Alkene hydroboration catalysis

Dicobalt octacarbonyl, in Pauson—Khand reaction homogeneous catalysis, 11, 340 metal-coupled promoters, 11, 339 non-oxidative promoter-assisted, 11, 338 oxidative promoter-assisted, 11, 337 physical promoters, 11, 339 solid-supported promoters, 11, 339 Dicobalt triple-decker sandwiches, preparation, 3, 14 (+)-Dictamnol, via [5+2]-cycloadditions, 10, 613-614 Dicyclohexylborane, for alkene hydroboration, 9, 150... 

Catalyzed Boration Reactions. Since the initial report by Mannig and Noth on the rhodinm catalyzed room temperature addition of HBcat to alkenes much effort has been devoted to the search for new catalytic hydroboration processes and other boration reactions (see Hydroboration Catalysis) The most intrigning aspect of transition-metal... 

Ammonia N-donor Ligands Asymmetric Synthesis by Homogeneous Catalysis Decarbonylation Catalysis Hydride Complexes of the Transition Metals Hydroboration Catalysis Hydrogenation Isomerization of Alkenes Hydrosilation Catalysis P-donor Ligands Rhodium Organometallic Chemistry. 

Hydroboration of alkenes with BH3 and R2BH proceeds at room temperature without a catalyst. As an exception, the reaction of the less reactive catecholborane (CBH, 529) proceeds at 70-100 °C. It was found that the slow reaction of CBH is accelerated by a Rh catalyst, and the catalyzed and uncatalyzed hydroborations give different products [200]. By the uncatalyzed reaction, the ketone in the enone 530 is attacked to form 531. On the other hand, the alkene is hydroborated to give 532 by the catalysis of RhCl(Ph3P)3 [201]. Different regio- and stereoselectivities are observed in the Rh-catalysed hydroboration of cyclohexenol (533) with catecholborane and uncatalysed reactions with 9-BBN-H [202],... 

Catalysis with LiBH4 Hydroboration of alkenes with catecholborane is generally slow, but can be affected at room temperature in about an hour when catalyzed by a small amount of LiBH4. The catecholborane can be generated in situ by reaction of BHj with catechol after evolution of H2 stops, the alkene (1 equiv.) and LiBH4 (0.1 equiv.) are added. Hydroboration is usually complete after stirring for 1 hour at 25°. Yields (after oxidation) are generally almost quantitative. 

Hydroboration of alkenes.2 Wilkinson s catalyst is effective for catalysis of hydroboration with catecholborane (CB), which can differ from the regioselectivity and stereochemistry of uncatalyzed hydroboration with 9-BBN. In the case of... 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

The hydroboration of alkenes is known to be activated either by pressure or catalysis. Consequently, the combination of these techniques might open the way for the hydroboration of particularly unreactive substrates. Maddaluno el al. recently investigated the hydroboration of some functionalized alkenes, comparing different reagents (catecholborane (CBH) versus pinacolborane (PBH)), and activation by Wilkinson catalyst (RhCl[PPh3]3) and pressure [24]. While bromoalkenes and al-lylamines were found to give the best results with CBH at ambient pressure, 2,3-dihydrofuran (52a, Scheme 7.17) was hydroborated most effectively by PBH in the... 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

In 1985, Manning and N6th first reported the hydroboration of alkenes catalyzed by Wilkinson s catalystJ Since this pioneering work, the development of transition-metal-catalyzed hydroboration has been investigated extensively. Burgess and Ohlmeyer demonstrated asymmetric catalysis with the use of BINAP and Diop-derived Rh-catalysts. Hayashi et al. later reported improvement of the enantioselectivity for the hydroboration of styrenes using Rh-BINAP complexes (up to 96% ee at -78 Other catalyst systems have also been shown to be effective... 

Early transition metal and lanthanide complexes also catalyze the hydroboration of alkenes. As shown in Equation 16.44, titanocene complexes catalyze the hydroboration of vinylarenes, and as shown in Equation 16.45, lanthanocene complexes catalyze the hydroboration of alkenes. Ligand-less lanthanides and zirconocene complexes have also been reported to catalyze these reactions, but the apparent catalysis of the hydroboration in these cases more likely occurs by catalytic generation of BH3 from catecholborane and uncatalyzed addition of BHj to the olefin. 

Somewhat related to cross-coupling reactions, the carbomagnesiation of terminal alkenes was shown to proceed efficiently in the presence of [(IMes)AgCl] and 1,2-dibromoethane, which was used as an oxidant. The hydroboration of terminal alkynes was also achieved using [(NHC)Ag] catalysts, and with higher regioselectivity compared to copper catalysis conditions. 

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