Diboration of alkynes

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

To realize milder reaction conditions, modification of the platinum catalyst system has been examined. A combined use of bis(catecholato)diboron with phosphine-free divalent platinum complex, PtCl2(cod), allows the diboration of alkynes to proceed at RT.42 The room-temperature diboration has also been achieved with a Pt(nbd)3-monophosphine (Pt/L= 1/1) catalyst.43... 

The transition-metal-catalyzed diboration of alkynes can be directed to give highly selective di-addition yielding 1,2-diborylated olefins (equation 24). Depending on the catalyst. 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

Proposed mechanisms of transition metal catalysed hydroboraton of alkenes and diboration of alkynes... 

Braunschweig H, Kupfer T, Lutz M, Radacki K, Seeler F, Sigritz R (2006) Metal mediated diboration of alkynes with [2]borametallocenophanes under stoichiometric, homogeneous, and heterogeneous conditions. Angew Chem Int Ed 45 8048... 

The mechanism of the processes where the alkene or alkyne is functionalized by two ER groups necessarily involves the insertion of the substrate into one M-ER bond. The actual mechanism depends on the specific reaction, type of substrate, and catalyst. Eor diboration or disilation of alkenes, theoretical studies have found this step rate determining when the catalyst is a Pt complex [176,179]. The insertion of acetylene into Pt-BR2 bonds is faster than the insertion of ethylene, and this step is not rate determiiung for diboration of alkynes [178,179]. As was pointed out before (see Section 6.4.1 (a)), insertion of an alkyne into the Pd-SnRs bond is preferred over insertion into the Pd-SiRa in the silylstannation of alkynes [177]. 

Silylboration and borylstannation of acetylenes have also been achieved easily in the presence of a catalyst of palladium complexes. In contrast, palladium complexes such as Pd(PPh3)4 or Pd(OAc)2-isocyanide were ineffective for diboration of alkynes. However, the development of another new ligand will solve this difficulty in the future. 

SchemeS.lS Gold-catalyzed diboration of alkynes. (Adapted with permission from ref. 87 2013 American Chemical Society.)...

Diboration of Unsaturated Hydrocarbons. B2pin2 adds to unsaturated hydrocarbons in the presence of a catalytic amount of a Pt complex to afford diborated products in high yield with excellent regio- and stereoselectivity. The reaction is recognized to proceed through a catalytic cycle, which involves (a) oxidative addition of the B-B bond to Pt , (b) insertion of the unsaturated hydrocarbon into the B-Pt bond, and (c) reductive elimination of the product to regenerate Pt (eq 1). The diboration of alkynes, allenes, conjugated dienes, methylenecyclopropanes, anda, -unsaturated carbonyl compounds is efficiently catal) ed by phosphine-based Pt complexes (eqs 2-6), whereas phosphine-free Pt complexes are favorable for the reaction of simple alkenes because of the low coordination ability of the alkene over phos-... 

Scheme 6 Proposed mechanism for the Pt(0)-diboration of alkynes with B2pin2.

Ishiyama T, Matsuda N, Miyaura N, Suzuki A. Platinum(0)-catalyzed diboration of alkynes.JAm Chem Soc. 1993 115 11018-11019. 

Chen Q, Zhao J, Ishikawa N, Yamamoto Y, Jin T. Remarkable catalytic property of nanoporous gold on activation of diborons for direct diboration of alkynes. Org Lett. 

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