The Development and Application of Rhodium‐Catalyzed Hydroboration of Alkenes

Abstract

3.1 IntroductionHydroboration, the addition of a boron -hydrogen bond across an unsaturated moiety, was fi rst discovered by H. C. Brown in 1956 [1] . Usually the reaction does not require a catalyst and the borane reagent, most commonly diborane (B 2 H 6 ) or a borane adduct (BH 3 · THF), reacts rapidly at room temperature to afford, after oxidation, the anti -Markovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms, as is the case in catecholborane (1,3,2 -benzodioxaborole) the electron density at boron is increased and elevated temperatures are needed for hydroboration to occur [2,3] .The development of a catalytic hydroboration process was aided by the observation of Kono and Ito in 1975 that Wilkinson ' s catalyst [Rh(PPh 3 ) 3 Cl] undergoes oxidative addition when treated with catecholborane (2) , Scheme 3.1 [4] . Subsequently, Westcott et al. reported the isolation of the oxidative addition adduct with the tri -isopropylphosphine derivative and its characterization by X -ray crystallography [5] . However, it was another decade before the idea of developing a rhodiumcatalyzed olefi n hydroboration process came to fruition with the fi rst examples reported by M ä nnig and N ö th [6] .The conversion of an alkene (1) into an organoborane intermediate (3) has made this a valuable synthetic technique, particularly since the development of enantioselective variants [7, 8] . They serve as synthons for numerous functional groups [9] and are often subject to a consecutive carbon -oxygen [10] , carbon -carbon [11 -13] boron -carbon [14] , boron -chlorine [5] , carbon -nitrogen [15] and other bondforming reactions, Scheme 3.2 . These and other approaches toward extending the synthetic scope of catalytic asymmetric hydroboration have been the subject of recent excellent reviews [16 -18] . Many metals including nickel [19] , ruthenium [20] , iridium [21] , lanthanum [22] , titanium [23] and zirconium [24] have been employed in this transformation with varying degrees of success, although rhodium has remained the metal of choice for transition metal hydroboration.

Electronic Tuning of a Carbene Center via Remote Chemical Induction, and Relevant Effects in Catalysis

“…Hydroboration of styrene: The rhodium-catalyzed hydroboration [29] of styrene with pinacol or catecholborane has been extensively studied with phosphine ligands, [30] but, to our knowledge, only once with NHC ligands.…”

A Selective and Functional Group-Tolerant Ruthenium-Catalyzed Olefin Metathesis/Transfer Hydrogenation Tandem Sequence Using Formic Acid as Hydrogen Source

“…Although the conventional hydrogenation offers the highest possible atom economy, it often requires the application of high hydrogen pressures and specialized laboratory equipment. Conversely, the choice of TH hydrogen donors includes cheap, easy to handle and store, low molecular weight organic compounds, typically isopropanol, formic acid, or its salts. − Moreover, both the hydrogen donors and their oxidized products are nontoxic and easier to remove from reaction mixtures as compared to reductions using moisture- and air-sensitive metal hydrides, silanes, , or boranes …”

Inorganometallics (Transition Metal–Metalloid Complexes) and Catalysis

“…Since then, there have been many excellent reviews over the past 35 years that addressed various aspects of this reaction and formation of specific products. ,,,− The chemoselectivity, regioselectivity, and enantioselectivity of the catalytic reactions vary depending on the metal center, ligands, and substrates. Rhodium is usually identified as the metal of choice to promote the efficiency of such reactions.…”