A comparison of acetyl- and methoxycarbonylnitrenes by computational methods and a laser flash photolysis study of benzoylnitrene

Article abstract of DOI:10.1021/jo048433y

Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand the chemical and reactivity differences between acetylnitrene (CH₃C(=O)N) and methoxycarbonylnitrene (CH ₃OC(=O)N) and related compounds,

Full text of DOI:10.1021/jo048433y

A Comparison of Acetyl- and Methoxycarbonylnitrenes by
Computational Methods and a Laser Flash Photolysis Study of
Benzoylnitrene
Jin Liu, Sarah Mandel, Christopher M. Hadad,* and Matthew S. Platz*
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
hadad.1@osu.edu; platz.1@osu.edu
Received September 6, 2004
Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand
3
the chemical and reactivity differences between acetylnitrene (CH C(dO)N) and methoxycarbon-
ylnitrene (CH OC(dO)N) and related compounds. CBS-QB3 theory alone correctly predicts that
3
acetylnitrene has a singlet ground state. We agree with previous studies that there is a substantial
N-O interaction in singlet acetylnitrene and find a corresponding but weaker interaction in
methoxycarbonylnitrene. Methoxycarbonylnitrene has a triplet ground state because the oxygen
atom stabilizes the triplet state of the carbonyl nitrene more than the corresponding singlet state.
The oxygen atom also stabilizes the transition state of the Curtius rearrangement and accelerates
the isomerization of methoxycarbonylnitrene relative to acetylnitrene. Acetyl azide is calculated
to decompose by concerted migration of the methyl group along with nitrogen extrusion; the free
energy of activation for this concerted process is only 27 kcal/mol, and a free nitrene is not produced
upon pyrolysis of acetyl azide. Methoxycarbonyl azide, on the other hand, does have a preference
for stepwise Curtius rearrangement via the free nitrene. The bimolecular reactions of acetylnitrene
and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to
have enthalpic barriers that are near zero and free energy barriers that are controlled by entropy.
These predictions were tested by laser flash photolysis studies of benzoyl azide. The absolute
bimolecular reaction rate constants of benzoylnitrene were measured with the following sub-
5
-1 -1
6
-1 -1
6
-1
strates: acetonitrile (k ) 3.4 × 10 M s ), methanol (6.5 × 10 M s ), water (4.0 × 10 M
-
1
5
-1 -1
s ), cyclohexane (1.8 × 10 M s ), and several representative alkenes. The activation energy for
the reaction of benzoylnitrene with 1-hexene is -0.06 ( 0.001 kcal/mol. The activation energy for
the decay of benzoylnitrene in pentane is -3.20 ( 0.02 kcal/mol. The latter results indicate that
the rates of reactions of benzoylnitrene are controlled by entropic factors in a manner reminiscent
of singlet carbene processes.
Introduction
the intervention of a free nitrene. There is also evidence
that the stepwise or concerted nature of the process
varies with the mode of azide decomposition.
Carbonyl-substituted azides are readily synthesized
and have been studied by physical and organic chemists
for several decades. Upon activation with heat or light,
both acyl and alkoxycarbonyl azides extrude molecular
nitrogen and form carbonyl nitrenes. These nitrenes can
3
1
Carbonyl-substituted nitrenes are efficiently inter-
cepted with a variety of reagents, and cis and trans
substituted alkene traps are particularly diagnostic trap-
ping agents. Extending the Skell-Woodworth rules from
undergo intramolecular rearrangement (Curtius rear-
4
rangement) to form isocyanates.2
carbenes to nitrenes allows deduction of the multiplicity
of the reactive intermediate that is captured. Product
studies with acylnitrenes and cis alkenes revealed reten-
5
tion of configuration in the aziridine product.
There is evidence that migration of RX can proceed in
concert with nitrogen extrusion in some cases, without
(
2) (a) Wallis, E. S. Org. React. 1946, 3, 267. (b) Bauer, E. Angew.
Chem., Int. Ed. Engl. 1974, 13, 376.
(
3) (a) Lwowski, W.; Tisue, G. T. J. Am. Chem. Soc. 1965, 82, 4022.
*
To whom correspondence should be addressed. Fax: (614) 292-
685 (C.M.H.); (614) 292-1685 (M.S.P.).
1) Lwowski, W. In Azides and Nitrenes; Scriven, E. F. V., Ed.;
Academic Press: New York, NY, 1984; p 205.
(b) Tisue, G. T.; Linke, S.; Lwowski, W. J. Am. Chem. Soc. 1967, 89,
6303. (c) Linke, S.; Tisue, G. T.; Lwowski, W. J. Am. Chem. Soc. 1967,
89, 6308.
1
(
(4) Skell, P. S.; Woodworth, R. C. J. Am. Chem. Soc. 1956, 78, 4496.
1
0.1021/jo048433y CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/11/2004
J. Org. Chem. 2004, 69, 8583-8593
8583

Liu et al.
This result is consistent with trapping of the singlet
state of the acylnitrene. Different results are obtained
with alkoxycarbonyl azides, which form mixtures of
aziridines upon photolysis, in the presence of alkenes.6
TABLE 1. Singlet-Triplet Energy Gap [∆H (298 K)
(
kcal/mol)] of Species 1, 2, and 3 by DFT, CCSD(T), and
a
CBS-QB3 Methods
level of theory
1
2
3
B3LYP/6-311+G**//B3LYP/6-31G*
CCSD(T)/6-311+G**//B3LYP/6-31G*
CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G*
CBS-QB3
4.9
1.9
13.9
12.2
12.0
5.7
3.7
4.4
3.6
1.3
1.6
-4.0
a
A negative sign denotes a singlet ground state.
red (TRIR) spectroscopy, is consistent with the prediction
of DFT calculations.13 A recent report by Pritchina,
Gritsan, and Bally have examined the chemical prefer-
ences of formylnitrene (HC(dO)N) and hydroxycarbon-
ylnitrene (HOC(dO)N.1
3b
To better understand the differences between acetylni-
trene (CH C(dO)N) and methoxycarbonylnitrene (CH -
3 3
OC(dO)N), we have performed calculations to predict
their ground state multiplicities and reactivity at con-
sistent levels of theory and have attempted to directly
compare alkylcarbonylnitrenes and alkoxycarbonyl-
nitrenes with the use of isodesmic equations. Herein, we
are pleased to report our results. Some of these predic-
tions are tested by Laser Flash Photolysis studies of
benzoylnitrene.
This result is consistent with the interception of both
the singlet and triplet states of an alkoxycarbonylnitrene.
The conclusions deduced from chemical trapping stud-
ies are also consistent with matrix spectroscopic experi-
ments. The triplet ESR spectra of alkoxycarbonylnitrenes
are observed upon low-temperature photolysis of alkoxy-
carbonyl azides,7 confirming that alkoxycarbonylni-
trenes have triplet ground states. However, triplet acylni-
trenes have never been detected by ESR spectroscopy
when acyl azides are irradiated at cryogenic tempera-
tures. Photolysis of benzoyl azide and related azides fails
to produce ESR spectra characteristic of a benzoylnitrene.
Taken together, the chemical trapping studies and matrix
spectroscopic studies indicate that acylnitrenes have
singlet ground states.
,8
Results
Computational Chemistry. A. Singlet-Triplet En-
ergy Splittings. The singlet-triplet energy splitting of
nitrenes 1 and 2 and carbene 3 were computed using
B3LYP, CCSD(T), and CBS-QB3 methodologies. In par-
ticular, the CBS-QB3 method relies on a B3LYP geom-
etry, a CCSD(T) energy as an estimate for electron
correlation, and an extrapolation to the infinite basis set
limit. As such, it is the most accurate method utilized in
this study. The results are given in Table 1.
Imidogen (NH),9 alkyl7,10 and arylnitrenes7,11 have
triplet ground-state multiplicities. Recent density func-
tional theory (DFT) and high level ab initio calculations,
however, indicate that other acylnitrenes have closed-
shell singlet ground states because of a bonding interac-
tion between the oxygen and nitrogen atoms of the
1
2,13
intermediate.
B3LYP and CCSD(T) methods predict that compounds
-3 will all have triplet ground states, as opposed to
1
The position of the carbonyl vibration of singlet ben-
zoylnitrene, observed by matrix and time-resolved infra-
CBS-QB3 theory, which indicates that whereas 2 and 3
have triplet ground-state multiplicities, acetylnitrene 1
is predicted to have a singlet ground state. This conclu-
sion is in agreement with G2 calculations by Faustov et
al. B3LYP predicts the triplet nitrenes to be too stable,
relative to CBS-QB3, by approximately 8-9 kcal/mol. In
the case of the carbene, the discrepancy is only 2.4 kcal/
(
5) Autrey, T.; Schuster, G. B. J. Am. Chem. Soc. 1987, 109, 5814.
6) (a) Lwowski, W.; Mattingly, T. W., Jr. J. Am. Chem. Soc. 1965,
(
12c
8
5
5
7, 1947. (b) Lwowski, W.; Woerner, F. P. J. Am. Chem. Soc. 1965, 87,
491. (c) McConaghy, J. S.; Lwowski, W. J. Am. Chem. Soc. 1965, 87,
490.
(
(
7) Wasserman, E. Prog. Phys. Org. Chem. 1971, 8, 319.
8) Sigman, M. E.; Autrey, T.; Schuster, G. B. J. Am. Chem. Soc.
1
988, 110, 4297.
(11) (a) Hrovat, D. A.; Waali, E. E.; Borden, W. T. J. Am. Chem.
Soc. 1992, 114, 8698. See also: Castell, O.; Garc ´ı a, V. M.; Bo, C.;
Caballolo, R. J. Comput. Chem. 1996, 17, 42-48. (b) Gritsan, N. P.;
Zhu, Z.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc. 1999, 121, 1202.
(12) (a) Gritsan, N. P.; Pritchina, E. A. Mendeleev Commun. 2001,
94. (b) Shapley, W. A.; Bacskay, G. B. J. Phys. Chem. A 1999, 103,
6625. (c) Faustov, V. I.; Baskir, E. G.; Biryukov, A. A. Russ. Chem.
Bull. Int. Ed. 2003, 52, 2328.
(13) (a) Pritchina, E. A.; Gritsan, N. P.; Maltsev, A.; Bally, T.; Autrey,
T.; Liu, Y.; Wang, Y.; Toscano, J. P. Phys. Chem. Chem. Phys. 2003, 5,
1010. (b) Pritchina, E. A.; Gritsan, N. P.; Bally, T. Russ. Chem. Bull.
Int. Ed. 2004, 53, in press.
(
9) (a) Fairchild, P. W.; Smith, G. P.; Crosly, D. R.; Jeffries, J. B.
Chem. Phys. Lett. 1984, 107, 181. (b) Engelking, P. C.; Lineberger, W.
C. J. Chem. Phys. 1976, 65, 4323.
(
10) (a) Travers, M. J.; Cowles, D. C.; Clifford, E. P.; Ellison, G. B.;
Engelking, P. C. J. Phys. Chem. 1999, 111, 5349. (b) Kemnitz, C. R.;
Ellison, G. B.; Karney, W. L.; Borden, W. T. J. Am. Chem. Soc. 2000,
1
1
1
22, 1098. (c) Barash, L.; Wasserman, E.; Jager, W. A. J. Chem. Phys.
967, 89, 3931. For a correction, see: Ferrante, R. F. J. Chem. Phys.
987, 86, 25. (d) Radziszewski, J. G.; Downing, J. W.; Wentrup, C.;
Kaszynski, P.; Jawdosiuk, M.; Kovacic, P.; Michl, J. J. Am. Chem. Soc.
1
985, 107, 2799.
8584 J. Org. Chem., Vol. 69, No. 25, 2004

Comparison of Acetyl- and Methoxycarbonylnitrenes
FIGURE 1. Geometries of 1-3 in their singlet and triplet states. Distances are shown in Å, and angles are in degrees.
mol. Scott et al. came to similar conclusions in their
studies of carbonyl carbenes.23
The geometries of 1-3 in their singlet and triplet states
are shown in Figure 1. The bond distance between the
oxygen and nitrogen atoms in the singlet state is about
set extrapolation predicted that this same species has a
singlet ground state.
To better understand why the ground-state multiplici-
ties of 1 and 2 differ, several isodesmic reactions were
calculated with DFT methods. To verify the accuracy of
DFT methods in this case, CBS-QB3 methods were also
applied to the isodesmic reactions I and III. Compared
to the CBS-QB3 method, the B3LYP results differ by less
than 1 kcal/mol and indicate that DFT is a reliable
method for this purpose. Upon consideration of reaction
I, it can be seen that singlet methoxycarbonylnitrene 5
is 13.1 kcal/mol more stable than acylnitrene 4 in the
singlet state. On comparing the triplet state, methoxy-
0
.46 Å shorter than that in the triplet state for acetyl-
nitrene, and the O-C-N bond angle in the singlet state
is also smaller than that in the triplet state. Therefore,
as noted in earlier studies,13 a bonding interaction
between the oxygen and nitrogen atoms provides an
explanation for the singlet ground state of acetylnitrene
1
. Methoxycarbonylnitrene 2 and carbene 3 have weaker
bonding interactions between oxygen and nitrogen or
between oxygen and carbon in their singlet states, as
demonstrated by the longer bond distances and larger
3
3
carbonylnitrene 5 is even more stable, relative to 4, by
7.1 kcal/mol (reaction III) than the singlet state ana-
1
logues. The difference in the singlet-triplet energy
splitting of 1 and 2 is explained by isodesmic equations
II and IV. Conjugation of oxygen with a C(dNH)OH
group is stabilizing by 15.7 kcal/mol (reaction II) as in
the carbonylnitrene singlet state, but conjugation of
bond angles (Figure 1). Our results are reminiscent of
the results obtained by Pritchina et al.13 in which B3LYP/
6
-31G* and CCSD methods, coupled with a complete
basis set extrapolation, predicted that H(CdO)N has a
triplet ground state, but CCSD(T) with a complete basis
•
oxygen to a C(dO)NH radical (reaction IV), as in the
(
(
14) Ziegler, T. Chem. Rev. 1991, 91, 651.
15) Jensen, F. Introduction to Computational Chemistry; Wiley:
Chichester, 1998.
(
(
16) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
17) (a) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (b) Gonzalez, C.;
Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(
18) (a) Cizek, J. Adv. Chem. Phys. 1969, 14, 35. (b) Barlett, R. J. J.
Phys. Chem. 1989, 93, 1963. (c) Raghavachari, K.; Trucks, G. W.; Pople,
J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989, 157, 479.
(
(
19) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
20) Montgomery, J. A., Jr.; Frisch, M. J.; Ochterski, J. W.; Peters-
son, G. A. J. Chem. Phys. 1999, 110, 2822.
(
(
21) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027.
22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople,
J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, 1998.
(
23) Scott, A. P.; Platz, M. S.; Radom, L. J. Am. Chem. Soc. 2001,
1
23, 6069.
J. Org. Chem, Vol. 69, No. 25, 2004 8585

Liu et al.
FIGURE 2. Energy surface [∆G (298 K) (kcal/mol)] for Curtius rearrangement of 1 (top) and 2 (bottom) at the B3LYP/6-311+G**//
B3LYP/6-31G* level in acetonitrile and cyclohexane and in the gas phase, as well as CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* and
CBS-QB3 levels. Distances are shown in Å, and angles are in degrees.
triplet nitrene, is even more stabilizing (by 18.1 kcal/mol).
Thus the methoxycarbonylnitrene has a triplet ground
state, and the acetylnitrene does not.
were calculated for the configurational isomers: ethoxy-
carbonylnitrene 5 and methoxymethylacylnitrene 4. The
transition state for Curtius rearrangement of 5 is 20-
30 kcal/mol more stable relative to the transition state
for Curtius rearrangement of its isomeric methoxy-
methylacylnitrene 4. The oxygen atom present in 5 is
responsible for an interaction that stabilizes the transi-
tion state for the Curtius rearrangement. This interaction
is not present in the transition state of the isomeric
acylnitrene 4. The extra oxygen atom stabilizes the
transition state of Curtius rearrangement more than it
stabilizes nitrene 5, and thus methoxycarbonylnitrene
should rearrange faster than acetylnitrene. Thus we
predict that Curtius rearrangement of singlet acetyl-
nitrene 1 will be much slower than that of singlet
methoxycarbonylnitrene 2, despite the greater exoergicity
of the former process. Furthermore, Curtius rearrange-
ment of 1 should not be competitive with bimolecular
reactions of this nitrene at ambient temperature. This
is in good agreement with the reports of Lwowski and
co-workers.3
B. Curtius Rearrangement. The reaction surfaces
for Curtius rearrangement of nitrenes 1 and 2, as
computed by DFT and CBS-QB3, are shown in Figure 2,
and these results are in agreement with previous
1
2c
calculations on the activation barrier for rearrange-
ment of 1. The Curtius rearrangement of 1 is substan-
tially more exoergic than that of 2 because a relatively
weak N-O bond is formed in the isocyanate product, but
surprisingly, rearrangement of 1 has a larger free energy
of activation. A substantial solvent effect is not predicted
by the polarizable continuum model (PCM) calculations
using either polar solvent acetonitrile or the nonpolar
solvent cyclohexane for the Curtius rearrangements.
The relative free energies of the two reactions can be
understood upon consideration of the following isodesmic
equations.
,5
We also considered a concerted pathway for Curtius
rearrangement from the precursor azides for isocyanate
formation (Figure 4). CBS-QB3 calculations find two
stable conformations of acetyl azide in a manner remi-
niscent of diazo carbonyl compounds.24 To the best of our
knowledge, experimental studies of the conformational
isomerism of methoxycarbonyl azide or acetyl azide have
not been reported; however, experimental and computa-
A C-O bond is simply much stronger than an N-O
bond, which makes the product of the Curtius rearrange-
ment of 1 much more stable than that of 2.
3
tional studies of fluorocarbonyl azide (F(CdO)N ) have
To understand the difference in the respective activa-
tion barriers, the isodesmic reactions shown in Figure 3
(
24) Regitz, M. Diazo Compounds: Properties and Synthesis; Aca-
demic Press: New York, NY, 1986.
8586 J. Org. Chem., Vol. 69, No. 25, 2004

Comparison of Acetyl- and Methoxycarbonylnitrenes
FIGURE 3. Energy surface [∆G (298K) (kcal/mol)] and isodesmic reactions for Curtius rearrangement of 5 and an isomeric
analogue 4 at the B3LYP/6-311+G**//B3LYP/6-31G* (top) and CBS-QB3 (bottom) levels of theory. Distances are shown in Å, and
angles are in degrees.
been reported25 and the conformer with syn relationship
between the carbonyl and the azide group is favored as
we predict for acetyl azide. In the gas phase for acetyl
azide, the less stable form (∆G ) 4.5 kcal/mol at the CBS-
QB3 level) positions the azide group anti to the carbonyl
group. The energy difference is slightly reduced in
cyclohexane (4.1 kcal/mol) and in acetonitrile (3.2 kcal/
mol). CBS-QB3 theory predicts that this conformer will
isocyanate. These effects are felt by the transition states
that lead to these products. Thus, formation of the
alkoxycarbonylnitrene is formed in the pyrolysis of the
alkoxycarbonyl azide.
This computational result is well-precedented as Ka-
plan, Meloy, and Mitchell26 have experimentally demon-
strated similar behavior in carbonyl-substituted diazo
compounds. Pyrolysis of syn diazo carbonyls leads to
Wolff rearrangement in concert with loss of nitrogen, but
heating compounds with anti-disposed carbonyl and diazo
groups produces the trappable carbene.
*
extrude nitrogen (∆G ∼32 kcal/mol) to form acetyl-
nitrene, which can subsequently isomerize to methyliso-
cyanate in the absence of trapping agents.
The barriers predicted for loss of nitrogen are consis-
tent with the activation energies to thermal decompostion
of a number of azides.27 The barriers to decomposition of
phenylacetyl azide and benzoyl azide in toluene are 21.5
and 27.1 kcal/mol, respectively.
C. Intramolecular C-H Insertion. The potential
energy surfaces for the intramolecular C-H insertion
reactions of 8 and 9 were calculated using DFT methods.
The insertion reaction of 9 is about 1.8 kcal/mol more
exoergic than that of 8 and has a smaller free energy of
activation barrier by about 0.7 kcal/mol (see Supporting
The lower energy conformer (by 4.5 kcal/mol) of acetyl
azide has a syn relationship between the carbonyl and
the azide groups. CBS-QB3 theory predicts that acetyl
azide will decompose by concerted migration of the
methyl group along with nitrogen extrusion and that the
free energy of activation for this concerted process is only
2
7 kcal/mol. Thus we predict that a free nitrene is not
produced upon pyrolysis of acetyl azide. This explains the
failure of Lwowski and co-workers to intercept piv-
aloylnitrene upon pyrolysis of pivaloyl azide. In this
3
Curtin-Hammett situation, the relative barriers between
concerted and stepwise Curtius rearrangement may, of
course, be very sensitive to substituents on the carbonyl
group. Indeed, for methoxycarbonyl azide (Figure 4b), the
stepwise process to generate the free nitrene is favored
over the concerted Curtius rearrangement. This is in good
agreement with experimental studies of alkoxycarbonyl
7,8
azides, which do form trappable alkoxycarbonylnitrenes
upon thermolysis. Essentially the alkoxy group stabilizes
the alkoxycarbonylnitrene and destabilizes the alkoxy-
(
25) Mack, H.; Della Vedova, C. O.; Willner, H. J. Mol. Struct. 1993,
2
91, 197.
J. Org. Chem, Vol. 69, No. 25, 2004 8587

Liu et al.
FIGURE 4. Energy surface [∆G (298 K) (kcal/mol)] for stepwise and concerted Curtius rearrangement of (a) acetyl azide and (b)
methoxycarbonyl azide at the B3LYP/6-311+G**//B3LYP/6-31G* (red, top) and CBS-QB3 (black, bottom) levels of theory. Distances
are shown in Å, and angles are in degrees.
8588 J. Org. Chem., Vol. 69, No. 25, 2004

Comparison of Acetyl- and Methoxycarbonylnitrenes
FIGURE 5. Energy surface [∆G (298 K) (kcal/mol)] for the reaction of singlet 1 with propane at the B3LYP/6-311+G**//B3LYP/
6-31G* (top) and CBS-QB3 (bottom) levels of theory. Distances are shown in Å, and angles are in degrees.
FIGURE 6. Energy surface [∆G (298 K) (kcal/mol)] for the reaction of singlet 1 with ethylene at the B3LYP/6-311+G**//B3LYP/
-31G* (top) and CBS-QB3 (bottom) levels of theory. Distances are shown in Å, and angles are in degrees.
6
Information). In each case, the free energy of activation
is positive but the enthalpy of activation is slightly
negative. The barriers of these reactions are entirely
entropic as found previously for the reaction of dichlo-
rocarbene with alkenes.28, 29
attack syn to the carbonyl oxygen could be located, but
it does not lead to an insertion product (see Supporting
Information).1 Instead, the syn transition state leads
to the Curtius rearrangement as verified by an IRC
calculation. In essence, syn attack corresponds to a
concerted Curtius rearrangement with the small effect
of solvation by an associated propane or ethylene mol-
ecule. However, for methoxycarbonylnitrene (2), transi-
tion states for true insertion into the C-H bond of
propane and into the CdC bond of ethylene could not be
located (see Supporting Information). Instead, the reac-
tions of 2 with propane and ethylene are barrierless
processes and proceed directly to products. Once again,
approach of the substrate to the nitrene occurs anti to
the carbonyl oxygen.
3b
D. Bimolecular Reactions. Energy surfaces for the
reactions of singlets 1 and 2 with propane, ethylene, and
methanol were calculated by both DFT and CBS-QB3
methods. The results are given in Figures 5-7. Both
methods predict that all of these reactions with singlet
acetylnitrene (1) and methoxycarbonylnitrene (2) are
extremely exoergic. The reactions of 1 with propane are
the most exoergic, -68.6 kcal/mol. The alkene cycload-
dition reactions are exoergic by about -55 kcal/mol, and
the O-H insertion reactions are the least exoergic, with
a change of free energy of approximately -40 kcal/mol.
Compound 2 reacts more exoergically than 1 in all three
reactions. The changes in activation enthalpies are given
in Figures 5-7, as well as the changes in free energies.
Indeed, transition-state structures could be located for
the propane and ethylene reactions with acetylnitrene
Two unique transition states for 1 and for 2 could be
located for reactions with methanol. These transition
states, once again, differ by the orientation of the
substrate relative to the carbonyl oxygen of the nitrene,
and in general, anti attack is preferred (see Figure 7 and
Supporting Information). The O-H insertion barrier for
methoxycarbonylnitrene (2) is lower than that of com-
pound 1 by ∼5 kcal/mol. Activation enthalpies are close
to zero and are even negative in certain cases. This is
reminiscent of the experimental findings of Moss and
Turro28 and the theoretical work of Houk and Rondan
that demonstrated that the free energy barriers to
bimolecular reactions of halocarbenes are dominated by
unfavorable entropic changes. In each case calculated
(
1) wherein the target reactant approaches anti to the
carbonyl oxygen of the nitrene. A transition state for
(
26) (a) Kaplan, F.; Meloy, G. K. J. Am. Chem. Soc. 1966, 88, 950.
(
b) Kaplan, F.; Mitchell, M. L. Tetrahedron Lett. 1979, 759.
(
27) (a) Abramovitch, R. A.; Kyba, E. P. The Chemistry of the Azido
Group; Patai, S., Ed.; Interscience, New York, NY, 1971. (b) L’Abbe,
G. Chem. Rev. 1969, 69, 345.
29
(
28) Turro, N. J.; Butcher, J. A.; Moss, R. A.; Guo, W.; Munial, R.
C.; Fedorynski, M. J. Am. Chem. Soc. 1980, 102, 7576.
29) Rondan, N. G.; Houk, K. N.; Moss, R. A. J. Am. Chem. Soc.
980, 102, 1770.
(
1
J. Org. Chem, Vol. 69, No. 25, 2004 8589

Liu et al.
FIGURE 7. Energy surface [∆G (298K) (kcal/mol)] for the reaction of singlet 1 and 2 with methanol at the B3LYP/6-311+G**//
B3LYP/6-31G* (top) and CBS-QB3 (bottom) levels of theory. Distances are shown in Å, and angles are in degrees.
here, the bimolecular reactions look like nucleophilic
attack on nitrogen.
agreement of the IR spectrum of benzoylnitrene in an
argon matrix with that predicted by theory for the singlet
ground state.13
Laser flash photolysis (LFP, XeCl excimer laser, 308
nm, 17 ns) of benzoyl azide 10 in CF
2 2
ClCFCl (Freon-
13) solvent produces the transient spectrum of ben-
1
zoylnitrene in its singlet ground state (Figure 8). The
transient spectrum observed is in good agreement with
the spectrum of this species obtained in an argon matrix
and with the predictions of CASSCF/CASPT2 proce-
dures.13
The calculations predict that bimolecular reactions of
singlet acetylnitrene and methoxycarbonylnitrene will be
faster than the Curtius rearrangements of these species,
particularly at low temperatures where the unfavorable
T∆S component of the free energy barrier is small.
Laser Flash Photolysis Studies. To test the predic-
tions of computational chemistry, laser flash photolysis
(
LFP) studies were performed. Benzoyl azide was studied
because it has previously received considerable attention,
is easier to handle than acetyl azide, and should have a
superior UV-vis chromophore to facilitate LFP studies.13
Chemical trapping studies are consistent with the inter-
mediacy of benzoylnitrene, and the bulk of the studies
are consistent with an intermediate with a singlet ground
We find that the lifetime of benzoylnitrene (11) in
deoxygenated Freon-113 is approximately 2 µs. The
lifetime of 11 in acetonitrile is 330 ns as measured by
time-resolved IR (TRIR) spectroscopy using relatively
concentrated samples.1
Benzoylnitrene (11) reacts with pyridine to form an
ylide (Supporting Information). The spectrum of the ylide
formed by reaction of 11 with p-cyanopyridine is par-
ticularly intense (Figure 9, λmax ) 380-390 nm), and ylide
3
0-33
state.
This conclusion is supported by the failure to
detect a triplet ESR spectrum upon photolysis of benzoyl
azide in a matrix at low temperature5 and the excellent
,7
3a
(30) Hayashi, Y.; Swern, D. J. Am. Chem. Soc. 1973, 95, 5205.
(31) Puttner, R.; Hafner, K. Tetrahedron Lett. 1964, 3119.
(32) Semenov, V. P.; Studenikov, A. N.; Bespalov, A. D.; Ogloblin,
K. A. J. Org. Chem. USSR (Engl. Transl.) 1977, 12, 2052.
33) (a) Inagaki, M.; Shingaki, T.; Nagai, T. Chem. Lett. 1981, 1419.
b) Inagaki M.; Shingaki, T.; Nagai, T. Chem. Lett. 1982, 9.
(
(
8590 J. Org. Chem., Vol. 69, No. 25, 2004

Comparison of Acetyl- and Methoxycarbonylnitrenes
FIGURE 8. Transient spectrum of benzoylnitrene (11) pro-
FIGURE 10. Plot of the observed rate constant for the
disappearance of benzoylnitrene (11) as a function of [aceto-
nitrile] in CF ClCFCl .
2 2
2 2
duced by 308 nm LFP of benzoyl azide in CF ClCFCl at
ambient temperature. The spectrum was recorded 10 ns after
the laser pulse over a window of 20 ns.
FIGURE 11. Plot of the log kobs for the decay of benzoylnitrene
11) in pentane versus 1/T (in K).
(
FIGURE 9. Transient spectrum of ylide 13 produced by 308
2 2
nm LFP of benzoyl azide in CF ClCFCl containing 0.1 M
TABLE 2. Absolute Rate Constants for Reaction of
Benzoylnitrene (11) with Various Quenchers at Ambient
Temperature
p-cyanopyridine. The spectrum was recorded over a 20 ns
window, 100 ns after the laser pulse.
k (M- s-1
1
)
13 is indefinitely stable in solution. The ylide structure
substrate
solvent
was confirmed by X-ray crystallography (Supporting
Information).
5
6
8
6
5
8
7
7
CH3CN
CF2ClCFCl2
CF ClCFCl
CF2ClCFCl2
CH3CN
CF2ClCFCl2
CH3CN
(3.4 ( 0.6) × 10
(6.5 ( 0.4) × 10
(1.7 ( 0.1) × 10
(4.0 ( 0.2) × 10
(1.8 ( 0.2) × 10
(6.6 ( 0.4) × 10
(4.2 ( 0.2) × 10
(2.2 ( 0.1) × 10
CH OH
3
2
2
CF3CH2OH
H2O
cyclohexane
CH3CH2CH2CH2NH2
1
2
1
-methylcyclohexene
,3-dimethylbutene
-hexene
CF2ClCFCl2
CF2ClCFCl2
CF2ClCFCl2
CF2ClCFCl2
CH3CN
7
8
(1.1 ( 0.04) × 10
6
trans-1,2-dichloroethylene
DABCO
p-cyanopyridine
(4.2 ( 0.5) × 10
8
(3.7 ( 0.1) × 10
CH3CN
(2.3 ( 0.03) × 10
apparent activation energy for the disappearance of
benzoylnitrene is -3.2 kcal/mol.
Benzoyl azide was photolyzed in pentane at 223 and
98 K, the extremes of the Arrhenius plot of Figure 11.
2
The products of photolysis were analyzed by GC-MS at
each temperature, and C-H insertion products were the
only products detected at both temperatures (see Sup-
porting Information). It is clear that C-H insertion is
the major reaction channel at 223 and 298 K but is not
necessarily the only process that consumes benzoylni-
trene in pentane. Thus, the negative activation energy
of -3.20 ( 0.02 kcal/mol is an effective activation energy
only and cannot be associated with an elementary
process.
The lifetime of 11 is reduced in the presence of
quenchers. A plot of 1/τ versus [acetonitrile] is linear
(Figure 10) with slope equal to the absolute rate constant
of reaction of benzoylnitrene (11) with acetonitrile.
Absolute reaction rate constants of benzoylnitrene ob-
tained in this manner are collected in Table 2.
The lifetime of benzoylnitrene in pentane decreases as
the temperature decreases. An Arrhenius treatment of
the temperature dependence of the pseudo-first-order
decay of benzoylnitrene is shown in Figure 11. The
To obtain the activation energy of an elementary
process, a more rapid quencher of benzoylnitrene was
J. Org. Chem, Vol. 69, No. 25, 2004 8591

Liu et al.
that of acetylnitrene because the oxygen atom stabilizes
the transition state of the Curtius rearrangement of
methoxycarbonylnitrene.
Bimolecular reactions of singlet acetylnitrene and
methoxycarbonylnitrene with propane, ethylene, and
methanol have enthalpic barriers that are close to zero
and free energy barriers that are completely entropic in
nature, as found previously for halocarbene singlet
states.2
0,21
Bimolecular reactions of the singlet states of
acetylnitrene and methoxycarbonylnitrene will be much
faster than the Curtius rearrangement, particularly at
low temperatures.
These predictions were tested by laser flash photolysis
LFP) studies of benzoylnitrene. Absolute rate constants
(
of the reaction of benzoylnitrene with several substrates
were determined at ambient temperature. The pseudo-
first-order rate constant of decay of benzoylnitrene in
pentane increases as the temperature decreases. The
apparent activation energy is -3.20 ( 0.02 kcal/mol. The
activation energy for the reaction of benzoylnitrene with
FIGURE 12. Plot of the log of the absolute rate constant for
reaction of benzoylnitrene (11) with 1-hexene in CF
versus 1/T (in K).
2 2
ClCFCl
studied. The absolute rate constant for reaction of ben-
zoylnitrene with 1-hexene in CFCl CFCl at a given
2
2
1
-hexene is essentially zero. The LFP data demonstrate
temperature was determined by measuring the observed
rate of decay of benzoylnitrene as a function of alkene
concentration. This process was repeated at several
temperatures. An Arrhenius treatment of the data is
given in Figure 12. The activation energy for the reaction
of singlet benzoylnitrene with 1-hexene is -0.06 ( 0.001
that entropic factors control the bimolecular reactivity
of benzoylnitrene as predicted by theory.
Experimental Methods
kcal/mol.
Synthesis of Benzoyl Azide. Benzoyl azide was synthe-
3
3
On the basis of previously reported30-33 product studies,
sized by the method of Barret et al. Briefly, benzoyl chloride
(
7.0 g, 0.05 mol) was dissolved in 12.5 mL of acetone. To this
mixture was slowly added a solution of NaN (3.7 g, 0.057 mol)
in 10 mL of water at 0 °C. When addition of the NaN solution
we expected that the kinetics of benzoylnitrene with
alkenes and alkanes correspond to typical addition and
insertion chemistry. This is consistent with GC-MS
analysis of the products formed in photolysis of benzoyl
azide in pentane containing 1-hexene (see Supporting
Information). The laser flash photolysis results indicate
that the enthalpy of activation for C-H insertion is
slightly negative and that for addition is essentially zero.
The latter result is in excellent agreement with the value
predicted by theory for the reaction of acetylnitrene with
ethylene. This demonstrates that the addition of ben-
zoylnitrene to 1-hexene is dominated by entropic factors
in a manner reminiscent of the addition of singlet
carbenes to olefins.
3
3
was complete, the reaction was allowed to mix at 0 °C for 30
min. Two layers formed and the top layer was removed and
dried with anhydrous magnesium sulfate. The dried solution
had the solvent removed in vacuo to yield a white solid. Mp:
-
1 13
2
4-25 °C. IR (NaCl, neat): 2135, 1693, 1600 cm
.
C NMR
(
CDCl ): 128.66, 129.47, 131.00, 135.01, 172.00 ppm. UV
3
max: 240 nm.
Materials. All quenchers were purchased at 99.5% purity
and used as received with the exception of pyridine, which was
distilled from KOH before using.
Laser Flash Photolysis of Benzoyl Azide. Apparatus.
A Nd:YAG laser (17 ns, 10 mJ, 266 nm) and a Lambda Physics
LPX (XeCl, 30 ns, 50 mJ, 308 nm) were used as the excitation
light sources.
Conclusions
Laser flash photolysis experiments with benzoyl azide were
performed using either a 266 or 308 nm pump and were probed
at 320 nm. The Ohio State University ns spectrometer has
been described elsewhere. Briefly, all samples were dissolved
in spectroscopic grade solvent to an absorption of 1 at either
CBS-QB3 calculations correctly predict that methoxy-
carbonylnitrene has a triplet ground state, whereas
acetylnitrene has a singlet ground state. B3LYP/6-
35
2
66 or 308 nm and placed in a 3-mL quartz cuvette. Samples
3
6
11+G**//B3LYP/6-31G*, CCSD(T)/6-311+G**//B3LYP/
were not purged before use. Transient UV spectra were
-31G*, and CCSD(T)/aug-cc-pDVZ//B3LYP/6-31G* cal-
3
5
obtained using a CCD camera as described elsewhere. For
variable temperature experiments, the temperature was con-
trolled by passing a thermostabilized nitrogen gas stream and
kept within 1 K of the reported temperature. In the Arrhenius
plots, at least three kinetic traces were obtained.
culations incorrectly predict that acetylnitrene has a
triplet ground state. Isodesmic reactions reveal that the
oxygen atom stabilizes both the singlet and triplet states
of the carbonyl nitrenes but stabilizes the triplet more
than the singlet state. Thus, methoxycarbonylnitrene has
a triplet ground state. Our calculations are consistent
Isolation of p-Cyanopyridine Benzoylnitrene Ylide.
Benzoyl azide (0.043 g, 0.29 mM) and p-cyanopyridine (0.037
g, 0.355 mM) were dissolved in 3 mL of dichloromethane. The
reaction was irradiated with 365 nm light for 2 h. The solvent
was then removed under reduced pressure to yield a yellow
oil. This oil was then purified on a silica gel column and eluted
with acetone and hexanes. The product was isolated in the
with that of earlier studies1
2,13
that indicated a significant
bonding interactions in the singlet state of acetylnitrene.
We find a comparable (but slightly weaker) interaction
in the singlet state of methoxycarbonylnitrene.
The Curtius rearrangement of singlet acetylnitrene is
much more exoergic than that of methoxycarbonyl-
nitrene. Nevertheless, the Curtius rearrangement of the
methoxycarbonylnitrene is predicted to be faster than
(
34) Barret, E. W.; Porter, C. W. J. Am. Chem. Soc. 1941, 63, 3434.
(
35) Gritsan, N. P.; Zhai, H. B.; Yuzawa, T.; Karweik, D.; Brooke,
J.; Platz, M. S. J. Phys. Chem. A 1997, 101, 2833.
8592 J. Org. Chem., Vol. 69, No. 25, 2004

Comparison of Acetyl- and Methoxycarbonylnitrenes
third fraction and identified by X-ray crystallography (Sup-
porting Information).
In some cases, CCSD(T)18 singlet-point energies were evalu-
ated using the B3LYP/6-31G* geometries, with the 6-311+G**
(6d) basis set as well as Dunning’s correlation consistent basis
Product Studies of Benzoyl Azide at 285 and 240 K. A
1
9
0.29 M solution of benzoyl azide in pentane in the presence or
set, aug-cc-pVDZ. For further comparison, Complete Basis
2
0
absence of 3.7 M 1-hexene was placed in a quartz cuvette and
irradiated with 254 nm mercury lamps in a Rayonet photo-
reactor until no more starting material was present. The
sample was then analyzed by GC-MS to determine products
and ratios. Samples irradiated at 240 K were kept in a dry
ice/acetonitrile bath maintained at 240 K during photolysis.
Set (CBS) methods have also been applied to increase the
accuracy of the results. The energies provided are CBS-QB3
enthalpies and free energies at 298.15 K. All stationary points,
including transition states, were re-characterized at the CBS-
QB3 level, complete with full geometry optimization at the
required level. To study the influence of solvent, polarizable
2
1
continuum model (PCM) calculations were used for the
Curtius rearrangement with acetonitrile and cyclohexane as
solvents. Population analyses were performed at the B3LYP/
6-311+G**//B3LYP/6-31G* level using the natural population
Computational Methods
Density functional theory (DFT)14 and ab initio methods15
have been applied in this study. The geometries were com-
pletely optimized at the B3LYP/6-31G* level. Analytical
vibrational frequencies were calculated at the same level for
each stationary point to verify a minimum energy structure
and to provide zero-point vibrational energy corrections, which
were scaled by a factor of 0.9806.16 Each transition state was
verified to connect to the respective reactant and product by
careful optimization (opt ) calcfc or calcall) after displacement
3
6
analysis (NPA) method of Weinhold and co-workers.
All DFT, CCSD(T), and CBS-QB3 calculations were per-
formed with Gaussian 9822 at the Ohio Supercomputer Center.
Acknowledgment. Support of this work by the
National Science Foundation and the Ohio Supercom-
puter Center is gratefully acknowledged.
(
typically 10%) along the reaction path for the normal coor-
dinate of the imaginary vibrational frequency. In some cases,
Supporting Information Available: Tables of energies,
Cartesian coordinates, vibrational frequencies, and selected
figures of computational and experimental results. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
7
intrinsic reaction coordinate (IRC) calculations were also
performed. The zero-point vibrational energies as well as
thermal and entropic contributions to the free energies were
obtained from the B3LYP/6-31G* frequency calculations; the
thermal and entropic corrections used the unscaled vibrational
frequencies. Single-point energy calculations of all stationary
points were performed at the B3LYP/6-311+G** level using
the corresponding B3LYP/6-31G* geometries. Six Cartesian
d functions were used for these calculations.
JO048433Y
(
36) Reed, A. E.; Weinhold, F.; Curtiss, L. A. Chem. Rev. 1988, 88,
899.
J. Org. Chem, Vol. 69, No. 25, 2004 8593