Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C−O Bond Cleavage

Abstract

Theauthors declare no conflict of interest.Keywords: CÀObond cleavage ·h omogeneous catalysis · hydroboration ·lanthanum hydride ·s aturation kinetics Scheme 3. (A) First-order dependence on [ester] indicates CÀOb ond cleavage occurs after the turnover-limiting step, whereas (B) half-order dependence on ester indicates CÀObond cleavage happensp rior to or during the turnover limiting step.Scheme 4. Mechanisms for 1-catalyzed hydroboration of esters. Mechanisms A and B are both consistent with kinetic experiments, but A is less likely based on reactivity of in situ generate hydridolanthanum species.

Manganese‐Catalyzed Hydroborations with Broad Scope

“… [12b] Catalytic hydroborations of carboxylates have recently been reported with transition metal catalysts, [13] bulky organo‐magnesium,[ 14 , 15 ] amidomagnesium,[ 7a , 16 ] and rare earth metal catalysts. [ 7d , 7e , 17 , 18 ] Catalytic Mn(hmds) 2 enabled clean ester reduction with HBpin at room temperature (Scheme 3 , center). Alkanoates and benzoates were reactive; halides, CF 3 , and heteroaryl substituents were tolerated.…”

Manganese‐Catalyzed Hydroborations with Broad Scope

“…( S )‐(+)‐2‐Methyl butyronitrile was reduced at 50 °C to the β‐chiral butylamine in near‐quantitative yield ( =6.2°, CH 3 OH) [12b] . Catalytic hydroborations of carboxylates have recently been reported with transition metal catalysts, [13] bulky organo‐magnesium, [14, 15] amidomagnesium, [7a, 16] and rare earth metal catalysts [7d–e, 17, 18] . Catalytic Mn(hmds) 2 enabled clean ester reduction with HBpin at room temperature (Scheme 3, center).…”

Rare‐earth metal‐catalyzed hydroboration of unsaturated compounds

“…Nevertheless, the Sadow group developed organolanthanum complexes immobilized on the surface of mesoporous silica nanoparticles, La[C(SiHMe 2 ) 2 ] n @MSN 700 ( 20a ) (Figure 4) and La[C(SiHMe 2 ) 2 ] n @MSN 550 ( 20b ), which significantly inhibited the migration of phenyl group in the hydroboration of stilbene oxides, giving sole product (Scheme 19). [ 33 ] Compared with the former homogenous catalysis, the solid‐supported catalyst exhibited equivalent capability and even better activity and regioselectivity in the case of styrene oxides and stilbene oxides, and more importantly, the latter could be reused for 3 cycles without compromising the performance. However, the mechanism of ring‐opening of epoxides is still not clear, although the authors believed that LaH 2 Bpin species should be responsible for the transformation (Scheme 20).…”

“…Indeed, esters could undergo the transformation smoothly promoted by the lanthanum complex La[C (SiHMe 2 ) 2 ] 3 (18) (Figure 4) under mild conditions, albeit subsequent studies showed La-H is not competent to the transformation (Scheme 14). [33] With only 0.01 mol% catalyst loading, ethyl acetate could be quantitatively converted into corresponding borate ester within 2 h at room temperature. A broad scope of functional groups was tolerated in the conditions, and unsaturated C-C double bonds were left intact.…”

“…[36] Nevertheless, together with further DFT studies, the involvement of a lanthanide hemiacetal species rather than La-H species is highly plausible in the catalytic cycle (Scheme 15), which is also proposed by Patnaik and Sadow. [33] Moreover, the isotopic experiment has showed that dideuterated borate has been observed, revealing that reverse Tishchenko mechanism is not possible while hemiacetal-based mechanism is more rational. Very intriguingly, the Xue group also reported the hydroboration of esters with the same catalyst independently at the same period.…”