Copper-catalyzed dehydrogenative borylation of terminal alkynes with pinacolborane

LCuOTf complexes (L = CAACs or NHCs) selectively promote the dehydrogenative borylation of C(sp)–H bonds at room temperature.


A. General Considerations
All reactions were performed under an atmosphere of argon using standard Schlenk or dry box techniques; solvents were dried over Na metal, or CaH 2 . Reagents of analytical grade were obtained from commercial suppliers, dried over 4Å molecular sieves and degassed before use. 1 H, 13 C, and 19 F NMR spectra were obtained with a Bruker Advance 300 MHz, and a Varian INOVA 500 MHz spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) relative to TMS, and were referenced to the residual solvent peak. NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, q = quartet, quin = quintet, sex sextet, m = multiplet, br = broad signal. Complexes (Et 2 CAAC)CuOTf (L 1 CuOTf), i (MenthylCAAC)Cu OTf (L 2 CuOTf), i (IPr)CuOTf (L 3 CuOTf) ii were prepared following literature procedures. Table S1. In a culture tube were added equipped with a magnetic stir bar cat. (x mol%) and 4tolylacetylene (0.087 ml, 0.69 mmol) were combined in a culture tube. y μL (determined according to the named concentration M) of Solvent was added followed immediately by appropriate amount of base additive (z mol%). After 1 minute stirring, HBPin (0.100 ml, 0.69 mmol) was added in a single portion and the resulting solution was allowed to stir for 2 hours at room temperature. NMR yields based on 1 equivalent addition of 1,4-dioxane after the 2 hour stirring period.

A General Protocol.
In a schlenk tube under an argon atmosphere were added C 6 H 6 (17 mL), the alkyne (1.82 mmol), pinacol borane (0.264 mL, 1.82 mmol), and triethylamine (0.012 mL, 0.091 mmol). Then, Et 2 CAACCuOTf (0.024 g, 0.045 mmol%) was added to the mixture and the reaction was left stirring at room temperature until the reaction had completed. After completion, all the volatiles were evaporated under vacuum and 5 mL of pentane was added. The mixture was passed, under argon, through a short column packed with dry neutral alumina (3 cm diameter x 5 cm high) using 30 mL of pentane as eluent. Evaporation of the volatiles under vacuum afforded the corresponding product.

A.
Kinetic measurements.
All the kinetic measurements were performed in a J-Young NMR tube at room temperature on a Bruker 300MHz NMR machine using the multi_zgvd command. Conversions were measured by following the growth of the characteristic signals for B 1 or D 1 with respect to the depleting signals for 1a. In each kinetic run, 0.45 mmol (0.057 mL) of 1a and 0.45 mmol (0.066 mL) of PinBH were used in combination with 2.5 mol% of the catalyst (either L 1 CuOTf: 6.8 mg or L 1 CuCCPh: 5.4 mg), the corresponding amount of Et 3 N (either 5 mol%: 0.0032 mL or 2.5 mol%: 0.0016 mL) and Et 3 NH.OTf (1.4 mg, 2.5 mol%) when needed. At the end of these experiments, the resulting mixture was transferred to a schlenk tube, the volatiles were evaporated under vacuum and 5 mL of pentane was added. The mixture was passed, through a short column packed with dry neutral alumina (3 cm diameter x 5 cm high) under an argon atmosphere using pentane as eluent. Evaporation of the volatiles under vacuum afforded the corresponding product as white solids.

H NMR spectra after 5 minutes.
BPin B.

Deuterium Labeling for the Dehydrogenative Transformation
In a schlenk tube fitted with a magnetic stir bar were added under an argon atmosphere CD 2 Cl 2 (17 mL), D-labeled phenyl acetylene (0.200 ml, 1.82 mmol), pinacol borane (0.264 mL, 1.82 mmol), and triethylamine (0.012 mL, 0.091 mmol). Then, Et 2 CAACCuOTf (0.024 g, 0.045 mmol%) was added to the mixture and the reaction was left stirring at room temperature for three hours. After this time NMR-analysis of the content showed the formation of a mixture of B 1 and styrene-D.

Evidence for the formation of A starting from F in the presence of Et 3 NH.OTf.
A J-Young NMR tube was charged with CDCl 3 (1 mL), F (20 mg), and Et 3 NH.OTf (5.3 mg, 0.5 eq). The NMR taken immediately after mixing the component together show a clean formation of the bis-copper A and are in agreement with the literature. i See NMR spectra bellow.