Photoelimination of Nitrogen from Diazoalkanes: Involvement of Higher Excited Singlet States in the Carbene Formation

Article abstract of DOI:10.1021/jacs.0c02221

Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photochemical formation of carbenes from diazoalkanes has not been proposed. Synergies of experiments and computations demonstrate the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazoalkanes, excitation to S1 results in nonreactive internal conversion to S0. On the contrary, excitation to higher-lying singlet states (Sn, n > 1) drives the reaction toward a different segment of the S1/S0 conical intersection seam and results in nitrogen elimination and formation of carbenes.

Full text of DOI:10.1021/jacs.0c02221

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Article
Photoelimination of Nitrogen from Diazoalkanes: Involvement
of Higher Excited Singlet States in the Carbene Formation
Tomislav Pitesa, Marija Aleskovic, Kristin Becker, Nikola Basaric, and Na#a Došli#
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2020
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Photoelimination of Nitrogen from Diazoalkanes: Involvement of
Higher Excited Singlet States in the Carbene Formation
Tomislav Piteša,‡ Marija Alešković,‖ Kristin Becker,‖ Nikola Basarić,‖* and Nađa Došlić‡*
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‡ Department of Physical Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia.
‖ Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia.
KEYWORDS Diazoalkanes, Carbenes, Anti-Kasha Photochemistry
ABSTRACT: Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photo-
chemical formation of carbenes from diazoalkanes has not been proposed. Synergy of experiments and computations demonstrates
the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazoalkanes, excitation to
S1 results in nonreactive internal conversion to S0. On the contrary, excitation to higher-lying singlet states (Sn, n > 1), drives the
reaction toward a different segment of the S1/S0 conical intersection seam and results in nitrogen elimination and formation of car-
benes.
Introduction
sibility of a wavelength dependent CH2 + N2 formation in dia-
zirine,24 but this possibility was subsequently dismissed.25 The
existence of an extended seam of conical intersections (CI)
along the C-N dissociation coordinate was revealed in diazo
compounds undergoing photoinduced Wolff rearrangement,
and segments of the seam were associated with different photo-
products.26,27 These findings, however, were not positioned in
the more general context of anti-Kasha reactivity.
Photoreactions taking place from higher excited states, known
as anti-Kasha photochemistry,1 are rare since not many photo-
chemical processes can compete with fast internal conversion
(IC) to the first excited electronic state (S1). Notable examples
involve formation of nitrenes by elimination of nitrogen from
higher excited singlet states (Sn) of the corresponding azides in
4-amino-3-nitrophenyl azide,2 or 2-naphthoyl azide.3 However,
anti-Kasha reactivity is not generally observed for all azides.
For instance, 1-naphthyl azide reacts from S1 due to a low en-
ergy barrier toward arylnitrene formation in that state, while the
S2 state is not reactive.4 Understanding ways to exploit the anti-
Kasha reactivity has fundamental and technological value as it
offers opportunities for a clever design of different photochem-
ical processes where the selectivity of a reaction can be tuned
by an appropriate choice of the excitation wavelength.
In this work we focus on diazoalkanes 1-3 (Chart 1) and demon-
strate experimentally and computationally the general im-
portance of higher excited singlet states in their photochemistry
and photophysics. Diazoalkanes are versatile reagents in or-
ganic synthesis,5 which can undergo thermally or photochemi-
cally induced elimination of nitrogen to give carbenes.6 Over
the years, photo-generated carbenes have been used for the
methylation of carboxylic acids, alcohols, phenols, heteroatoms
including nitrogen and sulfur, cyclopropanations, ring expan-
sions and cycloadditions.7 Photochemistry is particularly inter-
esting since spatial and temporal control over the formation of
carbenes may pave the road to applications in biological sys-
tems.8,9
Chart 1. Diazoalkanes 1-3, and the corresponding carbenes car-1-
car-3.
Results and Discussion
Fluorescence and Irradiation Experiments
The details on the synthesis of 2 and 3 and the description of
photochemical experiments are given in the Supporting Infor-
mation (SI).
Synthesis and photochemistry of 2 has been reported by Eguchi
et al.28 Photochemistry of 310-23 and a number of alkyl phenyl
carbene precursors has been investigated,29,30 but no evidence
of anti-Kasha photochemistry or fluorescence from higher ex-
cited states have been reported. We also investigated photo-
physics of 2 and 3, where 2 showed unexpected fluorescence
properties (Figs S1-S5 and Table S2 in the SI). Due to explosive
character, 1 was investigated only computationally.
Previous experimental studies were concerned with larger di-
azoalkanes, particularly diphenyl derivative 3 due to weaker ex-
plosive character and easy excitation. The chemistry of carbene
car-3 has been completely unraveled,10-23 but not much has been
learned on the photochemical elimination reaction. From the
computational side, an early CASSCF study suggested the pos-
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Fig.1 shows absorption and fluorescence spectra of 2 measured
Computational Methods
in benzene. Spectra measured in hexane, cyclohexane and ben-
zene are reported in the SI. The diazo compound is character-
ized by two absorption bands. The first band is weak (ε ≈ 30-50
M-1 cm-1) and centered at 500 nm (characteristic pink color),
whereas the second is intense (ε ≈ 11000 M-1 cm-1) and centered
at ≈ 300 nm. The diazo compound is fluorescent when excited
to Sn with the quantum yield of ΦF ≈ 0.04. Upon excitation to
S1 fluorescence could not be detected.
To gain insight into the photochemical mechanisms of N2 ex-
trusion following VIS or UV excitation, we performed exten-
sive ab initio static calculations and trajectory-surface-hopping
nonadiabatic dynamics simulations. Owing to the disparity in
the molecular size, we opted for an electronic structure method
capable of describing 1-3 with the same accuracy. The method
of choice is the algebraic diagrammatic construction up to sec-
ond-order (ADC(2)) method,32-35 which is a correlated single-
reference method characterized by relatively small mean abso-
lute error (0.22 eV) with respect to benchmark data.36-38 The
method, implemented in Turbomole,39 describes electronically
excited states with accuracy similar to the MP2 method for the
electronic ground state and has been employed in a number of
recent static and dynamics studies of organic molecules.40-42
However, system 1 is amenable to a more accurate treatment.
An extensive investigation of 1 was performed using bench-
mark extended multi-state complete active space self-consistent
field second-order perturbation theory (XMS-CASPT243), as
implemented in the BAGEL program package.44,45 A detail
comparison shows that the ADC(2) method reproduces very ac-
curately the electronic states of 1 not only in the Franck-Condon
(FC) region, but also along all investigated reaction pathways
(see Figures S6-S8) and, hence, can be used to investigate the
photochemistry of 1-3. For more details, see the SI.
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1.2x104
Absorbance
Fluorescence, ex = 310 nm
9.0x103
6.0x103
3.0x103
0.0
300
400
500
600
Wavelength / nm
Figure 1. Absorption spectrum for 2 and normalized fluorescence
spectrum (λex = 310 nm) in benzene. See Fig. S2 for comparison
with computed absorption spectrum.
Static View of the Photoreactions
Vertical excitation energies of 1-3 are compiled in Table 2. In
all three systems the first transition (S0 → S1) is weak, while the
second transition in 1 (S0 → S2) and third transition in 2 and 3
(S0 → S3) are characterized by large oscillator strength.
Prompted by the observation of fluorescence of 2 from a higher
excited state, we investigated if the molecule shows anti-Kasha
photochemistry. Preparative irradiations for 2 and 3 were per-
formed in cyclohexane and benzene upon excitation with visi-
ble light (cool white lamps, corresponding to the population of
S1) and at ≈ 300 nm giving rise to Sn (Schemes S3-S8 and Table
S1 in the SI).
Table 2. Vertical excitation energies in eV and oscillator
strengths (in parenthesis) computed at the XMS(4)-
CASPT2(12e,10o)/cc-pVDZ (for 1) and ADC(2)/cc-pVDZ
(for 1- 3) levels.
Table 1. Quantum yields (ΦR) for the photoelimination of
nitrogen from 2 and 3.a
XMS(4)-
CASPT2[12e,10o]
ADC(2)
b,c
b,d
Compound / Solvent
2 / cyclohexane
2 / benzene
Φ300
Φ500
0.48±0.04
0.49±0.02
0.36±0.01
0.46±0.06
0.048±0.001
0.010±0.001
0.020±0.001
0.020±0.001
State
S1
1
1
2
3
3.03
3.42
3.05
2.82
3 / cyclohexane
3 / benzene
(0.0001)
(0.0001) (0.0038) (0.0030)
6.66 4.86 4.57
(0.2789) (0.0408) (0.0401)
7.62 4.98 4.60
(0.0170) (0.1646) (0.1008)
S2
S3
6.47
a Measurement were conducted by irradiating the samples at 300
nm or 500 nm with a use of ferrioxalate actinometer ( = 1.25,
 = 0.9).31 Measurements were done in triplicate, the average
value is reported and the errors correspond to maximum absolute
deviations. b Quantum yields of compound decomposition, ΦR, was
calculated according to Eqs. S1-S3 in the SI. c Irradiation time 1
min. d Irradiation time 4 h.
(0.295)
8.63
(0.000)
From the natural transition orbital (NTO) pairs involved in the
excitations and compared in Figure 2, it is evident that the first
transition is almost analogous in 1-3. The S0 → S3 transition in
2 and 3 involves translocation of charge to the phenyl ring(s),
but locally, at the CNN moiety, bears a strong resemblance to
the S0 → S2 transition in 1.
Since preparative irradiations (Table S1) indicated that photoe-
limination was more efficient upon excitation at 300 nm then
by use of visible light, we measured quantum yields for the re-
action upon excitation to S1 and Sn (Table 1). The excitation to
Sn leads to 10-50 times more efficient nitrogen elimination then
excitation to S1, indicating that anti-Kasha photochemistry
takes place, which was verified by computations (vide infra).
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Chart 2. Coordinates describing the mutual orientation of the N2
and R1-C-R2 fragments of 1-3. The C-N distance, R, the C-N-N
bending angle φ (in the σ2 plane) and the angle δ between the σ1 and
σ2 planes. The planes σ1 and σ2 are defined by atoms R1-C-R2 and
C-N1-N2, respectively.
Figure 2. Comparison of dominant natural transition orbital (NTO)
pairs describing the lowest singlet excited states of 1-3. Computa-
tions have been performed at the ground state equilibrium geome-
tries using the ADC(2)/cc-pVDZ method.
Excitation of 1-3 to the S1(nπ1*) state leads to IC to the ground
state. To describe the main features of the reaction we first lo-
cated relevant CIs, such as the minimum-energy CI geometry
(MECI)47 and then constructed a linearly interpolated reaction
path (LIP) from the Franck-Condon geometry to the MECI.
Specifically, we generated a set of molecular geometries by lin-
ear interpolation in internal coordinates and performed single-
point energy calculations for the obtained geometries at the
ADC(2) level. Figure 4 shows the ADC(2) energy profiles of
the ground state (blue) and the diabatic nπ1* (orange) and nπ2*
(green) states along the LIPs connecting the Franck-Condon ge-
ometries of 1-3 and the corresponding S1/S0 CI.
To connect the nature of the initial excitation and the photochemi-
cal fate of the system it is useful to consider the difference in the
electron densities between the ground and the excited states.46 Fig-
ure 3 shows that electronic excitation of 1 to S1 involves transloca-
tion of negative charge from the carbon to the nitrogen atoms in the
xy-plane of the molecule.
Figure 3. Electron density difference map of 1. Translocation of
electron density upon electronic excitation to S1(nπ1*) (left) and
S2(nπ2*) (right). Areas of increased (reduced) electron density are
shown in pink (silver). The computations have been performed at
the ADC(2)/cc-pVDZ level.
As the nuclei follow the electrons, one can expect that nuclear
motion will initially take place in the plane of the molecule. The
S0 → S2 transition also increases the electron density at the mid-
dle N atom, but it does so perpendicularly to the xy-plane of the
molecule, suggesting an initial out-of-plane motion of the nitro-
gen atom. Owing to the comparable character of electronic ex-
citations in 1-3, it is plausible to expect that the initial nuclear
motion in the two larger systems will be stirred in the same di-
rection.
Before considering the photochemical reaction paths of 1-3 it is
useful to define the three coordinates (see Chart 2) which define
the mutual orientation of the N2 and carbene fragments: the C-
N distance R, the C-N-N bending angle φ, and the angle be-
tween the planes σ1 and σ2, denoted δ.
Figure 4. Relative energies of the ground (blue) and diabatic nπ1*
(orange), nπ2* (green) and nπ3* (red) states of 1 (top), 2 (middle)
and 3 (bottom) along the linearly interpolated paths (LIPs) from the
FC geometry to the S1/S0 CI. Left panels show the LIPs from the
FC geometry to the S1(nπ1*)/S0 MECI. Right panels show the LIP
from the FC geometry to S2(nπ2*)/S1(nπ1*) MECI (vertical line)
and to the closest S1(nπ2*)/S0 CI. For comparison with the LIPs for
1 computed using the XMS-CASPT2 method see Figure S7.
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Figure 5. Left: 2D rigid scan of the S1-S0 energy gap (in eV) along the C-N distance, R, and the angle, δ (see Chart 2). Superposed blue
ꢂꢃꢄ
arrows are scaled force vectors in the S1 state projected to the (R, δ) subspace, ꢀꢁ ꢂꢃ , ꢁ ꢂꢆ ꢇ. Right: One-dimensional cuts of the three
ꢄ
ꢂꢅ
lowest diabatic states for out-of-plane (top) and in-plane (bottom) dissociation. For analogous results computed using the XMS-CASPT2
method see Figure S8.
One immediately sees that all reaction paths are barrierless. Ex-
citation of 1 to the S1(nπ1*) state (Figure 4a and Figure S7) leads
to the increase of R from 1.33 to 1.56 Å and to the bending of
the C-N-N moiety by ≈ -60°, from 176.4° to 116.8°. As ex-
pected on the basis of the translocation of the electron density
(Figure 3), dissociation takes place in-the-plane of the mole-
cule. Very similar paths for in-plane N2 extrusion in the larger
systems are shown in Figures 4c and 4e.
than for out-of-plane (δ ≈ 90°) dissociation. The two one-di-
mensional potential energy cuts shown on the right, confirm
that out-of-plane N2 extrusion takes place on the nπ2* while in-
plane extrusion occurs on the nπ1* surface. The most important
difference, however, lies in the topography of the CIs. The su-
perimposed blue arrows in Figure 5 are the (scaled) force vec-
ꢂꢃ
tors with components ꢀꢁ ꢂꢃꢄ ꢇ and ꢀ– ꢂꢆ ꢇ. The force vectors
ꢄ
ꢂꢅ
were computed analytically39 in the S1 state, along which the
S1/S0 CI seam is approached. The arrows show that for out-of-
plane dissociation (δ ≈ 90°) along the diabatic nπ2* state an out-
ward force acts on the C-N bond, leading to an effective sepa-
ration of the CH2 and N2 fragments. On the contrary, for in-
plane dissociation in the diabatic nπ1* state, the forces are di-
rected inward (left arrows) and hinder the separation of the frag-
ments. Figure S8 shows that an analogous scan has been ob-
tained at the XMS-CASPT2 level.
On the other side, excitation of 1 to S2(nπ2*) and of 2 and 3 to
S3(nπ2*) (green) leads to out-of-plane extrusion of N2. The
mechanism is shown in the right panels of Figure 4. The first
part of the LIP connects the FC geometry and the
S2(nπ2*)/S1(nπ1*) MECI. These MECIs are non-planar (89.1° ≤
δ ≤ 90.9°) and characterized by elongated C-N bonds (1.50 Å ≤
R ≤ 1.59 Å). In the second part of the LIP, a slight elongation
of the C-N bond by 0.1 - 0.16 Å, stabilizes the nπ2* state below
the nπ1* state and leads to the S1(nπ2*)/S0 CI.
Dynamical View of the Photoreactions
Altogether, two different S1/S0 CIs exist in 1-3, one between the
diabatic nπ1* state and S0 which promotes in-plane fragmenta-
tion, and the other, between the diabatic nπ2* state and S0 lead-
ing to out-of-plane fragmentation. The decay through both CIs
should lead to the formation of carbene and N2, but the effi-
ciency of the reactions depends on whether the nπ1* or the nπ2*
state crosses with S0. This point is illustrated in Figure 5 where
the two-dimensional (2D) rigid scan shows the change in the
energy gap, ΔE, between the S1 and S0 states of 1 with respect
to the main reaction coordinates, R and δ (see Chart 2). The
value of φ was fixed to 130°. The colour scale (top) indicates
the size of the energy gap in eV. One sees that the S1/S0 CI seam
(black area) is reached along the C-N bond dissociation coordi-
nate, R. However, the CI seam is significantly curved and
reached at shorter CN distances for in-plane (δ ≈ 0° and 180°)
To verify that the anti-Kasha reactivity of 1-3 is governed by
the same underlying mechanism, we performed non-adiabatic
dynamics simulations. For each molecule, 70 trajectories were
launched from S1 and from either S2 or S3, depending on the
oscillator strength, and propagated until the S1/S0 CI was
reached (for details see S9-S12 in the SI). In all three systems
trajectories launched from the S1(nπ1*) state deactivated to the
ground state on the time scale of 50-70 fs. In systems 1 and 3,
trajectories initiated in the S2 or S2/S3 states returned to the
ground state in 150-200 fs. However, the decay of 2 is slower
and the ground state was completely repopulated only after 450
fs. The slower decay of 2 after photoexcitation to S2/S3 indicates
that the S2(nπ2*) surface is rather flat. In solution, where the
excess kinetic energy can be redistributed to the solvent on the
time scale of around 1 ps, this may result in a partial trapping of
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population in S2 and could potentially explain the weak fluores-
naturally continued in the ground state (for characteristic trajec-
tories see Figures S11 and S12). In ~1/3 of trajectories diazirine
was formed from a hot ground state of diazomethane confirm-
ing the 1994 prediction of Yamamoto et al.24
cence from S2 (ΦF =0.04) observed in the experiment (see Fig-
ure 1). For comparison of the excited state lifetimes of 1-3 see
Figure S9 and S10.
The most significant difference between the simulations initial-
ized in the S1 and S2 states concerns the geometries of the S1/S0
CIs. In all three systems, trajectories initialized in the S1 state
reached the ground state via a planar (δ ≈ 0° or 180°) S1/S0 CIs,
while those initiated in S2(S3) deactivated mostly via non-planar
S1/S0 CIs. The results are summarized in Figure 6 where the
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Conclusion
This work provides evidence of violation of the Kasha rule in
the formation of carbenes from diazoalkanes 1-3. We have
shown that diazomethane derivatives 1-3 undergo photochemi-
cal formation of carbenes and N2 by the same mechanism,
where excitation to higher-lying excited singlet states is neces-
sary for the elimination to take place efficiently. Excitation to
S1(nπ1*) leads to in-plane deactivation in a segment of the
S1(nπ1*)/S0 CI seam where the forces acting on the systems hin-
der the separation of the carbene and N2 fragments. In contrast,
excitation to S2 (S2/S3) for 1 (2-3) leads to non-planar CIs and
results in about 10-50 times more efficient N2 elimination.
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ASSOCIATED CONTENT
Supporting Information contains details on synthesis and photo-
chemistry of 2 and 3, photophysical properties of 2, computational
details including molecular coordinates and copies of H and 13C
1
NMR spectra of all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* ND E-mail: Nadja.Doslic@irb.hr; NB E-mail nbasaric@irb.hr.
Author Contributions
The manuscript was written through contributions of all authors.
MA and TP are both first authors who contributed equally.
Funding Sources
Croatian Science Foundation (HRZZ IP-2014-09-6312 and IP-
2016-06-1142).
ACKNOWLEDGMENT
These materials are based on work financed by the Croatian Sci-
ence Foundation (HRZZ IP-2014-09-6312 and IP-2016-06-1142).
The authors acknowledge generous support of Professors Peter
Wan and Cornelia Bohne from the University of Vicoria, Canada
BC and Centre for Advanced Materials and Related Technologies
(CAMTEC), University of Victoria, Canada BC for access to the
equipment. We thank our team members D. Babić, A. Ponzi and
M. Sapunar for constructive discussions and suggestions.
Figure 6. 2D projections of a set of structures of 1 (top), 2 (center)
and 3 (bottom) in the vicinity of the S1/S0 CI seam on the (R, δ)
subspace. Points originate from trajectories initialized in S1 (or-
ange) and in S2/S3 (green). Black points indicate the S1(nπ1*)/S0
MECIs (δ ≈ 0° and 180°) and S1(nπ2*)/S0 minimal distance CIs (
δ
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of 1-3 is very similar. Compared to the Franck-Condon geome-
try, all structures have elongated CN bonds, but the elongation
is significantly larger for out-of-plane N2 elimination (δ ≈ 90°)
and this happens only if the dynamics is initiated from a higher
excited state (green).
As a side result, let us mention that trajectories of 1 computed
using the XMS-CASPT2 method and initiated in S1(nπ1*) were
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