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PubMed2003

Synthesis, spectroscopy, and catalytic properties of cationic organozirconium adsorbates on "super acidic" sulfated alumina. "Single-site" heterogeneous catalysts with virtually 100 active sites.

Sulfated alumina (AlS), a highly Brønsted acidic sulfated metal oxide, is prepared by the impregnation of gamma-alumina with 1.6 M H(2)SO(4), followed by calcination at 550 degrees C for 3 h. (13)C CPMAS NMR spectroscopy of the chemisorbed (13)C(alpha)-enriched organozirconium hydrocarbyl Cp'(2)Zr((13)CH(3))(2) (2)/AlS (Cp' = eta(5)-(CH(3))(5)C(5)) reveals that the chemisorption process involves M[bond]C sigma-bond protonolysis at the strong surface Brønsted acid surface sites to yield a "cation-like" highly reactive zirconocenium electrophile, Cp'(2)Zr(13)CH(3)(+). In contrast, chemisorption of 2 on dehydroxylated alumina (DA) yields a similar cation via methide transfer to surface Lewis acid sites, while chemisorption onto dehydroxylated silica yields a mu-oxo Cp'(2)Zr((13)CH(3))-OSi[triple bond] species. Two complementary active site kinetic assays for benzene hydrogenation show that, unlike typical heterogeneous and supported organometallic catalysts, 97 +/- 2% of all Cp'ZrMe(3) (3)/AlS sites are catalytically significant, demonstrating that the species identified by (13)C CPMAS NMR is indeed the active species. 3/AlS mediates benzene hydrogenation with a turnover frequency of 360 h(-1) at 25 degrees C/1.0 atm H(2). Active site assays were also conducted for ethylene polymerization and reveal that 87 +/- 3% of 3/AlS sites are catalytically active, again demonstrating that nearly all zirconium sites are catalytically significant. Relative rates of ethylene homopolymerization mediated by the catalysts prepared via Cp(2)Zr(CH(3))(2) (1), Cp'(2)Zr(CH(3))(2) (2), Cp'Zr(CH(3))(3) (3), Zr(CH(2)TMS)(4) (4), and Zr(CH(2)Ph)(4) (5) (Cp = eta(5)-C(5)H(5)) chemisorption on AlS are 5/AlS > or = 4/AlS > or = 3/AlS > 2/AlS > or = 1/AlS for ethylene homopolymerization at 150 psi C(2)H(4), 60 degrees C. Under identical conditions, the polymerization rate for 3/DA is approximately 1/10th that for 3/AlS.

Journal Article↗
PubMed2003

Sulfonated microporous organic-inorganic hybrids as strong Bronsted acids.

It has been discovered that the use of excess zirconium in reactions with 4,4'-biphenyl and 4,4'-terphenylbis(phosphonic acid) in DMSO or DMSO-ethanol mixtures produces microporous inorganic-organic hybrids. Surface areas of 400 m(2)/g and pore sizes in the range of 10-20 A in diameter are routinely obtained. These materials are readily sulfonated with SO(3) under pressure to yield strong Bronsted acids. The acid strength, measured by (13)C NMR shifts of acetone and cyclopentanone in contact with the sulfonates, indicates an acidity close to that of 100% H(2)SO(4). Condensation and cracking reactions were obtained for both ketones under mild conditions. A working hypothesis is presented to account for the high surface area and microporosity. The combination of high surface areas and pore dimensions that are between those of zeolites and mesoporous silicas commends these materials for applications in separations, ion exchange, and catalysis.

Journal Article↗
PubMed2004

Stereochemistry of cyclopropane formation involving group IV organometallic complexes.

The reaction of (Z)-HDC=CHCH(OCH(3))C(6)H(5) (1) with Cp(2)Zr(D)Cl followed by BF(3).OEt(2) gave phenylcyclopropanes 3a and 3b, both having cis deuterium. This stereochemical outcome requires inversion of configuration at the carbon bound to zirconium and is consistent with a "W-shaped" transition state structure for cyclopropane formation. In a Kulinkovich hydroxycyclopropanation, trans-3-deutero-1-methyl-cis-2-phenyl-1-cyclopropanol (5) was formed stereospecifically from Ti(O-i-Pr)(4), ethyl acetate, EtMgBr, and trans-beta-deuterostyrene. This stereochemistry requires retention of configuration at the carbon bound to titanium and is consistent with frontside attack of the carbon-titanium bond on a carbonyl group coordinated to titanium. In a de Meijere cyclopropylamine synthesis, a 3:1 mixture of N,N-dimethyl-N-(trans-3-deutero-trans-2-phenylcyclopropyl)amine (6a) and N,N-dimethyl-N-(cis-3-deutero-cis-2-phenylcyclopropyl)amine (6b) was formed from Ti(O-i-Pr)(4), DMF, Grignard reagents, and trans-beta-deuterostyrene. This stereochemistry requires inversion of configuration at the carbon bound to titanium and is consistent with a W-shaped transition structure for ring closure.

Journal Article↗
PubMed2003

Chiral porous hybrid solids for practical heterogeneous asymmetric hydrogenation of aromatic ketones.

Chiral porous zirconium phosphonates containing Ru-BINAP-DPEN moieties were synthesized via a molecular building-block approach, and characterized by a variety of techniques including TGA, adsorption isotherms, XRD, SEM, IR, and microanalysis. These hybrid solids were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with remarkably high ee values of up to 99.2%. These solid catalysts can also be easily recycled and reused for eight times without the loss of activity and enantioselectivity. Ready tunability of such a molecular building-block approach will allow the optimization of these hybrid materials and promise to lead to other practically useful heterogeneous asymmetric catalysts.

Journal Article↗
PubMed2003

Stereoselective preparation of dienyl zirconocene complexes via a tandem allylic C-H bond activation-elimination sequence.

Several dienyl zirconocene derivatives were easily prepared, as unique geometrical isomers, from simple non-conjugated unsaturated enol ethers with (1-butene)ZrCp2 complexes. This new methodology is based on a tandem allylic C-H bond activation-elimination sequence and the mechanism has been mapped out by deuterium labeling experiments. The stereochemical outcome of this process was determined by addition of several electrophiles. Moreover, when the organometallic derivative is vinylic as well as allylic such as in 44-47Zr, an unexpected reversal of the stereochemistry has been found during the zirconium to copper transmetalation step.

Journal Article↗
PubMed2004

NOE and PGSE NMR spectroscopic studies of solution structure and aggregation in metallocenium ion-pairs.

The solution structures of the metallocenium homogeneous polymerization catalyst ion-pairs [Cp(2)ZrMe](+)[MeB(C(6)F(5))(3)](-) (1), [(1,2-Me(2)Cp)(2)ZrMe](+)[MeB(C(6)F(5))(3)](-) (2), [(Me(2)SiCp(2))ZrMe](+)[MeB(C(6)F(5))(3)](-) (3), [Me(2)C(Fluorenyl)(Cp)ZrMe](+)[FPBA](-) (FPBA = tris(2,2',2' '-nonafluorobiphenyl)fluoroaluminate) (4), [rac-Et(Indenyl)(2)ZrMe](+)[FPBA](-) (5), [(Me(5)Cp)(2)ThMe](+)[B(C(6)F(5))(4)](-) (6), [(Me(2)SiCp(2))Zr(Me)(THF)](+)[MeB(C(6)F(5))(3)](-) (7), [(Me(2)SiCp(2))Zr(Me)(PPh(3))](+)[MeB(C(6)F(5))(3)](-) (8), [(Me(2)SiCp(2))Zr(Me)(THF)](+)[B(C(6)F(5))(4)](-) (9), [(Me(2)Si(Me(4)Cp)(t-BuN)Zr(Me)(solvent)](+)[B(C(6)F(5))(4)](-) (solvent = benzene, toluene) (10), [(Cp(2)ZrMe)(2)(mu-Me)](+)[MePBB](-) (PBB = tris(2,2',2"-nonafluorobiphenyl)borane) (11), and [(Cp(2)Zr)(2)(mu-CH(2))(mu-Me)](+)[MePBB](-) (12), having the counteranion in the inner (1, 3, 4, 5, and 6) or outer (7, 8, 9, 10, 11, and 12) coordination sphere, have been investigated for the first time in solvents with low relative permittivity such as benzene or toluene by (1)H NOESY and (1)H,(19)F HOESY NMR spectroscopy. It is found that the average interionic solution structures of the inner sphere contact ion-pairs are similar to those in the solid state with the anion B-Me (1, 3) or Al-F (5) vectors oriented toward the free zirconium coordination site. The HOESY spectrum of complex 6 is in agreement with the reported solid-state structure. In contrast, in outer sphere contact ion-pairs 7, 8, 9, and 10, the anion is located far from the Zr-Me(+) moiety and much nearer to the Me(2)Si bridge than in 3. The interionic structure of 8 is concentration-dependent, and for concentrations greater than 2 mM, a loss of structural localization is observed. PGSE NMR measurements as a function of concentration (0.1-5.0 mM) indicate that the tendency to form aggregates of nuclearity higher than simple ion-pairs is dependent on whether the anion is in the inner or outer coordination sphere of the metallocenium cation. Complexes 2, 3, 4, 5, and 6 show no evidence of aggregation up to 5 mM (well above concentrations typically used in catalysis) or at the limit of saturated solutions (complexes 3 and 6), while concentration-dependent behavior is observed for complexes 7, 8, 10, and 11. These outer sphere ion-pairs begin to exhibit significant evidence for ion-quadruples in solutions having concentrations greater than 0.5 mM with the tendency to aggregate being a function of metal ligation and anion structure. Above 2 mM, compound 8 exists as higher aggregates that are probably responsible for the loss of interionic structural specificity.

Journal Article↗
PubMed2005

Mononuclear aluminum hydroxide for the design of well-defined homogeneous catalysts.

An unprecedented aluminum hydroxide LAlMe(OH) (5; L = HC[(CMe)(2,6-iPr2C6H3N)]2) has been prepared by the hydrolysis of LAlMeCl (4). For the preparation of 5, the reagents of KOH, water, and KH, as well as the two-phase ammonia/toluene system, were used. Further reactions of 5 with Cp2ZrMe2 (8) and Cp2ZrHCl in toluene lead to the intermolecular elimination of CH4 and H2 and the formation of mu-O-bridged dinuclear aluminum and zirconium complexes [LAlMe(mu-O)ZrMeCp2] (6) and [LAlMe(mu-O)ZrClCp2] (7), respectively, in high yields. The crystal structure reveals that 5 is a monomer with terminal OH and Me groups. The X-ray structure analysis shows that 6 and 7 contain a bent Al-(mu-O)-Zr core with terminal Al-Me and Zr-Me or Zr-Cl arrangements. The methylalumoxane (MAO)-activated compounds 6 and 7 exhibit high catalytic activity for the polymerization of ethylene. Under comparable polymerization conditions, the MAO/6 and MAO/7 catalyst systems show considerably higher activity and much lower MAO:catalyst ratios than that of MAO/8.

Journal Article↗
PubMed2005

Hydrogenated 1,4-insertion of butadiene in the copolymerization with propylene using an isospecific zirconocene catalyst.

Poly(propylene-ran-1,3-butadiene) that contained pendant vinyl groups derived from 1,2-inserted butadiene units was selectively synthesized by rac-dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dichloride (Ph-Ind) activated with modified methylaluminoxane (MMAO) in the presence of hydrogen. The copolymers obtained without hydrogen had 1,2-inserted and 1,4-inserted butadiene units. The addition of hydrogen to the copolymerization improved the activity by approximately 1000-fold and gave the copolymer only with 1,2-inserted butadiene units, of which the content was equal to the copolymer obtained without hydrogen. The 13C NMR analysis of the copolymers clarified that butadiene also inserted into the copolymer as a tetramethylene unit, of which the content was almost the same as that of 1,4-inserted butadiene units observed in the absence of hydrogen. No signal that could be assigned to cyclic structures or long branched side chains was observed. These results indicate that pi-allyl species of zirconocenes formed by 1,4-butadiene insertion at the growing polymer chain ends transformed to the tetramethylene chain end by hydrogenation and continued successive propylene insertion.

Journal Article↗
PubMed2005

Structural and spectroscopic demonstration of agostic C-C interactions in electron-deficient metallacyclobutanes and related cage complexes: possible implications for olefin polymerizations and metatheses.

The reaction of the half-open titanocene, Ti(C5H5)(c-C8H11)(PMe3) (c-C8H11 = cyclooctadienyl), with two equivalents of PhC2SiMe3 leads to their incorporation and coupling to the dienyl fragment. One alkyne inserts into a C-H bond of the central CH2 group of the c-C8H11 ligand's edge-bridge, while the second undergoes a 5+2 coupling with the dienyl fragment, yielding coordinated sigma-allyl and olefin fragments, as demonstrated by X-ray diffraction. Together with the C5H5 and PMe3 coordinations, this leads to a 14-electron count. While the very electron-deficient titanium center passes up potential pi coordination of the allyl fragment, it instead engages in interactions with one or two C-C bonds, and perhaps a C-H bond, as revealed from the structural and spectroscopic data. Similar interactions have been found in electron-deficient metallacyclobutane complexes of titanium and zirconium, but not in the 18-electron molybdenum and tungsten analogues. These and other observations may have implications relating to metatheses and polymerizations of olefins.

Journal Article↗
PubMed2005

Activation of a Si=Si Bond by eta1-coordination to a transition metal.

Reaction of the disilenide 1 with Cp2ZrCl2 yields 2, the first transition metal complex with an eta1-sigma-bonded disilene moiety. The compound 2 exhibits a strongly red-shifted UV-vis absorption, resulting in an intensely green color. The Si=Si bond in 2 is considerably activated as shown by the rearrangement to a silyl-substituted zirconocene 3. X-ray diffraction studies on 2 reveal that the Si=Si bond is significantly longer than that in the precursor 1. Conversely, the Zr-Si distance is considerably smaller than the corresponding distance in 3, suggesting a certain degree of electron donation toward the 16e- zirconium center.

Journal Article↗
PubMed1992

Design and synthesis of novel FKBP inhibitors.

Small molecule FKBP inhibitors were prepared with inhibitory activity ranging from micromolar to nanomolar. The design of these inhibitors derives from a structural analysis of the substrates for FKBP and cyclophilin. As a consequence of this analysis two key observations were made, namely: (1) amino ketone moieties are suitable as FKBP recognition elements at the P1-P1' site and (2) the P3'-P4' site will accept a trans-olefin as a suitable mimetic of a peptide moiety. The preparation of these non-peptide inhibitors is readily accomplished by a protocol which includes the synthesis of chiral propargylic amines and their subsequent conversion into vinyl zirconium reagents.

Amino Acid Isomerases↗
PubMed2000

Asymmetric aldol reactions using (S,S)-(+)-pseudoephedrine-based amides: stereoselective synthesis of alpha-methyl-beta-hydroxy acids, esters, ketones, and 1,3-Syn and 1,3-anti diols

A very efficient method for performing stereoselective aldol reactions is reported. The reaction of (S, S)-(+)-pseudoephedrine-derived propionamide enolates with several aldehydes yielded exclusively one of the four possible diastereomers in good yields, although transmetalation of the firstly generated lithium enolate with a zirconium(II) salt, prior to the addition of the aldehyde, is necessary in order to achieve high syn selectivity. The so-formed syn-alpha-methyl-beta-hydroxy amides were transformed into other valuable chiral nonracemic synthons such as alpha-methyl-beta-hydroxyacids, esters, and ketones. Finally, a stereocontrolled reduction procedure starting from the so-obtained alpha-methyl-beta-hydroxy ketones has been developed allowing the synthesis of either 1,3-syn- or 1,3-anti-alpha-methyl-1,3-diols in almost enantiopure form by choosing the appropriate reaction conditions.

Journal Article↗
PubMed2001

Highly diastereofacial anti-aldol reaction: practical synthesis of optically active anti-2-alkyl-3-hydroxycarboxylic acid ester units.

A variety of esters derived from commercially available norephedrine were used in diastereoselective anti-aldol reactions. The aldol reaction of designed 2-(N-2-methylbenzyl-N-2,4,6-trimethylbenzyl)amino-1-phenylpropanol esters 4a-d with aldehydes furnished anti-2-alkyl-3-hydroxycarboxylic acid esters in excellent diastereomeric ratios (>98:2) when LDA-Cp2ZrCl2 (0.3 equiv) was used for enolization, followed by transmetalation into the zirconium enolate for aldolization. The novel auxiliary 3 for the anti-aldol reaction does not exhibit the ordinary basicity of tertiary amines; 3 can be extracted from acidic media with organic solvents. Its use is, therefore, very advantageous not only for extraction of the aldol products from the acidic water solutions, but also for recovering the chiral auxiliary 3 after the reductive cleavage. Treatment of aldol or 3-protected aldol products with DIBAL-H or LiAlH4 affords the versatile synthons, 2-alkyl-propane-1,3-diols or those 3-protected diols in >98% ee's together with 3 in nearly quantitative recovery.

Journal Article↗
PubMed2002

A Suzuki coupling based route to 2,2'-bis(2-indenyl)biphenyl derivatives.

Because of the promising performance in olefin polymerization of 2,2'-bis(2-indenyldiyl)biphenyl zirconium dichloride, we developed a new and broadly applicable route to 2,2'-bis(2-indenyl)biphenyl derivatives. Reaction of the known 2,2'-diiodobiphenyl (26) with the new 2-indenyl boronic acid (23) did not result in the desired 2,2'-bis(2-indenyl)biphenyl (10); instead an isomer thereof, (spiro-1,1-(2,2'-biphenyl)-2-(2-indenyl)indane) (27), was obtained. It was found that compound 10 could be made via a palladium-catalyzed reaction of 2,2-biphenyldiboronic acid (31) with 2-bromoindene (21) under standard Suzuki reaction conditions. However, the yield of this reaction was low at low palladium catalyst loadings, due to a competitive hydrolysis reaction of 2,2-biphenyldiboronic acid (31). HTE techniques were used to find an economically viable protocol. Thus, use of the commercially available 1.0 molar solution of (n-Bu)(4)NOH in methanol with cosolvent toluene led to precipitation of the pure product in a fast and clean reaction, using only 0.7 mol % (0.35 mol % per C-C) of the expensive palladium catalyst.

Journal Article↗
PubMed2002

Direct addition of Zr-C bonds of alkylzirconocenes to activated alkenes.

Direct addition of alkylzirconocenes to activated alkenes was found, for the first time. Octyl- and decylzirconocene chloride reacted with benzylidenemalononitrile to give the corresponding addition products after hydrolysis in 86% and 79% yield, respectively. Zirconacyclopentanes showed a similar reactivity toward activated alkenes with a two-electron-withdrawing group. On the other hand, treatment of the reaction mixture of zirconacyclopentanes and ylidenemalononitriles with iodine afforded six-membered cyclic compounds in high yields. The diastereoselectivity of the cyclized compound was remarkably high and the high selectivity originated from the Zr-promoted cyclization. The structures of cyclic compounds 10b and the major diastereoisomer of 10d were determined by X-ray analysis. Zirconacyclopentenes reacted with ylidenemalononitrile with high chemoselectivity in which the sp(3)-carbon attached to zirconium reacted with ylidenemalononitrile.

Journal Article↗
PubMed2005

A controllable synthesis of homoallyl ketones and multiply substituted cyclopentadienes by direct insertion of aroyl cyanides to zirconacyclopentenes.

[reaction: see text] The direct reaction of aroyl cyanides with zirconacyclopentenes was achieved cleanly under controlled reaction conditions. This methodology provided an extremely efficient, one-pot, and high-yield route for the synthesis of homoallyl ketones when the reaction was carried out at -50 degrees C. Trapping of the zirconium intermediate by a variety of electrophiles afforded functionalized homoallyl ketones. Remarkably, the insertion reaction occurred with complete chemoselectivity, that means, the Zr-sp3 carbon bond reacted preferentially, which is different from Cu-mediated elaboration of zirconacycles. Surprisingly, when the reaction was done at room temperature, 1,2,3-trisubstituted cyclopentadiene derivatives were readily formed in high yields. The direct insertion reaction of zirconacyclopentanes with acyl cyanides was also described. When bicyclic zirconacyclopentanes were used, cyclopentanol derivatives were obtained with high stereoselectivity.

Journal Article↗
PubMed2005

Isolation, X-ray structures, and electronic spectra of reactive intermediates in Friedel-Crafts acylations.

[reaction: see text] Reactive intermediates in the Friedel-Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV-vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies.

Journal Article↗
PubMed2005

Zirconocene-mediated and/or catalyzed unprecedented coupling reactions of alkoxymethyl-substituted styrene derivatives.

Reactions of o-(alkoxymethyl)styrene derivatives with a stoichiometric amount of zirconocene-butene complex (zirconocene equivalent, "Cp(2)Zr") brought about an insertion of the zirconocene species into a benzylic carbon-oxygen bond. The oxidative insertion of Cp(2)Zr to the benzylic carbon-oxygen bond is a result of sequential reactions: (i) formation of zirconacyclopropane by the ligand exchange with o-(alkoxymethyl)styrene, (ii) elimination of the alkoxy group through an aromatic conjugate system giving metalated o-quinodimethane species, and (iii) transfer of zirconium metal to the benzylic position. Through use of a catalytic amount of "Cp(2)Zr", however, unprecedented homo-coupling reactions (dimerization) of o-(alkoxymethyl)styrene derivatives occurred to give a tetracyclic compound. On the other hand, reactions of o-(1-alkoxyisopropyl)styrene derivatives gave rise to the analogous tetracyclic compounds regardless of the amount of "Cp(2)Zr" (stoichiometric or catalytic). Heterocoupling product between o-(1-alkoxyisopropyl)styrene and styrene congeners was obtained in high cis stereo- and regioselectivity by treating o-(1-alkoxyisopropyl)styrene derivatives with "Cp(2)Zr" in the presence of an excess amount of styrene derivatives.

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