Photoinduced α-Cleavage of 2-Azido-2-phenyl-1,3-indandione at Cryogenic Temperatures

Upasana Banerjee, Sujan K. Sarkar, Jeanette A. Krause, William L. Karney, Manabu Abe,

Letter

Photoinduced α‑Cleavage of 2‑Azido-2-phenyl-1,3-indandione at

Cryogenic Temperatures

∥

and Anna D. Gudmundsdottir *

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sı Supporting Information

ABSTRACT: To expand the utility of α-cleavage at cryogenic

temperatures, we investigated the photoreactivity of 2-azido-2-

phenyl-1,3-indandione (1). EPR spectroscopy revealed that

irradiating 1 in 2-methyltetrahydrofuran (mTHF) matrices forms

alkylnitrene 2, which has zero-ﬁeld splitting parameters (D/hc =

−

−1

.5837 cm ; E/hc = 0.0039 cm ) typical of an alkylnitrene. IR

spectroscopy demonstrated that irradiating 1 in argon matrices

results in the concurrent formation of 2, imine 3, benzocyclobu-

tenedione 4, and benzonitrile 5.

he well-established Norrish type I photoreaction,

Scheme 1. Nonreversible α-Cleavage of α-Azido-substituted

involving α-cleavage of a C−C bond adjacent to a

Ketones

carbonyl group, allows the convenient formation of radical

pairs in solution. Due to the ease of forming radicals via the

photoinduced α-cleavage of ketones, this reaction is used in

various applications including polymerization, photoinitiation,

and photocuring. However, the utility of α-cleavage to form

radical pairs in conﬁned media (e.g., cryogenic matrices or

crystals) is limited because initial radical pairs generally

recombine to regenerate the starting material because they

cannot diﬀuse apart. However, α-cleavage of aliphatic ketones

in the solid state can be rendered nonreversible if the cleavage

of the second C−C bond to extrude CO is faster than the

recombination of the initial radical pair. Using this approach,

Garcia-Garibay and co-workers produced radical pairs in

crystals that, due to the rigid conﬁnement of the crystal lattice,

formed new C−C bonds stereoselectively, allowing the elegant

that the fate of the azido methyl radicals involves nitrogen

molecule extrusion to form imine radicals. Irradiating α-

azidoacetophenone derivatives in the solid state allows

synthesis of natural products.

Nonreversible α-cleavage of ketones can also be achieved by

placing an azido group at the α-position because a carbon-

centered radical adjacent to an azido moiety can undergo

trapping of these imine radicals to form products.

In contrast, irradiating α-azidoacetophenone in cryogenic

matrices yields only the corresponding triplet alkylnitrene

through intramolecular triplet sensitization of the azido group,

eﬀective nitrogen molecule extrusion to form an imine radical

(

Scheme 1). We have shown that irradiating α-azidoaceto-

phenone derivatives in solution yields the corresponding triplet

Received: August 21, 2020

excited ketones, which undergo α-cleavage to form benzoyl

6a,c

and azido methyl radicals. However, in competition with α-

cleavage, the triplet ketones also decay by energy transfer to

their azido chromophores, which yields triplet alkylnitrenes. In

solution, the benzoyl radicals are eﬃciently trapped by the

triplet alkylnitrenes to form the major product. We theorized

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

respectively, at 9.40 GHz. The calculated zero-ﬁeld splitting

Letter

−1

with no products attributable to α-cleavage. Thus, it is

unknown whether α-cleavage of azido alkyl derivatives can be

achieved at cryogenic temperatures or if the recombination of

the initial radical pair is more eﬃcient than nitrogen molecule

extrusion to form an imine radical.

parameters D/hc and E/hc were 1.5837 and 0.0039 cm ,

respectively. The D/hc value of 2 is similar to those of triplet

2-(λ -nitreno)-1,3-dioxo-2,3-dihydro-1H-indene-2-carbonitrile

(1.57 cm ) and triplet methylnitrene (1.595 cm ) and its

−

−1

derivatives.

Herein, we report the photoreactivity of azide 1 (CCDC

Furthermore, Wentrup and co-workers have demonstrated a

linear correlation between the calculated spin densities of

triplet nitrenes and measured D/hc values, and 2 follows the

970115 ) in cryogenic matrices. Using EPR, absorption, and

IR spectroscopies, we determined that alkylnitrene 2, imine 3,

benzocyclobutenedione 4, and benzonitrile 5 form via α-

cleavage of 1 in cryogenic matrices (Scheme 2), thus

same trend, with a calculated spin density of 1.81 on the N

atom (Supporting Information (SI)).

Because the formation of stable photoproducts or short-lived

diradicals cannot be determined by EPR spectroscopy, the

photoreactivity of 1 was studied in argon matrices using IR

spectroscopy. Irradiation (λ = 300−400 nm) of 1 in an argon

matrix resulted in the complete depletion of the IR bands for 1

Scheme 2. Photoreactivity of 1 in Cryogenic Matrices

−

at 2113, 1776, 1758, 1744, 1725, and 1268 cm .

Concurrently, new bands formed at 2238, 1817, 1788, 1754,

1708, 1697, 1692, 1600, 1595, 1566, 1250, 1187, 1135, 1070,

expanding the scope of α-cleavage for forming desirable

−

products or intermediates at cryogenic temperatures. However,

1059, 1021, 1002, 862, 764, 722, and 575 cm (Figure 2),

we cannot exclude the possibility that 2 forms through energy

which were assigned to 2, 3, 4, and 5 based on comparisons

transfer from the triplet excited state of the ketone (T ) of 1 to

with their calculated (B3LYP/6-311++G(3df,3pd) ) and

the triplet excited state of the azido moiety (T ).

scaled (0.9604) IR spectra. The bands at 1754, 1708,

−

EPR spectroscopy demonstrated that irradiation (λ = 300−

187, and 722 cm were assigned to 2, as they match well

00 nm) of 1 in an mTHF matrix at 80 K yields 2 (Figure 1).

with its calculated and scaled IR bands at 1755, 1719, 1197,

−

and 736 cm . The bands at 1697, 1692, 1600, 1595, 1566,

250, 1135, 1070, 1059, 1021, 1002, 862, and 764 cm⁻were

assigned to 3 based on a comparison with its computed

spectrum, which has major bands at 1681, 1670, 1566, 1560,

−

533, 1205, 1163, 1086, 1039, 1032, 993, 837, and 752 cm .

−

The bands at 1817 and 1788 cm were assigned to the

carbonyl stretches in 4 based on its reported IR bands at 1818

−

−1

and 1787 cm , with the additional band at 1768 cm

presumably resulting from matrix splitting. The carbonyl bands

−

for 4 were calculated to be at 1802 and 1778 cm , which

Figure 1. EPR spectrum obtained by irradiating 1 in an mTHF matrix

80 K).

corresponds well to the observed bands. Finally, we assigned

(

−1

the weak band at 2238 cm to the CN stretching of 5, which

−

has been reported to be at 2236 cm in argon matrices.

The spectrum showed H and H lines at 8138 and 8363 G,

To determine whether 2, 3, 4, and 5 form concurrently

from the same precursor or if 2 serves as the precursor to 3, 4,

Figure 2. (A) Diﬀerence IR spectrum obtained by subtracting the spectrum of 1 before irradiation from the spectrum after 150 min irradiation. (B)

−1

Calculated (B3LYP/6-311++G(3df,3pd)) and scaled IR spectra of 2, 3, 4, and 5. *The band at 1624 cm was attributed to matrix-isolated water.

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

after 15, 20, 30, and 40 min irradiation (Figure 3 ). As the IR

Letter

and 5, we compared the time proﬁles of the IR bands.

Diﬀerence spectra were obtained by subtracting the IR

spectrum obtained before irradiation from those collected

possibility that 2 is formed through energy transfer at the

exact same rate as 3, 4, and 5 are formed from 6 via α-

cleavage of 1.

Scheme 3. Proposed Mechanism for Forming 2, 3, 4, and 5

Figure 3. Diﬀerence IR spectra obtained by subtracting the IR

spectrum of 1 from those obtained after 2, 15, 20, 30, and 40 min

irradiation.

To support the proposed mechanism, DFT calculations were

performed using Gaussian16 at the B3LYP level of theory with

bands for 2, 3, 4, and 5 formed concurrently at the same rate

Figure S14 ), we theorized that these photoproducts have a

common precursor. Further, we employed absorption spec-

troscopy to support the formation of 2, 3, and 4 in mTHF

matrices using a conventional setup. Using the absorption

spectrum of 1 in an mTHF matrix before irradiation as the

baseline, absorption spectra were recorded at regular time

intervals following irradiation (LED, 400 ± 9 nm).

Upon irradiation, negative absorption was observed below

75 nm due to the depletion of 1 (Figure 4 ). However, a new

12,17

the 6-31+G(d) basis set.

The ground state (S ) of 1 was

optimized, and TD-DFT calculations gave the vertical

excitation energy of the ﬁrst singlet excited state (S ) and T

of 1, which were located 70 and 62 kcal/mol, respectively,

above S . The optimized T of 1 was located 55 kcal/mol

above its S , which is somewhat lower than the TD-DFT-

calculated energy (Figure 5). Furthermore, spin density

calculations on the T of 1 showed that the spin densities

are concentrated on the oxygen atoms of the CO groups

0.42 each).

Figure 4. UV−vis diﬀerence spectra of 1 in mTHF (77 K) before and

Figure 5. DFT (B3LYP/6-31+G(d)) optimized structures of 1, T

after irradiation, and calculated absorption spectra of 1, 2, 3, and 4.

, 2, 6, and 7. Numbers in parentheses are the energies (kcal/mol)

relative to the S of 1. Numbers near atoms are computed spin

densities (red: on O; gray: on C; blue: on N).

broad absorption band was observed between 275 and 520 nm,

which resembles the TD-DFT calculated spectra of 2 and 3.

Because all the signiﬁcant absorption bands of 4 overlap those

of 1 and because 5 does not absorb above 250 nm, it is diﬃcult

to detect the absorption of these products in mTHF matrices.

Therefore, the formation of 3, 4, and 5 was veriﬁed by HRMS

and GC-MS. Similar results were obtained when the mTHF

matrix of 1 was irradiated at 254 nm, indicating that the

photochemistry is not wavelength dependent.

Spin density calculations on the optimized structure of ³2

revealed spin localization on the N atom. The geometry of the

open-shell singlet conﬁguration ( 2) was optimized using DFT

calculation with the broken-symmetry method. The total

spin ⟨S ⟩ value of 1.0061 suggested signiﬁcant spin

contamination, so we performed CASPT2 calculations on a

similar alkylnitrene to better estimate the singlet−triplet gap

(SI). The estimated singlet−triplet gap of ∼32 kcal/mol

The EPR and absorption spectra demonstrate that

irradiating 1 yields 2, whereas the IR spectra verify that 3,

strongly suggested that 2 is unlikely to decay by intersystem

, and 5 form concurrently with 2. Thus, we propose that 2,

crossing (ISC). Optimization of 6 demonstrated that it

, 4, and 5 form simultaneously following α-cleavage of 1 to

contains benzoyl and benzyl carbon-centered radicals, which

was veriﬁed by the spin density calculations, as the unpaired

yield 6 (Scheme 3). However, we cannot exclude the

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

imine nitrogen and benzoyl carbon atoms (Figure 6 ). The

Letter

spins were mainly localized on the C atoms adjacent to the

phenyl rings. Similarly, optimization of 7 revealed that both

the imine and benzoyl radical moieties are conjugated to the

adjacent phenyl rings. Furthermore, spin density calculations

revealed that the unpaired spins are mainly located on the

Thus, we have demonstrated that α-cleavage of ketones with

α-azido substituents is feasible in cryogenic matrices. This

method can serve as a tool for forming heterocyclic

compounds and potentially triplet alkylnitrenes. Forming

triplet alkylnitrenes is challenging because the direct photolysis

of alkyl azides does not produce the corresponding triplet

alkylnitrenes but rather imine products, either through

concerted reactions on the singlet excited state surface of the

alkyl azides or through the formation of singlet alkylnitrenes.

If singlet alkylnitrenes are formed, they generate products

faster than ISC to their triplet conﬁguration occurs. Therefore,

triplet alkylnitrene formation is generally limited to the triplet

sensitization of alkyl azides. In our previous work on α-

azidoacetophenone in argon matrices, α-cleavage could not

compete with energy transfer to form triplet alkylnitrenes.

We theorize that energy transfer in 1 is less eﬃcient than that

in α-azidoacetophenone because the sensitizer moiety in 1 has

a signiﬁcantly lower triplet energy than the ketone

chromophore in α-azidoacetophenone (74 kcal/mol), allowing

6a,c

α-cleavage to occur eﬀectively at cryogenic temperatures.

In conclusion, 1 undergoes α-cleavage in argon matrices

because the rearrangement of 6 into 7 is suﬃciently eﬃcient

to compete with the reformation of the initial radical pair of l.

Thus, α-cleavage at cryogenic temperatures can potentially be

used for the formation of intermediates including triplet

alkylnitrenes and heterocyclic products.

ASSOCIATED CONTENT

sı Supporting Information

■

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.orglett.0c02794.

Experimental procedures, characterization of azide 1,

theoretical calculations, argon and mTHF matrices

(PDF)

Figure 6. Calculated stationary points on the triplet energy surface of

forming (a) 7 and (b) 2, 3, 4, and 5. Energy values (kcal/mol)

Accession Codes

calculated using B3LYP/6-31+G(d) and ωB97XD/6-31+G(d) are

represented in black and blue, respectively.

CCDC 1970115 contains the supplementary crystallographic

data for this paper. These data can be obtained free of charge

via www.ccdc.cam.ac.uk/data_request/cif , or by emailing

data_request@ccdc.cam.ac.uk , or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

energy of the optimized structure of 7 was similar to that of

, with a total spin ⟨S ⟩ value of 0.9729. Despite this large spin

contamination, it is reasonable to expect eﬃcient ISC from 7,

as the singlet−triplet gap is small.

Figure 6 displays the calculated stationary points (B3LYP/6-

1+G(d)) on the triplet surface of 1 for the formation of 2, 3,

AUTHOR INFORMATION

Corresponding Author

■

, and 5. The calculated barrier for α-cleavage of the T of 1 to

form 6 is 2 kcal/mol. Similarly, the computed barrier for

nitrogen molecule extrusion from 6 is small (5 kcal/mol). The

calculated barrier for 7 to form 2 is only 4 kcal/mol, whereas

that for 7 to cleave to form 5 and 8 is somewhat larger (16

Anna D. Gudmundsdottir − Department of Chemistry,

University of Cincinnati, Cincinnati, Ohio 45220-0172, United

States; orcid.org/0000-0002-5420-4098;

Email: Anna.Gudmundsdottir@uc.edu

kcal/mol). Therefore, it can be theorized that 7 undergoes

Authors

ISC to form 3, 4, and 5 and that the ISC rate is comparable to

the formation rate of 2. As we were unable to optimize the

Upasana Banerjee − Department of Chemistry, University of

Cincinnati, Cincinnati, Ohio 45220-0172, United States;

orcid.org/0000-0002-5580-4511

T₁_Aof 1 using B3LYP, we used a long-range-corrected

functional ωB97XD with the 6-31+G(d) basis set. The T₁_A

of 1 was located 45 kcal/mol above its S , and the calculated

barrier for forming 2 was less than 1 kcal/mol. The stationary

points on the triplet surface of 1 showed similar trends using

Sujan K. Sarkar − Department of Chemistry, University of

Cincinnati, Cincinnati, Ohio 45220-0172, United States;

orcid.org/0000-0002-8471-6297

this theory and B3LYP (Figure 6), except for the energy of the

Jeanette A. Krause − Department of Chemistry, University of

Cincinnati, Cincinnati, Ohio 45220-0172, United States

William L. Karney − Department of Chemistry, University of

San Francisco, San Francisco, California 94117, United States;

orcid.org/0000-0003-0976-742X

T of 1; however, B3LYP tends to underestimate the energies

of triplet ketones with (n,π*) conﬁguration. The ωB97XD-

computed IR and absorption spectra of 2 , 3 , 4 , and 5 are

similar to those obtained with B3LYP (Figures S26 −S29 ).

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Vinylnitrene into Triplet Alkylnitrene at Cryogenic Temperatures.

Letter

Manabu Abe − Department of Chemistry, Graduate School of

Org. Lett. 2019, 21, 7194−7198.

Science, Hiroshima University, Higashi-Hiroshima, Hiroshima

(10) (a) Ferrante, R. F. Spectroscopy of Matrix-Isolated Methylni-

39-8526, Japan; orcid.org/0000-0002-2013-4394

trene. J. Chem. Phys. 1987, 86, 25−32. (b) Carrick, P. G.; Brazier, C.

R.; Bernath, P. F.; Engelking, P. C. The Structure of the

Methylnitrene Radical. J. Am. Chem. Soc. 1987, 109 , 5100 −5102.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.0c02794

Author Contributions

∥_U_._B_._a_n_d_S_._K_._S_._c_o_n_t_r_i_b_u_t_e_d_e_q_u_a_l_l_y_.

Perfluoromethylnitrene in Cryogenic Matrixes. J. Phys. Chem. A 2001,

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Sankaranarayanan, J.; Muthukrishnan, S.; Chang, M.; Robinson, R.

M.; Lahti, P. M.; Ault, B. S.; Gudmundsdottir, A. D. Selective

Formation of Triplet Alkyl Nitrenes from Photolysis of β -Azido-

Propiophenone and Their Reactivity. J. Am. Chem. Soc. 2007, 129,

Notes

The authors declare no competing ﬁnancial interest.

11) (a) Kvaskoff, D.; Bednarek, P.; George, L.; Waich, K.; Wentrup,

6263−16272.

ACKNOWLEDGMENTS

■

We thank the NSF (CHE-01800140, CHE-1726092, and

CHE-1565793) and OSC (PES0597) for their support. U.B.

acknowledges a Doctoral Enhancement Fellowship and Ann P.

Villalobos/Lange and Laws Fellowships. We thank Dr. S. F.

Macha at UC for his technical help with HRMS. M. A.

gratefully acknowledges ﬁnancial support by JSPS KAKENHI

C. Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening

in 2-Quinazolylnitrenes. J. Org. Chem. 2006 , 71 , 4049 −4058.

(b) Wentrup, C. Flash Vacuum Pyrolysis of Azides, Triazoles, and

Tetrazoles. Chem. Rev. 2017 , 117 , 4562 −4623. (c) Wentrup, C.;

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