Titanocene(II)-Catalyzed Hydroboration of Carbonyl Compounds

Abstract

Titanocene bis(catecholborane), [Cp 2 Ti(HBcat) 2 ] (1), catalyzes the room-temperature hydroboration of carbonyl compounds by pinacolborane (HBpin) rapidly, cleanly, and chemoselectively. Aryl aldehydes and ketones produced alkoxypinacolboronate esters in moderate to high yields in 2 h, and facile hydrolysis of alkoxypinacolboronate esters over silica occurred cleanly to afford alcohols in good yields. Complex 1 demonstrated a preference for CO bonds over CC bonds in both conjugated and nonconjugated enones. Kinetic studies of the catalytic hydroboration of a series of acetophenones showed that electron-poor substrates undergo the reaction more quickly than electron-rich substrates. This result is consistent with the proposed mechanism, in which stronger π-acids should undergo CO bond cleavage more readily. Computational studies using benzophenone and benzaldehyde showed that the hydroboration is spontaneous and likely proceeds via intermediates that are best described as Ti metallacycles whose structures are not significantly altered by substrate steric differences. This result indicates that similarities in the electronic properties of benzophenone and benzaldehyde supersede their steric differences in determining reaction outcomes.

Catalytic Hydroboration of Organic Nitriles Promoted by Aluminum Complex

“…[9][10][11][12][13][14] As a result, innumerable procedures have been evolved to accomplish the addition of boranes to nitriles. All these procedures require more or less effective catalysts based on transition-metal complexes such as Co, Fe, Ru, [15][16][17][18][19][20][21][22][23] alkaline earth metal (Mg), [24][25][26][27][28][29][30][31][32][33][34][35] but the processes culminate in frustrated Lewis pairs. [36][37][38] On the other hand, while the reduction of aryl and alkyl nitriles can be achieved using stoichiometric quantities of maingroup reducing agents such as LiAlH 4 and NaBH 4 , [39] the combustible nature of these reagents and large quantities of inorganic waste by-products they generate render the process unattractive, and hence reductive hydroboration is preferable in order to provide further functionality to the resultant amine.…”

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

“…B, 7 Al, 8 Ge, Sn 9 ) elements, as well as first-row (e.g. Ti, 10 Mn, 11 Fe, 12 Ni 13 ), group 6 (Mo 14 ) and late (Re, 15 Ru 16 ) transition metals recently reported. Despite the prevalence of Rh catalysts for the hydroboration of alkenes, 17 to the best of our knowledge, there has been only one stoichiometric example of the use of a Rh complex in the hydroboration of a carbonyl compound (benzaldehyde) reported in the literature to date.…”

Hydroboration, Cyanosilylation, and Sequential Cyanosilylation and Hydroboration of Carbonyl Compounds in the Presence of a TiIV Amido Complex as an Efficient Catalyst

“…[13] However, these protocols have no tolerance for other active functional groups such as nitro groups, nitriles, and amides. In addition, cata- lysts based on transition metals such as titanium, [17][18][19][20][21] manganese, [22] iron, [23] molybdenum, [24] rhodium, [25] ruthenium, [26] copper, [27] and zinc, [28] as well as on main group metals such as aluminum, [29] gallium, [30] germanium, [31] and tin, [31] are known for the hydroboration of carbonyl compounds. In addition, cata- lysts based on transition metals such as titanium, [17][18][19][20][21] manganese, [22] iron, [23] molybdenum, [24] rhodium, [25] ruthenium, [26] copper, [27] and zinc, [28] as well as on main group metals such as aluminum, [29] gallium, [30] germanium, [31] and tin, [31] are known for the hydroboration of carbonyl compounds.…”

“…[10] Recent reports from the laboratories of Okuda, [14] Hill, [15] Sen, [16] and others have utilized new alkali metal and alkaline earth (Ae) metal catalysts for the hydroboration of carbonyl compounds. In addition, cata- lysts based on transition metals such as titanium, [17][18][19][20][21] manganese, [22] iron, [23] molybdenum, [24] rhodium, [25] ruthenium, [26] copper, [27] and zinc, [28] as well as on main group metals such as aluminum, [29] gallium, [30] germanium, [31] and tin, [31] are known for the hydroboration of carbonyl compounds. Very recently, the groups of Marks and of Shen reported on the use of active rare-earth metals La [32] and Yb [33] as catalysts for the hydroboration of carbonyl compounds.…”

“…Although various Ti IV alkoxide catalysts for the hydroboration of carbonyl compounds have been reported in the literature, no significant functional group tolerance has yet been achieved in this class ( Figure 1). [17][18][19][20][21] We recently prepared a number of Ti IV amido complexes and found that these complexes indeed support the hydroamination of different heterocumulenes under ambient conditions. [35] To extend the utility of the earth's second most abundant transition metal, in the form of Ti complexes, here we report the chemoselective hydroboration and cyanosilylation of carbonyl compounds under mild reaction conditions with the aid of new Ti IV complexes.…”