Photoinduced site-selective alkenylation of alkanes and aldehydes with aryl alkenes

The dehydrogenative alkenylation of C-H bonds with alkenes represents an atom- and step-economical approach for olefin synthesis and molecular editing. Site-selective alkenylation of alkanes and aldehydes with the C-H substrate as the limiting reagent holds significant synthetic value. We herein report a photocatalytic method for the direct alkenylation of alkanes and aldehydes with aryl alkenes in the absence of any external oxidant. A diverse range of commodity feedstocks and pharmaceutical compounds are smoothly alkenylated in useful yields with the C-H partner as the limiting reagent. The late-stage alkenylation of complex molecules occurs with high levels of site selectivity for sterically accessible and electron-rich C-H bonds. This strategy relies on the synergistic combination of direct hydrogen atom transfer photocatalysis with cobaloxime-mediated hydrogen-evolution cross-coupling, which promises to inspire additional perspectives for selective C-H functionalizations in a green manner.

The filtrate was concentrated, the residue was taken up in ether, washed with H2O, dried over MgSO4 and evaporated to dryness to obtain the diol as a white amorphous solid (452 mg, 94%).

Supplementary Discussion
Elucidation on whether styrene serves as a hydrogen acceptor Supplementary Fig. 3 Detection and quantitative analysis of generated hydrogen gas.
Dehydrogenative alkenylation of cyclooctane with styrene in a 10 mL microwave tube equipped with a penetrable septum. The upper atmosphere was analyzed by GC after 24 h of 370 nm LED irradiation. Plus. The gas phase in the headspace (headspace = 7.7 mL) of the reaction vessel was analyzed by gas chromatography. A gas-tight syringe was used to take a sample (0.10 mL) from the vessel.

Supplementary
Before each sampling, the gas in the syringe was replaced with the gas in the reaction vessel; 0.10 mL of gas in the headspace of the vessel was once taken and discharged. Then, 0.10 mL of gas in the vessel was newly taken and injected into a gas chromatograph.
In conclusion, hydrogen gas was produced in 64% yield by GC analysis of the crude product mixture, which is very close to the yield of alkenylated product . Based on GCMS and 1 H NMR results , it is not likely that styrene or styrene derivative serves as hydrogen acceptor since hydrogenated byproducts from olefins were not detected. Cobalt hydride catalyzed oligomerization of styrene was found to be the competing side reaction and consumes most of the styrene. It is also the reason that excess amount of styrene is required to achieve high yields in the dehydrogenative alkenylation reactions.   Even though we could not definitively rule out a radical-chain process, the data shown that any chain-propagation process must be short-lived. We measured the KIE from two parallel reactions under standard conditions for 12 h using cyclohexane and d12-cyclohexane as the substrate separately (Supplementary Fig. 14,top). We managed to obtain non-deuterated and deuterated products both in 53% yield. The KIE value was calculated to be 1.0. Next, we measured the KIE from intermolecular competition experiment by adding cyclohexane/d12-cyclohexane (1:1) into the same reaction mixture (Supplementary Fig. 14,bottom), the KIE value was calculated to be 2.0 based on GCMS and NMR analysis . These results indicated that C-H cleavage is not the rate-determining step, and different C-H cleavage rates between cyclohexane and d12-cyclohexane determined the product distribution in intermolecular competition experiment. A UV-Vis monitoring study of the reaction mixture revealed that two absorption bands at 440-500 nm and 550-700 nm appeared after 10 min light irradiation and exposure to air (Supplementary Fig.   17), which was in agreement with the formation of Co II and Co I intermediates, respectively. 13,14 It is well known that derivatives from [W10O32] 4photocatalyst, such as H + [W10O32] 5-, [W10O32] 5and

Supplementary
[W10O32] 6-, are deep blue with strong absorption bands in the 600-800 nm region. 15 However, these absorption bands would disappear quickly after exposure to air ( Supplementary Fig. 18). Quick disappearance of these bands in air was also observed by the Sorenson group. 16 Based on our finding and literature reports, the absorption bands at 440-500 nm and 550-700 nm were attributed to Co II and Co I intermediates respectively.

Calculation of bond dissociation energy
Theoretical calculations are performed using the Gaussian 16 Rev. A.03 software suite. 17 Bond dissociation enthalpies (BDE) was calculated by the formation enthalpy difference between the sum of two individual fragments formed after the homolysis of the involved bond and the corresponding species before the bond dissociation. 18 The geometries optimization and enthalpy calculations in this study were all performed at the (U)M062X/6-31G(d) level of theory, taking into account the solvent effect of acetonitrile using SMD solvent model. All frequency calculations gave no imaginary frequencies.
The reaction mixture was removed from light and quenched by stirring open to air for 5 minutes.
The solvent was removed on a rotary evaporator under reduced pressure and the residue was subjected to column chromatography isolation on silica gel to give the corresponding product as a colorless liquid (704 mg, 71% yield, eluent: hexane, Rf = 0.6).