Generation and trapping of the 4-biphenylyloxenium ion by water and azide: Comparisons with the 4-biphenylylnitrenium ion

Article abstract of DOI:10.1021/ja047488e

Generation of the 4-biphenylyloxenium ion, 1a, from hydrolysis of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, is demonstrated by common ion rate depression and azide trapping. The ion is less selective with a shorter lifetime (12 ns at 30 °C) in aqueous

Full text of DOI:10.1021/ja047488e

Published on Web 06/05/2004
Generation and Trapping of the 4-Biphenylyloxenium Ion by Water and
Azide: Comparisons with the 4-Biphenylylnitrenium Ion
Michael Novak*,† and Stephen A. Glover§
Department of Chemistry and Biochemistry, Miami UniVersity, Oxford, Ohio 45056, and DiVision of Chemistry,
School of Biological, Biomedical, and Molecular Sciences, UniVersity of New England,
Armidale, 2351, New South Wales, Australia
Received April 29, 2004; E-mail: novakm@muohio.edu
Mechanistic investigations of aryloxenium ions, 1, have been
limited.1-5 This is surprising because they have been invoked to
explain the products of electrochemical and chemical oxidations
of phenols,6-10 and the generation of useful polymers such as poly-
Figure 1. Dependence of log kobs for 2a and 2b on pH.
(2,6-dimethyl-1,4-phenylene oxide).11,12 Some stable, highly delo-
calized 1 have been characterized,9,10 but there are discrepancies
over the regiochemistry of reactions of transient 1 generated from
different sources, and the possible involvement of triplet ions.1,3,5,7
There is controversy over whether these ions are involved in all
the reported reactions.1,3,8,11,12 Only one detailed study of the aque-
ous solution chemistry of a sterically hindered transient exam-
ple has appeared.5 It is difficult to determine which of the reported
examples are genuine or how substituents affect the chemistry of
1.
We have initiated a study of 1 that is not highly stabilized by
extensive delocalization or by steric hindrance. Herein we describe
the chemistry of the transient ion 1a generated from 2a, and the
possible generation of the much less stable 1b from 2b. The
Figure 2. Common ion rate depression for 2a.
Scheme 1
reactivity, selectivity, and calculated properties of 1a and 1b are
compared to those of related nitrenium and carbenium ions.
Hydrolysis kinetics monitored by UV absorbance for 2a (30 °C)
and 2b (80 °C) were first order from pH 1 to 8 in 5 vol % CH3-
CN-H2O, µ ) 0.5 (NaClO4). The pH dependence of kobs is shown
in Figure 1. Kinetic data were fit to eq 1. The third term, kOH, is
only observed for 2b. The first term, ko, corresponds to spontaneous,
uncatalyzed decomposition of the ester. The nature of this reaction
will be described here. The acid- and base-catalyzed reactions will
be described elsewhere.13 Extrapolation of ko for 2a to 80 °C (4.5
× 10-3 s-1) shows that at 80 °C spontaneous decomposition of 2a
is 8.4 × 103-fold more rapid than that of 2b.
2). No rate depression is observed in formate or phosphate buffers.
kobs ) ko + kH[H+] + kOH[OH-]
(1)
The ratio kOAc/ks was determined from the dependence of kobs on
[OAc-]. In phosphate buffer at pH 7.0, addition of N3 leads to
-
In the absence of strong nonsolvent nucleophiles the decomposi-
tion product of 2a or 2b is the quinol 3a or 3b. These could be
derived by solvent trapping of 1a or 1b or by ester hydrolysis.
formation of 4 and 5 at the expense of 3a, with no change in the
magnitude of kobs. The ratio kaz/ks was determined from the
dependence of the yield of 3a on [N3-] (Figure 3).
-
Common ion rate depression and N3 trapping show that under
-
At 20 °C the related nitrenium ion 6a is trapped by N3 with a
conditions in which ko dominates, decomposition of 2a proceeds
via an oxenium ion path (Scheme 1). Hydrolysis proceeds with a
common ion rate depression in 1/1 HOAc/OAc- at pH 4.7 (Figure
diffusion-controlled rate constant, kaz, of 5.0 × 109 M-1 s-1, and
kaz/ks is 2.9 × 103 M-1 by both azide-clock and direct measure-
ments.14,15 Correction of kaz for 6a to 30 °C provides a value of
6.5 × 109 M-1 s-1, and for kaz/ks a value of 2.1 × 103 M-1 16,17
.
At
† Miami University.
-
§ University of New England.
30 °C 6a is ca. 27-fold more selective for reaction with N3 in
9
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J. AM. CHEM. SOC. 2004, 126, 7748-7749
10.1021/ja047488e CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
6a and 10a are nearly equivalent (1.2607 vs 1.2610 Å). The scaled22
HF stretching frequencies for the CdX bonds in 1a, 3a, 6a, and
10a are 1812.1, 1788.8, 1671.5, and 1731.5 cm-1, respectively. The
data suggest that 1a has a more distinctly “quinoidal” structure than
6a, resulting in appreciably more charge delocalization into the
distal ring. This is consistent with the 18% yield of 5. The calculated
∆E for the isodesmic reaction of eq 2 for 1a at HF level, pBP/
DN* level with ZPE corrections, and at pBP/DN* level with ZPE
and thermodynamic corrections are -12.2, -12.3, and -11.0 kcal/
mol, respectively. This suggests that 6a is considerably more stable
than 1a relative to their hydration products. This agrees with the
lifetimes of the two cations in H2O. For 1b the calculated ∆E are
-18.9, -19.2, and -18.4 kcal/mol, at the same levels as above,
indicating that stabilization by a π-donor is more important to
aryloxenium ions than to arylnitrenium ions. Results of more
detailed calculations will be presented elsewhere.13 Further work
will delineate in more detail substituent effects on aryloxenium ion
stability, and will attempt to determine whether these ions are
actually involved in previously reported cases.
Figure 3. Dependence of yield of 3a on [N3-].
H2O than is 1a. Reaction of 6a with N3- generates only 7,15 while
1a generates a ca. 18% yield of the product of attack on the distal
ring, 5, in addition to the major product 4 analogous to 7.
If kaz for 1a at 30 °C is diffusion-limited at 6.5 × 109 M-1 s-1
,
ks and kOAc for 1a are estimated as 8.4 × 107 s-1 and 2.8 × 108
M-1 s-1, respectively. The lifetime of 1a (1/ks) is ca. 12 ns. This
is considerably shorter than ∼0.3 µs, the estimated lifetime of 6a
at 30 °C, and its observed lifetime, 0.6 µs at 20 °C.14 However,
the lifetime of 1a is much longer than those estimated for 8 and 9,
0.1 and 0.5 ns, respectively, in 1/1 TFE/H2O at 20-25 °C.15,18 It
appears that in H2O aryloxenium ions are intermediate in stability
between nitrenium and carbenium ions of similar structure. The
only other reactive aryloxenium ion for which we know of a lifetime
estimate in H2O is 1c. The azide-clock estimate for this ion is 0.55
µs at 25 °C.5 Steric hindrance to nucleophilic attack must play a
considerable role in stabilizing this species. An estimate of the
equilibrium constant for the formation of 1a from 2a in aqueous
media, K+ ) ko/kOAc, is 8.9 × 10-14 M at 30 °C.
Acknowledgment. M.N. thanks Miami University for a sab-
batical leave and The University of New England for providing
facilities at which this work was accomplished.
Supporting Information Available: Experimental details, char-
acterization of 2a,b, 3a,b, 4, and 5, hydrolysis rate constants for 2a,b,
and product quantification. This material is available free of charge
via the Internet at http://pubs.acs.org.
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104-fold larger than ko for 2b, and it is likely that kOAc for both
ions at 80 °C approaches the diffusion controlled limit, K+ for 1a
must be at least 104-fold larger than for 1b. 6b is also much less
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