedy, S. A.; James, T. G. J. Org. Chem.
1995, 60, 8294.
(30) It is also possible that the substituent effect is due to changes in the
spin multiplicity of the cation, cf. refs 11 and 12.
(31) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys.
Chem. Ref. Data 1977, 6 (Suppl. 1), 1.
(32) See ref 22d. For a review on basicity of aromatics, see: Perkampus,
H.-H. AdV. Phys. Org. Chem. 1966, 4, 195.

11930 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Steenken et al.
Table 2. Rate Constantsa (M-1 s-1) for Reaction of 1-(p-R-C6H4)Benzenium Ions R-C6H4-Ar+ with Bases/Nucleophiles in HFIP at 20 ( 1 °C
R
Ar
HFIPb
MeOH
EtOH
2-PrOH
tBuOH
THF
1,4-diox.
Br-
H
H
H
H
Cl
Cl
Cl
Cl
Br
Br
Br
Br
2,4,6-Me3
2,4,6-Et3
2,4,6-iPr3
2,4,6-tBu3
2,4,6-Me3
2,4,6-Et3
2,4,6-iPr3
2,4,6-tBu3
2,4,6-Me3
2,4,6-Et3
2,4,6-iPr3
2,4,6-tBu3
1.2 × 105 (1 × 105)c
4.5 × 104 (7.4 × 104)d
4.5 × 104 (3.3 × 104)d
1.6 × 106 (1.4 × 104)d
1.5 × 105
1.2 × 107
1.0 × 107
1.3 × 107
1.4 × 107
1.9 × 107
1.2 × 107
1.5 × 107
1.7 × 107
1.7 × 107
1.5 × 107
1.0 × 107
1.7 × 107
8.3 × 106
8.5 × 106
3.9 × 107
1.7 × 107
1.0 × 107
1.4 × 107
3.5 × 107
5.1 × 106
3.0 × 106
1.7 × 106
1.0 × 107
7.1 × 106
3.5 × 106
4.4 × 106
1.1 × 107
3.5 × 109
2.7 × 109
2.9 × 109
6.5 × 109
5.2 × 109
3.3 × 109
4.9 × 109
5.1 × 109
2.1 × 107
1.7 × 107
1.3 × 107
5.3 × 104
4.8 × 104
1.7 × 106
2.2 × 105
6.2 × 104
4.5 × 104
2.5 × 106
MeO
MeO
MeO
MeO
H
Cl
Br
2,4,6-Me3
2,4,6-Et3
2,4,6-iPr3
2,4,6-tBu3
(Me)6
(Me)6
(Me)6
1.6 × 105
4.5 × 104
4.2 × 104
1.8 × 106
4.8 × 103
3.8 × 103
4.5 × 103
4.8 × 103
1.3 × 107
1.1 × 107
1.4 × 107
1.8 × 107
1.0 × 106
1.3 × 106
1.9 × 107
4.1 × 106 e
1.5 × 107
1.0 × 107
1.6 × 107
8.0 × 106
1.4 × 107
3.8 × 107
1.1 × 106
1.9 × 106
5.2 × 106
3.6 × 106
4.6 × 106
1.1 × 107
3.8 × 105
3.3 × 105
5.3 × 109
3.4 × 109
4.7 × 109
3.7 × 109
8.8 × 108
6.6 × 108
6.5 × 108
3.1 × 107 f
g1.7 × 108 g
1.7 × 109 h
MeO
(Me)6
1.1 × 106
2.0 × 106
3.9 × 105
a Determined at the λmax values of the cations (see Table 1). Error limits (15%. b ks. Units of s-1 c Rate constant for decay of H+ adduct of Ar,
.
obtained24 by photoprotonation. d Rate constant for decay of H+ adduct of Ar, obtained39 by photoprotonation. e Value refers to reaction with H2O
and was determined in the [H2O] range 0-0.1 M. f Value refers to F-. g Value refers to Cl-. h Value refers to I-.
roethanol (TFE), water, acetonitrile, dimethyl sulfoxide, tet-
rahydrofuran, 1,4-dioxane, cyclohexane, dichloromethane, 1,2-
dichloroethane, or n-butyl chloride. For the case of R ) MeO,
the quantum yields for depletion of the diazonium cation were
determined as described previously. The most acidic and least
nucleophilic solvent HFIP (pKa ) 9.3)35 is superior to the others
in promoting the photoinduced decomposition (φ ) 0.96,
determined via the depletion of the arenediazonium ions, vide
supra). The less acidic and more nucleophilic TFE (pKa )
12.4)35 is a factor of 2 less efficient (φ ) 0.52).36 A similar
value is obtained for water, for which the quantum yield (0.54)
is in excellent agreement with that21 obtained byproduct analysis
techniques. Since water is even less acidic and more nucleo-
philic than TFE, it is probably the higher polarity which supports
decomposition. In the polar solvent acetonitrile, photochemical
decomposition is also quite efficient (φ ) 0.30), as is the case
for dimethyl sulfoxide (0.42) and dioxane (0.45). In contrast,
in the nonpolar solvents cyclohexane and n-butyl chloride, the
diazonium cation appears to be photostable. No decrease of
OD was seen at 320 nm on photolysis (φ ≈ 0).37
photolyzed. A relatively weak transient was seen with λmax
≈
360 nm, which reacted rapidly with O2 but not with ethanol.
This is a behavior opposite to that observed in HFIP. The
conclusion is that this transient is a radical but not a cation. If
+
it is assumed that (a) MeOC6H4 is produced in TFE, (b) it
reacts with TPB with k g 5 × 109 M-1 s-1, and (c) the resulting
CHD+ has a lifetime g1 µs, the failure to see a CHD+ means
+
that the lifetime of MeOC6H4 in TFE is e200 ps.
4. Reactions of Phenylcyclohexadienyl Cations with Bases.
These reactions, eq 7b, were studied by adding small amounts
Figure 3 shows the effect on the quantum yield for depletion
+
of 4-MeOC6H4N2 and on the yield of CHD cation from the
(7)
+
reaction of 4-MeOC6H4 with TPB when increasing amounts
of n-butyl chloride are added to HFIP solutions of the reactants.
The n-butyl chloride suppresses the decomposition of the
diazonium cation very strongly and inhibits the formation of
the CHD cation even more efficiently.38
of alcohols, ethers, or bromide (as a tetra-n-butylammonium
salt) to the solutions containing a benzenediazonium ion and
one of the aromatic derivatives. Such addition resulted in (a)
An attempt was also made to produce CHD cations in TFE,
which is a stronger nucleophile than HFIP. For this purpose, a
+
(38) In mixtures containing, e.g., 80% n-butyl chloride, there is a
considerably delayed, first-order further formation of CHD+ on top of a
rapidly formed component. The rate of this delayed process was found to
be independent of the concentration of the aromatic in the range 0.09-1
M. The nature of this reaction, which was also seen with other aromatics
and other diazonium ions and in other chlorinated hydrocarbon solvents, is
presently unknown. In this connection, it may be interesting that benzene-
diazonium cations can form charge-transfer (CT) complexes with aromatics
(Koller, S.; Zollinger, H. HelV. Chim. Acta 1970, 53, 78. Becker, H. G. O.;
Schukat, G.; Kuzmin, M. G. J. Prakt. Chem. 1975, 317, 229), the formation
constants of which increase with decreasing polarity of the solvent. However,
our attempts to see CT complexes in HFIP were unsuccessful.
0.2 mM 4-MeOC6H4N2 solution containing 1 M TPB was
(34) It is likely that the extinction coefficients of phenyl cations are very
low, based on analogy with phenyl radicals. This decreases further the
chance of detecting the cations optically. Triplet-state phenyl cations have
recently been determined11 to have lifetimes of e15 ps.
(35) See: Schrems, O.; Oberhoffer, H. M.; Luck, W. A. P. J. Phys. Chem.
1984, 88, 4335.
(36) A similar value (φ ) 0.45) has been measured for decomposition
of 4-morpholinobenzenediazonium tetrafluoroborate (ref 11).
(37) The high value (φ ) 0.52) found for the weakly polar solvent
tetrahydrofuran is noticeable.

Making Photochemically Generated Phenyl Cations Visible
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11931
a decrease in the amplitude of the signal due to CHD cation
(measured ≈ 50 ns after the pulse) and (b) an increase in the
rate of decay of the CHD cation. The reduction in amplitude
is due to scavenging of the phenyl cation by the added reagent
presumably acting as a competing nucleophile. The increase
in rate must represent reaction directly with the CHD cation,
in most cases a simple deprotonation. The experimentally
observed rate of decay increased linearly with the concentration
of the base B (see inset c of Figure 1). From the slopes of the
plots of kobsd vs [B] the second-order rate constants for reaction
of the CHD cations with B were obtained. These values and
those for reaction of the CHD+’s with the solvent HFIP (kS,
see, e.g., eq 5) are collected in Table 2.
From an inspection of Table 2 the following statements can
be made and conclusions drawn:
1. The kS values are essentially independent of the substituent
R on the former phenyl cation. This is expected since a potential
electronic effect of R is insulated by the sp3-hybridized carbon
of the CHD cation.
lower than that of alcohols and ethers, this high reactivity
suggests that that Br- does not react as a base but rather as a
nucleophile, i.e., by addition to the cation. This has also been
observed in a previous study.24 No stable product incorporating
bromide has been observed, either in this work or the previous
one. The adduct with bromide would be a bromo-substituted
cyclohexadiene, and this likely rearomatizes at longer times by
eliminating HBr.
The values for the HMB-containing systems (≈7 × 108 M-1
s-1) are clearly below diffusion-controlled. It is interesting that
the cation exhibits selectivity toward the halides: The reactivity
(F- < Cl- < Br- < I-) increases with increasing nucleophilicity
but decreasing basicity (see the lower end of the last column of
Table 2).
5. The reactivity patterns of the CHD cations produced by
addition of a (substituted) phenyl cation to 1,3,5-trimethylben-
zene and to hexamethylbenzene are Very similar to those24 of
the corresponding H+ adducts, which means that replacement
of H at the methylene group of these CHD cations by phenyl
has little influence on reactivity. Whether this is also true for,
e.g., the corresponding tri-tert-butyl compounds is not known
since the nucleophilic reactivity of its H+ adduct has not yet
been studied. It is interesting that the reactivities with the
alcohols MeOH, EtOH, 2-PrOH, and tBuOH of the phenyl-CHD
cations from TMB are on average a factor of ≈6 larger than
that14c of the CHD cation produced by addition of the 9-fluorenyl
cation to TMB.
2. With respect to the substituents on the aromatic trap, the
reactivities kS with the weak nucleophile HFIP follow the order
i
Me > Et ∼ Pr < tBu. The rate constant decreases by a factor
of ≈3 in going from Me to Et, is essentially unchanged for iPr,
t
and then increases by a factor of 30-40 for Bu. Proceeding
from Me to tBu is expected to provide a small stabilization on
t
electronic grounds, Bu being slightly more electron donating
than Me. This however would be counterbalanced by a decrease
in planarity of the dienyl system due to the steric interaction of
the aryl group at C1 of the benzenium ion and the alkyl groups
at the two ortho positions. The result is a decrease in stability,
5. Summary and Conclusions. It has been demonstrated that
308-nm photolysis of a series of 4-substituted benzenediazonium
ions in the polar, acidic, and weakly nucleophilic solvent HFIP
leads to their decomposition with quantum yields close to unity.
This process is mainly if not completely a heterolysis41 to phenyl
cations which can be scavenged by aromatics giving cyclo-
hexadienyl cations. From experiments involving competition
for phenyl cations between HFIP and aromatics, it is deducible
that the substituents Me, MeO, and Cl stabilize the phenyl cation,
although this effect is not very strong (factor e 2). Such a
conclusion has so far not been possible since the substituent
effect on phenyl cation stability cannot be discerned from
solvolysis data of benzenediazonium ions.42 From competition
data relating to different 1,3,5-trialkylated benzenes, it is evident
that addition of the phenyl cations to the aromatics is subject
to steric hindrance which dominates over the counteracting
electronic influences.
The CHD cations produced by covalent bond formation
between the phenyl cation and the aromatic have characteristic
absorption spectra with bands at ≈200, 260-270, and 350-
390 nm and show a high reactivity with bases/nucleophiles such
as alcohols, ethers, and halides (k values of 106 to ≈5 × 109
M-1 s-1). With respect to both spectral and reactivity properties,
the “phenyl adducts” are very similar to the correspond-
ing22,23,24,28 “proton adducts”.
i
t
so that Pr and especially Bu have enhanced reactivity. In
agreement with this, kS values for the proton adducts (where
i
there is no steric interaction) follow the order Me > Et > Pr
> tBu (Table 2), as expected on the basis of only the electronic
effect.
There is a decrease in reactivity by a factor of ≈30 in going
from TMB to HMB. There are several possible explanations
for this. The cation from HMB is stabilized electronically since
it has more methyl substituents. There may also be a steric
effect, the HMB cation allowing decreased accessibility by
bases. In addition, the HMB cation cannot deprotonate directly
from the benzenium ring, although it could lose a proton from
one of the methyls to give a methylene-substituted cyclohexa-
diene. This process does not have the driving force of the
aromatization that occurs when the TMB cation loses its proton.
i
3. The trend with the alcohols (MeOH ∼ EtOH ∼ PrOH ∼
tBuOH) is the one expected for these compounds reacting as
bases; the order when these react as nucleophiles with carboca-
tions is MeOH > EtOH > iPrOH > tBuOH, with over an order
of magnitude decrease across the series.40 The similar reactivity
with tetrahydrofuran is also not surprising, since this has a
similar basicity. In contrast, the less basic 1,4-dioxane has rate
constants that are a factor of 3-5 lower. As in the case of
reaction with the solvent, the CHD cation from HMB reacts
less rapidly with MeOH, THF, and 1,4-dioxane.
Acknowledgment. We thank Marion Stapper for her com-
petent technical help.
4. With Br-, all systems except HMB gave rate constants
of ≈5 × 109 M-1 s-1, a value which is probably close to the
diffusion-controlled limit. Since the basicity of Br- is much
JA980712D
(41) A similar conclusion has been drawn11 for similar benzenediazonium
(39) Cozens, F. Unpublished results.
(40) McClelland, R. A. Tetrahedron 1996, 52, 6283.
salts.
(42) For a review of the solvolysis data, see ref 9a.