Copolymerization of Cyclohexene Oxide with CO2 by Using Intramolecular Dinuclear Zinc Catalysts

Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: a salen-cobaltate complex of an unusual binding mode

Salen-type ligands comprised of ethylenediamine or 1,2-cyclohexenediamine, along with an salicylaldehyde bearing a methyl substituent on its 3-position and a -[CR(CH(2)CH(2)CH(2)N(+)Bu(3))(2)] (R = H or Me) on its 5-position, unexpectedly afford cobalt(III) complexes with uncoordinated imines. In these complexes, two salen-phenoxys and two 2,4-dinitrophenolates (DNPs), which counter the quaternary ammonium cations, coordinate persistently with cobalt, while two other DNPs are fluxional between a coordinated and an uncoordinated state in THF at room temperature. The complexes of this binding mode show excellent activities in carbon dioxide/propylene oxide copolymerization (TOF, 8300-13,000 h(-1)) but with some fluctuation in induction times (1-10 h), depending on how dry the system is. The induction time is shortened (<1.0 h) and activity is increased approximately 1.5 times upon the replacement of the two fluxional DNPs with 2,4-dinitrophenol-2,4-dinitrophenolate homoconjugation ([DNP...H...DNP](-)). Imposing steric congestion either by replacing the methyl substituent on the salicylaldehyde with tert-butyl or by employing H(2)NCMe(2)CMe(2)NH(2) instead of ethylenediamine or 1,2-cyclohexenediamine results in conventional imine-coordinating complexes, which show lower activities than uncoordinated imine complexes.

Binding of 4-(N,N-dimethylamino)pyridine to Salen- and Salan-Cr(III) cations: a mechanistic understanding on the difference in their catalytic activity for CO2/epoxide copolymerization

The coordination chemistry of 4-(N,N-dimethylamino)pyridine (DMAP) with Salen- or Salan- (where the tetradentate N,N'-disubstituted bisaminophenoxide is designated as Salan, a saturated version of the Schiff-base Salen ligand) chromium complexes was studied by electrospray ionization mass spectrometry (ESI-MS). The relative stabilities of mono DMAP adducts of these chromium complexes were characterized by collision-induced dissociation (CID) and further discussed with regard to the activity in catalyzing CO(2)/epoxide copolymerization. [SalenCr](+) cations preferably bind two DMAP molecules to form six-coordinated complex ions, while [SalanCr](+) cations usually bind one molecule of DMAP to form five-coordinated complex ions, which were found to be relatively unstable. The remarkable difference in the coordination of DMAP to these two chromium complexes resulted in a significant difference in catalytic activity for the alternating copolymerization of CO(2) and propylene oxide. In the presence of 1 equiv of DMAP, the activity of the chromium-Salan complex 2a was up to 86 h(-1) of TOF at ambient temperature, which was about 30 times that of the corresponding chromium-Salen complex 1a. In sharp contrast to a long induction period up to 2 h with the use of 1a in conjunction with DMAP as catalyst, no initiation time or a very short one was observed in the binary 2a/DMAP catalyst systems. The initiator role of DMAP was confirmed by continuous determination of the propagating polymer species at various intervals using ESI-MS, which in combination with a kinetic study by means of infrared spectroscopy resulted in a mechanistic understanding on the difference in activity of the two catalyst systems for CO(2)/epoxide copolymerization.

Direct asymmetric Michael addition to nitroalkenes: vinylogous nucleophilicity under dinuclear zinc catalysis

Under dinuclear catalysis, the direct conjugate addition of 2(5H)-furanone to nitroalkenes involves the gamma-position of the nucleophile. The synthetically versatile Michael adducts are prepared in good yields, with high levels of diastereo- and enantioselectivity. A model is presented to rationalize the observed stereoselectivity.

Dinuclear Zinc-Catalyzed Enantioselective Aza-Henry Reaction

The dinuclear zinc catalyst 1a was found to catalyze the addition of nitroalkanes to carbamate-protected imines. This aza-Henry reaction proceeds with high enantioselectivity when various carbamate-protected imines are used. alpha,beta-Unsaturated imines proved to be a particularly useful class of substrate routinely giving the alpha-nitro amine products in high enantiomeric excess.

A Heterodinuclear Asymmetric Catalyst for Conjugate Additions of α-Hydroxyketones to β-Substituted Nitroalkenes

[Structure: see text] The bis-ProPhenol ligand was designed to facilitate formation of hetereodinuclear complexes based upon the large difference in pKa of the one phenolic OH group to the tertiary OH groups. In exploring the first example of hydroxyacetophenones as donors in asymmetric Michael reactions with nitroalkene acceptors, the best stereocontrol was observed with a zinc/magnesium dinuclear complex where enantiomeric excesses ranged up to 92% for the major anti diastereomer.

Bimetallic Fluorine-Substituted Anilido−Aldimine Zinc Complexes for CO2/(Cyclohexene Oxide) Copolymerization

Regioselective nucleophilic aromatic substitution of an o-fluorine occurs to afford fluorine-substituted o-phenylene-bridged bis(anilido-aldimine) compounds o-C6H4[(C6H2R2)N=CH-C6F4-(H)N(C6H3R'2)]2 when Li(H)N-C6H3R'2 (R' = iPr, Et, Me) is reacted with o-C6H4[(C6H2R2)N=CH-C6F5]2 (R = iPr, Et, Me) in a nonpolar solvent such as diethyl ether or toluene. Successive additions of Me2Zn and SO2 gas to the bis(anilido-aldimine) compounds afford quantitatively dinuclear mu-methylsulfinato zinc complexes o-C6H4[[(C6H2R2)N=CH-C6F4-N(C6H3R'2)-kappa2N,N]Zn(mu-OS(O)Me)]2 (R = iPr, R' = iPr, 3a; R = iPr, R' = Me, 3c; R = Et, R' = (i)Pr, 3d; R = Et, R' = Et, 3e; R = Et, R' = Me, 3f; R = Me, R' = iPr, 3g; R = Me, R' = Et, 3h; R = Me, R' = Me, 3i). The molecular structure of 3c was confirmed by X-ray crystallography. Fluorine-substituted complexes 3a-i show significantly higher TOF (turnover frequencies) than the unfluorinated analogues for CO2/(cyclohexene oxide) copolymerization. The TOF is highly sensitive to the substituents R and R', and the highest TOF (2480 h(-1)) is obtained with 3g (R = Me, R' = iPr). Complex 3g is less sensitive to the residual protic impurities present in the monomers and shows activity at such a low catalyst concentration as [Zn]:[cyclohexene oxide] = 1:50,000, at which the unfluorinated analogue is completely inactive. By realizing the activity at such an extremely low [Zn]:[cyclohexene oxide] ratio, we achieve a high TON (turnover number) up to 10,100. High-molecular-weight polymers (M(n), 100,000-200,000) are obtained with a rather broad molecular-weight distribution (M(w)/M(n), 1.3-2.5). The obtained polymers are not perfectly alternating, and variable carbonate linkages (65-85%) are observed depending on the N-aryl ortho substituents R and R' and the polymerization conditions.