Reactions of the formyl radical with alkyl radicals

Solvent-Controllable C-F Bond Activation for Masked Formylation of α-Trifluoromethyl Alkenes via Organo-Photoredox Catalysis

A solvent-controllable organo-photoredox-catalyzed C-F bond activation for masked formylation of α-trifluoromethyl alkenes with low-priced 1,3-dioxolane as masked formyl radical equivalent has been described. Consequently, a diversity of masked formylated gem-difluoroalkenes and monofluoroalkenes are constructed in moderate to high yields. This approach merits readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this approach has been highlighted by the one-pot masked formylation/hydrolysis sequence to form γ,γ-difluoroallylic aldehydes and late-stage modification of pharmaceutical and natural product derivatives.

Formyl Radical Generation from α-Chloro N-Methoxyphthalimides Enables Selective Aldehyde Synthesis

The aldehydes installation by radical formylation constitutes an attractive synthetic strategy. However, the generation of formyl radicals for organic synthesis applications remains unknown. Herein we report the first formyl radical generation from α-chloro N-methoxyphthalimides, which selectively synthesize aldehydes by alkene hydroformylation under mild photoredox conditions. The aldehydes can be installed on acrylates, acrylamides, vinyl sulfones, vinyl ketones, and complex steroids by radical hydroformylation in excellent chemoselectivity and regioselectivity. The concerted hydrochloride elimination for the formyl radical generation from α-chloro methoxy radicals is established by experimental and computational approaches.

Experimental and Theoretical Investigation of Reactions of Formyl (HCO) Radicals in the Gas Phase: (I) Kinetics of HCO Radicals with Ethyl Formate and Ethyl Acetate in Tropospherically Relevant Conditions

Formyl (HCO) radicals were generated in situ in the gas phase via the photolysis of glyoxal in N₂ at 248 nm using the pulsed laser photolysis-cavity ring-down spectrometry technique, and the absorption cross-section of the radical was measured to be σ_HCO = (5.3 ± 0.9) × 10^-19 cm² molecule^-1 at 298 K and 615.75 nm, which was the probing wavelength. The kinetics of the reactions of HCO radicals with ethyl formate (EF) and ethyl acetate (EA) were investigated experimentally in the temperature range of 260-360 (±2) K at a pressure of 60 Torr/N₂. The absolute rate coefficient for the reaction between HCO and EF was measured to be kHCO+EFExpt(298 K) = (1.39 ± 0.30) × 10^-14 cm³ molecule^-1 s^-1 at ambient temperature, whereas that for the reaction of HCO with EA was measured tobe kHCO+EATheory(298 K) = (2.05 ± 0.43) × 10^-14 cm³ molecule^-1 s^-1. The reaction of HCO with EA was faster than that with EF, which might be due to the greater stability of the formed radical intermediate due to hyperconjugative and inductive effects. The dependency of the measured kinetics on experimental pressures and laser fluences was examined within a certain range. To complement the experiments, kinetics of the title reactions in the temperature range of 200-400 K were deciphered theoretically via the canonical variational transition-state theory with small-curvature tunneling and interpolated single-point energy (CVT/SCT/ISPE) method using a dual-level approach at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory/basis set. A good degree of agreement was detected between the rate coefficients measured experimentally and those calculated theoretically both at room temperature and throughout the range of temperatures studied. The kinetic branching ratios and thermochemistry of the reactions were investigated to understand the thermodynamic feasibility and kinetic lability of each pathway throughout the studied temperatures. Atmospheric implications of these reactions of HCO radicals are also discussed.

RADICAL-MOLECULE REACTIONS HCO/HOC + C2H4: A MECHANISTIC STUDY

A detailed computational study is performed on the radical-molecule reactions between HCO/HOC and ethylene ( C 2 H 4) at the Gaussian-3//B3LYP/6-31G(d) level. For the HCO + C 2 H 4 reaction, the most favorable pathway is the direct C -addition forming the intermediate H 2 CCH 2 CHO , followed by a 1,2- H -shift leading to H 3 CCHCHO . Subsequently, there are two highly competitive dissociation pathways for H 3 CCHCHO : one is the formation of the direct H -extrusion product H 2 CCHCHO + H , and the other is the formation of C 2 H 5 + CO via the intermediate H 3 CCH 2 CO . The overall reaction barrier is 14.1 and 14.6 kcal/mol respectively, at the G3B3 level. The quasi-direct H -donation process to produce C 2 H 5 + CO with the barrier 16.5 kcal/mol is less competitive. Thus, only at higher temperatures, the HCO + C 2 H 4 reaction could play a role. In contrast, the HOC + C 2 H 4 reaction just need to overcome a small barrier 2.0 kcal/mol to generate C 2 H 5 + CO via the quasi-direct H -donation mechanism. This is suggestive of the potential importance of the HOC + C 2 H 4 reaction in combustion processes. However, the direct C -addition channel is much less competitive. The present kinetic data and orbital analysis show that the HCO radical has much higher reactivity than HOC , although the latter is more energetic. Till now, no kinetic study on the HOC radical has been reported, the present study can provide useful information on understanding the reactivity and depletion mechanism of the energetic HOC radical.

Ab initio methods for reactive potential surfaces

Case studies of ten reactions using a variety of standard electronic structure methods are presented. These case studies are used to illustrate the usefulness and shortcomings of these standard methods for various classes of reactions. Limited comparisons with experiment are made. The reactions studied include four radical-radical combinations, H + CH(3)--> CH(4), CH(3) + CH(3)--> C(2)H(6), H + HCO --> H(2)CO and CH(3) + HCO --> CH(3)CHO, three abstraction reactions, H + HO(2)--> H(2) + O(2), H + HCO --> H(2) + CO and CH(3) + HCO --> CH(4) + CO, a radical-molecule addition, H + HCCH --> C(2)H(3), and two molecular decompositions, H(2)CO --> H(2) + CO and CH(3)CHO --> CH(4) + CO. The electronic structure methods used are DFT, MP2, CCSD(T), QCISD(T), CASSCF, CASPT2, and CAS+1+2+QC.

Product Study of the Reaction of CH3 with OH Radicals at Low Pressures and Temperatures of 300 and 612 K†

The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS) connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser pulse (lambda = 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen. Helium was used as the bath gas; the total density was held constant at 1.2 x 10(17) cm(-3). At 300 K the observations were consistent with singlet methylene ((1)CH(2)) and water as the main product channel with a small contribution of methanol. In contrast, at about 610 K three channels-formaldehyde isomers and methanol in addition to (1)CH(2) + H(2)O-are formed with similar yields. When acetone-d(6) was used, the production of both CHDO and CD(2)O was observed, indicating that two different formaldehyde-producing channels are operating simultaneously. These experimental results are compared with RRKM and master equation calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio study.

Mechanism of HCS + O2 reaction: Hydrogen- or oxygen-transfer?

In spite of the potential importance of the HCS radical in both combustion and interstellar processes, its chemical reactivity has not been tackled previously. In the present paper, the oxidation reaction of the HCS radical is theoretically investigated for the first time at the CCSD(T)/6-311++G(3df,2p)//BH&HLYP/6-311++G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels. It is shown that the most feasible pathway is the O2 addition to the HCS radical forming the intermediate SC(H)OO which can undergo a subsequent O-extrusion leading to SC(H)O + 3O. This features an indirect O-transfer mechanism with the overall barrier of 4.4 and 3.5 kcal mol(-1), respectively, at the two levels. However, formation of the H-transfer product CS + HO2 is kinetically much less feasible, i.e., the direct mechanism has barriers of 14.3 and 8.7 kcal mol(-1), whereas the indirect mechanism has barriers of 12.6 and 10.7 kcal mol(-1), respectively. This result is in sharp contrast to the analogous HCO + O2 reaction, where the direct (with a barrier of 2.98 kcal mol(-1)) and indirect (2.26 kcal mol(-1)) H-transfer processes are highly competitive over the indirect O-transfer process (the least endothermicity is 19.9 kcal mol(-1)). The possible explanations and implications of the present results are provided.