Research Update: a Hafnium-based Metal-organic Framework as a Catalyst for Regioselective Ring-opening of Epoxides with a Mild Hydride Source Understanding Hydrogen Sorption in a Polar Metal-organic Framework with Constricted Channels

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4][25] Anti-Markovnikov products of epoxide ring-opening, such as primary alcohols, are critical to the chemical and pharmaceutical industry and significant effort has gone into developing reagents and methodologies to obtain regioselective products. 22With metal hydrides, Lewis acids, such as transition-metals, 26 AlCl 3 , 27 or BH 3 , 28 are added to the reaction to shift regioselectivity to the anti-Markovnikov product.While extremely effective, these reagents are added in stoichiometric amounts and, in most cases, lack functional-group tolerance.Since Hf-NU-1000 acted as such a remarkable catalyst for regioselective ring-openings of epoxides with other mentioned nucleophiles, 21 we sought to use Hf-NU-1000 as a catalyst to ring-open epoxides with hydrides to form 1 • alcohols.Since a strong hydride would preferentially react with the acidic protons on the Hf 6 node, a mild hydride reagent is needed to prevent non-productive formation of hydrogen.Unlike other more reactive hydrides, cyanoborohydride is stable in water at pH 7 and above, but is unreactive toward epoxides. 29Indeed, ring-opening of epoxides with cyanoborohydride has been carried out at low pHs (pH 3-4), by the addition of BF 3 • etherate, 30 ZSM-5, 31 or by carrying out the reaction in neat acetic acid. 32Therefore, sodium cyanoborohydride was selected as our hydride source for the ring-opening of styrene oxide.Combining four equivalents of sodium cyanoborohydride with styrene oxide at room temperature in the presence of a catalytic amount (10 mol.%) of Hf-NU-1000 produces the anti-Markovnikov product, 2-phenylenthanol, in 60% yield with over 98% selectivity (Table I, entry 1).(See Figure 2 for a proposed mechanism.)In the absence of Hf-NU-1000, no product was obtained (Table I, entry 2) indicating that Hf-NU-1000 is a necessary component of the reaction.The catalyst was reused three times without decrease in activity or loss of crystallinity as shown by powder X-ray diffraction (PXRD) measurements (Figure S6). 33To gauge the importance of the Brønsted acid sites in this reaction, we also investigated a dehydrated form of  the MOF, Hf-NU-1000-dehydrated (Table I, entry 3).The dehydrated MOF is structurally identical to Hf-NU-1000 with the exception that there are no metal bound H 2 O or OH molecules; only linker carboxylate groups and bridging oxo ligands are found at the Hf core.With the dehydrated MOF as the candidate catalyst, no conversion of styrene oxide was obtained, thereby illustrating the importance of Brønsted acid sites for activation of cyanoborohydride.A dramatic loss in both yield and selectivity occurred when the reaction was carried out in refluxing tetrahydrofuran (Table I, entry 4).We attribute this loss to the deprotonation of Hf-bound acid sites by the hydride to generate H 2 .Therefore, a 1 H NMR experiment was conducted to assess the stability of Hf-NU-1000 under conditions that simulated reaction conditions in the presence of sodium cyanoborohydride, both at room temperature and at elevated temperatures.Under a N 2 FIG. 2. Proposed catalytic cycle for ring-opening of styrene oxide with sodium cyanoborohydride, catalyzed by Hf-NU-1000.The epoxide is activated by H-bonding with the acidic protons of the Hf 6 core.The hydride attacks the benzylic carbon in a S N 2 fashion.The alkoxide then attacks BH 2 CN moiety.The catalytic cycle is complete when a new molecule of styrene oxide H-bonds with Hf-NU-1000.For clarity, the bottom third of the hafnium core and the ligand carboxylate groups have been omitted.atmosphere, a J. Young NMR tube was loaded with Hf-NU-1000 and sodium cyanoborohydride.Tetrahydrofuran-d 4 was added, and the NMR tube was agitated to facilitate mixing and was left to stand for 24 h.A NMR spectrum was recorded (Figure S2) and a small peak was observed at δ = 4.53 ppm which corresponds to molecular hydrogen.The same sample was heated for 24 h at 60 • C and another NMR spectrum was recorded using the same parameters as the room temperature spectrum.The relative peak integration at δ = 4.53 ppm is larger at elevated temperature which supports our hypothesis that deprotonation of Hf-NU-1000 occurs at elevated temperatures.
We also investigated the ring-opening of propylene oxide (Table I, entries 5 and 6).A yield similar to that for styrene oxide was obtained for propylene oxide, but the regioselectivity of the reaction was reversed, with 2-propanol as the only product.Based on literature precedent 30 and our results with styrene oxide, we expected instead to see 1-propanol as the dominant product.This is an intriguing result since cyanoborohydride is generally unreactive to epoxides; Hf-NU-1000 clearly is required for the ring opening to occur.Identification and validation of the mechanism responsible for exclusive formation of 2-propanol is a focus of ongoing work.
In conclusion, Hf-NU-1000 serves as a catalyst for ring-opening epoxides with sodium cyanoborohydride.Although the product yields are modest, the regioselectivities obtained for both substrates are excellent.Hf-coordinated Brønsted acid groups appear to be important to the catalytic function of Hf-NU-1000, as a dehydrated version of the MOF is ineffective as a catalyst.Hf-NU-1000 is reusable as a catalyst for ring-opening, with no loss of activity and no evidence for structural deterioration after three cycles of reuse.These promising results warrant further investigation of MOFs as catalysts for hydride based ring-opening reactions and related reactions.
O.K.F. and J.T.H. gratefully acknowledged the financial support from the ISEN (Institute for Sustainability and Energy at Northwestern) and from the Department of Chemistry at Northwestern University.The German Research Foundation (DFG) is also acknowledged for the Postdoctoral Research Fellowship Award to M.H.B. Acquisition of NMR and gas chromatograph instruments used in the IMSERC facility of Northwestern University was made possible by Grant Nos.CHE-1048773 and CHE-0923236 from the National Science Foundation.

TABLE I .
Results from the Hf-NU-1000 catalyzed reactions of epoxides with various hydride sources.General reaction conditions: 0.2 mmol substrate, 0.8 mmol hydride, and 0.02 mmol catalyst were stirred for 24 h at room temperature under a N 2 atmosphere in 1.9 ml dry tetrahydrofuran (THF).See Ref.21for details of its preparation and characterization.
b Determined by 1 H NMR. c d Carried out in refluxing tetrahydrofuran.e Determined by GC-TOF.