Activation of Epoxides by a Cooperative Iron–Thiolate Catalyst: Intermediacy of Ferrous Alkoxides in Catalytic Hydroboration

Abstract

This paper describes a cooperative iron–thiolate catalyst Cp*Fe­(1,2-Ph2PC6H4S)­(NCMe) (Cp*– = C5Me5 –, [1(NCMe)]) for regioselective hydroboration of aryl epoxide by pinacolborane (HBpin). The critical catalytic step involves the direct addition of epoxide to the catalyst rather than activation of the B–H bond of HBpin. Through iron–thiolate cooperation, [1(NCMe)] opens the aryl epoxide rings affording ferrous–alkoxide compounds. Notably, the ferrous–alkoxide intermediate (4) was structurally characterized after its isolation from the reaction of [1(NCMe)] with trans-2,3-diphenyloxirane. The more Lewis acidic hydroboranes such as H3B·THF and 9-BBN (BBN = bora­bicyclo­nonane) can also be captured by [1(NCMe)]. The resulting iron–borane adducts [1H­(BH2)] and [1H­(BBN)] feature an agnostic Fe···B–H interaction. DFT calculations indicate that the addition of HBpin across the iron–thiolate sites is endergonic by 12.9 kcal/mol, whereas it is exergonic by 20.2 kcal/mol with BH3 and 4.6 kcal/mol with 9-BBN. Combining the experimental data with theoretical studies, a mechanism of the substrate activation by [1(NCMe)], followed by HBpin addition, is proposed for the catalysis.

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

“…[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. [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 corresponding alcohol in high isolated yield (Table 1). [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 corresponding alcohol in high isolated yield (Table 1).…”

“…Insoluble 2 reacts quantitatively with 3equiv of acetophenone to afford soluble 11 BNMR spectrum of this mixture,a nd HBpin was not present in the in situ generated samples of 2, ruling out consumption of acetophenone by hydroboration in these experiments.T he product of 2 and acetophenone was assigned as {La(OCHMePh) 3 } n on the basis of hydrolysis which gives HOCHMePh. 0.2 equiv with respect to initial 1), and 1 HNMR spectra of ketone-quenched materials revealed unidentified aliphatic signals.T he latter problem is avoided by in situ precipitation of 2 from 1 and 3equiv of HBpin.…”

“…Insoluble 2 reacts quantitatively with 3equiv of acetophenone to afford soluble 11 BNMR spectrum of this mixture,a nd HBpin was not present in the in situ generated samples of 2, ruling out consumption of acetophenone by hydroboration in these experiments.T he product of 2 and acetophenone was assigned as {La(OCHMePh) 3 } n on the basis of hydrolysis which gives HOCHMePh. Reaction of 1 and 6equiv of HBpin provides amixture of (Me 2 HSi) 3 CBpin and HBpin (1:0.9 ratio) as determined by 11 BNMR spectroscopy.C ompound 2 does not precipitate under these conditions.E vaporation of the volatile components,i ncluding HBpin, provides am aterial that is insoluble in benzene and appears equivalent to 2.A pparently,t he presence of HBpin solubilizes 2 in benzene (Scheme 1, center). [14] This approach was previously used to assign reactive magnesium hydride.…”

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

“…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 corresponding alcohol in high isolated yield (Table 1). Halogen-, nitro-, alkoxy-, and trifluoromethyl-substituted benzoate esters are reduced without affecting the functional group.O lefins are untouched, whereas La{N(SiMe 3 ) 2 } 3 is reported to catalyze addition of alkenes and HBpin.…”

“…0.2 equiv with respect to initial 1), and 1 HNMR spectra of ketone-quenched materials revealed unidentified aliphatic signals.T he latter problem is avoided by in situ precipitation of 2 from 1 and 3equiv of HBpin. Insoluble 2 reacts quantitatively with 3equiv of acetophenone to afford soluble 11 BNMR spectrum of this mixture,a nd HBpin was not present in the in situ generated samples of 2, ruling out consumption of acetophenone by hydroboration in these experiments.T he product of 2 and acetophenone was assigned as {La(OCHMePh) 3 } n on the basis of hydrolysis which gives HOCHMePh. Compound 2 is tentatively assigned as containing LaH 3 on the basis of the stoichiometry of its formation and reaction with ketones.…”