Elisheva Genizi

Group IV complexes of an amine bis(phenolate) ligand featuring a THF sidearm donor: From highly active to living polymerization catalysts of 1-hexene

Abstract A new amine bis(phenolate) ligand bearing a THF donor on a sidearm and several dialkyl complexes of the group IV triad are introduced. The crystal structures of a zirconium dibenzyl complex and a titanium dibenzyl complex were solved, and revealed a strong binding of the oxygen donor of the THF group to the metal center. Upon activation with tris(pentafluorophenyl)borane the zirconium and the hafnium dibenzyl complexes lead to highly active 1-hexene polymerization catalysts. In comparison, titanium complexes (a dibenzyl complex and a dimethyl complex) lead to slower but living polymerization of 1-hexene at room temperature. Polymerization of neat 1-hexene under high dilution conditions was found to be living for an exceptionally long time of 6 days, leading to poly(1-hexene) of Mw=816 000 and PDI=1.09. A block copolymerization of 1-hexene and 1-octene at room temperature could be obtained using the titanium catalysts.

Reactions of coordinated cyclic polyolefins. The mechanism of the pericyclic [4,4]-sigmahaptotropic rearrangement; a kinetic study. Crystal structure of tricarbonyl[(2,3,4,5-η)bicyclo[4.2.1]nona-2,4-diene-7,7,8,8-tetracarbonitrile]iron

Abstract The novel [4,4]-sigmahaptotropic rearrangement II → IV, in which a σ-bonded group and a Fe(CO) 3 unit exchange bonding sites antarafacially across a four carbon skeleton, has been shown by kinetic data to involve a concerted, non-synchronous, one step process. First order rate constants for the rearrangement at 23°C are k 1.25 × 10 −6 s −1 (acetone) and k 2.2 × 10 −5 s −1 (methanol), with activation parameters Δ H # 21 kcal mol −1 and Δ S # −15 e.u. (acetone). The moderate value of the ratio of k in methanol to that in acetone, viz. 18, indicates that although a minor charge separation develops upon activation, no intermediate is formed. The least motion pathway mechanism is shown to involve a Berry pseudorotation about the metal, which maintains the bonding interaction between the metal and the organic fragment orbitals during the rearrangement. Topologically this rearrangement corresponds to a [σ2 a + (σ2 s + π2 a )] thermally allowed pericyclic reaction. The structure of the rearranged complex IV was determined by single-crystal X-ray diffraction.

The mechanism of pericyclic organometallic reactions *. Competitive (4,4)-sigmahaptotropic rearrangement versus metal-hydride shift

The anti-2-2H labeled 3+2 adduct of tricarbonyl(η4-cycloheptatriene)iron and tetracyanoethylene undergoes a thermal rearrangement to the 6 + 2 isomeric complex, exclusively labeled at the anti position of the methylene group. Rate studies reveal no kinetic isotope effect. These observations exclude a hydrogen transfer mechanism, and suggest a pericyclic [4,4]-sigmahaptotropic (ση) rearrangement.

Pericyclic organometallic reactions. Cycloaddition reactions of (η4-cycloheptatriene)Ru(CO)3. Crystal structure of tricarbonyl[(2,3,4,9-η)-bicyclo[4.2.1]non-2-ene-4,9-diyl-7,7,8,8-tetracarbonitrile]ruthenium☆

(η4-Cycloheptatriene)Ru(CO)3 reacts with tetracyanoethylene (TCNE), 4-phenyltriazoline-3,5-dione (PTAD) and (carbomethoxy)maleic anhydride (CMA) to give stable 3 + 2 σ,π-allylic adducts. The 3 + 2 adduct with TCNE equilibrates via a [4,4]-sigmahaptotropic rearrangement with the less stable 6 + 2 adduct, which decomposes under the reaction conditions to the demetallated 6 + 2 adduct. It is concluded that σ,π-allylic adducts are in general more stable than their isomeric η4-π counterparts. The structure of the 3 + 2 TCNE adduct was determined by a single-crystal X-ray diffraction study.

Pericyclic organometallic reactions. The reactions of (cht)Fe(CO)2L (L=CO, P(OPh)3, PPh3) and (carbomethoxy)maleic anhydride revisited ☆

Kinetic studies of the reaction of (cht)Fe(CO)2L (L=CO, P(OPh)3, PPh3) and (carbomethoxy)maleic anhydride reveal, contrary to our previous report, that the reactions proceed by a tandem 3+2-cycloaddition-[2,2]-sigmahaptotropic rearrangement, to give the corresponding η2,η2-Diels–Alder adducts as main products, in equilibrium with the primary η1,η3-σ,π-allylic kinetic isomers. The labile primary isomers were characterized by 1H-, 13C- and 31P-NMR spectra. Substitution of carbonyl by phosphorus ligands increased the rates of both cycloadditions and rearrangements in the order PPh3>P(OPh)3>CO.

Peptide conjugation: unexpected ring opening of azacrown ether nucleophiles in the oxazolone-based coupling

Unexpected cleavage of the macrocylic ring of secondary azacrown ethers when interacting with the Aib8 (Aib = alpha-aminoisobutyric acid) oxazolone indicates the possibility for a new mechanism of peptide racemization due to transformations of the oxazolones formed from the N-derivatives of alpha-amino acids in peptide synthesis.

Photochemistry of bridged cycloheptadienes. Multiplicity dependent pericyclic rearrangements of excited bicyclo[4.2.1]nona-2,4-diene-7,7,8,8-tetracarbonitrile

Abstract Bicyclo[4.2.1]nona-2,4-diene-7,7,8,8-tetracarbonitrile rearranges selectively under direct irradiation in acetonitrile to tricyclo[3.2.2.0 2,4 ]non-6-ene-8,8,9,9-tetracarbonitrile, and upon acetone sensitization to bicyclo[3.2.2]nona-2,6-diene-8,8,9,9-tetracarbonitrile. Both products are derived from sigma bond cleavage, in contrast to the parent unsubstituted diene where only the π-bonds react.