Iridium(I)‐catalyzed C−H Borylation of α,β‐Unsaturated Esters with Bis(pinacolato)diboron

Abstract

Abstract:A new process has been developed for the iridium(I)-catalyzed vinylic C-H borylation of α,β-unsaturated esters with bis(pinacolato)diboron. These reactions proceeded in octane at temperatures in the range of 80-120 °C to afford the corresponding alkenylboronic compounds in high yields with excellent regio-and stereoselectivities. The presence of an aryl ester led to significant improvements in the yields of the acyclic alkenylboronates. Crossover experiments involving deuterated substrates as well as a mixture of stereoisomers confirmed that this reaction proceeds via a 1,4-addition/ β-hydride elimination mechanism. Notably, this reaction was also used to develop a one-pot borylation/SuzukiMiyaura cross-coupling procedure.

Mechanistic Investigation of Iridium-Catalyzed C–H Borylation of Methyl Benzoate: Ligand Effects in Regioselectivity and Activity

“…Iridium-catalyzed borylation with bis­(pinacolato)­diboron (B 2 pin 2 ) or pinacolborane (HBpin) is probably one the best methods to undertake the C–H bond activation and functionalization on organic substrates . One of the most active catalysts to carry out these reactions is [Ir­(OMe)­COD] 2 in combination with bipyridine ligands such as 2,2′-bipyridine or 4,4′-di- t Bu-2,2′-bipyridine (dtbpy) or phenanthroline derivatives . In contrast, the regioselectivity derived from these catalysts is mainly controlled by steric effects, and, thus, the selective borylation in the ortho -position of a substituted arene is practically impossible to achieve .…”

Inorganometallics (Transition Metal–Metalloid Complexes) and Catalysis

“…Such reaction conditions give borylated esters with isolated yields of 29−56%. 241 Although the literature does not provide the mechanism of olefin borylation supported by unquestionable evidence, the results strongly indicate that the mechanism involves olefin insertion into the M−B bond, followed by product formation and activation of the C−H bond via β−H elimination. The mechanism for the iridium-based catalytic system has been proposed by Szabo on the basis of earlier literature data 230 and results of labeling and kinetic studies (Scheme 94).…”

“…242 However, it should be noted that an alternative mechanism has been proposed in the literature, which assumes oxidative addition of the olefinic C−H bond to the metal center. 241 Efficient palladium-catalyzed olefin borylation has first been described by Szabo and co-workers. 243 Cycloolefins, allyltrimethylsilane, and vinylboronic acid pinacol ester undergo efficient borylation with B 2 pin 2 in the presence of a palladium pincer complex and [bis(trifluoroacetoxy)iodo]benzene as oxidant (Scheme 95).…”

“…Optimized conditions for the iridium-catalyzed borylation of α,β-unsaturated esters with B 2 pin 2 involve [{IrCl­(cod)} 2 ] or [{Ir­(OMe)­(cod)} 2 ] and AsPh 3 as the ligand of choice. Such reaction conditions give borylated esters with isolated yields of 29–56% …”

“…Similar mechanisms have been proposed for rhodium complexes and palladium–pincer complexes . However, it should be noted that an alternative mechanism has been proposed in the literature, which assumes oxidative addition of the olefinic C–H bond to the metal center …”