Diboration reactions

Cui, Q., Musaev, D. G., Morokuma, K., 1998a, Why do Pt(PR3)2 Complexes Catalyze the Alkyne Diboration Reaction, but Their Palladium Analogues Do Not A Density Functional Study , Organometallics, 17, 742. 

Diboration provides another means of obtaining organoboranes. Studies [30-36] have been focused on the diboration reactions of alkynes and olefins with pinacol ester derivatives catalyzed by Pd(0) and Pt(0) metal complexes. Interestingly, it has been shown that Pt (0) complexes catalyze cis addition of the B-B bond in pinacol ester derivatives to alkynes but not to olefins. On the other hand, Pd (0) complexes do not catalyze diboration reactions, neither for alkynes nor for olefins. 

Figure 9. The reaction mechanism of Pt(0)-catalyzed alkyne and alkene diboration reactions calculated by Morokuma and co-workers.

For the diboration reactions of alkynes catalyzed by Pt(0) complexes, the reaction mechanism involves the oxidative addition of diborane to the Pt(0) center, followed by the insertion of alkyne into the Pt-B bond and reductive... 

The sulfur-containing diborane compounds 39 were considered of potential interest as reagents in metal-catalyzed diboration reactions <2000AXC440>. 

Borate complexes have been utilized by Brigl and Griiner47 to effect partial esterification. Anhydrous D-glucose and metaboric acid dissolved in acetone give a complex which exhibits the analysis of a diborate. Reaction of the latter with an excess of benzoyl chloride gives 2,6-di-O-benzoyl-D-glucose (XL). D-Mannitol likewise forms a diborate, which produces the 1,6-di-O-benzoyl derivative (XLI) upon benzoylation. In the presence of boric acid, D-glucose diethyl thioacetal yields the 6-benzoate (XLII). In the non-aqueous medium the formation of complexes... 

Haloboration Reactions. The haloboration of carbon-carbon triple bonds provides another entry point for the synthesis of organoboranes. A wide variety of haloboranes including BBrs, 9-BBN-Br, and 9-BBN-l has been found to react with terminal alkynes to produce (Z)-2-halo-l-aIkenylboranes. The reaction occurs in a stereo-, regio-, and chemoselective fashion specifically with terminal alkynes and has been used to synthesize numerous substituted olefins and related compounds. Diboration reactions of alkynes with B2CI4 are also well known. However, more convenient transition-metal-catalyzed procedures with the less reactive aUcoxy substituted diboranes B2(OR)4 have recently been developed. 

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. ... 

Diborane(4) compounds contain unsupported two-centre, two-electron B-B bonds examples of which, such as the diboron tetrahalides, have been known for many years.1 However, due in large part to the recent work of many groups in the field of transition metal catalysed diboration reactions,2 there has been a resurgence of interest in their synthesis and reactivity. Herein, we describe some of our own work in this area. 

Scheme 1 Postulated activation of CI2B-BCI2 with olefins toward diboration reaction.

A theoretical study has been carried out by Morokuma for the mechanism of Pt(0)-catalyzed alkyne and alkene diboration reactions with the B3LYP density functional method. The complexation energy between (0H)2B-B(0H)2 and Pt(PH3)2 has been calculated to be 3.7 kcal/mol, where the B-B and P-Pt-P axes are perpendicular to each other. The next step is the activation of the B—B bond, and the activation barrier has been calculated to be 12.5 kcal/mol, relative to the molecular complex (OH)2B— B(OH)2-Pt(PH3)2 or 8.8 kcal/mol relative to the reactants.In parallel, Sakaki theoretically investigated the insertion of Pt(PH3)2 into X2B—BX2 (X = H or OH) with the ab initio MO/MP4SDQ, SD-CI, and coupled cluster with double substitution methods. They found that this reaction proceeds with a moderate of 15 kcal/mol and a considerable of 20 kcal/mol for (OH)2B—B(OH)2. They noted that the B—B bond undergoes the insertion reaction of Pt(PH3)2 much more easily than does the C—C bond. Nowadays, the Pt-catalyzed diboration of terminal... 

A key step in many transition metal-catalyzed diboration reactions is oxidative addition of the B—B bond of a diborane(4) compound (R2B-BR2) to a low-valent transition metal center affording a metal bisboryl complex of the form LkM(BR2)2 ... 

Within the last decade, another source of diboron activation has been promoted in basis to the efEcient application to catalysis. This is the use of nanoparticles which can interact with the diboron reagent and develop an enhanced performance toward diboration reaction. Fernandez observed in 2008 that the in situ formation of Au nanoparticles, from Au(I) complexes, could not only activate the B2cat2 but also deliver the boryl units on alkenes with total chemoselectivity. " The gold nanoparticles were estimated to have a mean crystaUite size of 10.5 0.3 nm. The gold nanoparticles were stabilized by 2,2 -bis-(diphenylphosphino)-l-l -binaphthyl (BINAP), diphenylphosphinoethane (DPPE), and L-glutathione. The core size and size distribution ofBINAP-Au nanoparticles were examined by transition electron microscopy (TEM), and the image shows disperse nanopartides 6.9 3.0 nm in diameter (Fig. 6). 

Pure BINAP-Au nanoparticles with smaUer diameter (1.7 0.3nm) were alternatively synthesized following the Fujihara protocol, from HAUCI4 + BINAP in the presence of NaBH4, and eventuaUy used in the catalytic diboration reaction. The activity and chemoselectivity observed were... 

Studied the activation of B2pin2 and the catalytic diboration of terminal alkynes, they found a remarkably increased activity with completed diborated reactions in 1.5 h, giving the corresponding ds-adduct as the major isomer with a small amount of trans-zdduct. It is worth noting that AuNPore was active only in toluene as solvent. The leaching experiments and ICP-MS analysis indicated that no gold catalyst was leached. 

Cui Q, Musaev DG, Morokuma K. Molecular orbital smdy of the mechanism of platinum(0)-catalyzed aUtene and alkyne diboration reactions. Organometallics. 1997 16 1355-1365. 

Dang L, Lin Z, Marder TB. Understanding the higher reactivity ofB2cat2 versus B2pin2 in copper(I)-catalyzed alkene diboration reactions. Organometallia. 2008 27 1178-1186. 

Corberan R, RaimrezJ, Poyatos M, Peris M, Fernandez E. Coinage metal complexes with N-heterocychc carbene hgands as selective catalysts in diboration reaction. Tetrahedron Asymmetry. 2006 17 1759-1762. 

Bonet A, PubiU-UUdemolins C, Bo C, Gulyas H, Fernandez E. Transition-metal-free diboration reaction by activation of diboron compounds with simple Lewis bases. Angew Chem Int Ed Engl. 2011 50 7158-7161. 

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