Amination of Pyridylketenes
A R T I C L E S
Scheme 1. Hydrogen Bonding in (2-Pyridyl) Acetic Acid and
of 5 with Et2NH, the rate constant for amination exceeds that
for tautomerization of the amide enol 6b to the amide by a factor
of 22.4 From the experimental evidence, a differentiation
between an amide enol structure 6b and a zwitterionic structure
(2-Pyridyl) Acetophenone
c
4
c
could not be made. There have been no theoretical studies of
nucleophilic additions to arylketenes to 1d further elucidate the
structural factors on the reactivity and the structures of the
intermediates.
3
than 10 greater enol content in H O as compared to that of
2
6
e
PhCH COPh.
2
Results
For the elucidation of the effect of the 2-pyridyl group on
the reactivity of 2-7, structures and energies for the addition of
NH3 to phenylketene and 2-pyridylketene were calculated at
the B3LYP/6-31G*//B3LYP/6-31G* level using Gaussian 98.7
These include the reactants, transition structures, and products
for the addition of one and two ammonia molecules to the Cd
O bond, amide enol intermediates, and product arylacetamides.
The complete results are given in the Supporting Information,
and summary reaction profiles are shown in Figures 1 and 2
for 2-pyridylketene and for phenylketene, respectively. These
show two NH3 molecules, as favored for the gas-phase reaction
and for the tautomerization of the amide enols, and for
Pyridylketenes 75 provide an interesting comparison to
phenylketene. The pyridyl ring is strongly electron withdrawing
as compared to phenyl; for example, the σp values of the 2-, 3-,
5
e
and 4-pyridyl groups are 0.33, 0.23, and 0.27, respectively,
5
f
while the σR value for the aza group of pyridine is 0.236.
2
-pyridylketene (2-7) include an initial complex 10, transition
state 11, an initial amide enol intermediate 12a, a more stable
amide enol 12b, a transition state 13, and the amide 14a. For
phenylketene (5), there is a complex 15, transition state 16,
amide enol 17, transition state 18, and amide 19. Reaction
2
-Pyridylketene (2-7) has been generated by pyrolysis of ethyl
5a
profiles for the addition of single NH molecules, and more
2
-pyridyl acetate and was observed in an argon matrix by IR
bands at 2123 and 2132 cm for E and Z conformations,
respectively, when formed by the photochemical Wolff rear-
rangement of diazo ketone 2-8.5b The zwitterionic species 9 was
also formed, as identified by the IR spectrum, and rearranged
to 2-7 (eq 3).5b 3-Pyridylketene was formed in a matrix by both
Wolff rearrangement and addition of carbon monoxide to the
carbene. Pyridylketenes have also been recently generated and
3
-
1
detailed profiles showing two additional transition states between
1
2a and 12b, and other transition states and intermediates
located along the reaction pathway are given in the Supporting
Information (Figures S-1-S-4).
Laser flash photolysis (LFP) of the diazo ketones 8 using a
8
2
48 nm output of an excimer laser in CH3CN with TRIR
5c
detection gave the ketenes 2-, 3-, and 4-7 as identified by their
-
1
5d
IR bands at 2127, 2115, and 2128 cm , respectively. Similar
photolysis of 2-, 3-, and 4-8 in the presence of n-BuNH2 showed
the decay of the transient ketenes which was monitored by IR
spectroscopy, and growth and decay of transient amide enols,
monitored by UV (eq 4).
observed as monolayers on platinum surfaces.
The study of 2-pyridylketene is of particular interest because
of the possibility of direct interaction between the ring nitrogen
and the adjacent ketenyl group. The acidity of 2-pyridylacetic
acid (2-PyCH2CO2H), the hydration product of the ketene, is
significantly lower than that for the 3- and 4- isomers, a result
Second-order rate constants obtained from measurements with
different [n-BuNH2] are reported in Table 1. Photolysis of
6
a,b
attributed to intramolecular hydrogen bonding (Scheme 1).
4c
diazoacetophenone has already been shown to form phe-
Similarly, (2-pyridyl)acetophenone (2-PyCH2COPh) exists mainly
nylketene (5) under these conditions, with an IR band at 2118
as the enol in nonpolar solvents (Scheme 1)6 and has a more
c,d
-1
cm . We have carried out kinetic measurements on this ketene
with n-butylamine using UV detection, as is also reported in
(
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1
3
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(
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1
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J. AM. CHEM. SOC.
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VOL. 124, NO. 46, 2002 13791