Ultrafast time-resolved infrared spectroscopy study of the photochemistry of N,N-diethyldiazoacetamide: rearrangement in the excited state

Article abstract of DOI:10.1021/ja904009x

(Graph Presented) Ultrafast infrared spectroscopy shows that in chloroform, β-lactam is formed immediately after the laser pulse but γ-lactam is formed from both slow and fast processes. It is concluded that β-lactam is formed from the diazoamide excited state via the rearrangement in the excited state (RIES) mechanism and that γ-lactam is formed from both RIES and carbene. In methanol, both carbene decay and the rise of amide ether product are observed directly. Predictions from density functional theory calculations are consistent with these observations.

Full text of DOI:10.1021/ja904009x

Published on Web 06/24/2009
Ultrafast Time-Resolved Infrared Spectroscopy Study of the Photochemistry
of N,N-Diethyldiazoacetamide: Rearrangement in the Excited State
Yunlong Zhang,† Gotard Burdzin´ski,‡ Jacek Kubicki,‡ and Matthew S. Platz*,†
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210, and Quantum
Electronics Laboratory, Faculty of Physics, Adam Mickiewicz UniVersity, 85 Umultowska, Poznan 61-614, Poland
Received May 17, 2009; E-mail: platz.1@osu.edu
Photolysis of N,N-diethyldiazoacetamide (DZA) produces a
nearly equal mixture of ꢀ- and γ-lactams (Scheme 1),1,2 and the
reaction has potential application in the synthesis of penicillins.3
The formation of ꢀ-lactams contrasts with the photochemistry of
diazoesters, which fail to produce lactones.1 Rando1,2 pointed out
that diazocarbonyl compounds are planar4 and that diazoamides
necessarily have an alkyl group in proximity to the incipient carbene
center promoting cyclization, but the single alkyl group on the ester
is disposed away from it, making the alkoxyl group unavailable
for cyclization.2 Further studies by Tomioka et al.5,6 using carbene
quenchers demonstrated that the γ-lactam product was formed from
both a trappable singlet carbene and a second, untrappable
intermediate. As methanol, an excellent carbene trap, failed to
depress the yield of ꢀ-lactam, it must be formed entirely from a
non-carbene pathway. Tomioka et al.5 proposed that the ꢀ-lactam
was produced from the singlet excited state of the diazoamide
(Scheme 1). Similar conclusions have been made concerning the
photochemical Wolff rearrangement,7 and such processes were
named by Liu as examples of “rearrangement in the excited state”
(RIES).8 The mechanism of Scheme 1 predicts that the ꢀ-lactam
and a portion of the γ-lactam are produced on different time scales
(faster via RIES and slower with carbene intermediacy). As these
two lactams have distinct carbonyl vibrations, we were encouraged
to test that prediction using ultrafast time-resolved IR spectroscopy.
transient absorption spectra produced with 270 nm laser pulses (300
fs) are given in Figure 1. Significantly, these two lactams are formed
at different rates. Within 2 ps of the laser pulse, ꢀ-lactam was
detected as a broad band at 1760-1700 cm-1 that subsequently
narrowed and shifted to the blue within 50 ps, a result typical of
vibrational cooling (VC) of newly born species.9 ꢀ-Lactam was
formed instantaneously (<0.4 ps), and the long growth time constant
of 29 ps probed at 1745 cm-1 was due to VC (Figure S2a). After
50 ps, the VC subsided, and the ꢀ-lactam band centered at 1745
cm-1 maintained the same intensity over 3 ns. However, the
mechanism for γ-lactam formation is different. The γ-lactam band
observed at 1669 cm-1 was formed on two time scales: a fast
component was present within 50 ps of the laser pulse, and a slow
component gradually grew over 50-1000 ps with a time constant
of 190 ( 42 ps (Figure S2b). According to calculations (Figure
S1, Table S1), the singlet carbene overlaps with γ-lactam and thus
also significantly contributes to the instantaneous rise of the band
at 1669 cm-1 (see below).
Scheme 1. Reaction Pathway for N,N-Diethyldiazoacetamide
(DZA) upon Photolysis
The FT-IR absorption spectra of DZA in chloroform solution
before and after 266 nm photolysis are shown in Figure S1 in the
Supporting Information along with their calculated frequencies. The
band at 1745 cm-1 is easily attributed to the ꢀ-lactam on the basis
of its characteristic absorption at this high wavenumber, consistent
with calculations and the literature.1,2 The band at 1669 cm-1 is
confidently assigned to the γ-lactam upon comparison with the
spectrum of an authentic sample. The ultrafast time-resolved
Figure 1. Transient IR spectra produced upon 270 nm photolysis of DZA
in chloroform. The dashed curves are the FTIR spectra of authentic γ-lactam.
A DZA solution in CH3OD was bleached with 266 nm light,
and the resulting FT-IR spectra are shown in Figure S3 along with
the calculated frequencies. The carbonyl stretching bands for both
ꢀ- and γ-lactams in CH3OD were slightly red-shifted (relative to
those in chloroform) and split into doublets, which are attributed
to mono- and dihydrogen-bonded carbonyl stretching bands, as
reported by Minato.10 In addition, a strong band at 1640 cm-1 was
† The Ohio State University.
‡ Adam Mickiewicz University.
9
9646 J. AM. CHEM. SOC. 2009, 131, 9646–9647
10.1021/ja904009x CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
observed (Figures S1 and S3), which is assigned to the amide ether
(AE) product formed from the reaction of the singlet carbene with
methanol.2 The time-resolved transient IR spectra produced with
270 nm laser pulses (300 fs) in CH3OD are shown in Figure 2.
calculations reported in Table S1, the yield of γ-lactam formed
from the RIES pathway is ∼30% (see the transient absorption
spectrum at 3050 ps in Figure 2), which is consistent with the
previously reported chemical analyses.6,7
Significantly, a new transient species with a band peaking at 1706
cm-1 was observed in CH3OD. Even though this band overlapped
one of the ꢀ-lactam doublet peaks, it had a decay time constant of
102 ( 12 ps (Figure S6b), and remarkably, its decay correlated
with the growth of the AE band at 1640 cm-1 (109 ( 10 ps, Figure
S7). The carrier of the transient band at 1706 cm-1 is assigned to
the singlet carbene on the basis of computational results (Figure
S3). Another assignment of the carrier of this band is the cation
formed by protonation of the singlet carbene, but this is not
consistent with theory (Table S1). The singlet carbene was readily
detected in CH3OD relative to chloroform because (1) quantum-
mechanical calculations predict that the vibrational band of the
singlet carbene in methanol is less obscured by other species
(Figures S1 and S2), (2) this band has a larger intensity in methanol
than in chloroform (Table S1), and (3) VC is faster in methanol
than in chloroform and obscures the carbene spectra to a lesser
extent.
The transition states for ꢀ-lactam and γ-lactam formation from
the singlet carbene were located with density functional theory
(DFT) methods (Figure S8, Tables S2 and S3). The energy barrier
for γ-lactam formation (4.4 kcal/mol) is predicted to be 1.8 kcal/
mol less than that for ꢀ-lactam (6.2 kcal/mol), consistent with the
selective formation of γ-lactam from singlet carbene. Transition
state theory predicts that the lifetime of the relaxed singlet carbene
is 271 ps at 298 K. This is consistent with the 190 ( 42 ps growth
time constant for γ-lactam in chloroform.
In summary, our experiments indicate that in chloroform, both
ꢀ- and γ-lactams are formed from the diazoamide precursor via
RIES, and γ-lactam is also formed by isomerization of relaxed
singlet carbene. In methanol-O-d both carbene decay and the rise
of amide ether product are observed directly. The predictions of
DFT calculations are consistent with these experimental observations.
Figure 2. Transient IR spectra produced upon 270 nm photolysis of DZA
in CH3OD. The dashed curves are FTIR spectra of authentic γ-lactam in
this solvent.
The ꢀ-lactam band centered at 1733 cm-1 was again formed
immediately after the laser pulse, and it had constant intensity
between 30 ps and 3 ns. The rising time constant of 5.8 ps at 1733
cm-1 (Figure S5a) indicates that VC was faster in methanol than
in chloroform. One of the vibrational bands of the γ-lactam severely
overlapped the AE band centered at 1640 cm-1. However, the
intensity of γ-lactam probed at 1665 cm-1 remained constant from
10 to 3050 ps (Figure S5b), in contrast to the slow growth observed
in chloroform (Figure S2b). Thus, our data indicate that there are
two pathways for the formation of γ-lactam: the fast formation in
both chloroform and CH3OD is from the excited state of diazoamide
via the RIES mechanism, and the slow growth observed only in
chloroform is from singlet carbene (Scheme 1). The latter mech-
anism is absent in CH3OD, indicating that alcohol efficiently traps
the carbene species.6,7,12,13
Acknowledgment. This work was performed at The OSU
Center for Chemical and Biophysical Dynamics. G.B. thanks MF
EOG and FNP for a “Homing” Grant in 2009.
Supporting Information Available: Descriptions of the spectro-
meters, Figures S1-S8, and Tables S1-S3. This material is available
free of charge via the Internet at http://pubs.acs.org.
Singlet carbonyl carbenes have orthogonal geometries, in contrast
to their planar precursors.11 The orthogonal conformation assists
γ-lactam formation because the empty carbene p orbital points
toward a γ-CH bond (Tables S2 and S3). We speculate that in
methanol, solvent molecules surround the nascent singlet carbene
as a result of hydrogen bonding. This provides a solvent shell that
insulates the carbene center from the reactive C-H bond. Thus,
cyclization is suppressed, and OH insertion to solvent is preferred.
Singlet carbene rotation from the nascent planar to the relaxed
orthogonal conformation has been observed in chloroform.14
Our data also indicate that ꢀ-lactam is entirely formed from the
excited state of the diazoamide precursor, consistent with studies
of Tomioka et al.6 Moreover, Tomioka demonstrated that methanol
does not completely suppress γ-lactam formation relative to
cyclohexane (67% of the γ-lactam is carbene-derived, and 33% is
formed by a RIES mechanism).6,7 If we assume that the extinction
coefficients of γ-lactam and AE are similar, as predicted by the
References
(1) Rando, R. R. J. Am. Chem. Soc. 1970, 92, 6706.
(2) Rando, R. R. J. Am. Chem. Soc. 1972, 94, 1629.
(3) Corey, E. J.; Felix, A. M. J. Am. Chem. Soc. 1965, 87, 2518.
(4) Kaplan, F.; Meloy, G. K. J. Am. Chem. Soc. 1966, 88, 950.
(5) Tomioka, H.; Kitagawa, H.; Izawa, Y. J. Org. Chem. 1979, 44, 3072.
(6) Tomioka, H.; Kondo, M.; Izawa, Y. J. Org. Chem. 1981, 46, 1090.
(7) Burdzinski, G. T.; Wang, J.; Gustafson, T. L.; Platz, M. S. J. Am. Chem.
Soc. 2008, 130, 3746.
(8) Bonneau, R.; Liu, M. T. H.; Kim, K. C.; Goodman, J. L. J. Am. Chem.
Soc. 1996, 118, 3829.
(9) Hamm, P.; Ohline, S. M.; Zinth, W. J. Chem. Phys. 1997, 106, 519.
(10) Minato, H. Bull. Chem. Soc. Jpn. 1963, 36, 1020.
(11) Scott, A. P.; Platz, M. S.; Radom, L. J. Am. Chem. Soc. 2001, 123, 6069.
(12) The fast component observed in CHCl3 is consistent with the observation
that singlet carbene is observed in CHCl3 and overlaps with γ-lactam.
(13) We posit that the diazo excited state abstracts a hydrogen atom from a
C-H bond to form a singlet biradical, which cyclizes to form the two
lactams. The putative biradicals were not observed.
(14) Zhang, Y.; Kubicki, J.; Wang, J.; Platz, M. S. J. Phys. Chem. A 2008, 112,
11093.
JA904009X
9
J. AM. CHEM. SOC. VOL. 131, NO. 28, 2009 9647