Regioselective Markovnikov hydrodifluoroalkylation of alkenes using difluoroenoxysilanes

Alkene hydrodifluoroalkylation is a fruitful strategy for synthesizing difluoromethylated compounds that are interesting for developing new medicinal agents, agrochemicals, and advanced materials. Whereas the anti-Markovnikov hydrodifluoroalkylation to linear-type products is developed, employing radical-based processes, the Markovnikov synthesis of branched adducts remains unexplored. Herein, we describe acid-catalyzed processes involving carbocation intermediates as a promising strategy to secure the Markovnikov regioselectivity. Accordingly, the Markovnikov hydrodifluoroalkylation of mono-, di-, tri-, and tetrasubstituted alkenes using difluoroenoxysilanes, catalyzed by Mg(ClO4)2·6H2O, is achieved. This allows the diversity-oriented synthesis of α,α-difluoroketones with a quaternary or tertiary carbon at the β-position that are otherwise difficult to access. The method is applied to the modification of natural products and drug derivatives. The resulting α,α-difluorinated ketones could be converted to the corresponding α,α-difluorinated esters or alcohols, or organofluorine compounds featuring a CF2H or CF2CF2Ph moiety. Mechanistic studies support that Mg(ClO4)2·6H2O functions as a hidden Brønsted acid catalyst.


Selected condition optimization
The performance of several typical Brønsted acids in the coupling of 2-phenyl-1-butene 1a with difluoroenoxysilane 2a was examined by running the reaction in ClCH 2 CH 2 Cl (Supplementary Table   1). All the following condition optimization reactions were run in air, except that mediated by HOTf.
Two superacids were first examined. HOTf afforded the desired branch product 3a in 36% yield (entry 1), and HClO 4 , used as 70% aqueous solution, produced 3a in a higher 55% yield, albeit with longer reaction time, possibly due to the presence of some water (entry 2). Ordinary strong acids exhibited much lower catalytic properties. Only H 2 SO 4 gave 3a in only 15% yield (entry 3), p-toluenesulfonic acid and CF 3 CO 2 H were unable to mediate this reaction (entries 4 and 5). Then we tried a range of metal triflates, and found Fe(OTf) 3 , Sc(OTf) 3 , Ph 3 PAuOTf and Ga(OTf) 3 could mediate this reaction (entries [6][7][8][9]. In particular, Ga(OTf) 3 afforded 3a in 75% yield (entry 9). However, the highly hydroscopic nature of metal triflates decreased the reproducibility of the reaction. Because the use of HClO 4 (aq. 70%) mediated the reaction well, we speculated the presence of some water might facilitate the reaction via scavenging the trimethyl group. In light of this, we considered using easy-to-handle metal perchlorate hydrates as the catalyst. We are pleased to find that identified to be best choice, delivering 3a in 80% and 82% yield, respectively (entries 2 and 5). Then we examined the solvent effects by using Mg(ClO 4 ) 2 ‧6H 2 O as the catalyst. Only halogenated solvents could mediate the reaction, ClCH 2 CH 2 Cl afforded 3a in better yield than CH 2 Cl 2 (entry 5 vs 6). Other screened solvents, toluene, EtOAc, CH 3 CN and THF, could not produce product 3a at all (entries [7][8][9][10]. Further increasing the loading of Mg(ClO 4 ) 2 ‧6H 2 O to 10 mol%, the reaction time could be decreased to 7 h, and the yield of 3a could be increased to 92% (entry 11).

The correlation of serial number of alkenes and their structure
Because the space limitation of the manuscript, we showed the structure of each alkene substrate and its serial number in this section. Table 3. The structure and serial number of 1,1-disubstituted olefins 1.

The preparation of unknown alkenes
To an oven-dried flask were added MePPh 3 Br (1.5 equiv.) and anhydrous THF (0.5 M), followed by the addition of t BuOK (1.5 equiv.). After being stirred at room temperature for 1 h, the indicated ketones (1.0 equiv.) was added to the resulting mixture, and continue to stir at room temperature overnight. The reaction mixture was then filtered through a short pad of silica gel, and the residue was washed with hexanes. The filtrates were evaporated under reduced pressure to give the crude residue, which was purified by flash column chromatography with PE to afford the desired alkenes. To an oven-dried flask were added Ph 3 PCH 2 RBr (1.2 equiv.), anhydrous THF (0.5 M). The resulting mixture was cooled to 0 °C (under ice-water bath condition), and n-BuLi (1.2 equiv., 2.5 M in hexanes) was added slowly over 15 min at 0 °C. After being stirred at 0 °C for 0.5 h, to the resulting orange mixture was added a solution of indicated ketones (1.0 equiv.) in anhydrous THF (1.0 M) by dropwise over 15 min at the same temperature. After stirring at room temperature overnight, the reaction mixture was quenched with saturated aqueous NH 4 Cl, and then extracted with pentane or diethyl ether for three times. The combined organic phases were washed with brine, dried over Na 2 SO 4 , and concentrated under reduced pressure. The crude residue was purified by flash column chromatography to afford the desired alkenes. 5-Phenylhex-5-enenitrile (1ag) was prepared from 5-oxo-5-phenylpentanenitrile 23 in 44% yield as light yellow oil. 1

Substrate scope for the Markovnikov hydrodifluoroalkylation of simple alkenes
All the reactions were carried out in air. Unless otherwise noted, 10 mol% of Mg(ClO 4 ) 2 6H 2 O and 1.5 equiv. of 2 were used. To a 5.0 mL vial were added Mg(ClO 4 ) 2 6H 2 O (0.03 mmol, 10 mol%) and anhydrous ClCH 2 CH 2 Cl (3.0 mL), followed by the sequential addition of alkenes (0.3 mmol, 1.0 equiv.) and fluorinated silyl enol ethers 2 (0.45 mmol, 1.5 equiv.). The resulting mixture was stirred at room temperature until full consumption of alkenes by TLC analysis. The reaction mixture was then concentrated under reduced pressure. The crude residue was purified by flash column chromatography using the indicated eluent to afford the desired products.

Transformations of products 3a and 3b
To a solution of 3a (57.7 mg, 0.2 mmol), t BuOH (2.0 mL) in a 10 mL sealed tube was added t BuOK (449.0 mg, 4.0 mmol), and the resulting mixture was stirred at 80 o C. After full consumption of 3a by TLC analysis (about 2 h), EtOAc (10 mL) and saturated aqueous NH 4 Cl (5.0 mL) were added subsequently. The mixture was transferred to a separating funnel. The organic layer was separated, and the aqueous phase was extracted with EtOAc (2  10 mL). The combined organic layers were dried over Na 2 SO 4 and concentrated under reduced pressure. The crude residue was purified by flash column chromatography with PE to provide 17 as colorless oil (28.7 mg, 78% yield).  2976, 1500, 1464, 1391, 1368, 1204, 1117, 1057

Biological activities study
Given that α,α-difluoroketones are interesting targets for the development of pharmaceutical agents, we initially evaluated the in vitro cytotoxic activity of the newly synthesized difluorinated ketones in human colorectal cancer cells (HCT116) and human osteosarcoma cells (Saos2) using CCK-8 assay. We first treated HCT116 cells with ten difluorinated products for 72 h and then measured the cell viability using CCK-8 assay. All these compounds showed good growth inhibitory activity at 30 µM, as shown in Supplementary Table 9. The IC 50 values of cytotoxic effects was further measured in HCT116 cells, which ranged from 4.22 to 58.25 µM. Among them, products 3ag, 3ai and 12 exhibited the most potent inhibitory effects with IC 50 of 6.7 µM, 5.7 µM and 4.2 µM, respectively. Meanwhile, the preliminary investigation of the cytotoxic activity of five of selected compounds in Saos2 cells using CCK-8 assay also provided good growth inhibitory activity. These preliminary results demonstrate that our difluorinated ketones are potentially valuable in anticancer drug development. General procedure: HCT116 cells or Saos2 cells were seeded in 96-well plates at 3000/well overnight and treated with every synthesized compound at 30 µM. The control cells were treated with the vehicle DMSO. After 72 h incubation, 10 µL of CCK8 solution (5 g/L; Yeasen) in the media was added to each well and incubated for additional 4 h. Finally, the optical density (OD) was measured at 450 nm using a microplate reader (spectraMax M5/M5e, Sunnyvale, CA, USA) and a reference wavelength at 620 nm. As for the compounds with significant growth inhibitory effects, the IC 50 value was determined by testing the inhibitory effects of the compound with 10 gradient-dilution concentrations with at least three replicates per concentration.

X-ray Crystallography Analysis Preparation and structure confirmation of product 3s'
To a 10 mL sealed tube were added TsNHNH 2 (93.1 mg, 0.5 mmol), 3s (143.2 mg, 0.5 mmol), followed by the addition of MeOH

Structure confirmation of product 8k
Data intensity of 8k was collected using a XtaLAB PRO MM003 (Cu radiation) at 100.00(10) K in a nitrogen stream. Data collection and reduction were done by using the XtaLAB software package.