Mechanism of reaction of an arenediazonium ion in aqueous solutions of acetamide, N-methylacetamide, and N,N-dimethylacetamide. A potential method for chemically tagging peptide bonds at aggregate interfaces

Article abstract of DOI:10.1021/ja980556a

The mechanism of dediazoniation of 2,4,6-trimethylbenzenediazonium ion, 1-ArN₂⁺, in concentrated aqueous solutions of acetamide, N-methylacetamide, and N,N-dimethylacetamide (peptide bond models) was probed by a combination of techniques including HPLC, GC/MS, and H₂¹⁸O isotopic labeling. The kinetics and product distributions are completely consistent with the heterolytic dediazoniation mechanism, i.e., rate-determining loss of N₂ followed by trapping of the aryl cation intermediate, 1-Ar+, by H₂O and the oxygens and nitrogens of the amides. Aryl imidates formed from trapping by amide O hydrolyze rapidly into aryl ester/amine and amide/phenol product pairs. The results were used to estimate the selectivity of 1-Ar+ toward the amide oxygens and nitrogens versus H₂O. 1-Ar+ is only 10-40% more selective toward H₂O than amide O, but it is more than 10 times more selective toward H₂O than the amide N. 1-Ar+ is slightly more selective toward the N of acetamide than N-methylacetamide. However, within the HPLC detection limit, 1-At+ does not give a product from reaction with the N,N-dimethylacetamide nitrogen. The selectivities are interpreted by using a preassociation model, i.e., selective solvation by the different nucleophiles of the reactive diazonio group in the ground state. These results indicate that chemical tagging (trapping by N) and cleaving (trapping by O) of the peptide bonds and the weakly basic side chains of polypeptides and proteins bound to association colloids, vesicles and biomembranes, and emulsions may provide new information on their topologies and orientations at the aggregates' interfaces.

Full text of DOI:10.1021/ja980556a

10046
J. Am. Chem. Soc. 1998, 120, 10046-10054
Mechanism of Reaction of an Arenediazonium Ion in Aqueous
Solutions of Acetamide, N-Methylacetamide, and
N,N-Dimethylacetamide. A Potential Method for Chemically Tagging
Peptide Bonds at Aggregate Interfaces
Laurence S. Romsted,* Jianbing Zhang, and Lanzhen Zhuang‡
†
Contribution from the Department of Chemistry, Wright and Rieman Laboratories, Rutgers,
The State UniVersity of New Jersey, New Brunswick, New Jersey 08903
ReceiVed February 18, 1998. ReVised Manuscript ReceiVed July 7, 1998
+
Abstract: The mechanism of dediazoniation of 2,4,6-trimethylbenzenediazonium ion, 1-ArN2 , in concentrated
aqueous solutions of acetamide, N-methylacetamide, and N,N-dimethylacetamide (peptide bond models) was
1
8
probed by a combination of techniques including HPLC, GC/MS, and H2 O isotopic labeling. The kinetics
and product distributions are completely consistent with the heterolytic dediazoniation mechanism, i.e., rate-
determining loss of N2 followed by trapping of the aryl cation intermediate, 1-Ar+, by H2O and the oxygens
and nitrogens of the amides. Aryl imidates formed from trapping by amide O hydrolyze rapidly into aryl
ester/amine and amide/phenol product pairs. The results were used to estimate the selectivity of 1-Ar+ toward
the amide oxygens and nitrogens versus H2O. 1-Ar+ is only 10-40% more selective toward H2O than amide
O, but it is more than 10 times more selective toward H2O than the amide N. 1-Ar+ is slightly more selective
toward the N of acetamide than N-methylacetamide. However, within the HPLC detection limit, 1-Ar+ does
not give a product from reaction with the N,N-dimethylacetamide nitrogen. The selectivities are interpreted
by using a preassociation model, i.e., selective solvation by the different nucleophiles of the reactive diazonio
group in the ground state. These results indicate that chemical tagging (trapping by N) and cleaving (trapping
by O) of the peptide bonds and the weakly basic side chains of polypeptides and proteins bound to association
colloids, vesicles and biomembranes, and emulsions may provide new information on their topologies and
orientations at the aggregates’ interfaces.
Introduction
zenediazonium ion.7 The logic of chemical trapping is grounded
in the pseudophase model of aggregate effects on chemical
We have developed a novel method for estimating the
compositions of the interfacial regions of amphiphilic aggregates
based on product yields from chemical trapping by an aggregate-
bound arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzene-
diazonium ion, 16-ArN2 . Recently the chemical trapping
method was used to estimate chloride ion concentrations at the
surfaces of zwitterionic phospholipid micelles and vesicles,
hydration numbers of nonionic micelles, alcohol distributions
in reverse microemulsions, halide concentrations at the surfaces
8
,9
reactivity.
The success of the method stems from the low
selectivity of dediazoniation reactions toward weakly basic
nucleophiles such that reasonable product yields are obtained
from competing reactions with each nucleophile, including H2O,
+
1
1
present in the interfacial region. Arenediazonium ions are
2
known to react with very weak nucleophiles, including N2 and
1
0
3
CO, but an extensive search of the current literature and
1
1-19
4
reviews and classic texts
for information on reactions of
5
of anionic micelles, and, with the short-chain analogue,
+
6
(7) Bravo-Diaz, C.; Romsted, L. S.; Harbowy, M.; Romero-Nieto, E.;
Gonzalez-Romero, E. J. Phys. Org. Chem., in press.
1
-ArN2 , degrees of ionization of cationic micelles. The basic
protocols were also used to obtain new information on the pH
dependence of CuCl2-catalyzed dediazoniation of p-nitroben-
(
8) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem.
Res. 1991, 24, 357.
9) Bunton, C. A.; Yao, J.; Romsted, L. S. Curr. Opin. Colloid Interface
(
*
To whom correspondence should be addressed. E-mail: romsted@
Sci. 1997, 2, 622.
(10) Zollinger, H. J. Org. Chem. 1990, 55, 3846.
(11) Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic
Compounds; VCH Publishers: Weinheim, 1994; p 1.
(12) Zollinger, H. Acc. Chem. Res. 1973, 6, 335.
rutchem.rutgers.edu. Web: http://chmwww.rutgers.edu/faculty/romsted/
romsted.html.
†
E-mail: jiazhang@rutchem.rutgers.edu.
Current address: Columbia University, 630 W. 168th St., Room 11-
‡
4
01, New York, NY 10032. E-mail: lz90@columbia.edu.
1) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem.
Soc. 1993, 115, 8351.
2) Jain, M. K.; Rogers, J.; Yu, B.-Z.; Yao, J.; Romsted, L. S.; Berg, O.
G. Biochemistry 1997, 36, 14512.
(13) Zollinger, H. Angew. Chem., Intl. Ed. Engl. 1978, 17, 141.
(14) Hegarty, A. F. In The Chemistry of the Diazonium and Diazo
Groups, Part 2; Patai, S., Eds.; John Wiley & Sons: New York, 1978; p
511.
(15) Wulfman, D. S. In The Chemistry of Diazonium and Diazo Groups,
Part 1; Patai, S., Ed.; John Wiley and Sons: New York, 1978; p 247.
(16) Ambroz, H. B.; Kemp, T. J. Chem. Soc. ReV. 1979, 8, 353.
(17) Zollinger, H. In Supplement C: The Chemistry of Triple-Bonded
Functional Groups, Part 1; Patai, S., Rappoport, Z., Eds.; John Wiley &
Sons: New York, 1983; p 603.
(
(
(
3) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425.
(4) Yao, J.; Romsted, L. S. Colloids Surf. A: Physicochem. Eng. Aspects
1
997, 123-124, 89.
5) Cuccovia, I. M.; Agostihno-Neto, A.; Wendel, C. M. A.; Chaimovich,
H.; Romsted, L. S. Langmuir 1997, 13, 5302.
6) Cuccovia, I. M.; da Silva, I. N.; Chaimovich, H.; Romsted, L. S.
Langmuir 1997, 13, 647.
(
(
(18) Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd
ed.; Edward Arnold: London, 1985.
S0002-7863(98)00556-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/19/1998

Arenediazonium Ion in Aqueous Acetamide Solutions
J. Am. Chem. Soc., Vol. 120, No. 39, 1998 10047
Scheme 1
arenediazonium ions with amide bonds drew a blank. Here we
The formation of the ester product, 1-ArOAc, from all three
amides, which was isolated from the reaction mixture and
characterized (see Experimental Section), was initially a surprise.
The most sensible explanation for formation of 1-ArOAc is that
amide O traps 1-Ar+ to give 2,4,6-trimethylphenyl imidates,
1-ArOI, as reactive intermediates that hydrolyze to give
report the results of a study of the mechanism of reaction of
+
1
-ArN2 with some simple amides in aqueous solution, propose
a preassociation model for interpreting the results, and suggest
how chemical trapping might be used to obtain information on
polypeptide topology and orientation at aggregate interfaces.
1
-ArOAc/amine and 1-ArOH/amide product pairs, consistent
Results
21-26
with published reports of the chemistry of aryl imidates.
1
-ArOI is represented as a cation because aryl imidates are weak
Scheme 1 summarizes the basic mechanism for dediazoniation
of 1-ArN2 in aqueous solutions of acetamide, N-methylacet-
21
+
acids; e.g., for protonated p-tolyl imidates, pKa g 6.7. The
rate constant for the spontaneous hydrolysis of p-tolyl N,N-
dimethylacetimidate, which is structurally similar to 1-ArOI
amide, and N,N-dimethylacetamide that is consistent with our
results. Evidence for this mechanism comes from dediazonia-
tion kinetics, identification of products by FT-IR, NMR, HPLC,
(which has two additional ortho methyls on the phenyl ring), is
-
1
21
18
0.056 min at 50 °C. We estimate the half-life for this
reaction to be ca. 50 min at 40 °C, which is close to the half-
life for dediazoniation at this temperature, t1/2 ) 35.2 min (see
below). Because dediazoniations were carried out for g24 h,
no 1-ArOI should be observed in the HPLC chromatograms.
Tables 1-3 list normalized product yields obtained from
and GC/MS, and O isotopic labeling experiments. A detailed
interpretation is given in the Discussion. Scheme 1 is based
on the well-established heterolytic pathway for reactions of
1
1
arenediazonium ions with weakly basic nucleophiles. The
rate-determining step is loss of N2 to generate an aryl cation,
-Ar+,20 followed by product formation in a subsequent fast
1
HPLC chromatograms for products formed from reaction of
step. 2,4,6-Trimethylphenol, 1-ArOH, is formed from reaction
with H2O. N-(2,4,6-Trimethylphenyl)acetamide and N-(2,4,6-
trimethylphenyl)-N-methylacetamide, 1-ArNAc, are formed by
reaction with the nitrogen (amide N) of acetamide and N-
methylacetamide, respectively, but no product was observed
from reaction with the amide N of N,N-dimethylacetamide.
Trapping of 1-Ar+ by the amide N of N,N-dimethylacetamide
should give acetyl N,N-dimethyl-2,4,6-trimethylanilinium ion
as an intermediate that should hydrolyze rapidly to acetic acid
and N,N-dimethylaniline, 1-ArNMe2. No 1-ArNMe2 was
observed in the product mixtures above the detection limit of
the HPLC, about 0.05%. Control experiments were run to check
+
1-ArN2 with H2O and acetamide, N-methylacetamide, and N,N-
dimethylacetamide, respectively. The major product is 1-ArOH,
which is typically g92% of the total normalized yield. All other
products are formed in ca. 1-5% yield. Products yields are
measured in triplicate, with a reproducibility of (5% or better.
Normalized product yields are used in all calculations instead
of total measured yields because of weighing errors in the small
amounts of 1-ArN2BF4 used in preparation of stock solutions,
because all relevant products and side products could be
accounted for, because formation of these products does not
significantly affect the normalized yields of the phenol, amide,
and ester products (see below), and because the reproducibility
+
for possible competing reactions between 1-ArN2 and 1-ArNMe2
that would remove 1-ArNMe2 from the product mixture, but
none were found (see Experimental Section).
(21) Satterthwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974, 96, 7031.
(22) Satterthwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974, 96, 7018.
(23) Satterthwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974, 96, 7045.
(
19) Zollinger, H. Color Chemistry: Syntheses, Properties and Applica-
tions of Organic Dyes and Pigments, 2nd ed.; VCH: Weinheim, 1991.
20) The plus sign is on the line (not superscripted) to represent the
(24) Lee, Y.-N.; Schmir, G. L. J. Am. Chem. Soc. 1978, 100, 6700.
(25) Lee, Y.-N.; Schmir, G. S. J. Am. Chem. Soc. 1979, 101, 3026.
(26) Jencks, W. P. Catalysis in Chemistry and Enzymology; Dover: New
York, 1987.
(
2
positively charged sp orbital that remains after N2 is lost.

10048 J. Am. Chem. Soc., Vol. 120, No. 39, 1998
Romsted et al.
Table 1. Normalized Product Yields for Reaction of 1-ArN +
with Acetamide in Water at 40 ( 0.1 °Ca
2
1
03[1-ArN
(M)
2
+]c
1-ArNHAc
(%)
1-ArOH total
(%)
1-ArOH d
h
1-ArOH d
w
1-ArOAc
(%)
estere
(%)
b
N
W
/N
A
(%)
(%)
2
2
2
2
4
4
4
4
1.00
0.50
4.76
2.00
1.00
0.50
4.76
2.00
2.70
2.68
2.99
2.70
1.82
2.06
1.97
1.52
94.22
94.23
94.22
94.71
96.26
96.19
96.28
96.70
25.16
25.16
25.16
25.29
16.46
16.45
16.46
16.54
69.16
69.16
69.16
69.52
79.80
79.74
79.82
80.16
3.08
3.08
2.79
2.60
1.92
1.75
1.75
1.78
10.9
10.9
10.0
9.3
10.4
9.6
9.6
9.7
a
b
Normalized yield: %1-ArX (X ) OH, OAc, NHAc) ) 100{%(1-ArX)}/{%(1-ArOH) + %(1-ArOAc) + %(1-ArNHAc)} The molar ratio of
c
-3
+ d
2
water to acetamide in each solution. Cyclohexane was layered over all solutions except those containing 4.76 × 10 M 1-ArN
. Normalized
e
yields from hydrolysis of 1-ArOI, 1-ArOH
h
, and by direct trapping of water, 1-ArOH
w
.
Yield of ester from hydrolysis of 1-ArOI: % ester )
1
00{%(1-ArOAc)}/{%(1-ArOAc) + %(1-ArOH
h
)}. Average ) 10.1 ( 0.6%.
Table 2. Normalized Yields for Reaction of 1-ArN +
with N-Methylacetamide in Water at 40 ( 0.1 °Ca
2
1
03[1-ArN
(M)
2
+]
1-ArNMeAc
(%)
1-ArOH total
(%)
1-ArOH c
h
1-ArOH c
w
1-ArOAc
(%)
esterd
(%)
b
N
W
/N
A
(%)
(%)
2
2
2
2
2
4
4
4
4
4
7.81
7.81
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
1.18
1.30
1.57
1.84
1.94
1.13
0.88
1.10
0.90
1.13
92.90
92.72
92.45
92.11
91.97
95.43
96.09
95.76
96.10
95.93
17.74
17.71
17.66
17.59
17.57
10.40
10.47
10.44
10.47
10.46
75.16
75.01
74.79
74.52
74.40
85.03
85.62
85.32
85.62
85.47
5.92
5.98
5.98
6.05
6.09
3.44
3.02
3.13
3.00
2.95
25.0
25.2
25.3
25.6
25.7
24.9
22.4
23.1
22.3
22.0
a
b
Normalized yield: %1-ArX (X ) OH, OAc, NMeAc) ) 100{%(1-ArX)}/{%(1-ArOH) + %(1-ArOAc) + %(1-ArNMeAc)}. The molar ratio
c
d
of water to N-methylacetamide. Normalized yields from hydrolysis of 1-ArOI, 1-ArOH
h
, and by direct trapping of water, 1-ArOH
w
. Yield of
h
ester from hydrolysis of 1-ArOI: % ester ) 100{%(1-ArOAc)}/{%(1-ArOAc) + %(1-ArOH )}. Average ) 24.2 ( 0.34%.
+
Table 3. Normalized Yields for Reaction of 1-ArN
N,N-Dimethylacetamide in Water at 40 ( 0.1 °C
2
with
methyl groups on the amide nitrogen increases the percent yield
of ester.
a
Values of kobs for dediazoniation of 1-ArN2+ were determined
by standard methods under the experimental conditions used
for measuring product distributions; i.e., at two different amide-
to-H2O ratios, NW/NA, at 40 °C to check for composition and
medium effects on the rate-determining step (see Supporting
1
-ArOH
3
+]c total 1-ArOH
d
d
1-ArOAc esterf
N
N
W
/ 10 [1-ArN
2
h
1-ArOH
(%)
w
b
A
(M)
(%)
(%)
(%)
(%)
2
4.76
4.76
3.75
4.71
4.73
4.76
4.76
3.75
4.71
92.70
92.41
92.58
93.64
95.51
95.19
95.05
95.24
95.98
16.41
16.36
16.39
16.57
9.07
9.04
9.03
76.29
76.05
76.19
77.07
86.44
86.15
86.02
86.19
86.86
7.30
7.59
7.42
6.36
4.49
4.81
4.95
4.76
4.02
30.8
31.7
31.2
27.7
33.1
34.7
35.4
34.5
30.6
2
2
2
4
4
4
4
4
-
4
e
Information). The average value of kobs, (3.28 ( 0.33) × 10
-
1
s , is constant within 10% and a little lower than kobs in water,
-
4
-1
1
ca. 5 × 10
s
at 40 °C. The value of kobs increases slightly
with the concentration of H2O and decreases slightly with added
N-methyl groups on the amide. This insensitivity of kobs to
amide type and concentration is consistent with the characteris-
tic insensitivity of thermal dediazoniations to solvent polar-
9.05
9.12
e
a
Normalized yield: %1-ArX (X ) OH, OAc) ) 100{%(1-ArX)}/
b
{
%(1-ArOH) + %(1-ArOAc)}. The molar ratio of water to N,N-
1,27,28
ity.
c
dimethylacetamide. Cyclohexane was layered only on solutions with
-
3
+
2
. Normalized yields from hydrolysis of
e
d
Estimating the selectivity of the 1-Ar+ toward the amide O
relative to H2O and the amide N (see below) requires determi-
nation of the yield of 1-ArOI. However, the yield of 1-ArOI
could not be measured directly because it hydrolyzes too rapidly,
and the phenol from hydrolysis of 1-ArOI, 1-ArOHh, cannot
be measured by HPLC because its peak area in the chromato-
grams cannot be separated from the peak area for phenol formed
by direct reaction of 1-Ar+ with H2O. A series of experiments
3
1
.75 × 10 M 1-ArN
-ArOI, 1-ArOH , and by direct trapping of water, 1-ArOH
(see Experimental Section).
Ester yield from hydrolysis of 1-ArOI: % ester ) 100{%(1-ArOAc)}/
%(1-ArOAc) + %(1-ArOH )}. Average ) 32.2 ( 0.64%.
h
w
. Ex-
periments aimed at detecting 1-ArNMe
2
f
{
h
is excellent at the same molar ratio of H2O, NW, to amide, NA,
but different 1-ArN2 concentrations. The greatest variations
+
occur for products formed in low yields. The largest average
deviation is ca. (15% for 1-ArNMeAc yields at NW/NA ) 2,
Table 2. Tables 1-3 also list the yields of 1-ArOH obtained
from direct trapping of 1-Ar+ by H2O, 1-ArOHw, and from
1
8
were carried out in 43.85% H2 O to determined the yield of
-ArOH from hydrolysis of 1-ArOI. Scheme 2 shows the
1
1
8
expected distribution of the O label for trapping of amide O
and H2O. The underlined numbers are the molecular weights
of the products whose yields were measured by GC/MS and
HPLC. The variables, x, y, and z, are needed in product yield
calculations (see Supporting Information). The amount of the
phenol produced by each pathway was determined from analyses
18
hydrolysis of 1-ArOI, 1-ArOHh, which were determined by
O
isotopic labeling experiments combined with HPLC results (see
below). The last column lists the percent yield of ester ob-
tained from 1-ArOI hydrolysis. No trends are observed in the
+
product yields with variation of the 1-ArN2 concentration, but
increasing the concentration of H2O, i.e., increasing the H2O to
amide molar ratios, reduces the amount of amide and ester
products compared to 1-ArOH. Increasing the number of
(
27) Swain, C. G.; Sheats, J. E.; Harbison, K. G. J. Am. Chem. Soc.
1
975, 97, 783.
(28) Szele, I.; Zollinger, H. HelV. Chim. Acta 1978, 61, 1721.

Arenediazonium Ion in Aqueous Acetamide Solutions
J. Am. Chem. Soc., Vol. 120, No. 39, 1998 10049
Scheme 2
Table 4. GC/MS Peak Abundance Ratios, hm/z,a for Reaction of 5
dance ratios for the base peak, h121/h123, and the parent ion,
h136/h138, for 1-ArOH are very similar, and triplicate runs gave
good reproducibility, typically (3% or better. The average peak
abundance ratios, K, are used in calculations of selectivities (see
below). The minimum value of K is 1.28; i.e., the 1-ArOH:1-
-
3
+
2
×
10 M 1-ArN
in Aqueous Amide Solutions Containing
1
8
4
3.85% H O at 40 ( 0.1 °C
2
b
Kd
amides
acetamide
acetamide
N-methylacetamide
N-methylacetamide
N,N-dimethylacetamide
N,N-dimethylacetamide
N
W
/N
A
h
121/h123
h
136/h138
c
c
2.10, 2.15c 2.11
2
4
2
4
2
4
2.10, 2.10
1.75, 1.74
1.82
1.53
1.75
1.78, 1.73c 1.75
18
Ar OH ratio ) (100% - 43.85%)/43.85%, for 1-ArOH
1.82
1.59
1.79
1.54
1.82
1.56
1.77
1.52
+
produced only by reaction of 1-ArN2 with 43.85% labeled H2O.
All measured values of K are greater than 1.28, showing that a
significant fraction of unlabeled 1-ArOH is produced by the
hydrolysis of 1-ArOI.
1.51
a
Peak abundance at particular mass-to-charge ratios, m/z. b Molar
c
ratio of water to amide. A different preparation of 1-ArN
2
BF
4
at a
The selectivities of 1-Ar+ toward amide O and toward amide
N compared to H2O, eqs 1 and 2 respectively, were calculated
from the stoichiometric molar ratio NW/NA and the normalized
yields of 1-ArOHw, 1-ArOI (amide O), and 1-ArNAc (amide
N) listed in Tables 1-3.
-
4
d
different concentration, 3.62 × 10 M. K is the average peak
abundance ratios of h121/h123 and h136/h138
.
of peak abundance ratios for the base and molecular ion peaks
18
of 1-ArOH and 1-Ar OH obtained by GC/MS and analyses of
total phenol yields by HPLC of the same samples.
%(1-ArOI)
NW
Table 4 lists phenol peak abundance ratio results for de-
O
W
S )
(1)
(2)
+
diazoniation of 1-ArN2 in aqueous amide solutions containing
N
%(1-ArOHw)
A
1
8
O-labeled H2O, 43.85%. Control experiments demonstrated
that the peak abundance ratios for the molecular ions and base
peaks are very reproducible; i.e., they gave the same percentage
%
(1-ArNAc) N
N
W
S )
W
18
%(1-ArOH ) N
w
A
of O as that of the labeled water within experimental error
1
8
and provided reliable estimates of the amount of O in the
phenol product (see Supporting Information). The peak abun-
The selectivity of 1-Ar+ toward the amide O compared to the

10050 J. Am. Chem. Soc., Vol. 120, No. 39, 1998
Romsted et al.
Table 5. %(1-ArOH
h
) Yields from 1-ArOI Hydrolysis and Average Selectivities Determined by Chemical Trapping at Two Water/Amide,
N
W
/N
A
, Molar Ratios at 40 °C
)a
S
O b
W
S
N c
W
S
Od
N
amide
N
W
/N
A
% (1-ArOH
h
acetamide
acetamide
N-methylacetamide
N-methylacetamide
N,N-dimethylacetamide
N,N-dimethylacetamide
2
26.7
17.1
19.2
10.9
17.7
9.5
0.81 (4)
0.92 (4)
0.63 (5)
0.64 (5)
0.62 (4)
0.63 (4)
0.080 (4)
0.092 (4)
0.042 (4)
10.4
9.94
15.1
13.3
4
2
4
2
4
0.048 (4)
(e0.0001)e
a
, from 1-ArOI hydrolysis and from reaction with water. b Defined by eq 1. Number of averaged values is given
Based on total phenol, 1-ArOH
T
in parentheses. Defined by eq 2. Number of averaged values is given in parentheses. Defined by eq 3. e Estimate based on the detection limit of
c
d
the HPLC.
Scheme 3
O
N
O
W
N
W
N
amide N, S , is given by the S /S ratio, eq 3. Values of
the detection limit of the HPLC, about 0.05%, SW can be no
%(1-ArNAc) are obtained directly from HPLC data.
greater than 0.0001, about 100 times smaller than that for
acetamide. Note that the amide O is trapped more efficiently
O
W
O
S
S
%(1-ArOI)
than the amide N; i.e., S is about 10 and 14 for acetamide and
N
O
N
S )
)
(3)
N-methylacetamide, respectively.
N
W
%(1-ArNAc)
Discussion
%(1-ArOHw) is equal to the total phenol yield obtained by
Arenediazonium ions have a rich complex chemistry that is
HPLC minus the amount of unlabeled phenol produced by
hydrolysis of 1-ArOI, Scheme 2. %(1-ArOI) is equal to the
sum of the yields of 1-ArOAc and 1-ArOHh. Calculations of
1
1,14
still not completely understood.
Dediazoniation, i.e., loss
of N2 during product formation, proceeds by both homolytic
1
1
and heterolytic pathways. In the absence of strongly basic
nucleophiles, in aqueous solutions containing acid and O2, and
in the dark, arenediazonium ions lose N2 spontaneously in a
slow step to give a highly reactive aryl cation that is trapped in
a subsequent fast step by a nucleophile, Scheme 3 (see below).
Zollinger discussed in detail the evidence for the formation of
two intermediates leading to product formation, a solvated aryl
cation‚N2 pair and a free (solvated) aryl cation leading to product
%(1-ArOI) and %(1-ArOHw) for each amide require defining
the selectivities in terms of the average peak abundance ratios,
K, listed in Table 4 and the total product yields obtained by
HPLC (see Supporting Information).
O
W
N
W
O
N
Table 5 lists the average values of S , S , and S obtained
for acetamide, N-methylacetamide, and N,N-dimethylacetamide
for all the results in Tables 1-3. Table 5 also gives the average
percent yields of 1-ArOHh from aryl imidate hydrolysis based
on the average total phenol yield, %(1-ArOHT). Several trends
are apparent. The values of %(1-ArOHh) decrease with added
H2O, indicating an increase in the fraction of %(1-ArOHT) that
comes from reaction with H2O. %(1-ArOHh) decreases with
the number of N-methyl groups, indicating that adding methyl
groups makes the amine a better leaving group from the
tetrahedral intermediate, Scheme 2. The selectivities for each
1
1
formation. Sidhu and Thibblin used the concept of a rate-
determining, irreversibly formed carbocation‚nucleophile pair
to explain the competitive formation of nucleophilic substitution
and elimination products in solvolyses of compounds with good
leaving groups that form tertiary carbocation intermediates.2
The single most compelling piece of evidence for rate-
determining loss of N2 followed by fast subsequent product-
forming steps is the extraordinary insensitivity of the rates of
heterolytic dediazoniation reactions to solvent polarity.1
For example, kobs for dediazoniation of benzenediazonium
tetrafluoroborate varies by only 9-fold in solvents such as
methylene chloride, dioxane, H2O, fuming sulfuric acid, and
9,30
amide at NW/NA ratios of 2 and 4 are essentially the same, and
,11,27,28
O
W
adding one or two methyl groups reduces S modestly, about
2
0%. However, adding methyl groups has a much greater effect
N
N
on S . One methyl reduces S about 50%, and there is no
W
W
evidence of trapping of the amide N of N,N-dimethylacetamide.
Based on the assumption that the yield of %(1-ArNMe2) from
reaction of 1-Ar+ with N,N-dimethylacetamide was equal to
(
29) Thibblin, A.; Sidhu, H. J. Phys. Org. Chem. 1993, 6, 374.
(30) Sidhu, H.; Thibblin, A. J. Phys. Org. Chem. 1994, 7, 578.

Arenediazonium Ion in Aqueous Acetamide Solutions
J. Am. Chem. Soc., Vol. 120, No. 39, 1998 10051
MeOH that vary from about 2 to over 100 in their dielectric
constants. This extreme insensitivity of reactivity to solvent
polarity stands in dramatic contrast to the extraordinary solvent
sensitivity of other reactions, such as decarboxylations that vary
The remainder of Scheme 3 illustrates rate-determining forma-
tion of the selectively solvated aryl cation‚nucleophile inter-
mediates which collapse in a subsequent fast steps to give stable
products. Selective solvation of the diazonio group is repre-
8
31,32
N
O
by ∼10
and solvolyses of alkyl halides or arenesulfonates
that vary by a factor of ∼10 .
sented by the equilibrium constants, KW and K . Product
W
16 33
yields are proportional to the concentration of each nucleophile
in solution and to the equilibrium constants for formation of
each arenediazonium cation‚nucleophile pair. The value of kobs
reflects the rate of spontaneous decomposition of the entire
ensemble of ground-state arenediazonium ion‚nucleophile pairs.
The lifetimes of the aryl cation intermediates are currently
unknown. If their lifetimes are significantly longer than the
diffusion-controlled limit, then exchange of nucleophiles be-
tween these intermediates is possible, as shown for the aryl
cation‚nucleophile intermediates, Scheme 3. However, because
charge distributions in the ground, transition, and intermediate
states are very similar (see above), the distributions of nucleo-
philes in the aryl cation‚nucleophile pairs are assumed to be
the same as in the ground state. Relationships between
measured selectivities and equilibrium constants for nucleophile
exchange have been derived and applied to alcohol and halide
The almost complete insensitivity of kobs to medium effects
suggests that selective solvation of the ground state, in particular
its interactions with its immediate environment of ions and
neutral molecules, remains unchanged in the vicinity of the
transition state and the aryl cation intermediate; i.e., the free
energy of activation is not significantly affected by solvent.
Recent ab initio calculations of bonding in benzenediazonium
ions in the gas phase34 and electron density analyses of gas-
phase heterolytic dediazoniations35 support this interpretation.
The calculated binding energy for N2 to the aryl cation is in
excellent agreement with experiment,34 and virtually all the
positive charge is distributed among the hydrogens and carbons
of the aromatic ring, indicating that the solution chemistry of
the benzendiazonium ion depends on its intrinsic properties and
not its interactions with the medium.34 Glaser, Horan and
Zollinger conclude that “...[d]iazonium ions can best be thought
of as carbenium ions closely associated with an N2 molecule
1
ion exchange with water. The relationships indicate that
measured selectivities are equal to the exchange constants,
provided the rate constants for collapse of the aryl cation‚
nucleophile pairs to products in the fast steps are the same, i.e.,
kN ) kW ) kO.
δ-
δ+ 35
that is internally polarized in the fashion NR -Nâ .”
Substantial evidence suggests that the aryl cation must be
3
6-38
extremely short-lived, probably less than 500 ps,
consistent
with the reactivity-selectivity principle.39 The formation of a
highly reactive charged delocalized cation is also consistent with
the extremely low selectivity typically shown by arenediazonium
The interaction of 1-ArN2+ with weakly basic nucleophiles
can be viewed as the formation of a complex and the selectivities
as a measure of relative basicities of the nucleophiles. In this
2
+
ions toward competing nucleophiles. The selectivity of 1-ArN2
sense, H O and the amide O solvate the diazonio group to about
toward alcohols versus H2O is about 0.3-0.6,1,3 and it is only
O
the same extent, i.e., SW ≈ 0.6-0.9, consistent with the
1
,27
about 10 times more selective toward anions over H2O. The
results in Table 5 show that the selectivities toward amide O
compared to H2O are a little less than 1 and amide N compared
to H2O are about 10 times less.
similarities in their interactions with a proton; i.e., the pKa of
water is -1.74, and those of amides are in the range from 0 to
39
+
-
4. The greater solvation of 1-ArN2 by amide O over amide
O
N, i.e., S g 10, is consistent with significant contribution
N
Scheme 3 is an adaptation of Zollinger’s ion‚molecule pair
from the charged resonance form; i.e., a negative charge on O,
a positive charge on N, and a double bond between C and N.
Recent semiempirical and ab initio calculations seem at variance
with this picture because, regardless of the basis set used, the
calculations always give greater negative charge density on the
mechanism for dediazoniation to reactions in aqueous amide
solutions.4
0,41
The selectivities of the aryl cation‚N2 pair and
the free aryl cation toward nucleophiles are assumed to be the
same. A preassociation equilibrium42 in the ground state is
included to describe the selectivities of the dediazoniation
reaction toward different nucleophiles. This approach is
consistent with Rys’s detailed analysis of the factors that may
contribute to chemical selectivity, including nonstoichiometric
reactant distribution within the solvent cage of an encounter
complex.43 The equilibrium distributions of arenediazonium
cation‚nucleophile pairs represent selective solvation of the
44
amide N than on the amide O. However, the negative charge
density on the amide O is always greater than that of the entire
-
NH2 group, which in some basis sets has a net charge near
zero. Thus, these calculations are consistent with the amide O
+
solvating 1-ArN2 better than the -NH2 and probably better
than the -NHMe and -NMe2 groups.
The results with N,N-dimethylacetamide are strikingly dif-
ferent in that no product appears to be formed from reaction
with amide N. Chemical trapping of 1-Ar+ by the nitrogen of
N,N-dimethylacetamide should give the acetyl N,N-dimethyl-
+
reactive diazonio group of 1-ArN2 by the nucleophiles, water,
amide N, and amide O in the ground state. Nucleophile
exchange is assumed to occur at the diffusion-controlled limit.
2
,4,6-trimethylanilinium ion. This ion should be a reactive
(
(
31) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305.
32) Kemp, D. S.; Cox, D. D.; Paul, K. G. J. Am. Chem. Soc. 1975, 97,
4
5
intermediate, similar to acyl pyridinium ions, and hydrolyze
rapidly into acetic acid and 1-ArNMe2, in its protonated or
unprotonated form. The absence of a significant yield of
1-ArNMe2 suggests that the -NMe2 group may be sufficiently
hydrophobic that it does not complex (solvate) the diazonio
7
312.
(
(
(
33) Szele, I.; Zollinger, H. J. Am. Chem. Soc. 1978, 100, 2811.
34) Glaser, R.; Horan, C. J. J. Org. Chem. 1995, 60, 7518.
35) Glaser, R.; Horan, C. J.; Zollinger, H. Angew. Chem., Intl. Ed. Engl.
1
997, 36, 2210.
(
(
(
(
36) Scaiano, J. C.; Kim-Thuan, N. J. Photochem. 1983, 23, 269.
37) Chateauneuf, J. E. J. Chem. Soc., Chem. Commun. 1991, 1437.
38) Cacace, F. Science 1990, 250, 392.
+
group of 1-ArN2 significantly, and no product is formed from
reaction with 1-Ar+. Alternatively, 1-Ar+ may not react with
tertiary amides because concerted loss of a proton is a
fundamental requirement for aryl-N bond formation. This
explanation is consistent with our chemical trapping results in
39) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry, 3rd ed.; Harper and Row: New York, 1987.
40) Hashida, Y.; Landells, R. G. M.; Lewis, G. E.; Szele, I.; Zollinger,
H. J. Am. Chem. Soc. 1978, 100, 2816.
41) Ravenscroft, M. D.; Takagi, K.; Weiss, B.; Zollinger, H. Gazz. Chim.
Ital. 1987, 117(b), 353.
(
(
(44) Estiu, G. L. Theochem 1997, 401, 157.
(
42) Jencks, W. P. Chem. Soc. ReV. 1981, 10, 345.
(45) Carey, R. A. AdVanced Organic Chemistry. Part A: Structure and
Mechanism, 3rd ed.; Plenum Press: New York, 1993.
(43) Rys, P. Acc. Chem. Res. 1976, 9, 345.

10052 J. Am. Chem. Soc., Vol. 120, No. 39, 1998
Romsted et al.
nonionic oligooxyethylene monoalkyl ether micelles and in
aqueous oligooxyethylene glycol solutions.3 Product is formed
from reaction with the terminal -OH group which has a
removable proton, but no products are observed from trapping
by ether oxygens within the oligooxyethylene chains; i.e., no
C-O bond cleavage is observed.
which peptide bonds are located in the interfacial region of the
aggregates and provide information on the topology and
orientation of the original polypeptide at the aggregate surface.
Small yields of multiple peptide fragments can be determined
by mass spectroscopy or by HPLC using fluorescence detection
after tagging the amino terminus of the cleaved peptide bond
with a fluorescent tag such as 9-fluorenylmethyl chloroformate
The mechanism of hydrolysis of imidate esters has been
2
1-26
59
studied in detail.
The yield of ester formed by hydrolysis
(FMOC-Cl).
of 1-ArOI, Scheme 2, at H2O:amide molar ratios of 2 and 4 is
independent of solution composition but increases with the
number of methyl groups on the amide nitrogen, Tables 1-3.
The average ester yields for acetamide, N-methylacetamide, and
N,N-dimethylacetamide are respectively 10.1 ( 0.6%, 24.2 (
Conclusions
Chemical trapping of acetamide and N-methylated acetamides
with 2,4,6-trimethylbenzenediazonium ion, 1-ArN2 , in aqueous
+
0
.34%, and 32.2 ( 0.64%. This increase in ester yield with
solution works. Observed rate constants and product yields are
completely consistent with the heterolytic dediazoniation mech-
anism, i.e., rate-determining loss of N2 to give a highly reactive/
unselective aryl cation, 1-Ar+, that traps the weakly basic
nucleophiles in a separate, subsequent step, in aqueous amide
solutions: H2O, amide N, and amide O. Recent ab initio
calculations support this model. Trapping of the amide nitrogen
gives stable aryl amides with acetamide and N-methylacetamide,
However, no observable product is obtained from trapping of
the nitrogen in N,N-dimethylacetamide, suggesting either that
the -NMe2 group is too nonpolar to solvate the cationic
diazonio group or that a removable proton is required for product
formation. Trapping of amide oxygens gives aryl imidate
intermediates that hydrolyze within the dediazoniation reaction
time to give ester/amine and phenol/amide product pairs,
consistent with literature results. The traditional heterolytic
dediazoniation mechanism is combined with a preassociation
mechanism to describe the competitive solvation of the diazonio
the number of methyl groups compares favorably with the results
of Satterthwait and Jencks21 for the hydrolysis of p-tolyl
N-methylacetimidate (∼10% ester) and p-tolyl N,N-dimethyl-
acetimidate (∼20% ester) in dilute aqueous solution between
pH 3 and 6, in which the aryl imidate is in its protonated form
at 25 °C. Thus, the mechanism for aryl imidate hydrolysis in
aqueous amide solutions and in dilute aqueous solution is
probably the same. The high reproducibility of 1-ArOHh and
1
-ArOAc yields shows that selectivities determined by the
chemical trapping method from these data, Table 5, are reliable.
Chemical Trapping of Aggregate-Bound Polypeptides
Structures and orientations of polypeptides at aggregate
surfaces are difficult to determine by CD, 2D NMR, IR,
4
6-51
electrophoretic mobility, gel filtration, and calorimetry
because their structures are often not fixed by multiple cross-
links and internal hydrogen bonds but are induced by multiple
hydrophobic, electrostatic, hydration, and hydrogen bonding
interactions at interfaces.46,49 Chemical tagging of aggregate-
bound polypeptides should identify those sections of the
polypeptides in the interfacial region of the aggregate. Chemical
tagging of proteins at interfaces has been attempted by using a
group by H O, amide O, and amide N. The chemical trapping
2
method provides a potentially novel approach for determin-
ing the topologies and orientations of aggregate-bound poly-
peptides.
Experimental Section
52-54
55-57
variety of reagents,
including arenediazonium ions,
but
Methods. 1H and 13C NMR spectra were recorded on a Gemini-
200 spectrophotometer. Mass spectra of synthesized compounds were
obtained on a HP 5890 Series II GC interfaced with a HP 5971 mass-
selective detector using a 12-m × 0.2-mm HP-1 capillary column (cross-
linked methyl silicone on fused silica), 0.33-mm particle size. IR
spectra were recorded on a Mattson Genesis Series FT-IR spectropho-
tometer. Product yields were determined on a Perkin-Elmer LC-235
HPLC equipped with a Ranin Microsorb-MV C-18 reversed-phase
column (5-mm particle size, 4.6 mm i.d. × 25 cm), a 200-µL sample
loop, a LC-235 diode array detector, and a PE-Nelson 900 series
interface attached to a Ultra PC computer. The mobile phase for
without much success. We recently demonstrated by HPLC
that amphiphilic 16-ArN2 gives significant yields of products
+
from reaction with the terminal carboxylate groups and amide
O (but not amide N) in micelles of sodium lauroylsarcosinate
and sodium lauroylglycinate. These surfactants have N-meth-
ylglycine and glycine headgroups, respectively, and are reason-
5
8
able models of peptide bonds at aggregate interfaces. 16-
+
ArN2 should tag amino acid side chains, and reaction with
the peptide bond should give fragments after hydrolysis (O
cleavage). The tagging and fragmentation data should identify
2
product separation was 60%/40% MeOH/H O with a flow rate of 0.8
(
46) DeGrado, W. F.; Wasserman, Z. R.; Lear, J. D. Science 1989, 243,
mL/min for all runs. HPLC chromatograms were analyzed by using
PE Nelson Turbochrom 3 software. Absorbances were measured at
220 nm, and reported peak areas are averages of triplicate injections.
Kinetics were monitored on Perkin-Elmer UV/visible 559A spectro-
photometer fitted with electronic temperature control. Melting points
were determined by using a Mel-Temp apparatus and are uncorrected.
Materials. HPLC grade MeOH, 2,4,6-trimethylphenol, 1-ArOH
6
2
1
22.
(47) Shon, K.-J.; Kim, Y.; Colnago, L. A.; Opella, S. J. Science 1991,
52, 1303.
(48) de Jongh, H. H. J.; Goormaghtigh, E.; Killian, J. A. Biochemistry
994, 33, 14521.
(
49) Taylor, J. W.; Osapay, G. Acc. Chem. Res. 1990, 23, 338.
(50) Buser, C. A.; Sigal, C. T.; Resh, M. D.; McLaughlin, S. Biochemistry
1
994, 33, 13093.
51) Flach, C. R.; Brauner, J. W.; Taylor, J. W.; Baldwin, R. C.;
Mendelsohn, R. Biophys. J. 1994, 67, 402.
52) Aggeler, R.; Chicas-Cruz, K.; Cai, S.-X.; Keana, J. F. W.; Capaldi,
R. A. Biochemistry 1992, 31, 2956.
(
99%), acetamide (99+%), N-methylacetamide (99+%), N,N-dimethyl-
acetamide (99.9+%), 1,3,5-trimethylbenzene (98%), 1-ArH, N,N-
dimethylaniline (99.5+%), 1-ArNMe , ethyl acetate, EA, petroleum
ether, PE, and inorganic reagents were purchased from Aldrich, and
(
(
2
(53) Davison, M. D.; Findlay, J. B. C. Biochem. J. 1986, 234, 413.
(54) Andres, S. F.; Atassi, M. Z. Biochemistry 1973, 12, 942.
(55) Prochaska, L.; Bisson, R.; Capaldi, R. A. Biochemistry 1980, 19,
18
4
3.85% H
2
O was purchased from Icon Services, Inc. (Summit, NJ).
All reagents were used as received. Preparations of 2,4,6-trimethyl-
3
1
174.
2 4
benzenediazonium tetrafluoroborate, 1-ArN BF , and 2,4,6-trimethyl-
,3
(56) Ludwig, B.; Prochaska, L.; Capaldi, R. A. Biochemistry 1980, 19,
fluorobenzene, 1-ArF, have been described.1 Dediazoniation products
516.
(
(
57) Eytan, G. D.; Broza, R. J. Biol. Chem. 1978, 253, 3196.
58) Unpublished results.
(59) Einarsson, S.; Folestad, S.; Josefsson, B.; Lagerkvist, S. Anal. Chem.
1986, 58, 1638.

Arenediazonium Ion in Aqueous Acetamide Solutions
J. Am. Chem. Soc., Vol. 120, No. 39, 1998 10053
have been prepared previously, and literature references are given. All
aqueous solutions were prepared by using distilled water which was
passed over activated carbon and deionizing resin and then redistilled.
prepared and characterized compounds. One significant new peak
consistently appeared in the HPLC chromatograms. Product mixtures
+
from reaction of 1-ArN
extracted with Et
2
in neat liquid N-methylacetamide were
60
Methyl 2,4,6-Trimethylphenyl Ether, 1-ArOMe. The ether was
2
O, and GC/MS analysis showed that this peak was
prepared by alkylation of 1-ArOH with MeI according to the procedure
of Miller et al. The crude product was purified by column chroma-
tography on alumina eluted with n-pentane giving a colorless liquid;
1-ArOAc. This assignment was confirmed by isolating 1-ArOAc from
the product mixture by column chromatography and characterizing it
6
1
1
13
by H and C NMR, and by a spiking experiment with independently
synthesized 1-ArOAc. In previous work, we found that some products,
particularly aryl halide products, have significant vapor pressures and
that their yields decrease on standing.1 This problem was solved by
layering small amounts of cyclohexane on the top of the solutions to
dissolve volatile products. The cyclohexane method was used here to
check for product loss; small increases in the total yield were found,
but the product ratios were unchanged.
-
1
6
1
2
6
5% yield: IR (neat) νmax (cm ) 2938, 1599, 1484, 1451, 1225, 1144,
1
019, 853, 762; H NMR (CDCl ) δ (ppm) 6.92 (2H, s), 3.82 (3H, s),
.31 (3H, s), 2.38 (6H, s); GC/MS 150 (m/e), 135, 119, 105, 91, 79,
3
5.
N-(2,4,6-Trimethylphenyl)acetamide, 1-ArNHAc.62 The aryl amide
was prepared by acylation of 2,4,6-trimethylaniline, 1-ArNH
2
, with
acetyl chloride according to a procedure for the preparation of
63
benzanilide. The crude product was recrystallized twice from EtOH/
O to give white crystals; 80% yield (mp 215-216 °C): IR (film in
Several other products (not shown in Scheme 1) are observed in the
HPLC chromatograms. All have been identified previously and
H
CH
2
-
1
1
2
Cl
2
) νmax (cm ) 3425.5, 3019.1, 1668.7, 1215.9, 770.2; H NMR
:DMSO-d 1:1) δ (ppm) 8.99 (1H, s), 6.81 (2H, s), 2.21 (3H,
1,3
synthesized, if not available commercially: 2,4,6-trimethylbenzene,
(CDCl
3
6
1
1
-ArH, 2,4,6-trimethylfluorobenzene, 1-ArF, and 2,4,6-trimethylanisole,
-ArOMe. The observed yields of these products are small and variable.
s), 2.10 (6H, s), 2.04 (3H, s); GC/MS 177 (m/e), 135, 120, 91, 77, 65,
3, 39.
N-(2,4,6-Trimethylphenyl)-N-methylacetamide, 1-ArNMeAc.64
4
Typical yields are as follow: 1-ArH, <2%; 1-ArF, <2%; and 1-ArOMe,
67
<
2 4
3%. 1-ArF is formed via the Schiemann reaction from 1-ArN BF
The aryl methyl amide was prepared by the phase-transfer catalysis
3
in the solid state or in solution. 1-ArOMe is formed in the MeOH
procedure of Kalkote et al. for the N-alkylation of aryl amides.65
stock solutions and possibly in the reaction mixture. Formation of these
1
-ArNHAc was methylated with dimethyl sulfate in a stirred mixture
of aqueous NaOH/K CO and toluene at ca. 30 °C for ca. 2 h using
tetra-n-butylammonium hydrogen sulfate as the catalyst. The white
flat crystals were recrystallized from EtOH/H O and further purified
by chromatography on silica (eluting solvent PE:EA:Et O 1:1:1) and
by preparative HPLC (C-18 reversed-phase, 40% H O/60% MeOH,
+
two products reduces the amount of 1-ArN
2
available to react but
2
3
does not affect the relative yields of 1-ArOH, 1-ArNAc, and 1-ArOAc.
+ 3
1
-ArH is formed by reaction of 1-ArOH with unreacted 1-ArN , but
2
2
its effect on the total phenol yield is small, and it was ignored in all
calculations. Typical retention times in minutes for dediazoniation
products are as follow: 1-ArNHAc, 6-7; 1-ArOH, 11-12; 1-ArN-
MeAc, 12-13; 1-ArOAc, 17-18; 1-ArOMe, 25-26; 1-ArH, 45-48;
and 1-ArF, 52-54. Concentrations of products were obtained from
peak areas using standard calibration curves obtained from independ-
ently prepared or commercial products. Percent yields reported in the
Supporting Information are based on the concentration of added
2
2
64
2
5
1
20 nm). The overall yield was 80% (mp 53-54 °C, lit. mp 52-
-1
2 2
2.5): IR (film in CH Cl ) νmax (cm ) 3053.6, 1650.3, 1264.9, 1064.9,
1
034.2, 896.0, 738.0; H NMR (CDCl ) δ (ppm) 6.93 (2H, s), 3.11
3
(
1
3H, s), 2.30 (3H, s), 2.16 (6H, s), 1.73 (3H, s); GC/MS 191 (m/e),
76, 148, 134, 119, 105, 91, 77, 65, 56, 43.
,4,6-Trimethylphenyl Acetate, 1-ArOAc.6 The ester was pre-
pared from 1-ArOH and acetyl chloride using a procedure based on
6
2
1
2 4
-ArN BF . Normalized yields based on the total yields of phenol,
ester and amide products were used in the calculations of selectivities.
6
3
the preparations of phenyl acetate and phenyl benzoate. Vacuum
distillation gave an impure product (GC/MS), which was further purified
by column chromatography on silica (eluting solvent EA:hexane 1:6
The Search for N,N-Dimethylaniline. Dediazoniations were carried
W
out in two different aqueous N,N-dimethylacetamide solutions with N /
-
3
N
A
molar ratios of 2 and 4 containing 4.71 × 10 M 1-ArN
2.5% MeOH v/v) at 40 °C for 24 h. No peak was observed at the
retention time for 1-ArNMe , but the areas of all other peaks were
2 4
BF (ca.
or CH
2
Cl
2
:PE 1:3). The overall yield of the colorless liquid was 90%
-
1
(
1
bp 85-90 °C, 3-4 mmHg): IR (neat) νmax (cm ) 3011.5, 1754.6,
1
2
607.1, 1370, 1222.7, 1191.0, 1137.1, 910.6, 854.5, 733.0; H NMR
) δ (ppm) 6.91 (2H, s), 2.36 (3H, s), 2.30 (3H, s), 2.15 (6H, s);
C NMR, broad-band decoupled (CDCl ) δ (ppm) 169.6, 146.4, 135.8,
consistent with earlier results (see Table 3). In a separate set of
(
CDCl
3
+
1
3
experiments, 0.05, 0.1, and 1 equiv (compared to 1-ArN
2
) of
3
-
3
+
1
-ArNMe
2
were added to 5 × 10 M 1-ArN
O:N,N-dimethylacetamide at 40 °C. Increasing the amount of
did not affect the total product yield or the 1-ArOH:1-ArOAc
2
in a 2:1 molar ratio of
1
30.1, 129.7, 21.3, 21.0, 16.7; GC/MS 178 (m/e), 136, 121, 91, 77,
H
2
6
5, 43.
1
-ArNMe
2
HPLC Experiments/Product Yields. Products were identified and
their yields obtained by the following general procedures. Reaction
was initiated by injecting small amounts, ca. 20 µL, of freshly prepared,
2 4
ice cold, stock solutions of 0.1 M 1-ArN BF in MeOH into aqueous
amide solutions of varying molar ratios in 5-10-mL volumetric flasks
thermostated at 40 °C. Note that MeOH was used as the solvent for
product yield ratio significantly within experimental error, i.e., (5%.
A special gradient eluting system was used to minimize broadening of
the 1-ArNMe
7
8
2
peak: 0-7 min, 0.05% TFA/80% H
-37 min, the ratio was changed linearly to 0.05% TFA/20% H
0% MeCN and held constant for 5 min, followed by wash cycles,
2
O/20% MeCN;
2
O/
and returned to the original condition. Typical retention times were
1
-ArN
2 4
BF instead of MeCN because the peak in the chromatograms
+
as follow: 1-ArNMe , 10 min; 1-ArOH, 30 min; 1-ArOAc, 36 min;
for amide product formed from reaction of 1-ArN
2
with MeCN has
2
and 1-ArOMe, 39.5 min. The smallest peak observable by our detector
the same retention time as that of some products from reaction with
3
has an area of ca. 5 × 10
3
µV‚s, which is equivalent to ca. 2.49 ×
the amides. MeOH concentations in the reaction mixtures varied from
-4
-3
+
2
,
10-6 M 1-ArNMe2 or about 0.05% of the initial concentration of
a low of 0.5% (v/v) at 5 × 10 M to 8.2% at 7.81 × 10 M 1-ArN
see Tables 1-3. After 24-72 h (the half-life for dediazoniation is ca.
5 min at 40 °C), products were separated by HPLC.
-ArOH, 1-ArNAc, and other products (see below) were identified
in HPLC chromatograms by spiking experiments using independently
1-ArN BF in the aqueous N,N-dimethylacetamide reaction mixture.
2
4
Isotopic Labeling Experiments. Experiments using 43.85% H218O
were prepared by adding successively to 500-µL cone-shaped flasks
(Weaton), 45 µL of H218O, sufficient weight or volume (based on
densities) of the amide, i.e., acetamide, N-methylacetamide, or N,N-
3
1
(
(
(
60) Miller, B. J. Org. Chem. 1977, 42, 1402.
61) Miller, J. M.; So, K. H.; Clark, J. H. Can. J. Chem. 1979, 57, 1887.
62) Bradshaw, J. S.; Knudsen, R. D.; Loveridge, E. L. J. Org. Chem.
dimethylacetamide, to give the needed N
W A
/N molar ratio (see Table
4
), and sufficient volume of a freshly prepared stock solution of
1
970, 35, 1219.
63) Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G.;
1-ArN
10 M 1-ArN
2
BF
4
in MeOH (0.05 M) to give a final concentration of 5 ×
-
3
+
(
. The final concentration (by volume) of MeOH in
2
Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 4th ed.;
Longman: London, 1979; p 1368.
the solutions was about 10%. The solutions were thermostated at 40
C overnight and then analyzed by GC/MS. Reported peak abundances
°
(
(
64) Volz, H.; Ruchti, L. Justus Liebigs Ann. Chem. 1972, 763, 184.
65) Kalkote, U. R.; Choudhary, A. R.; Natu, A. A.; Lahoti, R. J.;
were average values of triplicate injections.
Ayyangar, N. R. Synth. Commun. 1991, 21, 1889.
66) Baciocchi, E.; Rol, C.; Mandolini, L. J. Org. Chem. 1977, 42, 3682.
(
(67) Swain, C. G.; Rogers, R. J. J. Am. Chem. Soc. 1975, 97, 799.

10054 J. Am. Chem. Soc., Vol. 120, No. 39, 1998
Romsted et al.
Acknowledgment. We appreciate Jianping Lou’s assistance
Supporting Information Available: Appendix S1, showing
with the synthesis of some of the analytical standards, Arabinda
Chaudhuri for preparing 1-ArOMe; helpful discussions with
Santanu Bhattacharya, Clifford Bunton, Douglass G. Dalgleish,
Robert Moss, Ron Sauers, John Taylor, and Heinrich Zollinger;
and very useful comments by the reviewers. L.Z. thanks Rutgers
University for the Bevier Fellowship that provided partial
support of her Ph.D. research. L.S.R. thanks the National Science
Foundation, Organic Dynamics Division (CHE 95-26206), NSF-
U.S.-Brazil Cooperative Science Program (INT-9722458). This
is publication no. D10535-1-98 of the New Jersey Agricultural
Experiment Station, supported by State Funds and the Center
for Advanced Food Technology (CAFT).
the derivation of the equations used to analyze the product yields
18
in the O labeling experiments; Tables S1-S3, containing peak
areas, measured and normalized product yields, and HPLC
calibration curves for Tables 1-3; Table S4, completely listing
of phenol yields and selectivities for Table 5; Table S5, listing
the dediazoniation rate constants and the experimental method
for measuring kobs; and Table S6, summarizing the control
1
8
experiments for the O labeling experiments (8 pages, print/
PDF). See any current masthead page for ordering information
and Web access instructions.
JA980556A