Dehydrogenative borylation

Similarly, hydroboration of p-methoxystyrene with HBcat using zwitterionic [(P-P)Rh(//6-catBcat)] ((4), P-P = dppe, dippe) or [(dippe)Rh(r/3-2-IVIc-a 11 y 1)] (dippe= l,2-bis(diisopropylphosphine)ethane) gave 99% of the internal hydroboration product. Complications arose in studying reactions of hindered styrenes due to the appearance of not only the hydroboration products (A and B), but also the dehydrogenative borylation products C and D (Scheme 5).32 The ratio of products was found to be highly catalyst dependent (Table 2).33... 

Table 2 Ratios of hydroboration/dehydrogenative borylation products shown in Scheme 4 found with a range of rhodium catalysts.33,34...

The reaction of pinacolborane with styrenes 127 in the presence of bis(chloro-l,5-cyclooctadienylrhodium) at room temperature provides styrenyl pinacol boronate 128 <1999TL2585, 2002BCJ825>. While hydroboration of alkenes is the predominant reaction with phosphine-containing rhodium catalysts such as Wilkinson s catalyst and Rh(PPh3)2COCl, dehydrogenative borylation dominates over hydroboration in the presence of phosphine-free... 

Smith and co-workers reported the dehydrogenative borylation of ethylene with catecholborane in the presence of decamethyltitanocene Cp2 Ti catalyst to produce 5-vinyl catechol boronate 137. Reaction of Cp2 Ti with ethylene leads to the formation of metallacyclopropane species 135 that undergoes reaction with catecholborane 38a to... 

The titanium complex Cp2 Ti(C2H4) catalyzes the dehydrogenative borylation of ethylene,43... 

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. ... 

Scheme 2 A general proposed mechanism for the rhodium-catalyzed dehydrogenative borylation of alkenes using HBcat...

The complex shown in Scheme 511 is formed by the reaction of Cp 2Ti(CH2=CH2) with the corresponding borane in appropriate conditions. These complexes are intermediates in the dehydrogenative borylation of ethylene through the selective conversion of a titanium-olefin bond to vinyl borate esters.1303... 

Caballero A, Sabo-Etienne S (2007) Ruthenium-catalyzed hydroboration and dehydrogenative borylation of linear and cyclic alkenes with pinacolborane. Organometallics 26 1191... 

Smith and Marder reported the dehydrogenative borylation of arenes, yielding arylboronates, with pinacolborane in the presence of rhodium and iridium catalysts such as Cp Rh(7/ -C6Me6), CpIrPMes, and [RhClP( Pr)3]2N2 (eq 22). Toluene and other methyl substituted arenes react with pinacolborane in the presence of [RhClP(Tr)3]2N2 and furnish benzylboronates via benzylic C-H activation and dehydrogenative borylation (eq 23). 

Despite these successes in stoichiometric reactions, productive (catalytic) functionalization of simple alkanes via oxidative addition is almost restricted to CO insertion and dehydrogenation by the RhCl(CO)(P(CH3)3)2- v system (9-11) (vide infra) and the dehydrogenative borylation (eq. (5)) (12). 

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