Expanding Zirconocene Hydride Catalysis: In Situ Generation and Turnover of ZrH Catalysts Enabling Catalytic Carbonyl Reductions

Abstract

Despite the wide use and popularity of metal hydride catalysis, methods utilizing zirconium hydride catalysts remain underexplored. Here, we report the development of a mild method for the in situ preparation and use of zirconium hydride catalysts. This robust method requires only 2.5−5 mol % of zirconocene dichloride in combination with a hydrosilane as the stoichiometric reductant and does not require careful air-or moisture-free techniques. A key finding of this study concerns an amine-mediated ligand exchange en route to the active zirconocene hydride catalyst. A mechanistic investigation supports the intermediacy of an oxo-bridged dimer precatalyst. The application of this method to the reduction of a wide variety of carbonylcontaining substrates, including ketones, aldehydes, enones, ynones, and lactones, is demonstrated with up to 92% yield and exhibits broad functional group tolerability. These findings open up alternative avenues for the catalytic application of chlorozirconocenes, potentially serving as the foundation for broader applications of zirconium hydride catalysis.

Multimetallic Permethylpentalene Hydride Complexes

“… 8 Notably, Schwartz’s reagent (Cp 2 Zr( H )Cl) has proved to be a powerful tool in organic synthesis, allowing for hydrozirconation and subsequent chemoselective C–C, C–N, and C–X bond formation with good functional group tolerance. 9 Related polyhydride complexes not only possess interesting structural motifs but have also shown to demonstrate remarkable power in activating challenging bonds; 10 Hou and co-workers have reported both dinitrogen reduction and cleavage 10c and C–C activation of benzene 10b by the trimetallic heptahydride cluster {(η 5 -C 5 Me 4 SiMe 3 )Ti} 3 (μ 3 -H)(μ 2 -H) 6 . Noncyclopentadienyl-supported group 4 metal hydrides are comparatively rarer; 11 Okuda and co-workers have reported the preparation of [{Zr(Me 3 TACD)(μ-H) 2 } 2 ][A] 2 (Me 3 TACD = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane; A = Al{OC(CF 3 ) 3 } 4 , B{3,5-C 6 H 3 (CF 3 ) 2 } 4 , B(3,5-C 6 H 3 Cl 2 ) 4 , and BPh 4 ) by hydrogenolysis (50 bar H 2 ) of the corresponding neosilyl complexes.…”

Zirconium-catalysed hydrosilylation of esters and depolymerisation of polyester plastic waste

“…By overcoming the inherent low reactivity of these polymer materials due to their lack of solubility, inhomogeneity and relatively low purity, we provide an example of chemical upcycling by transforming waste materials into value-added monomer products. [18][19][20] Based on the work of Bayeh-Romero et al and our previous work on the reduction of secondary amides, 16,17 initial conditions were applied to the reduction of ethyl 4-methylbenzoate 2a, using 10 mol% of Cp 2 Zr(H)Cl (1) with 2.1 equiv. of DMMS in THF-d 8 at 80 °C (Table 1, entry 1).…”

“…Under the catalytic conditions, the aldehyde (not detected) is immediately reduced by complex 1 to the Zr alkoxide complex 3 Zr . 16 The two alkoxyzirconocene complexes 3 Zr and 4 Zr perform a σbond metathesis with the hydrosilane to form the desired silyl ethers 3 Si and 4 Si , respectively, with concomitant regeneration of the catalyst 1. 16,17 Using the optimised conditions defined above for ethyl 4-methylbenzoate 2a (Table 1, entries 2 and 8), the generality of this system was explored through a substrate scope (Scheme 4).…”

“…16 The two alkoxyzirconocene complexes 3 Zr and 4 Zr perform a σbond metathesis with the hydrosilane to form the desired silyl ethers 3 Si and 4 Si , respectively, with concomitant regeneration of the catalyst 1. 16,17 Using the optimised conditions defined above for ethyl 4-methylbenzoate 2a (Table 1, entries 2 and 8), the generality of this system was explored through a substrate scope (Scheme 4). The benzoate esters 2a and 2b and methyl phenylacetate 2c were cleanly reduced to the corresponding alcohols.…”

“…Under the catalytic conditions, the aldehyde (not detected) is immediately reduced by complex 1 to the Zr alkoxide complex 3 Zr . 16 The two alkoxyzirconocene complexes 3 Zr and 4 Zr perform a σ-bond metathesis with the hydrosilane to form the desired silyl ethers 3 Si and 4 Si , respectively, with concomitant regeneration of the catalyst 1 . 16,17…”