MPV reduction using AlIII–calix[4]arene Lewis acid catalysts: Molecular-level insight into effect of ketone binding

Recent applications of chiral calixarenes in asymmetric catalysis

“…In 2011, Katz et al developed two optically pure aluminum complexes of inherently chiral calix[4]arene 111 and 112 bearing also an asymmetric carbon center on the phenylethylamine substituent ( Scheme 33 ) as catalysts for asymmetric Meerwein–Ponndorf–Verley (MPV) reduction of acetophenone derivatives using R -(−)-2-methylbutanol as hydride source [ 71 ]. As shown in Scheme 34 , independent of which calix[4]arene diastereomer ( 111 or 112 ) was employed, when ortho -fluorobenzophenone was used as substrate, the enantioselectivity remained at 20%.…”

Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation

“…Aluminum complexes are widely used in ring-opening polymerization. − Aluminum precatalysts with only one active alkoxide chain are facile to prepare, making it easy to control the polymerization and investigate the mechanism. − Thus, an aluminum alkoxide complex, (thiolfan*)­Al­(O t Bu) ( 1 red , thiolfan* = 1,1′-di­(2,4-di- tert -butyl-6-thiophenoxy)­ferrocene), based on an OSSO ligand architecture, ,− was synthesized and studied in order to extend the redox-switchable ROP strategy to a broad substrate scope and to understand the mechanism. Herein, we report the polymerization of l -lactide (LA), ε-caprolactone (CL), δ-valerolactone (VL), cyclohexene oxide (CHO), and trimethylene carbonate (TMC, Chart ) for the formation of their respective homopolymers and copolymers.…”

Trapped Intermediate of a Meerwein–Pondorf–Verley Reduction of Hydroxy Benzaldehyde to a Dialkoxide by Titanium Alkoxides

“…For most MPV mechanistic pathways, an Al-alkoxide is used as the catalyst with a sacrificial isopropanol (HOPr i ) as the H source. ,,, Other alkoxides have also been shown to promote the reduction of CO moieties by an MPV route including (i) boron alkoxides, − , (ii) assumed in situ generated [Ln­(OPr i ) 3 ] (Ln = La 0 , Ce 0 , Sm 0 , Yb 0 ) in the presence of I 2 , or (iii) the combination of LiOPr i with [Ln­(OPr i ) 3 ] (Ln = La, Ce) prepared from the LnCl 3 . Group 4 alkoxides supported by salicylaldiminato ligands were found to reduce an imine moiety to an amido complex by the MPV pathway .…”

“…For the system presented in this report, the initial metathesis of the OH with the parent OR would generate free HOR that may be necessary for an MPV reduction. ,,, The complexation of the aldehyde carbonyl to the metal is anticipated with a subsequent β-H transfer from the OPr i or ONep ligands, generating acetone or pivaldehyde, respectively, as a byproduct. The metal center is coordinatively unsaturated and binds to another similar moiety to satiate the Ti atoms coordination sphere.…”

“…These products were characterized by single crystal X-ray diffraction experiments as [(py)­(OR) 2 Ti­(κ 2 (O,μ-O′)­(OC 6 H 4– x (CH 2 O)-2)­(L) x ] (OR = OPr i , x = 1, L = H ( 1a ), OMe-3 ( 2a ), Br-5 ( 3a ·py), NO 2 -5 ( 4a ·4py), x = 2, Bu t -3,5 ( 5a ), I-3,5 ( 6a ); OR = ONep, x = 1, L = H ( 1b ), OMe-3 ( 2b ), Br-5 ( 3b ·py), NO 2 -5 ( 4b ), x = 2, Bu t -3,5 ( 5b ), I-3,5 ( 6b ·py)). Elucidation of the potential mechanism for this process on the basis the above reactivity was found to be consistent with a Meerwein–Pondorf–Verley (MPV)-like reduction pathway. − The synthesis, characterization, and attempts to elucidate the reduction pathway of this unprecedented aldehyde reduction by a simple [M­(OR) 4 ] is presented.…”