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Emil B Lobkovsky

Publications and source records attributed to Emil B Lobkovsky.

30 records · Page 2Linked to original sources
PubMed2004

The course of (R2R'SiO)3TaCl2 (R = tBu, R' = H, Me, Ph, tBu (silox); R = iPr, R' = tBu, iPr) reduction is dependent on siloxide size.

Various sized siloxides (Cy(3)SiO > (t)Bu(3)SiO > (t)Bu(2)PhSiO > (t)Bu(2)MeSiO approximately (i)Pr(2)(t)BuSiO > (i)Pr(3)SiO > (t)Bu(2)HSiO) were used to make (R(2)R'SiO)(3)TaCl(2) (R = (t)Bu, R' = H (1-H), Me (1-Me), Ph (1-Ph), (t)Bu (1); R = (i)Pr, R' = (t)Bu (1-(i)Pr(2)); R = R' = (i)Pr (1-(i)Pr(3)); R = R' = (c)Hex (Cy)). Product analyses of sodium amalgam reductions of several dichlorides suggest that [(R(2)R'SiO)(3)Ta](2)(mu-Cl)(2) may be a common intermediate. When the siloxide is large (1-(t)Bu), formation of the Ta(III) species ((t)Bu(3)SiO)(3)Ta (6) occurs via disproportionation. When the siloxide is small, the Ta(IV) intermediate is stable (e.g., [((i)Pr(3)SiO)(3)Ta](2)(mu-Cl)(2) (2)), and when intermediate sized siloxides are used, solvent bond activation via unstable Ta(III) tris-siloxides is proposed to occur. Under hydrogen, reductions of 1-Me and 1-Ph provide Ta(IV) and Ta(V) hydrides [((t)Bu(2)MeSiO)(3)Ta](2)(micro-H)(2) (4-Me) and ((t)Bu(2)PhSiO)(3)TaH(2) (7-Ph), respectively.

Journal Article↗
PubMed2003

Symmetry and geometry considerations of atom transfer: deoxygenation of (silox)3WNO and R3PO (R = Me, Ph, (t)Bu) by (silox)3M (M = V, NbL (L = PMe3, 4-picoline), Ta; silox = (t)Bu3SiO).

Deoxygenations of (silox)(3)WNO (12) and R(3)PO (R = Me, Ph, (t)Bu) by M(silox)(3) (1-M; M = V, NbL (L = PMe(3), 4-picoline), Ta; silox = (t)Bu(3)SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd) = -47 kcal/mol), Ph) but not (t)Bu(3)PO (85 degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaG degrees (rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)(3)V-OPR(3) (3-OPR(3)) with OPR(3). 1-Nb(4-picoline) (S = 0) and 1-NbPMe(3) (S = 1) deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaG degrees (rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (t)Bu(3)PO. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of approximately 17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier approximately 2 kcal/mol) are competent. Once binding occurs, significant mixing with an (1)A(1) excited state derived from population of a sigma-orbital is needed to ensure a low-energy intersystem crossing of the (3)A(2) (reactant) and (1)A(1) (product) states. Correlation of a reactant sigma-orbital with a product sigma-orbital is required, and the greater the degree of bending in the (silox)(3)M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and (t)Bu(3)PO to attain a bent 90 degree angle M-O-E due to sterics explains their slow or negligible deoxygenations. Syntheses of relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe(3) (90 degree angle V-O-P = 157.61(9) degrees), 3-OP(t)Bu(3) ( 90 degree angle V-O-P = 180 degrees ), 1-NbPMe(3), and (silox)(3)ClWO (9).

Journal Article↗
PubMed2003

Mechanism of the alternating copolymerization of epoxides and CO2 using beta-diiminate zinc catalysts: evidence for a bimetallic epoxide enchainment.

A series of zinc beta-diiminate (BDI) complexes and their solid-state structures, solution dynamics, and copolymerization behavior with CO(2) and cyclohexene oxide (CHO) are reported. Stoichiometric reactions of the copolymerization initiation steps show that zinc alkoxide and bis(trimethylsilyl)amido complexes insert CO(2), whereas zinc acetates react with CHO. [(BDI-2)ZnOMe](2) [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] and (BDI-1)ZnO(i)Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] react with CO(2) to form [(BDI-2)Zn(mu-OMe)(mu,eta(2)-O(2)COMe)Zn(BDI-2)] and [(BDI-1)Zn(mu,eta(2)-O(2)CO(i)Pr)](2), respectively. (BDI-2)ZnN(SiMe(3))(2) inserts CO(2) and eliminates trimethylsilyl isocyanate to give [(BDI-2)Zn(mu-OSiMe(3))](2). [(BDI-7)Zn(mu-OAc)](2) [(BDI-7) = 3-cyano-2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] reacts with 1.0 equiv of CHO to yield [(BDI-7)Zn(mu,eta(2)-OAc)(mu,eta(1)-OCyOAc)Zn(BDI-7)]. Under typical polymerization conditions, rate studies on the copolymerization exhibit no dependence in [CO(2)], a first-order dependence in [CHO], and orders in [Zn](tot) ranging from 1.0 to 1.8 for [(BDI)ZnOAc] complexes. The copolymerizations of CHO (1.98 M in toluene) and 300 psi CO(2) at 50 degrees C using [(BDI-1)ZnOAc] and [(BDI-2)ZnOAc] show orders in [Zn](tot) of 1.73 +/- 0.06 and 1.02 +/- 0.03, respectively. We propose that two zinc complexes are involved in the transition state of the epoxide ring-opening event.

Journal Article↗
PubMed2002

Single-site beta-diiminate zinc catalysts for the ring-opening polymerization of beta-butyrolactone and beta-valerolactone to poly(3-hydroxyalkanoates).

Polymerization of beta-butyrolactone (BBL) and beta-valerolactone (BVL) using the zinc alkoxide initiator (BDI-1)ZnO(i)()Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] proceeds very rapidly under mild conditions to produce poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV), respectively. For the monomer-to-initiator ratio 200:1, PHB number-average molecular weights (M(n)) are proportional to conversion throughout the reaction and polydispersity indices (PDIs) are narrow, consistent with a living polymerization. Higher monomer-to-initiator ratios can be used to achieve high molecular weight PHB (M(n) > 100 000). End-group analysis verifies that the polymerization of BBL follows a coordination-insertion mechanism, where complexes of the form (BDI-1)ZnOCH(Me)CH(2)CO(2)R are the active species. Variable temperature NMR experiments show that (BDI-1)ZnO(i)()Pr is monomeric in benzene-d(6) solution. In contrast, (BDI-2)ZnO(i)()Pr [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] is a poor initiator at room temperature because it prefers to form a bis-mu-isopropoxide dimer in solution. According to kinetic studies, propagation by (BDI-1)ZnO(i)()Pr is first order in both monomer as well as (BDI-1)ZnO(i)()Pr concentration. These results lead us to propose a monometallic active species. Several results suggest that elimination side reactions are slowly catalyzed by zinc alkoxides, leading to degradation of the polymer.

4-Butyrolactone↗
PubMed2002

High-activity, single-site catalysts for the alternating copolymerization of CO2 and propylene oxide.

Despite recent advances regarding catalysts for CO2/epoxide copolymerization, the development of high-activity catalysts for alternating polymerization of CO2 and commodity epoxides, such as propylene oxide, remains a challenge. A new class of unsymmetrically substituted beta-diiminate zinc complexes is reported that exhibits unprecedented activity for CO2/propylene oxide copolymerization. The polymers formed are of high molecular weight (Mn approximately 35 kg/mol) and have narrow polydispersities (PDI approximately 1.1), consistent with a living polymerization.

Journal Article↗
PubMed2002

Synthesis of beta-lactones: a highly active and selective catalyst for epoxide carbonylation.

A new highly active and selective catalyst for the synthesis of beta-lactones from CO and epoxides is reported. The catalyst, [(N,N'-bis(3,5-di-tert-butylsalicylidene) phenylenediamino)Al(THF)2][Co(CO)4] ([(salph)Al(THF)2][Co(CO)4]) is easily prepared from the corresponding (salph)AlCl and NaCo(CO)4. At 50 degrees C and 880 psi of CO, the catalyst (1 mol %) carbonylates epoxides such as propylene oxide, 1-butene oxide, epichlorohydrin, and isobutylene oxide to the lactones beta-butyrolactone, beta-valerolactone, gamma-chloro-beta-butyrolactone, and beta-methyl-beta-butyrolactone in high yield. (R)-Propylene oxide was carbonylated to (R)-beta-butyrolactone with retention of stereochemistry.

Catalysis↗
PubMed2002

Interplay of orbital symmetry and nonstatistical dynamics in the thermal rearrangements of bicyclo[n.1.0]polyenes.

CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.

Journal Article↗
PubMed2002

Structure rationalization and topology prediction of two-distinct-component organic crystals: the role of volume fraction and interface topology.

We consider here small-length-scale crystal structures with two clearly different molecular components (e.g., hydrophobic and hydrophilic). Using a perspective developed by studies on large-length-scale block copolymers and liquid crystals, we focus on the crystalline interface between the two components. We examine four types of two-component crystals: aromatic ammonium carboxylates, aromatic oligo(ethylene oxides), cyclohexylammonium carboxylates, and ether-thioether compounds. Of the 111 crystal structures found in the Cambridge Structure Database (CSD), 108 adopt one of the five generic topologies found in diblock copolymers: spheres, columns, perforated layers, layers, and bicontinuous structures. As in diblock copolymers, a key factor controlling the interfacial topology is shown to be the volume ratio of the two components. When the volume fraction of one component is less than 30% of the whole, more than five-sixths of the examined crystal structures are of columnar or spherical type. For volume fractions between 40 and 50% more than three-quarters are of lamellar or bicontinuous type. We use this model to predict the topologies of small-length-scale two-component crystals. We predict the crystal topolgies of six new crystal structures: three are predicted to be columnar, and the other three, lamellar or bicontinuous. The crystal structures of these systems were then determined by single-crystal X-ray methods. Five of the structures form in topologies consistent with the predictions: three in columns and two in layers. The remaining one forms as a perforated layer instead of the predicted columnar structure. Such predictive accuracy is consistent with the statistics of the CSD investigation.

Journal Article↗