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

Abstract

The high catalytic reactivity of homoleptic tris(alkyl) lanthanum La{C(SiHMe 2 ) 3 } 3 is highlighted by C À Ob ond cleavage in the hydroboration of esters and epoxides at room temperature.T he catalytic hydroboration tolerates functionality typically susceptible to insertion, reduction, or cleavage reactions.T urnover numbers (TON) up to 10 000 are observed for aliphatic esters.L anthanum hydrides,g enerated by reactions with pinacolborane,a re competent for reduction of ketones but are inert toward esters.Instead, catalytic reduction of esters requires activation of the lanthanum hydride by pinacolborane.Strategies that rely on highly nucleophilic metal hydrides to affect catalytic reduction and C À Ob ond cleavage of unsaturated oxygenates are limited by the stability of the corresponding metal alkoxide intermediates,which impedes regeneration of the metal hydride (Scheme 1A). [1] More reactive catalysts are needed for conversions of inert substrates,s uch as esters and ethers,a nd new mechanisms are needed to ameliorate the thermodynamic barriers associated with breaking strong M À Ob onds.O ne approach that could avoid M À Obond formation involves an activating interaction of the hydride source with the electrophilic catalytic center (Scheme 1B).Pinacolborane (HBpin) is promising in this regard because it generates metal hydrides upon reaction with organometallic compounds,a nd it also forms adducts with nucleophilic metal hydrides. [1, 2] Fore xample,h ydride species Cp* 2 LaH (Cp* = h 5 -C 5 Me 5 )a nd {HC(CMeNDipp) 2 }MgH (Dipp = 2,6-C 6 H 3 iPr 2 )a re proposed catalytic intermediates in ketone,aldehyde,imine,carbodiimide,nitrile,and pyridine hydroborations. [2f, 3] Alternatively,i solable pinacolborohydrides To M Mg{HXBpin} (To M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate;X= H, alkoxide) are proposed in hydroboration of esters,and the postulated mechanism avoids M-O intermediates. [4] In contrast, pinacolborohydride compounds are also proposed as off-cycle states or as precursors to catalyst deactivation. [2f,h, 3c] Early trivalent lanthanide hydrides often provide high catalytic activity. [5] Fore xample,L a{N(SiMe 3 ) 2 } 3 and LaCp 3 , which may provide entry to lanthanum hydrides upon reaction with HBpin, afford highly active catalysts for hydroboration of ketones and aldehydes. [6] While hydroboration of aldehydes and ketones is catalyzed by alarge number of metal complexes, [7] fewer catalysts,m ainly magnesium [2f, 4, 8] and molybdenum compounds, [9] mediate hydroboration of esters. Moreover,h ydroboration of epoxides is uncommon, with reports of only an ickel catalyst (involving C À Oo xidative addition) [10] and an iron catalyst limited to aryl-substituted substrates. [11] In the present contribution, we demonstrate that the easily synthesized tris(alkyl)lanthanum compound La{C-(SiHMe 2 ) 3 } 3 (1) [12] catalyzes the hydroboration of esters and epoxides.W ith 5mol % 1 at room temperature,a lkyl esters, alkyl benzoates,a nd aryl benzoates are reduced to boryl esters.H ydrolysis gives the...

Silica-Supported Organolanthanum Catalysts for C–O Bond Cleavage in Epoxides

“…Both materials 1@MSN550 and 1@MSN700 proved to be effective precatalysts at 5 mol % La loading (relative to epoxide) for ring opening C-O bond cleavage of epoxides via hydroboration to give borate esters, and both performed similarly in terms of activity and product yield ( Table 4). These reaction performance metrics are comparable, and often superior, to the soluble catalyst precursor La{C(SiHMe2)3}3; 12 in several cases the supported catalyst out-performed the homogeneous precursor. These organolanthanide catalysts also are advantaged over the state-of-the art transition-metalbased catalysts, which are limited to conversions of styrenyl oxides.…”

“…In addition, the supported organolanthanide also provided improved selectivity in the catalytic ring-opening hydroboration of cis-stilbene oxide, giving 1,2diphenylethanol, whereas the soluble catalyst precursor La{C(SiHMe2)3}3 afforded a mixture of 1,2-diphenylethanol and a rearranged 2,2-diphenylethanol product. 12 The catalytic activity of 1@MSN was solely associated with the solid material, even though soluble 1 is a viable catalyst in its own right. Styrene oxide ring-opening hydroboration reactions, in which half of the supernatant solution was separated from the reaction mixture and filtered after 1 h (at ca.…”

“…[4][5][6][7][8] Lanthanide oxophilicity can also enable C-O bond cleavage in ethers or in transesterification reactions; however, reductive catalytic C-O bond cleavage processes are typically inhibited by the stabilization of intermediates containing Ln-O bonds (Ln = lanthanides and group 3). A few examples of hydroboration of ketones and aldehydes, using the homoleptic lanthanum catalysts La{N(SiMe3)2}3 and Cp3La, [9][10][11] or hydroboration and C-O bond cleavage of esters catalyzed by La{C(SiHMe2)3}3 or La{N(SiMe3)2}3, [12][13] suggest strategies for overcoming the Ln-O bond strength to make use of highly electrophilic lanthanide centers. In fact, La{C(SiHMe2)3}3 represents the most active and versatile catalyst for the ringopening hydroboration of epoxides to date.…”

“…In fact, La{C(SiHMe2)3}3 represents the most active and versatile catalyst for the ringopening hydroboration of epoxides to date. 12,[14][15] We note that a related approach to breaking strong M-O bonds of oxophilic centers is enabled by redox-active metal centers, for example with Ti complexes under homogeneous conditions, [16][17] or in metal-organic frameworks (MOFs), 18 as well as Ce-containing MOF catalyzed hydroboration of alkenes and pyridines. 19 The oxophilic lanthanide centers also form strong bonds to oxygen in metal oxides, which is a useful property for attaching these sites onto supports in heterogeneous catalysts.…”

La[N(SiMe3)2]3‐Catalyzed Hydroboration of Esters and Other Challenging Unsaturated Groups

“…Besides, Nembenna's group disclosed that magnesium amide complexes could be served as highly efficient catalysts for hydroboration of a broad range of esters with pinacolborane (HBpin) . Very recently, Sadow and co‐workers further found that the homoleptic tris(alkyl)lanthanum complex La{C(SiHMe 2 ) 3 } 3 was competent to catalyze the hydroboration of esters and epoxides at room temperature . Apart from aforementioned work, the ester is only acted as sole example of substrates that has been sporadically reported …”