[Pt(PPh3)4]-Catalyzed Selective Diboration of Symmetrical and Unsymmetrical 1,3-Diynes

A straightforward, efficient, and selective method for the preparation of novel boryl-functionalized enynes or dienes via [Pt(PPh3)4]-catalyzed diboration of a broad spectrum of symmetrical and unsymmetrical 1,3-diynes was developed. The catalytic cycle of diboration was proposed on the basis of low-temperature 31P NMR studies. An alternative isolation method via product condensation on a cold finger was developed, which, in contrast to previous literature reports, eliminates the need for the additional transformation of rapidly decomposing enynyl pinacol boronates to more stable silica-based column chromatography derivatives during the separation step. To prove the efficiency of this simple catalytic protocol, bisboryl-functionalized enynes were synthesized in a gram scale and tested as useful building blocks in advanced organic transformations, such as hydrosilylation and Suzuki and sila-Sonogashira couplings. The presence of silyl, boryl, as well as other functions like halogen or alkoxy in their structures builds a new class of multifunctionalized enynes that might be modified in various chemical reactions.


Bisboryl-functionalized enynes (3a-z)
The reaction mixture was evaporated to remove all volatiles. Subsequently, the crude product was dissolved in n-pentane and filtered through the syringe filter (0.2 µm). After evaporation of n-pentane, the product was heated (approx. 70-130 °C) and condensed at cold-finger trap under vacuum (<10 -3 mbar) (see Scheme 1). The products were obtained as solids (symmetrical substituted 1,3-diynes, excluding 3b) or oils (unsymmetrical substituted 1,3-diynes). Products 3n and 3q could not be condensed on a cold finger trap and were analyzed after the filtration step.
Purification of 3h-i: The reaction mixture was evaporated to remove all volatiles.
Subsequently, the crude product was dissolved in n-pentane and filtered through the syringe filter (0.2 µm) and placed in the freezer (-18 °C) in 24 hours. The 3h-i precipitate as white solids.
Purification of 3d-e and 4d-e: The reaction mixture was evaporated to remove all volatiles.
Subsequently, the crude product was dissolved in n-pentane and filtered through the syringe filter (0.2 µm) and excess of diyne or B2pin2 was removed after the filtration step by heating at (C) -condensed product (3a) on a cold finger trap.

Synthesis of alkynyl bromides
The alkynyl bromides were prepared according to the literature with some modification 2 : To a solution of alkyne (5 mmol) in acetone (50 mL), N-bromosuccinimide (6 mmol) and silver nitrate (0.5 mmol) were successively added. The reaction mixture was stirred without light access at room temperature over 18 h before adding water (100 mL). The resulting mixture was extracted with hexanes (3 x 100 mL) and the combined organic layers were washed with brine (100 mL), dried over MgSO4, filtered through a pad of silica and concentrated to give a colourless or slightly yellow liquids.
Caution: All synthesized alkynyl bromides are strong lachrymators. The isolation should be performed under the hood.

Hydrosilylation (for product 8)
To the one-neck round bottom flask, 3a (0.1 mmol), toluene (0.1 mL), triethylsilane (0.12 mmol) were added and heated to 100 °C (oil bath). Subsequently, the Pt2(dvs)3 (10 -4 mmol) was added and the reaction mixture was stirred for 24 h. Afterwards, crude reaction mixture was analyzed by GC-MS and 1 H NMR analyzes and purified according to the procedure in 2.1.2. subsection.

Mechanistic studies
To the Young's NMR tube the 1 (15,8 mg, 0.0625 mmol) and Pt(PPh3)4 (7,8mg, 0.00625 mmol) were added and kept under the vacuum for 1 h. After that, the 0.7 mL of dry toluene -d8 was added and heated over 1.5 h at 100 °C (oil bath) under Ar atmosphere. The mixture was cooled down to -50 °C (NMR chiller) and low temperature 31 P NMR was performed.  4. NMR spectra Figure S1. 1 H NMR spectrum of (bromoethynyl)benzene. Figure S2. 13 C NMR spectrum of (bromoethynyl)benzene.