Matrix-IR spectroscopic investigations of the thermolysis and photolysis of diazoamides

Article abstract of DOI:10.1021/jo401695x

Matrix photolysis of N,N-dialkyldiazoacetamides 1a-d at 7-10 K results in either the formation of C-H insertion products (in case of N,N-dimethyl and N,N-diethyl diazoamides) or almost exclusive Wolff rearrangement to ketenes (in the case of the cyclic di

Full text of DOI:10.1021/jo401695x

Article
pubs.acs.org/joc
Matrix-IR Spectroscopic Investigations of the Thermolysis and
Photolysis of Diazoamides
Curt Wentrup,*,† Herve Bibas,† Arvid Kuhn,† Ullrich Mitschke,† and Mark C. McMills‡
́
†School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia
‡Department of Chemistry and Biochemistry, Ohio University, 380 Clippinger Laboratories, Athens, Ohio 45701, United States
S
* Supporting Information
ABSTRACT: Matrix photolysis of N,N-dialkyldiazoacetamides 1a−d at 7−10 K
results in either the formation of C−H insertion products (in case of N,N-dimethyl
and N,N-diethyl diazoamides) or almost exclusive Wolff rearrangement to ketenes
(in the case of the cyclic diazoamides N-(diazoacetyl)azetidine and N-
(diazoacetyl)pyrrolidine). This can be ascribed to higher activation barriers for
the approach of the singlet carbene p orbital in 5 (or of the diazo carbon in an
excited state of 1) to the stronger and “tied back” nature of the C−H bonds in the
cyclic substituents. In contrast, flash vacuum thermolysis (FVT) of diazoamides
1a−d, in which reactions of excited states are excluded, gives rise to clean C−H
insertion with only minor Wolff rearrangement to ketenes.
catalyzed reactions; varying ratios of β- or γ-lactams have been
obtained from substituted diazoamides under these conditions.6
The C−H insertion reaction usually dominates for diazoamides,
reducing the Wolff rearrangement to trace amounts, if it is
present at all.3,4,7 In this regard diazoamides differ from the
related diazo esters, which readily form ketenes both photo-
chemically and thermally. In spite of a statement to the
contrary,1b alkoxy groups do migrate in thermal Wolff
rearrangements under FVT conditions,8,9 and we will show
that amino groups migrate as well.
INTRODUCTION
■
Diazoacetamides 1 may be expected to undergo photochemi-
cally or thermally induced Wolff rearrangement1 (WR) to form
ketenes 2 (Scheme 1).2 However, insertion into substituents at
Scheme 1. Wolff Rearrangement (WR) and C−H Insertion
Reactions of Diazoamides
Rando3 explained the preference for diazoamides to undergo
intramolecular C−H insertion on photolysis with the
conformational differences between the ester and the amide
bonds: the planar peptide-type bond in the diazoamides should
bring one N substituent into sufficient proximity to the carbene
center and hence facilitate intramolecular insertion.
The photochemical decomposition of N,N-diethylacetamide
(1b) with near-UV light in various solvents afforded almost
quantitative transformation to the two C−H insertion products
1-ethyl-4-methyl-2-azetidinone (3b) and 1-ethyl-2-pyrrolidi-
none (6b).3b In protic solvents such as methanol, the formation
of γ-lactam 6b decreased sharply, mainly in favor of the O−H
insertion product 7 (Scheme 2). In addition, ester 8 was also
found, thereby indicating a non-negligible contribution of WR
to ketene 2b under these conditions (Scheme 2).
Tomioka et al. refined these observations mechanistically and
posited the participation of a noncarbenic process in the
intramolecular C−H insertion.4 Investigating the relative ratio
of all products under various conditions, in particular by
the amide nitrogen may compete with the Wolff rearrange-
ment,3,4 leading to the formation of β- and γ-lactams 3 and 6
(Scheme 1). Both reactions may proceed either via the s-Z or s-
E conformers of 1 or via the carbenes 5. The formation of β-
lactams is synthetically interesting and has been used in the
preparation of methyl penicillanate (4)5 and many related
compounds, especially by employing rhodium- or ruthenium-
Received: August 2, 2013
© XXXX American Chemical Society
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excluded. In order to cast more light on these reactions, we
have investigated the thermolyses and photolyses of diazo-
amides 1a−d (Scheme 4) by matrix-isolation IR spectroscopy.
Scheme 2. Products of Solution-Phase Photolysis of
Diazoamide 1b
a
Scheme 4. Diazoamides Investigated in This Paper
a
No stereochemical preferences are implied (s-E and/or s-Z); see text.
RESULTS
■
examining the effects of sensitizers and quenchers on the
product distributions, the authors concluded that the β-lactam
3b and the Wolff rearrangement product 8 are formed directly
from the singlet excited state of N,N-diethyldiazoacetamide
(11b*), whereas the formation of γ-lactam 6b and the O−H
insertion product 7 should involve a free singlet carbene.4a
Recent femtosecond time-resolved laser flash photolysis
studies demonstrated that both the β- and the γ-lactams 3b and
Photochemical Decomposition of Matrix-Isolated
Diazoamides 1a−d. Diazoamides can exist in s-E and s-Z
conformations (Scheme 1), but in contrast to the diazo esters,
the s-Z conformers are energetically preferred, although the
calculated energy difference is small (2.75 kcal/mol for N,N-
dimethyldiazoacetamide (1a) at the B3LYP/6-31G** level (see
the Supporting Information)). This can be ascribed to a steric
interaction between the N2 moiety and an alkyl group in the s-E
form.The diazo bands in the matrix IR spectra of the
diazoamides 1a−d are uniformly sharp, thereby indicating
that one major conformer is present (see Figure 1 and
1
6b are in fact formed from 1b*, i.e. by reaction in the excited
state (RIES process), but the γ-lactam is also formed from the
singlet carbene 15. The product of methanol addition, 7, is also
formed from 15, but there was no information on the ketene 2b
or its methanol trapping product 8.10
In contrast, femtosecond time-resolved infrared studies on
the naphthyl-substituted diazo ester 9 clearly revealed that both
the ketene 12 and the β-lactone 11 are formed entirely from
carbene 10 with very little if any contribution from the excited
singlet state 19* (Scheme 3).11 The singlet carbene 110 is born
in the planar structurelike the diazo compound 9but
1
relaxes to the nonplanar form 10′ on a femtosecond time
scale.10
Almost nothing is known about the thermal decomposition
of diazoamides, where RIES reactions can obviously be
Scheme 3. Photolysis of Diazo Ester 9
Figure 1. IR difference spectrum after irradiation of N,N-
dimethyldiazoacetamide (1a) (2115, 1649, 1496, 1409, 1397 cm−1)
at λ ≥305 nm for 3.5 h in Ar matrix at 7 K: (bottom) disappearing
bands of 1a; (top) appearing bands of N-methylazetidinone (3a)
(1780 cm−1) and N,N-dimethylaminoketene (2a) (2132 cm−1
;
calculated value 2137 cm−1 (Table 1)). Peaks due to ketene
CH2CO (K) at 2142 cm−1 and CO at 2138 cm−1 arise from the
photochemical decomposition of 3a.
subsequent figures). The IR spectra of the matrix-isolated 1a−d
agree best with the calculated IR spectra of the s-Z conformers
(see the Supporting Information), which are therefore the likely
source of the observed chemistry. In the FVT reactions
described in Thermal Decomposition of Diazoamides 1a−d,
however, the small energy barrier between the two conformers
is easily overcome, so that either conformer may be involved.
N,N-Dimethyldiazoacetamide (1a). Photolysis of matrix-
isolated 1a at λ ≥305 nm results in the formation of N-
methylazetidinone (3a) (strong absorption at 1780 cm−1)
B
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Scheme 5. Matrix Photolysis of Dizoamide 1a and Trapping of Carbene 5a with CO
together with a small amount of dimethylaminoketene (2a)
(2132 cm−1; Scheme 5 and Figure 1). The identity of the β-
lactam 3a was ascertained by deposition of an authentic sample.
The IR spectra of all the aminoketenes 2a−d examined here are
dominated by the strong CCO stretch around 2130 cm−1
(absolute intensities 500−600 km/mol), so that all other
absorptions at lower frequencies have relative intensities below
6% of the ketene band (see Table 1 and the Supporting
Information).
Scheme 6. Products of Photodecomposition of β-Lactams
3a,b
Table 1. Comparison of Calculated (B3LYP/6-31G(d)) and
Observed (Ar Matrix, 10 K) Characteristic IR Bands of
Ketenes 2, Azetidinones 3, and Pyrrolidinones 6
benes and arylcarbenes from diazo compounds.9,11a,12 Stabi-
lized diaminocarbenes do not react with CO, but the more
reactive acylaminocarbenes do.13 Here, the photolysis of 1a in a
carbon monoxide matrix yielded both s-cis and s-trans α-
oxoketene conformers 13a together with an approximately
equal amount of 3a (Figure 2). This result is compatible with
(but does not demand) the coexistence of carbenic and
noncarbenic photochemical processes, the latter leading to C−
H insertion. Prolonged irradiation caused complete conversion
of s-trans-13a to the s-cis-13a conformer, in which there is less
steric interaction between ketene and dimethylamino groups
(Figure 2).
a
νcalcd/cm−1 (I/km mol−1
)
νexptl/cm−1
Ketenes
b
c
2a
2137 (579)
2104/2101 (499/490)
2133 (660)
2132
2132
2133
2134
2b
d
2c
e
2d
2135 (632)
Azetidinones
1806 (503)
3a
3b
3c
3d
1782
1775
1799 (512)
1813 (401)
1806 (458)
1785
1715
Pyrrolidinones
1737 (348)
N,N-Diethyldiazoacetamide (1b). The preference for C−H
insertion is also observed for the N,N-diethyl analogue 1b. The
photolysis of matrix-isolated 1b yielded β-lactam 3b as the
predominant product together with smaller amounts of γ-
lactam 6b and ketene 2b (2132 cm−1) (Scheme 7 and Figure
3). Larger-scale Figures S1−S3 in the Supporting Information
clearly show the bands of 3b, 6b, and 2b developing during the
first 15 min of photolysis (λ ≥290 nm). Already after 15 min of
photolysis a peak due to CO (2137 cm−1) appears next to the
ketene peak (2132 cm−1) due to photochemical decomposition.
Further irradiation at λ ≥290 nm orfasterat λ ≥190/254
nm (low-pressure Hg lamp without filter) causes decom-
position of the β-lactam 3b to two sets of [2 + 2]
cycloreversion products together with increased formation of
CO (Scheme 6 and Figure 4).14
6b
6d
1804 (289)
a
b
Wavenumbers scaled by 0.9613. No band at lower frequency with
c
intensity I > 6% of the ketene band. Two different conformers; no
bands at lower frequency with I > 6%. No bands at lower frequency
with I > 2%. No bands at lower frequency with I > 3% of ketene band.
d
e
Prolonged irradiation at λ ≥305 nm or short irradiation at
shorter wavelength (254 nm) gives rise to increased intensities
of signals at 2142 and 2138 cm−1, which are assigned to ketene
(CH2CO) and carbon monoxide, respectively (Figure 1).
Other bands characteristic of ketene were present at 2086,
1380, 114, and 973 cm−1, and the compound was characterized
securely by direct comparison with an authentic sample. Ketene
is most probably formed in a photochemically induced [2 + 2]
cycloreversion of β-lactam 3a (Scheme 6). (Further informa-
tion on the cycloreversions is given in Thermal Decomposition
of Diazoamides 1a−d.) The presence of carbene 5a was
confirmed by trapping with CO. This procedure has been used
previously to demonstrate the formation of alkoxycarbonylcar-
Thus, new bands at 2142 cm−1 and a series of bands at 2289,
2279, and 2266 cm−1 are assigned to ketene (CH2CO) and
ethyl isocyanate, respectively.14 The ketene band is seen as a
weak shoulder on the high-frequency side of CO (2138 cm−1).
N-(Diazoacetyl)azetidine (1c). In sharp contrast to the clear
prevalence of C−H insertion in the photolyses of open-chain
C
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Figure 3. IR difference spectrum after irradiation of N,N-
diethyldiazoacetamide (1b) (2112, 1639 cm−1) at λ ≥290 nm for 15
min in an Ar matrix at 8 K: (bottom) disappearing bands of 1b; (top)
appearing bands of 1-ethyl-4-methyl-2-azetidinone (3b) (1775, 1755
cm−1), 1-ethyl-2-pyrrolidinone (6b) (1716 cm−1), and N,N-
diethylaminoketene (2b) (2132 cm−1).
Figure 2. Course of the photochemical decomposition of N,N-
dimethyldiazoacetamide (1a) (2119, 1638 cm−1) in a CO matrix at 8
K at λ ≥305 nm: (a) deposition spectrum; (b) spectrum after 1 h
irradiation at λ ≥305−310 nm; (c−e) spectra after an additional 10
min (c), 21 min (d), and 35 min (e) of irradiation at λ 254 nm. The
assignment of the bands is supported by B3LYP/6-31G(d)
calculations. The disappearance of 1a (2119, 1638 cm−1) and
appearance of 3a (1770 cm−1), s-trans-13a (2131, 1683 cm−1)
(appearing first), and s-cis-13a (2155, 1654 cm−1) (appearing later) are
indicated by arrows. The absorption of the ketene function of s-trans-
13a (calculated at 2130 cm−1) is hidden under the neighboring CO
band (see the Supporting Information for calculated data).
Scheme 7. Matrix Photolysis of Diethyldiazoacetamide (1b)
Figure 4. IR difference spectrum after irradiating the mixture of β-
lactam 3b, γ-lactam 6b, and ketene 2b (Figure 3) for another 3 h at λ
190/254 nm in an argon matrix at 8 K: (top) appearing bands of
carbon monoxide (CO), ethyl isocyanate (EI), and propene (P);14
(bottom) disappearing bands of 3b, 6b, and 2b (2132 cm−1).
signal could be hidden under the CO band, the experiment was
repeated with a matrix composed of 10% CO in Ar. The same
results were obtained, and ketene 2c was not observable. This
makes it possible that carbene 5c is involved in the
photochemical WR to 2c.
N-(Diazoacetyl)pyrrolidine (1d). The high strain energies of
the bicyclo[2.2.0] and [2.1.1] derivatives 3c and 6c and/or the
more difficult approach of the diazo carbon or carbene to the
tied-back methylene groups apparently disfavor the intra-
molecular C−H insertion (see further in the Discussion).
Therefore, it was of interest to examine the pyrrolidine
analogue 1d. Here, a weak absorption at 1785−1761 cm−1
(Figure 7) suggests the formation of a minor amount of β-
lactam 3d, which has been indentified securely in the FVT
reaction of 1d, where it is formed in much larger amount (see
Figure 11 below). Compound 6d is unknown, but the
calculated carbonyl absorptions of 3d and 6d are identical
N,N-dialkyl diazoamides 1a,b, the incorporation of the amide
nitrogen into a small heterocycle such as N-(diazoacetyl)-
azetidine (1c) causes predominant conversion to the WR
product, the aminoketene 2c (Scheme 8 and Figure 5).
The possible C−H insertion products, viz. the strained
bicyclic β-lactam 3c and 1-azatricyclo[2.1.1]hexan-2-one (6c),
were not detected. The existence of carbene 5c as an
intermediate was again proved in a trapping experiment with
CO (Scheme 8 and Figure 6). At first, the resulting
azetidinylcarbonylketene 13c is formed in the s-trans
conformation, which is gradually transformed to the s-cis
conformer upon prolonged irradiation. Notably, the amino-
ketene 2c was not observable in this experiment. Because the
D
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Scheme 8. Matrix Photolysis of Diazoamide 1c and Trapping
of Carbene 5c with CO
Figure 6. Course of the photochemical decomposition of N-
diazoacetylacetidine (1c) (2120, 2112, 1646 cm−1) in a CO matrix
at 10 K: (a) deposition spectrum; (b−e) spectra after 5 min (b), 15
min (c), 45 min (d), and 135 min (e) irradiation at λ 254 nm.
Prolonged irradiation causes the complete conversion of the
azetidinylcarbonylketene s-trans-13c (2125, 1663 cm−1) to s-cis-13c
(2153, 1647 cm−1). Decreasing absorptions of 1c and increasing
absorptions of s-trans-13c and s-cis-13c are indicated with arrows.
Scheme 9. Matrix Photolysis of Diazoamide 1d
Figure 5. IR difference spectrum obtained after irradiation of N-
diazoacetylazetidine (1c) (2113, 1658, 1407 cm−1) at λ 254 nm for 10
min in an Ar matrix at 10 K: (bottom) disappearing bands of 1c; (top)
appearing bands ascribed to azetidinylketene (2c) (2133 cm−1;
calculated value 2133 cm−1 (Table 1)).
within 2 cm−1 (see Table 1 and the Supporting Information).
Therefore, it cannot be excluded that a trace of 6d is hidden
under the weak absorption due to 3d, but if so, the amount
would be very small. Since ketene 2d with its most prominent
band at 2134 cm−1 is obviously the main product, the Wolff
rearrangement route is clearly favored in this case as well
(Scheme 9 and Figure 7).
In summary, the photochemical decomposition of diazo-
amides 1a−d and matrix-IR spectroscopic investigation of the
products reveal a crucial dependence of the rearrangement path
on the molecular structure. Thus, open-chain N,N-dialkyldia-
zoacetamides 1a,b unambigously prefer intramolecular C−H
insertion and show only little if any contribution of the Wolff
rearrangement. In contrast, the incorporation of the amide
nitrogen into a four- or five-membered ring in diazoamides 1c,d
effectively suppresses insertion reactions in favor of the Wolff
rearrangement. The presence of free carbenes as intermediates
in these reactions was substantiated by trapping with carbon
monoxide, although this does not exclude the possibility that
some rections may also occur by RIES.
Thermal Decomposition of Diazoamides 1a−d. FVT of
N,N-dimethyldiazoacetamide 1a at 330−630 °C with Ar matrix
isolation of the products gave a mixture of carbon monoxide,
ketene (CH2CO), N-methylmethanimine, methyl isocyanate,
and ethylene (Scheme 10 and Figure 8). All compounds were
identified by comparison with matrix spectra of reference
samples or previously reported matrix spectra.14 Similarly to the
photodecomposition of 3a outlined in Scheme 6, these
products most likely stem from the corresponding thermal
cleavage of N-methylazetidinone (3a). [2 + 2] Cycloreversion
of 3a can lead either to ketene and N-methylmethanimine or to
methyl isocyanate and ethylene (Scheme 10). The relative
ratios of the various decomposition products vary only slightly
with increasing temperature. However, the formation of carbon
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Figure 7. IR difference spectrum showing the photolysis of N-
diazoacetylpyrrolidine 1d matrix-isolated in argon at 10 K: (bottom)
disappearing bands of 1d (2112, 1430 cm−1); (top) appearing bands of
pyrrolidinylketene (2d) (2134 cm−1; calculated value 2135 cm−1
(Table 1)). Starting decomposition to CO gives rise to the peak at
2137 cm−1 on the high-frequency side of 2d. Weak bands at 1785−
1761 cm−1 are tentatively assigned to the C−H insertion product 3d
(cf. Figure 11). It is possible that a trace of 6d is hidden under the
weak signal due to 3d (see text).
Figure 8. IR spectrum (argon, 14 K) of the products of FVT of N,N-
dimethyldiazoacteamide (1a) at 500 °C. The inset shows an expansion
of the 2160−2120 cm−1 region. Legend: K, ketene (CH2CO); MI,
methyl isocyanate; E, ethylene; I, N-methylmethanimine. 2a appears at
2132 cm−1.14
sufficiently high vapor pressure of the sample. Since the
diazoamide 1a does not meet this requirement, a variation of
the method was devised. When 1a is heated in about 750 hPa
of Ar, the yellow diazoamide starts to decompose at around 150
°C to form a more volatile compound, which enters the gas
phase as a fine colorless mist and condenses on any cold
surface. The thermolysis was performed in a glass tube attached
to the pulsed valve, and the resulting gaseous product was
deposited onto the 10 K KBr target (this is not a pulsed
pyrolysis). The most prominent feature in the resulting IR
spectrum is a characteristic set of carbonyl absorptions at 1781,
1775, and 1761 cm−1, belonging to N-methylazetidinone (3a)
(Figure 9a). (This is in accord with the photolysis experiment
depicted in Figure 1.) The correct assignment of these bands
was proved by deposition of an authentic sample of 3a (Figure
Scheme 10. Thermal Formation of 3a and 2a and
Decomposition of 3a by CO Elimination and [2 + 2]
Cycloreversion
monoxide and the [2 + 2] cycloreversion to methyl isocyanate
and ethylene gain more importance at the expense of the
alternative cleavage to ketene and N-methylmethanimine (in
the photochemical reaction, formation of ketene and CO was
preferred; vide supra). Formation of a small amount of
aminoketene 2a is indicated by a weak absorption at 2132
cm−1, which also gains intensity with increasing temperature
(Figure 8).
As azetidinone 3a cannot be isolated from the FVT reaction
under these conditions, an alternative technique was sought. In
pulsed pyrolysis experiments the sample is mixed with a carrier
gas, e.g. argon, at a pressure of about 1 atm and then introduced
through a pulsed valve into a short capillary pyrolysis tube at
1000−1200 °C. Just after the pyrolysis zone the resulting
supersonic argon beam is allowed to expand into the high
vacuum of the cryostat, thereby rapidly cooling the thermally
excited primary pyrolysis products and allowing their
observation in the deposited Ar matrix.15 This technique is
known to allow isolation of primary pyrolysis products which
would not survive FVT conditions.16 As the sample has to be
mixed with the carrier gas, an essential prerequisite is a
Figure 9. Matrix IR spectra (argon, 10 K): (a) N-methylazetidinone
(3a) after thermolysis of N,N-dimethyldiazoacetamide (1a) at 150 °C
in Ar with pulsed deposition; (b) same as (a) but with pulsed FVT
prior to deposition; (c) authentic sample of 3a; (d) pulsed FVT of
authentic 3a. Legend: K, ketene (CH2CO); MI, methyl isocyanate; B,
unidentified carbonyl compound (1702 cm−1).14
F
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9c). Both the standard FVT of 1a and pulsed pyrolysis of the
gaseous product of the 150−170 °C thermolysis yield only
decomposition products, largely methyl isocyanate and ketene
(Figure 9b). N-Methylazetidinone (3a) is volatile enough for
pulsed pyrolysis, and the same bands due to decomposition
products are again obtained (Figure 9d).
In contrast to 1a, FVT of N,N-diethyldiazoacetamide (1b) at
400−500 °C with matrix isolation of the pyrolyzates gives
detectable amounts of β- and γ-lactams. Thus, a mixture of the
C−H insertion products 3b and 6b, carbon monoxide, and the
products of thermal decomposition is obtained (Scheme 11 and
decomposition of 1b starts at approximately 350 °C under
FVT conditions. The amounts of both lactams 3b and 6b
decrease with increasing temperature, and their cleavage was
complete on FVT at 600 °C. Interestingly, also the proportion
of the [2 + 2] cycloreversion reaction to ketene and N-
ethylethylidenamine decreases and vanishes at 600 °C. The [2
+ 2] cycloreversion to ethyl isocyanate and propene dominates
at all investigated temperatures (400−610 °C). The intensity of
the CO signal increases with increasing temperature and
thereby masks any possible formation of 2b.
While photolysis of the matrix-isolated N-diazoacetylazetidine
1c yielded aminoketene 2c as a result of Wolff rearrangement,
this ketene was not identifiable under thermal conditions.
Unfortunately, FVT of 1c above 300 °C leads to complete
decomposition, with carbon monoxide as the only identified
product.
Scheme 11. Formation and Thermal Decomposition of
Lactams 3b and 6b
Thermolysis of diazoacetylpyrrolidine (1d) at 150−170 °C
in about 1 atm of Ar with pulsed deposition of the resulting
gaseous product as described for 1a above leads to complete
decomposition and development of a new band at 1785 cm−1
with shoulders at 1779 and 1761 cm−1, which all belong to the
β-lactam 3d (Scheme 12 and Figure 11). Because of the very
Scheme 12. Thermal Formation and Decomposition of β-
Lactam 3d and Possibly 6d
Figure 10. IR spectrum (argon, 7 K) of the products of FVT of N,N-
diethyldiazoacteamide (1b) at 500 °C: residual 1b, 2112 cm−1; 3b,
1775 cm−1; 6b, 1715 cm−1. The inset shows an expansion of the
2160−2090 cm−1 region. Legend: K, ketene (2142 cm−1); EI, ethyl
isocyanate (2280, 2265 cm−1); P, propene (1464, 1453, 1441 cm−1);
CO, 2137 cm−1.14
Figure 10; see also Figure S4 in the Supporting Information).
The ability to detect 3b under conditions where 3a decomposes
completely may be ascribed to the greater number of degrees of
freedom in 3b (and likewise in 6b); i.e., these molecules are
better able to dissipate the excess vibrational energy.
Very little if any diethylaminoketene (2b) is formed. At best,
a shoulder at ca. 2132 cm−1 on the stronger CO peak (2137
cm−1) may be due to 2b (Figure 10). The ratios of all observed
compounds depend strongly on the temperature. The
Figure 11. IR spectra (Ar, 10 K): (a) the product of thermolysis of N-
diazoacetylpyrrolidine 1d in Ar with pulsed deposition, largely 3d
(inset: bands due to ketene (CH2CO) and CO at 2142 and 2139/
2149 cm−1);14 (b) synthesized β-lactam 3d (1785, 1779, 1761 cm−1).
G
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Figure 12. Relative calculated energies (kcal mol−1, B3LYP/6-31G(d)) of products and transition states arising from N,N-dimethyl- and N,N-
diethyldiazoacetamides 1a,b. In each series the respective singlet carbene 5 is set to 0 kcal mol−1; the value given for 2b corresponds to its most
stable conformer. Note: the carbene C−H bond is perpendicular to the planar amide moiety (see the Supporting Information for computational
details).
similar calculated carbonyl absorption frequencies of 3d and 6d
(Table 1), it cannot be excluded that the γ-lactam 6d is present
as well, but if so, it would only be a minor amount. Further
strong bands at 2139/2149, 2142, and 1629 cm−1 reveal the
presence of CO, ketene (CH2CO), and 1-pyrroline (14),
respectively. These compounds are formed by thermal
fragmentation of 3d, as demonstrated in separate experiments
by FVT of a synthesized sample of 3d at 500−700 °C. The
decomposition of 3d was complete only at 700 °C. The identity
of 1-pyrroline (14) was confirmed by comparison with the
matrix IR spectrum of an authentic sample. Ketene (CH2CO)
and CO absorbed as usual at 2142 and 2137 cm−1, respectively.
Again, in sharp contrast to the matrix photolysis, there is no
clear evidence for the participation of a thermally induced Wolff
rearrangement in the FVT of 1d, although it is possible that a
weak absorption due to aminoketene 2d at 2134 cm−1 could be
hidden under the massif between 2130 and 2150 cm−1 (Figure
11). Characteristic experimental and calculated wavenumbers of
compounds 2, 3, and 6 are compared in Table 1.
DISCUSSION
■
The interpretation of the experimental results is complicated by
two factors: the possible occurrence of reactions in excited
states (RIES) in the photochemical reactions (but not in the
thermal ones) and the possibility of concerted Wolff rearrange-
ments (WR) from diazoamide conformers with an s-Z
relationship between the diazo and carbonyl groups.17 Unlike
the diazo esters,1,10 the lowest energy conformer of the
diazoamides 1 is the s-Z conformation (Scheme 1), which could
favor concerted WR in the matrix photolysis experiments, but
the barrier between s-E and s-Z conformers may easily be
overcome in FVT experiments. In the following discussion we
will primarily consider the reactions of the singlet carbenes.
N,N-Dimethyldiazoacetamide (1a) decomposes both photo-
chemically and thermally to give predominantly N-methyl-
azetidinone (3a) and only a small amount of N,N-
dimethylaminoketene (2a). This suggests that C−H insertion
is energetically favored with respect to the Wolff rearrange-
ment, and at least in the FVT reactions it is a carbene process.
Trapping of carbene 5a with CO indicates that at least some
carbenes are formed in the photolysis reactions, and the
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Figure 13. Relative calculated energies (kcal mol−1, B3LYP/6-31G(d)) of products and transition states arising from diazoamides 1c,d. For each
series the respective singlet carbene 5 is set to 0 kcal mol−1. Note: the carbene C−H bond is perpendicular to the planar amide moiety (see the
Supporting Information for computational details).
absence of aminoketene 2c in the analogous trapping of 5c with
CO suggests that WR may in fact be a carbene process. DFT
calculations for the potential carbene reactions show that the
azetidinone 3a is some 26 kcal mol−1 lower in energy than
aminoketene 2a (Figure 12). Nevertheless, formation of the
ketene is possible, since the difference in activation barriers is
only ca. 6 kcal/mol for the singlet and zero for the triplet
carbene rearrangements (Figure 12). The calculated structures
of carbene 5a (in both its singlet and triplet states) are in favor
of the insertion reactions: one hydrogen atom of the planar
amide group comes close to a p orbital at the carbene center, in
which the carbenic hydrogen atom is out of plane. Such
nonplanar carbene structures have also been calculated for
Chart 1. Relative energies of β-lactam 3a and its
fragmentation products in kcal/mol (numbers in
parentheses)
10
1
carbenic esters, e.g. 10′ (Scheme 3). No minimum was
found for a geometry with the amide methyl groups out of
plane for either the singlet or the triplet state (B3LYP/6-
31G(d) and UB3LYP/6-31G(d), calculations, respectively).
The fact that azetidinone 3a could not be isolated in standard
FVT experiments suggests that it is generated in a vibrationally
excited state: i.e., chemically activated. This excess energy is
sufficient for the fragmentation to the observed cycloreversion
products, which lie ca. 12 and 33 kcal/mol above 3a,
respectively (Chart 1). In the presence of 1 atm of argon the
excess vibrational energy can be removed collisionally, and thus
the primary thermolysis product 3a can be observed. Since
RIES is excluded, 3 must be formed in a C−H insertion
reaction of the carbene in this case. Both singlet and triplet
insertions are possible (Figure 12). The carbene has a
calculated triplet ground state, but the singlet−triplet splitting
is very small (Figure 12).
The results for N,N-diethyldiazoacetamide 1b are analogous
to those for the N,N-dimethyl case with C−H insertion to β-
and γ-lactams 3b and 6b prevailing and the WR to aminoketene
2b occurring only to a minor extent on photolysis. Although
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the less strained five-membered-ring compound 6b is
thermodynamically favored by a calculated energy difference
of 13 kcal mol−1 (Figure 12), the four-membered-ring β-lactam
3b predominantes in the photolysis, but 6b dominates in the
thermolysis in accord with the thermochemistry (Figure 10 and
Figure S4 (Supporting Information)).
overcome under the FVT conditions employed here, but the
reaction would not be expected to occur under the static
thermolysis conditions in Ar at 150 °C. A third possibility is
that the ketenes disappear due to elimination of CO. Such
processes are known as well, but they normally require
temperatures above 500−600 °C in our apparatus,9,20 and
again they would not be expected under the static thermolysis
conditions in Ar at 150 °C. It is noted that the putative RWR of
ketenes 5 can proceed in two directions, to the original
carbenes 5 and to the aminocarbenes 15 (Scheme 13).
Furthermore, 5 and 15 can in principle interconvert via an
oxirene. Nothing is known about FVT reactions of either
aminoketenes or aminocarbenes, which are desirable topics for
further investigation.
In sharp contrast to the diazoacetamides 1a,b, the
incorporation of the amide nitrogen into a four- or five-
membered ring in diazoamides 1c,d effectively suppresses the
C−H insertion reactions, thus leading to the formation of
ketenes by WR on matrix photolysis. Since the intermediacy of
carbenes in these reactions was confirmed by carbon monoxide
trapping of 5a,c, the WR is assumed to be at least in part a
carbene reaction under these conditions. In contrast to the
open chain alkyl-substituted carbenes, the approach of a C−H
bond to the carbenic carbon is disfavored by the tied-back
conformations of the methylene groups. Moreover, smaller ring
C−H bonds have a higher degree of s character and are
therefore stronger. The outcome is higher activation barriers for
C−H insertion in 5c (Figure 13). For the five-membered-ring
system 5d these effects are smaller, and the activation energies
of WR and C−H insertion approach each other (Figure 13).
The experimental results are in accord with this, since at least a
trace of the C−H insertion products 3d can be detected,
whereas no C−H insertion is observed for 5c.
CONCLUSION AND OUTLOOK
■
The photolyses of matrix-isolated diazoamides 1a−d reveal a
crucial dependence of the rearrangement path on the
underlying molecular structure. Open-chain N,N-dialkyldiazoa-
mides 1a,b clearly prefer intramolecular C−H insertion and
show only little formation of Wolff rearrangement (WR)
products. In contrast, the incorporation of the amide nitrogen
into a four- or five-membered ring in diazoamides 1c,d
effectively suppresses the insertion reactions in favor of the
Wolff rearrangement to ketenes 2. Calculations at the B3LYP
level support the idea that the “tying-back” of the methylene
groups, especially in the carbene 5c, impedes C−H insertion,
thereby favoring the WR energetically.
Scheme 13. Wolff (WR) and Potential Retro-Wolff (RWR)
Rearrangements
The presence of free carbenes in the photochemical reactions
was substantiated by trapping with carbon monoxide.
Under FVT conditions the near-exclusive formation of C−H
insertion products (lactams 3 and 6) and/or their decom-
position products arising from [2 + 2] cycloreversions is
observed for diazoamides 1a,b,d, and only 1a affords a small
amount of the aminoketene 2a. This is in accord with the
higher calculated activation barriers for WR relative to C−H
insertion. Thus, the reactions reported here can be understood
in terms of carbene reactivities, although it is emphasized that
these experiments do not provide any information on the
potential occurrence of reactions in the excited state (RIES) in
the photochemical reactions or the concertedness of WR. We
will show in forthcoming publications that suitably substituted
diazoamides can in fact undergo substantial WR to amino-
ketenes under both photochemical and thermal conditions.
The near-exclusive formation of C−H insertion products in
the pyrolyses of diazoacetamides 1a,b,d demonstrates that
purely thermal C−H insertion is perfectly feasible under
conditions where RIES is excluded. The invariable formation of
C−H insertion products in preference to ketenes is in
agreement with the activation barriers for the carbene reactions,
which are 5−7 kcal/mol higher for formation of ketenes 2a,b,d
than for the corresponding lactams (Figures 12 and 13).
Unfortunately, compound 1c decomposed on thermolysis, so
that neither ketene nor lactam was observable. Only in the case
of lactam 6d is the activation barrier for its formation higher
than that for the WR to aminoketene 2d, and there was in fact
no clear evidence for the formation of 6d; instead, the
energetically more favorable formation of lactam 3d dominates
(Figure 13). In addition to the lower activation barriers, the
lactams also have significantly lower energies than the ketenes.
Therefore, thermodynamic control of the ractions would be
another reason for the low yields of thermal ketene formation.
However, thermodynamic control would require that ketene
formation be thermally reversible. This would correspond to a
retro-Wolff rearrangement (Scheme 13). Retro-Wolff rear-
rangements (RWR) are known in cyclic iminoketenes, which
under FVT conditions rearrange to cyclic iminocarbenes with
calculated activation barriers of 37−44 kcal/mol.15,18 It was
shown recently that diaminoketenes can undergo RWR at room
temperature.19 However, in the present case, the calculations
indicate activation barriers of the order of 50 kcal/mol for RWR
(Figures 12 and 13). Although high, such barriers may be
EXPERIMENTAL SECTION
■
General Methods. Argon matrices were prepared by vacuum
deposition of samples with argon (99.999%) onto a spectroscopic KBr
target at about 30 K. Closed-cycle helium cryostats were used for
matrix experiments. In FVT experiments, a mixture of argon and
sample was led through a quartz tube (10 cm length, 0.8 cm inner
diameter, equipped with heating wire and a thermocouple), and the
products were subsequently isolated on the cold target at ca. 10 K.
Procedures for pulsed pyrolysis have been described.15,16 For
thermolysis experiments under static argon pressure (750 hPa) the
sample was placed in a glass tube. One end of the glass tube was
connected to an electromagnetically operated pulsed valve, which was
directly mounted on the head of the cryostat. The other end of the
glass tube was connected to an argon reservoir. The sample was heated
at the required temperature, and the resulting gaseous product was
deposited onto the cold target by pulsing the electromagnetic valve.
Photolyses were carried out using either a 1000 W Xe/Hg lamp in
combination with cutoff filters or a low-pressure Hg arc lamp. Spectra
were recorded at 7−10 K.
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Materials. N,N-Dimethyldiazoacetamide (1a), N,N-diethyldiazoa-
cetamide (1b), N-(diazoacetyl)azetidine (1c), and N-(diazoacetyl)-
pyrrolidine (1d) were prepared according to literature proce-
dures.2,3,21,22 N-Methylazetidinone (3a),23 1-azabicyclo[3.2.0]heptan-
7-one (6b),24 and 1-pyrroline (14)25 were prepared as reported.
Matrix Experiments. N,N-Dimethyldiazoacetamide (1a). IR (Ar,
7 K): 2963 (vw), 2949 (vw), 2938 (vw), 2919 (vw), 2898 (vw), 2880
(vw), 2860 (vw), 2115 (vs), 1677 (w), 1649 (m), 1638 (w), 1614 (w),
1496 (m), 1460 (w), 1441 (w), 1409 (m), 1397 (m), 1338 (vw), 1271
(vw), 1171 (w), 1127 (w), 1095 (vw), 1066 (vw), 1064 (vw), 915 (w),
913 (w), 910 (w), 908 (vw), 743 (m), 741 (w), 723 (w) cm−1.
Photolysis of N,N-Dimethyldiazoacetamide (1a) in an Argon
Matrix. A sample of 1a was codeposited with Ar at 25 K. Broadband
photolysis at λ ≥305−310 nm and 7 K yielded predominantly N-
methylazetidinone (3a) accompanied by minor amounts of N,N-
dimethylaminoketene (2a), carbon monoxide, ketene, and possibly
further decomposition products. IR (argon, 7 K): 2960 (w), 2948 (w),
2919 (vw), 2892 (vw), 2142 (ketene, vw), 2138 (CO, w), 2132 (2a,
vw), 1780 (3a, s), 1776 (3a, w), 1774 (3a, w), 1761 (3a, w), 1683 (w),
1677 (w), 1388 (w), 1273 (vw), 1234 (vw), 1156 (vw), 1095 (vw),
1072 (w), 969 (vw), 915 (w), 913 (w), 910 (w), 908 (vw), 744 (m),
741 (w) cm−1. Prolonged irradiation or, more rapidly, reduction of the
excitation wavelength to λ 254 nm intensified the absorbances at 2142
(ketene), 2138 (CO), and 2132 cm−1 (2a) with respect to the still
dominant band of 3a at 1780 cm−1.
Photolysis of N,N-Dimethyldiazoacetamide (1a) in a Carbon
Monoxide Matrix. A sample of 1a was codeposited with carbon
monoxide at 30 K. Photolysis at 8 K and λ ≥305−310 nm or, more
rapidly, at 254 nm yielded a mixture of N-methylazetidinone (3a) and
both the s-cis and the s-trans conformers of N,N-dimethylaminocarbo-
nylketene (13a). IR (CO, 8 K): 3a, 1770 (s), 1392 (w), 1081 (vw)
cm−1; s-cis-13a, 2155 (s), 1636 (m) cm−1; s-trans-13a, 1654 (m) cm−1.
The absorption of the ketene function in s-trans-13a is covered by the
CO bands. Irradiation also brings about a conformational change in
favor of s-cis-13a.
FVT of N,N-Dimethyldiazoacetamide (1a). FVT of 1a at various
temperatures (330−630 °C) followed by Ar matrix isolation of the
pyrolyzates gave mixtures of carbon monoxide, ketene, N-methyl-
methanimine, methyl isocyanate, and ethylene. IR (Ar, 14 K): carbon
monoxide, 2138 (s) cm−1; ketene, 3063 (m), 2142 (vs), 2086 (w),
1380 (m), 1114 (w), 973 (vw) cm−1; N-methylmethanimine, 3014
(vw), 2964 (vw), 2955 (vw), 2940 (vw), 2902 (vw), 2886 (w), 2878
(vw), 2856 (vw), 2850 (w), 2839 (w), 2818 (vw), 2776 (vw), 1470
(m), 1441 (m), 1222 (w), 1027 (m), 950 (w) cm−1; methyl
isocyanate, 2309 (m), 2292 (vs), 2284 (w), 2265 (m), 2263 (m) cm−1;
ethylene, 1441 (m), 948 (m), 915 (w), 913 (m), 910 (w), 743 (m),
741 (m) cm−1; starting from ∼450 °C, N,N-dimethylaminoketene
(2a), 2132 (w) cm−1.
Thermolysis of N,N-Dimethyldiazoacetamide (1a) in Argon with
Pulsed Deposition. A sample of 1a was placed in a glass tube, which
was attached to the cold head of a cryostat via an electromagnetic
valve. After an atmosphere of about 750 hPa of Ar was applied to the
system, the sample was heated to 150−170 °C, and the resulting
gaseous product was deposited onto the cold window to yield N-
methylazetidinone (3a). IR (argon, 10 K): 3a, 1841 (vw), 1782 (vs),
1775 (m), 1762 (m), 1573 (vw), 1557 (vw), 1496 (vw), 1474 (vw),
1417 (vw), 1388 (m), 1273 (w), 1236 (vw), 1072 (m), 968 (w), 811
(vw), 743 cm−1; unknown carbonyl compound, 1702 (m), 913 (w)
cm−1.
FVT of N-Methylazetidinone (3a) produced by Thermolysis of 1a
in Argon. The gaseous product from the aforementioned experiment
containing 3a was pulse-pyrolyzed, and the products were condensed
on the cold window to yield ketene, N-methylmethanimine, methyl
isocyanate and ethylene. IR data: see the data under FVT of 1a.
N-Methylazetidinone (3a). An authentic sample of 3a was
codeposited with Ar at 10 K. IR (Ar, 10 K): 2959 (vw), 2911 (vw),
2892 (vw), 1841 (vw), 1782 (vs), 1775 (m), 1762 (s), 1747 (w), 1739
(w), 1572 (vw), 1557 (vw), 1492 (vw), 1474 (vw), 1416 (vw), 1388
(m), 1273 (w), 1236 (vw), 1086 (vw), 1071 (m), 968 (w), 811 (vw),
743 (vw), 547 (vw) cm−1.
FVT of N-Methylazetidinone (3a). Pyrolysis of an authentic sample
of 3a followed by Ar matrix isolation of the products gave ketene, N-
methylmethanimine, methyl isocyanate, and ethylene. IR (Ar, 10 K):
ketene, 3064 (m), 2142 (vs), 1381 (m), 1115 (w), 525 (m); N-
methylmethanimine, 1470 (m), 1441 (m), 1221 (w), 1027 (m);
methyl isocyanate, 2309 (m), 2293 (s), 2266 (m); ethylene, 1441 (m),
948 (m) cm−1.
N,N-Diethyldiazoacetamide (1b). IR (Ar, 8 K): 2986 (w), 2939
(w), 2112 (vs), 1639 (s), 1625 (sh), 1616 (sh), 1483 (m), 1461 (m),
1450 (m), 1442 (s), 1432 (s), 1382 (w), 1351 (m), 1311 (vw), 1257
(m), 1230 (w), 1167 (m), 1134 (m), 1098 (w), 1089 (w), 1077 (w),
952 (vw), 813 (vw), 779 (vw), 725 (w), 621 (vw), 417 (w) cm−1.
Photolysis of N,N-Diethyldiazoacetamide (1b). A sample of 1b
was codeposited with Ar at 25 K. Broad-band photolysis (Xe/Hg
lamp) at 8 K yielded a mixture of 1-ethyl-4-methyl-2-azetidinone (3b),
1-ethyl-2-pyrrolidinone (6b), and N,N-diethylaminoketene (2b). IR
(argon, 8 K): 2985 (w), 2940 (w), 2903 (vw), 2880 (vw), 2132 (2b,
w), 1775 (3b, vs), 1755 (3b, sh), 1716 (6b, m), 1593 (w), 1495 (vw),
1463 (w), 1446 (w), 1427 (w), 1411 (w), 1406 (w), 1396 (w), 1381
(w), 1355 (w), 1348 (w), 1332 (vw), 1315 (w), 1286 (w), 1271 (w),
1263 (w), 1241 (vw), 1214 (w), 1170 (vw), 1129 (vw), 1100 (vw),
1085 (w), 1070 (w), 1060 (w), 965 (vw), 942 (vw), 898 (vw), 850
(vw), 830 (vw), 812 (w), 788 (vw), 770 (vw), 654 (vw) cm−1.
Prolonged irradiation or, more rapidly, reduction of the wavelength to
λ ≥190 nm gave rise to new bands at 2289 (w), 2279 (w), 2266 (all
due to ethyl isocyanate, w), 2142 (ketene, sh), 2138 (CO, s), 1680
(vw), 1658 (possibly ethylethylidenamine, vw), 1602 (m), 1593 (s),
1591 (s), 1453 (propylene, vw), and 1438 (propylene, vw) cm−1.
FVT of N,N-Diethyldiazoacetamide (1b). FVT of a sample of 1b at
400−600 °C with Ar matrix isolation of the pyrolyzates gave a
complex, temperature-dependent mixture of 1-ethyl-4-methyl-2-
azetidinone (3b), 1-ethyl-2-pyrrolidinone (6b), carbon monoxide,
ketene, N-ethylethylideneamine, ethyl isocyanate, and propene. IR
(argon, 7 K; intensity decreasing (↓) or increasing (↑) with increasing
temperature): 2988 (m), 2947 (w), 2925 (w), 2881 (w), 2858 (vw),
2292 (ethyl isocyanate, sh), 2280 (ethyl isocyanate, vs), 2265 (ethyl
isocyanate, s), 2228 (ethyl isocyanate, w), 2142 (ketene, ↓), 2138
(CO, ↑), 1775 (3b, ↓), 1715 (6b, ↓), 1693 (br, ↑), 1683 (br, ↑), 1496
(w), 1464 (m), 1453 (propene, m), 1441 (propene, m), 1427 (m),
1382 (w), 1350 (w), 1332 (vw), 1315 (vw), 1287 (m), 1272 (m),
1243 (vw), 1222 (vw), 1168 (vw), 1134 (vw), 1089 (vw), 978 (vw),
909 (vw), 850 (vw), 808 (vw), 793 (vw), 742 (vw), 650 (vw), 578
(vw), 524 (vw) cm−1.
1-Azabicyclo[3.2.0]heptan-7-one (6b). IR (Ar, 6 K): 2994 (w),
2982 (w), 2899 (w), 1784 (vs), 1778 (m/sh), 1761 (w/sh), 1487
(vw), 1461 (vw), 1452 (vw), 1417 (vw), 1345 (w), 1324 (w), 1290
(w), 1262 (vw), 1214 (w), 1209 (w), 1201 (w), 1002 (w), 609 (w),
549 (vw), and 456 (vw) cm−1.
1-Pyrroline (14). IR (At, 8 K): 3022 (vw), 2979 (s), 2963 (s), 2937
(s), 2877 (m), 1629 (s), 1474 (m), 1435 (m), 1321 (m), 1305 (w),
1280 (w), 1214 (m), 1081 (m), 1021 (m), 947 (m), 915 (m), 862
(w), 813 (w), 789 (w), and 412 (w) cm−1.
FVT of 1-Azabicyclo[3.2.0]heptan-7-one (6b). FVT at 500−700
°C with deposition of the products in an Ar matrix gave mixtures of
undecomposed 6b, ketene (CH2CO), 1-pyrroline (14), and CO. The
bands due to 6b disappeared completely only on FVT at ca. 800 °C.
N-(Diazoacetyl)azetidine (1c). IR (Ar, 10 K): 3017 (vw), 2991
(vw), 2970 (vw), 2949 (vw), 2908 (w), 2890 (w), 2885 (vw), 2113
(vs), 1658 (s), 1653 (m), 1636 (w), 1618 (w), 1600 (w), 1453 (w),
1439 (m), 1423 (m), 1407 (s), 1362 (vw), 1325 (vw), 1302 (vw),
1271 (vw), 1243 (vw), 1217 (vw), 1161 (w), 1157 (w), 1124 (vw),
1120 (vw), 1027 (w), 895 (vw), 815 (vw), 722 (w), 541 (vw) cm−1.
Photolysis of N-(Diazoacetyl)azetidine (1c) in an Argon Matrix. A
sample of 1c was codeposited with Ar at 25 K. Photolysis at λ ≥305−
310 nm and 10 K or, more rapidly, irradiation at 254 nm and 10 K
yielded azetidinylketene (2c). IR (argon, 10 K): 2133 (vs), 2131 (s)
cm−1. Photolysis of 1c at 254 nm was partially accompanied by
decomposition of 2c and gave rise to new signals at 2138 (CO, m),
2102 (w), 2098 (w), and 1718 (m) cm−1.
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Photolysis of N-(Diazoacetyl)azetidine (1c) in a Carbon
Monoxide Matrix. A sample of 1c was codeposited with carbon
monoxide at 25 K. Photolysis at 10 K and λ 254 nm yielded a mixture
of the s-cis and the s-trans conformers of azetidinylcarbonylketene
(13c). IR (CO, 10 K): s-trans-13c, 2125 (s), 1663 (m), 1658 (m),
1391 (w), 1384 (w) cm−1; s-cis-13c, 2153 (s), 1647 (m), 1428 (w)
cm−1. Prolonged irradiation caused the complete conversion of s-trans-
13c to s-cis-13c.
FVT of N-(Diazoacetyl)azetidine (1c). FVT of 1c at various
temperatures (300−600 °C) followed by argon matrix isolation of the
pyrolyzates resulted in complete and unspecific decomposition, with
carbon monoxide being the only identifiable product.
N-(Diazoacetyl)pyrrolidine (1d). IR (Ar, 10 K): 2992 (w), 2880
(w), 2114 (vs), 1642 (m), 1630 (sh), 1616 (m), 1455 (sh), 1440 (m),
1429 (s), 1258 (vw), 1231 (vw), 1204 (vw), 1175 (vw), 1160 (vw),
1124 (vw), 1040 (vw), 1006 (vw), 966 (vw), 915 (vw), 881 (vw), 732
(w), 518 (vw) cm−1.
Photolysis of N-(Diazoacetyl)pyrrolidine (1d). A sample of 1d was
codeposited with argon at 10 K. After photolysis with λ 254 nm ketene
2d was observed at 2134 cm−1 (Ar, 10 K).
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FVT of N-(Diazoacetyl)pyrrolidine (1d). The FVT was carried out
under the same conditions as described for 1a above. IR (argon, 10 K):
3064 (ketene, w), 2984 (w), 2878 (w), 2149 (m), 2142 (ketene, vs),
2139 (CO, s), 1785 (s), 1779 (m), 1760 (sh), 1600 (w), 1380 (ketene,
w), 1290 (m), 1160 (w), 913 (vw), 749 (m), 590 (vw), 525 (vw)
cm−1.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures S1−S4, showing additional IR spectra from the
photolysis of 1b, and figures and tables giving standard
orientations, dipole moments, thermochemical data, and IR
spectra of optimized structures at the (U)B3LYP/6-31G(d)
level (thermochemistry at the B3LYP/6-311+G(3df,2p)) level.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(13) Siemeling, U.; Farber, C.; Bruhn, C.; Leibold, M.; Selent, D.;
̈
Baumann, W.; von Hopffgarten, M.; Goedecke, C.; Frenking, G. Chem.
Sci. 2010, 1, 697−704. Siemeling, U. Aust. J. Chem. 2011, 64, 1108.
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2626. Methyl isocyanate: (i) Dunkin, I. R.; Shields, C. J.; Quast, H.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for C.W.: wentrup@uq.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are indebted to Professor M. S. Platz (Ohio State
University), who was instrumental in creating the collaboration
between our laboratories, for discussions, and for providing
samples of 1c. U.M. was a Feodor Lynen postdoctoral fellow
and thanks the Alexander von Humboldt-Stiftung (Germany)
for financial support. This work was also supported by the
Australian Research Council and the National Computing
Infrastructure facility (NCMAS g01) supported by the
Australian Government.
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