Vibration-mediated long-wavelength photolysis of electronegative bonds beyond S0–S1 and S0–T1 transitions

Photolysis is an attractive method in organic synthesis to produce free radicals through direct bond cleavage. However, in this method, specific irradiation wavelengths of light have been considered indispensable for excitation through S0–Sn or S0–Tn transitions. Here we report the photoinduced homolysis of electronegative interelement bonds using light at wavelengths much longer than theoretically and spectroscopically predicted for the S0–Sn or S0–Tn transitions. This long-wavelength photolysis proceeds in N–Cl, N–F, and O–Cl bonds at room temperature under blue, green, and red LED irradiation, initiating diverse radical reactions. Through experimental, spectroscopic, and computational studies, we propose that this “hidden” absorption is accessible via electronic excitations from naturally occurring vibrationally excited ground states to unbonded excited states and is due to the electron-pair repulsion between electronegative atoms.


Experimental Section 2.1. C(sp 2 )-H chlorination of aldehydes (Fig. 2a)
Representative procedure for the C(sp 2 )-H chlorination of aldehydes (3a, Fig. 2) Method A: Benzaldehyde 2a (10.6 mg, 0.10 mmol), freshly passed through a basic alumina pad, N-chlorosuccinimide 1a (14.7 mg, 0.11 mmol), and CCl4 (0.20 mL) were placed in a 3 mL screw cap vial.The vial was capped and wrapped with a Teflon seal.The mixture was stirred at room temperature under Ar with blue LEDs irradiation for 18 h.To the resulting mixture were added triethylamine (0.30 mL) and a THF solution of dimethylamine (2 mol/L, 0.50 mL) and the mixture was stirred at room temperature under Ar for an additional period of 2 h.The final mixture was concentrated under reduced pressure, and to the residue were added 1,3,5-trimethylbenzene (as an internal standard) and CDCl3.NMR crude yield was analyzed by 1 H NMR. Finally, the crude product was purified by PTLC to afford the desired product 3a.

Aminochlorination of alkenes (Fig. 2b)
Representative procedure for the aminochlorination of olefins (5aa, 5dc Fig. 2) Method A: 1,2-dihydrofuran 4a (35.1 mg, 0.50 mmol), N-chlorosuccinimide 1a (73.4 mg, 0.55 mmol), and CH2Cl2 (1.0 mL) were placed in a 3 mL screw cap vial.The vial was capped and wrapped with a Teflon seal.The mixture was stirred at room temperature under Ar with blue LEDs irradiation for 18 h.The final mixture was concentrated under reduced pressure and to the residue were added 1,3,5-trimethylbenzene (as an internal standard) and CDCl3.NMR crude yield was analyzed by 1 H NMR. Finally, the crude product was purified by PTLC to afford the desired product 5aa.
Method B: Cyclohexene 4d (8.2 mg, 0.10 mmol), N-chlorosaccharin 1c (23.9 mg, 0.11 mmol), and CH2Cl2 (0.2 mL) were placed in a 3 mL screw cap vial.Then, the remaining procedure was executed as in method A except by using green LEDs irradiation to afford the desired product 5dc.

C(sp 2 )-H fluorination of aldehydes (Fig. 2c)
Representative procedure for the fluorination of aldehydes (6a Fig. 2): Benzaldehyde 2a (21.2 mg, 0.20 mmol), freshly passed through a basic alumina pad, Nfluorobenzenesulfonimide 1b (63.1 mg, 0.20 mmol), and MeCN (0.4 mL) were placed in a 3 mL screw cap vial.The vial was capped and wrapped with a Teflon seal.The mixture was stirred at room temperature under Ar with blue LEDs irradiation for 18 h.The final mixture was concentrated under reduced pressure and to the residue were added 1,3,5-trimethylbenzene (as an internal standard) and CDCl3.Then NMR yield was analyzed by 1 H NMR. Finally, the crude product was purified by column chromatography to afford the desired product 6a.

Computational details
All calculations were carried out with the Gaussian 16 program package. 20The hybrid density functional method based on (U)M06 [21][22][23][24] with a standard 6-31+G* basis set was used for preliminary geometry optimizations and in the BDE calculations.The 6-311+G** basis set was used to calculate the single-point energies and transitions for NCS, NCP, NFSI, t-BuOCl, and N-methylsuccinimide because it was envisaged that this strategy would provide greater accuracy with regard to the energetic information.Geometry optimization and vibrational analysis were performed at the same level.All stationary points were optimized without any symmetry assumptions and characterized by normal coordinate analysis at the same level of theory (number of imaginary frequencies, NIMAG, 0 for minima).Excitation wavelengths via S0-S1 transitions and oscillator strengths were obtained at the density functional level using the time-dependent perturbation theory (TD-DFT) approach.

Theoretical prediction of S0-S1 and S0-T1 transition (Table 1)
Supplementary Table 2. (TD)-DFT calculated transitions from the ground state (S0) to singlet (S1) and triplet (T1) excited states and respective X-Y bond dissociation energies.(TD-)DFT calculations were performed at (U)M06/6-31+G* levels of theory unless otherwise noted.BDE = bond dissociation energy.For such small molecules with only N-Cl bonds, carbonyl groups, and phenyl groups, the S0-S1 transition is limited to wavelengths below 320 nm, as expected, and the spin-restricted S0-T1 transition was assigned to UV-A (320-400 nm) for 1e, 1f, 1h and 1i (N-chlorophthalimide, NCP), or visible light (> 400 nm) for 1c.In addition, compounds A, B, and C can absorb UV-A if the spinrestricted S0-T1 transitions occur.The transitions of compounds D, E, F, and G are limited to wavelengths below 300 nm.NFSI 1b with an N-F bond cannot absorb visible light or UV-A.

S20
In Fig. 4b, at approximately 1.5 Å of the N-F bond the T1 energy increases.To better understand this specific energy point within the analyzed surface we recalculated the surrounding points with a finer scan (0.025 Å intervals instead of the initial 0.050 Å).Supplementary Table 9. Sum of electronic and zero-point energies and imaginary frequencies for the finer rescan (0.025 Å) of a section of Fig. 4b   From the results, we can infer that this energy point probably represents an outlier on the analyzed energy surface.There are several factors that could influence this outcome, such as changes in the initial structure, level of theory, or basis set.In addition to the possibility of an outlier, there might be a very small activation barrier for the N-F bond cleavage in the T1 state.In any case, this potential energy surface clearly supports our conclusion that the present reactions proceed via a VMP pathway.Supplementary Table 10.Sum of electronic and zero-point energies and imaginary frequencies for

Vibrational analysis
Since the photoexcitation of NCS is known to lead to N-Cl bond homolysis, in this work we focused on analyzing the energy surfaces (ground and excited) on the electronegative interelement bond axis, as shown.In the case of NCS, when considering the N-Cl vibrational mode, for example, excitation energies followed by their respective Boltzmann factors (at room temperature and 60ºC) for each vibrational state were calculated as follows.
The N-Cl vibrational mode frequency was estimated to be 648.1086cm -1 (@M06/6-311+G**), which is equivalent to 1.853 kcal mol -1 .By assuming that the other states of this mode are harmonic, the corresponding energies will be equivalent to multiples of 1.853.Moreover, by approximating each of the obtained energy surfaces (S0, S1, and T1) to a polynomial trendline (order = 3), it is possible to calculate the S0-S1 or S0-T1 transitions for each state of the N-Cl vibrational mode as shown below.
Supplementary Table 16.Approximated excitation energy (values in blue represent transitions in the visible range.)and Boltzmann factors (at 27 ºC and 60ºC, values in yellow and red respectively) corresponding to each vibrational state of the N-Cl vibrational mode for NCS (at M06/6-311+G**).Furthermore, when considering the excitations achievable from each of the other vibrational modes, the transition energies and Boltzmann factors can be also calculated as shown below.17.Approximated excitation energy (values in blue represent transitions in the visible range.)and Boltzmann factors (at 27 ºC and 60 ºC, values in yellow and red respectively) corresponding to each vibrational mode of NCS (at M06/6-311+G**).

Fig. 4c .
The point numbers indicate the distance from the original O-Cl bond times 0.05Å.The names preceded by T1 indicate the corresponding triplet.

Table 3 .
Sum of electronic and zero-point energies and sum of electronic and thermal free energies for Table1 and Supplementary

Table 2 .
The dotted names indicated the corresponding N or O-centered radical, after the dissociation of Cl, F (fluorine) or Me radical.The names followed by T1 indicate the corresponding triplet.

Table 4 .
TD-DFT vertical one-electron excitations for

Table 5 .
Sum of electronic and zero-point energies and imaginary frequencies forFig.4a.The point numbers indicate the distance from the original N-Cl bond times 0.05 Å.The names preceded by T1 indicate the corresponding triplet.
Supplementary Table 7. Sum of electronic and zero-point energies and imaginary frequencies forFig.4b.The point numbers indicate the distance from the original N-F bond 0.05 Å.The names preceded by T1 indicate the corresponding triplet.

Table 12 .
Sum of electronic and zero-point energies and imaginary frequencies for Fig. 4d.The point numbers indicate the distance from the original N-C bond times 0.05Å.The names preceded by T1 indicate the corresponding triplet.

Table 14 .
Potential energy surface, transition values, sum of electronic and zeropoint energies and imaginary frequencies for the calculated transitions from unstable conformations in the N-Cl, axis of NCP (1i) at the (U)M06/6-311+G** level; complementary to Fig. 4. ΔE in kcal mol -1 .The point numbers indicate the distance from the original N-Cl bond times 0.05 Å.The names preceded by T1 indicate the corresponding triplet.

Table 15 .
TD-DFT vertical one-electron excitations for Supplementary