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

Abstract

A facile ruthenium‐catalyzed regio‐ and stereoselective hydroboration of symmetrical, aromatic 1,3‐diynes with pinacolborane towards 2‐boryl‐1,4‐diaryl‐buta‐1‐en‐3‐ynes and their further transformation into potassium trifluoroborate salts is presented. The reaction proceeded efficiently for 1,4‐diphenylbutadiynes with various substituents on the phenyl rings and heterocyclic 1,4‐di(thiophen‐3‐yl)buta‐1,3‐diyne. The resulting products were isolated and fully characterized (1H, 13C, 11B and 1D NOESY NMR, IR, GC‐MS, HRMS or elemental analysis). Moreover, the crystal structure of an enynyl boronate was determined, proving the addition of boryl moiety at the internal carbon in the diyne, according to the anti‐Markovnikov rule. The results presented here are the first examples of selective catalytic monohydroboration of conjugated diynes. The products obtained due to the presence of the boryl group and the unsaturated bonds are potential synthons in the synthesis of natural compounds or active pharmaceutical ingredients (API). The model Suzuki coupling of 2‐boryl‐1,4‐diphenyl‐buta‐1‐en‐3‐ynes with iodobenzene was carried out to illustrate the utility of the resulting compounds.

Hydroelementation of diynes

“…Hydroboration of conjugated 1,3-diynes is the simplest procedure for the synthesis of boryl-substituted 1,3-enynes or bisboryl-substituted 1,3-dienes, but the transition metalcatalysed selective addition of the B-H bond to the CRC bond is limited only to three recently published examples. 72,124,125 The first selective hydroboration of 1,3-diynes was reported by Zweifel and Ponso. Noncatatalytic reduction of alkylsubstituted buta-1,3-diynes 13a-c was carried out using disiamylborane (bis(3-methyl-2-butyl)borane) 14 or less bulky dicyclohexylborane 15.…”

“…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.…”

“…The reaction occurred with the retention of the configuration and (Z)-1,2,4triphenylbut-1-en-3-yne 36a-b was formed with high yields 71% and 78%, respectively (Scheme 9). 72 Ge et al reported an interesting method for the synthesis of boryl-substituted enynes from unsymmetrical and symmetrical 1,3-diynes 1c, 1e, 37a-t in the presence of a cobalt catalyst generated from inexpensive and stable Co(acac) 2 38 and bidentate phosphine ligands 39a-c. The authors showed that the regioselectivity of the process was dependent on the bidentate phosphine ligand used.…”

“…Using this catalytic system, different regioisomers were formed with the boryl group attached to the internal carbon bond in comparison to the previously described works on noncatalytic or Ru-H catalysed hydroboration. 72,126,130 The obtained products were subsequently derivatised by Suzuki-Miyaura coupling with iodobenzene 34 using PdCl 2 64/dppf 39b as a catalyst and KOH as a base, as well as deborylated to enynes with acetic acids. Moreover, it was possible to carry out selective desilylation of silylboryl-substituted enynes with K 2 CO 3 /MeOH while Bpin remained unreactive under applied process conditions.…”

“…[63][64][65][66][67][68][69][70][71] Working on the hydroboration and hydrosilylation of various unsaturated compounds and especially on the reactivity of conjugated 1,3-diynes in these processes, we have found that literature information is often scattered, with no detailed procedures or much discussion on the process optimisation or methodology. [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87] Therefore, we have decided to build a comprehensive and critical compendium focused on this subject, which will systemise the existing knowledge on the hydroelementation of diynes in relation to the formation of different products. We will also show the possible applications of the obtained products in the synthesis of fine chemicals and materials.…”