An Effective Catalytic Hydroboration of Alkynes in Supercritical CO₂ under Repetitive Batch Mode

Abstract

A facile method for the synthesis of unsaturated organoboron compounds by hydroboration of terminal and internal alkynes in supercritical (sc) CO2 has been presented for the first time. The reactions performed in scCO2 in the presence of Ru catalysts (selectivity, productivity) are compared with those in a traditionally used solvent or without it. Moreover, the catalytic systems based on the application of scCO2 have been successfully tested for the processes under repetitive batch conditions. This strategy permitted us to obtain boryl‐substituted olefins with high yields and selectivities up to the 16th catalytic cycle if using Ru(CO)Cl(H)(PPh3)3 as a catalyst. The process applied the self‐dosing catalyst, which is gradually released to the reaction mixture under supercritical conditions in the individual batch. Such an approach has a positive impact on process sustainability and economy.

Recyclable Hydroboration of Alkynes Using RuH@IL and RuH@IL/scCO2 Catalytic Systems

“…In contrast to the model reaction, hydroboration of nonaromatic compounds results in deterioration of selectivity, which was also identified in our previous work. 32 The major products were (E)-substituted alkenyl boronates in all batches, irrespective of the ionic liquid used (approximately: 85% for silyl-substituted and 55% for alkylsubstituted alkenyl boronate). On the other hand, for internal alkynes, the major products were (Z)-substituted alkenyl boronates.…”

“…The same alkynes have been previously tested for the reactions in monophasic scCO 2 system. 32 For terminal alkynes, 10 catalytic runs were performed in IL8 and IL9 without a significant decrease in the reaction yield. In contrast to the model reaction, hydroboration of nonaromatic compounds results in deterioration of selectivity, which was also identified in our previous work.…”

“…One study investigated alkene hydroboration under homogeneous conditions by employing Rh catalyst with modified perfluorinated phosphines as ligands 45 and the second is our study on hydroboration of a series of terminal and internal alkynes catalyzed by self-dosing Ru(CO)Cl(H)(PPh 3 ) 3 , which is gradually released to the reactor under high pressure in supercritical conditions (170−180 bar, 100 °C, d = 0.40−0.43 g/cm 3 ). 32 The higher pressure the more soluble catalyst was in CO 2 , due to its stronger solvation power. During the extraction process of the products, low volatile catalyst remained in the reactor, giving the possibility for its reuse in the following repetitive batches with high efficiency.…”

Regio‐ and Stereoselective Synthesis of Enynyl Boronates via Ruthenium‐Catalyzed Hydroboration of 1,4‐Diaryl‐Substituted 1,3‐Diynes

“…It should be emphasized that the steric hindrance of the substituents did not influence the process selectivity. Additionally, [Ru 3 (CO) 12 ], which was previously applied as an effective catalyst for the hydroboration reaction of internal alkynes, also gave a mixture of mono‐ and bisadducts. Better results for the hydroboration of 1,4‐diaryl‐1,3‐butadiynes than alkyl analogues suggest that the process in the catalytic system that was applied depend significantly on the electronic properties of the diyne.…”

Hydroelementation of diynes

“…Compound 28 has previously been described as an active catalyst in hydroboration of terminal-or internal monoalkynes in conventional and novel, green reaction media (supercritical CO 2 (scCO 2 ), ionic liquids (ILs), polyethylene glycol, (PEG)). 72,75,76,128,129 The reaction proceeded effectively for various diynes possessing electron-withdrawing or electron-donating substituents on the aryl ring, as well as for heterocyclic 1,4-di(thiophen-3-yl)buta-1,3-diyne 27c. Alkyl-substituted diynes yielded boryl-substituted enynes by cis-addition of borane to the CRC bond, but the postreaction mixture also consisted of other monoborylated enynes, bisborylfunctionalised dienes, and some undefined products.…”