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PubMed1997

Variable-Valent Re&tbd1;NAr Species. A Family of Re(VI)NAr Amide Complexes and Their Re(V)NAr Imine Precursors Related by Oxygen Atom Transfer.

Imide complexes of type Re(V)Cl(3)(X-SB)(NC(6)H(4)Y(p)), with X, Y= H, Me, OMe, Cl have been synthesized where X-SB is the Schiff base of pyridine-2-carboxaldehyde (the corresponding complex is 4), 2-acetylpyridine (5), 2-benzoylpyridine (6), and anilines, p-XC(6)H(4)NH(2). Treatment of 4 or 5 (but not 6) with aqueous nitric acid in acetonitrile afforded Re(VI)Cl(3)(X-PA)(NC(6)H(4)Y(p)), 7, via oxygen atom transfer (X-PA = monoanionic picolinamide). In the structures of 5(X=Cl,Y=Cl), 6(OMe,OMe), and 7(Me,Me), the chlorine atoms are meridionally disposed in a ReCl(3)N(3) coordination sphere. The trans influence of the imide nitrogen considerably lengthens the Re-N(pyridine) bond. The ReNC(6)H(4)Y(p) group has the triple-bonded linear moiety, Re&tbd1;N-C. The amide group in 7(Me,Me) is planar. In 6(OMe,OMe) the two aryl rings on the imine function block water attack and hence amide formation. The rhenium(VI)-rhenium(V) E(1/)(2) values for 4-6 (0.7-1.0 V vs SCE) are much higher than that for 7 (E(1/2) approximately 0.15 V), which displays the rhenium(VII)-rhenium(VI) couple near 1.6 V. Six EPR hyperfine lines are observed for solutions of 7 at room temperature (g(iso) approximately 1.91; A(av) approximately 490 G). Crystal data for the complexes are as follows: 5(Cl,Cl), empirical formula C(19)H(15)Cl(5)N(3)Re, crystal system monoclinic, space group P2(1)/c, a = 13.360(6) Å, b = 12.110(3) Å, c = 14.954(9) Å, beta = 111.41(4) degrees, V = 2252.4(1.7) Å(3), Z = 4; 6(OMe,OMe), empirical formula C(26)H(23)Cl(3)N(3)O(2)Re, crystal system orthorhombic, space group Pbca, a= 12.079(5) Å, b = 17.083(9) Å, c = 26.049(9) Å, V = 5375.4(4.0) Å(3), Z = 8; 7(Me,Me), empirical formula C(20)H(18)Cl(3)N(3)ORe, crystal system monoclinic, space group P2(1)/c, a = 7.071(2) Å, b = 17.541(6) Å, c = 16.857(8) Å, beta = 100.59(3) degrees, V = 2055.3(1.3) Å(3), Z = 4.

Journal Article↗
PubMed1999

Chemistry of Diazaphospholephosphines. 2. Exocyclic Phosphine-Sulfido, -Selenido, and -Imido Derivatives of a Diazaphospholephosphine System. Crystal and Molecular Structures of Two Diazaphospholephosphine Imines: 4-(Difluoro((p-cyanotetrafluorophenyl)imino)phosphorano)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole and 4-(Bis(dimethylamino)(((pentamethylcyclopentadienyl)dichlorotitanio)imino)phosphorano)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole.

The substituted-exo-phosphine (X = F, NMe(2), OCH(2)CF(3)) diazaphospholephosphines are exclusively oxidized at this center with either chalcogens (S, Se) or azides to phosphoranodiazaphospholes. Oxidation imparts a dramatic upfield shift of the phosphorus NMR signals and an increase in the (1)J(PC) coupling constants within the ring. (Difluorophosphino)diazaphosphole was also oxidized with selected amines using diethyl azodicarboxylate (DAD) as the coupling agent. Bulky amines (e.g., 2,4,6-tri-tert-butylaniline (mes)) gave the monomeric iminophosphorane whereas less bulky amines (p-toluidine) formed mostly the cyclic diazadiphosphetidine. The crystal and molecular structure of 4-(difluoro((p-cyanotetrafluorophenyl)imino)phosphorano)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole was determined: triclinic, P&onemacr; (No. 2), a = 7.2744(15) Å, b = 10.087(4) Å, c = 10.566(2) Å, alpha = 66.62(2) degrees, beta = 77.60(2) degrees, gamma = 78.14(3) degrees, V = 688.8(4) Å(3), Z = 2. Final indices are R(1) = 0.0368 and wR(2) = 0.0968, and for all data, R(1) = 0.0478, wR(2) = 0.1033, and GOF = 1.067. The structure revealed two planar ring systems consisting of the diazaphosphole and the p-tetrafluorophenyl (tfbn) ring with an angle of 26.3 degrees between the rings. The angle about the phosphine imine nitrogen (i.e., P=N-tfbn) is relatively open (141.2(2) degrees ), and the P=N bond length is relatively short (1.514(2) Å). (((Trimethylsilyl)imino)(bis(dimethylamino))phosphorano)diazaphosphole gave, with CpTiCl(3), [(eta(5)-C(5)Me(5))TiCl(2)(N=P(NMe(2))(2)(2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphol-4-yl))], which was also characterized structurally: monoclinic, P2(1) (No. 4), a = 11.9477(11) Å, b = 8.4757(6) Å, c = 12.7567(11) Å, beta = 108.824(8) degrees, V = 1222.7(2) Å(3), Z = 2. Final indices are R(1) = 0.0630 and wR(2) = 0.1593, and for all data, R(1) = 0.0768, wR(2) = 0.1973, and GOF = 1.081. The Ti-N-P angle of 161.0(5) degrees was large, and the P=N distance (1.592(6) Å) and the Ti-N distance (1.781(6) Å) were both slightly shorter than those in similar titanium complexes. The P-N single bond distances between the exo-phosphorus atom and the attached dimethylamino groups were also short (1.649 Å (average)). These short values suggest delocalized bonding character throughout the metal-ligand framework, possibly a consequence of additional conjugation through the diazaphosphole ring.

Journal Article↗
PubMed2002

Consequences of correlated solvation on the structures and reactivities of RLi-diamine complexes: 1,2-addition and alpha-lithiation reactions of imines by TMEDA-solvated n-butyllithium and phenyllithium.

6Li and (13)C NMR spectroscopic studies were carried out on [(6)Li]n-BuLi and [(6)Li]PhLi (RLi) in toluene-d(8) containing the following diamines: N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetraethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipiperidinoethane, N,N,N',N'-tetramethylpropanediamine, trans-(R,R)-N,N,N',N'-tetramethylcyclohexanediamine, and (-)-sparteine. Dimers of general structure (RLi)(2)S(2) (S = chelating diamine) are formed in each case. Treatment of RLi with two different diamines (S and S') affords homosolvates (RLi)(2)S(2) and (RLi)(2)S'(2) along with a heterosolvate (RLi)(2)SS'. Relative binding constants and associated free energies for the sequential solvent substitutions are obtained by competing pairs of diamines. The high relative stabilities of certain heterosolvates indicate that solvent binding to the RLi dimer can be highly correlated. Rate studies of both the 1,2-addition of RLi/TMEDA to the N-isopropylimine of cyclohexane carboxaldehyde and the RLi/TMEDA-mediated alpha-lithiation of the N-isopropylimine of cyclohexanone reveal monomer-based transition structures, [(RLi)(TMEDA)(imine)], in all cases. The complex relationships of solvent binding constants and relative reactivities toward 1,2-additions and alpha-lithiations are discussed.

Diamines↗
PubMed2001

An allyltitanium derived from acrolein 1,2-dicyclohexylethylene acetal and (eta(2)-propene)Ti(O-i-Pr)(2) as a chiral propionaldehyde homoenolate equivalent that reacts with imines with excellent stereoselectivity. An efficient and practical access to optically active gamma-amino carbonyl compounds.

A chiral allyltitanium compound 2, prepared in situ by the reaction of optically active acrolein 1,2-dicyclohexylethylene acetal (3) with (eta(2)-propene)Ti(O-i-Pr)(2) (1), reacts with a variety of acyclic and cyclic imines 4 in a regiospecific way to afford alpha-addition products 5 as a mixture of the E- and Z-isomers in good combined yield, where the former is predominant in a ratio of 92:8 to >95:5. The mixture of (E)- and (Z)-5 and pure (E)-5 which could be isolated in several cases were respectively converted to the corresponding beta-amino ester 6 to confirm the absolute configuration and enantiomeric purity. The ee of the newly formed asymmetric center of 5 is more than 78% for the mixture of (E)- and (Z)-5 and more than 96% for pure (E)-5. By taking advantage of the versatility of the vinyl ether moiety in 5, optically active gamma-amino aldehydes 8, gamma-amino aldehyde acetals 7 and 10, gamma-amino acids 9, beta-amino esters 6, and pyrrolidinoisoquinolines 12 were readily prepared. In the reaction of 2 with optically active alpha-silyloxyimine 4n, remarkable double stereodifferentiation was observed; thus, the reaction of 2 derived from (S,S)- or (R,R)-3 provided syn- and anti-5n in a ratio of 55:45 or 0:100, respectively. Meanwhile, the stereochemistry of the product in the reaction of 2 with beta-silyloxyimine 4o was controlled mainly by 2. Thus, the reaction of beta-silyloxyimine 14 with 2 derived from 1 and (R,R)-3 afforded gamma-silyloxyimine 15 with 92% diastereoselectivity, from which 4-amino-6-hydroxypentadecanal dimethyl acetal (13), a key intermediate for the synthesis of batzelladine D, was synthesized.

Journal Article↗
PubMed2001

Cu(0) nanoclusters derived from poly(propylene imine) dendrimer complexes of Cu(II).

A family of diaminobutane core, poly(propylene imine) dendrimers coordinated to Cu(II), DAB-Am(n)-Cu(II)x (n = 4, 8, 16, 32, 64, x = n/2), was studied by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies. The geometry of the dipropylene triamine (dpt)-Cu(II) end-group complexes for all dendrimer generations is reported for the first time and is found to be that of a square-based pyramid with each Cu ion bound to three nitrogen atoms (Cu-N distance approximately 2.03 A) of the dpt end group of the dendrimer. An oxygen atom residing 1.96 A from the Cu ion also occupies the equatorial plane, and the pyramid is completed by an axial oxygen at approximately 2.65 A. In addition, we report for the first time that reduction of the Cu(II)-dendrimer complexes with NaBH4 yields DAB-Am(n)-Cu(0)(cluster) species. Transmission electron microscopy (TEM) studies of the reduced species demonstrate that there is a systematic decrease in the size of the generated Cu clusters with increasing dendrimer generation. Additionally, it was found that the size of the nanoclusters is a function of the n/x ratio of the DAB-Am(n)-Cu(II)x precursor, with highly monodisperse, extremely small nanoclusters (r(cluster) = 8.0 +/- 1.6 A) obtained with n = 64 and x = 16. EXAFS and XANES measurements on the reduced DAB-Am(n)-Cu(0)(cluster) corroborate the TEM data, and provide additional information on the possible encapsulation of the Cu nanoclusters by the dendrimers.

Aziridines↗
PubMed2002

Ruthenium-catalyzed intramolecular oxidative amination of aminoalkenes enables rapid synthesis of cyclic imines.

[RuCl2(CO)3]2/dppp is shown to be a highly effective catalyst system for the first intramolecular oxidative amination of a variety of aminoalkenes when it is used concomitantly with K2CO3 and allyl acetate in N-methylpiperidine, to give the corresponding cyclic imines and indoles in excellent yields. For example, the reaction of 2,2-diphenyl-4-pentenyl-1-amine performed in the presence of 2 mol % of [RuCl2(CO)3]2, 4 mol % of dppp, K2CO3, and allyl acetate in N-methylpiperidine at 140 degrees C for 8 h gives 4,4-diphenyl-2-methyl-1-pyrroline in quantitative (>99%) yield.

Journal Article↗
PubMed2002

Detecting the adsorption of dye molecules in homogeneous poly(propylene imine) dendrimer monolayers by surface plasmon resonance sensor.

In this work, we studied guest-host interactions between various dye molecules and the fifth-generation poly(propylene imine) (PPI-5) dendrimers in aqueous solutions using a surface plasmon resonance (SPR) sensor. The effect of the properties of guest and host molecules (e.g., charge and shape) and media (e.g., pH and ion strength) on affinity between guest and host molecules was investigated. Based on an immobilized homogeneous monolayer of PPI-5 dendrimer tethered to carboxyl-terminal self-assembled monolayers, the adsorption behavior of a group of dye molecules in PPI-5 was obtained. Results show that the strong affinity of PPI-5 to Rose Bengal and erythrosine B is attributed to the good match in charge and shape between the cavities of the dendrimer and the dye molecules. Maximum adsorption around a pH value of 7 was observed. The kinetic behaviors of different dye molecules in dendrimers were also studied. A fundamental understanding of guest-host interactions in dendrimers will guide the design of new-generation sensors and drug delivery carriers.

Journal Article↗
PubMed2002

Fluorine- and trimethylsilyl-containing phenoxy--imine Ti complex for highly syndiotactic living polypropylenes with extremely high melting temperatures.

A fluorine- and trimethylsilyl-containing phenoxy-imine titanium complex was synthesized and the structure was determined by an X-ray analysis. The complex on activation with MAO initiates highly controlled syndiospecific living propylene polymerization to form extremely high Tm syndiotactic polypropylenes (Mw/Mn = 1.05-1.08, Tm = 156-152 degrees C) at 0 or 25 degrees C. Moreover, at 50 degrees C, the complex afforded monodisperse syndiotactic polypropylene with very high Tm's of 149, 150 degrees C. In contrast, complexes having a t-Bu group instead of the silyl group provided lower tacticity polymers with much lower Tm's. In addition, we revealed the substituent effect that plays a key role for the highly controlled syndiospecific polymerization displayed by the catalyst.

Journal Article↗
PubMed2004

Metal coordination and imine-amine hydrogen bonding as the source of strongly shifted adenine pKa values.

The X-ray crystal structure of a Pt(II) complex of composition trans-[(NH(3))(2)Pt(1,9-DimeA) (1,9-DimeAH)](ClO(4))(3) (2) with 1,9-DimeA = 1,9-dimethyladenine and 1,9-DimeAH(+) = 1,9-dimethyladeninium) is presented. Complex 2 forms upon deprotonation of one of the exocyclic amino groups of the adeninium ligands in trans-[(NH(3))(2)Pt(1,9-DimeAH)(2)](ClO(4))(4) (1), where the two nucleobases are in a head-tail arrangement. The low pK(a1) of 1 (4.1 +/- 0.2) is a consequence of a combination of the effects of metal coordination to N7 of the purine base and efficient stabilization of the deprotonated species. This feature is supported by the results of the structure determination of 2, which displays a head-head orientation of the two bases and intramolecular H-bonding between the imine group of 1,9-DimeA and the amino group of 1,9-DimeAH. In the fully deprotonated species trans-[(NH(3))(2)Pt(1,9-DimeA)(2)](ClO(4))(2) (3), the two nucleobases are again in a head-tail arrangement. The findings are of relevance with regard to the concept of "shifted pK(a) values" of nucleobases. This concept is applied to rationalize acid-base catalysis reactions involving nucleobases of DNA and RNA which occur in the near-physiological pH range.

Adenine↗
PubMed2004

Donor-stabilized cations and imine transfer from N-silylphosphoranimines.

A series of donor-stabilized N-silylphosphoranimine salts [DMAP.PCl2=NSiMe3]+X- (DMAP = 4-(dimethylamino)pyridine) were prepared by the reaction of Cl3P=NSiMe3 with DMAP in the presence of silver salts AgX (X = OSO2CF3, BF4, and SbF6). Repeating the reaction in the absence of AgX gave the chloride salt [DMAP.PCl2=NSiMe3]Cl which has been shown to be in equilibrium with free DMAP and Cl3P=NSiMe3. Attempts to stabilize a N-silylphosphoranimine cation with phosphine donors led to unexpected imine transfer chemistry. For example, Cl3P=NSiMe3 reacts with phosphines, R3P (R = nBu and Ph), to produce the metathesis products PCl3 and R3P=NSiMe3 which subsequently react together to afford the N-phosphinophosphoranimines R3P=N-PCl2 and ClSiMe3 as a byproduct.

Journal Article↗
PubMed2005

Discotic liquid crystalline poly(propylene imine) dendrimers based on triphenylene.

The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting of the commercially available poly(propylene imine) (PPI) dendrimers (G = 1-5), PPI-(NH(2))(n)() (n = 4, 8, 16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with the second to fifth generations giving a hexagonal columnar mesophase (Col(h)) and the first generation a rectangular columnar mesophase (Col(r)). The X-ray study reveals that these mesophases show a highly ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the terminal amino groups of the dendrimers were functionalized with a discotic unit.

Journal Article↗
PubMed2004

Living copolymerization of ethylene with norbornene catalyzed by bis(pyrrolide-imine) titanium complexes with MAO.

Bis(pyrrolide-imine) Ti complexes in conjunction with methylalumoxane (MAO) were found to work as efficient catalysts for the copolymerization of ethylene and norbornene to afford unique copolymers via an addition-type polymerization mechanism. The catalysts exhibited very high norbornene incorporation, superior to that obtained with Me(2)Si(Me(4)Cp)(N-tert-Bu)TiCl(2) (CGC). The sterically open and highly electrophilic nature of the catalysts is probably responsible for the excellent norbornene incorporation. The catalysts displayed a marked tendency to produce alternating copolymers, which have stereoirregular structures despite the C(2) symmetric nature of the catalysts. The norbornene/ethylene molar ratio in the polymerization medium had a profound influence on the molecular weight distribution of the resulting copolymer. At norbornene/ethylene ratios larger than ca. 1, the catalysts mediated room-temperature living copolymerization of ethylene and norbornene to form high molecular weight monodisperse copolymers (M(n) > 500,000, M(w)/M(n) < 1.20). (13)C NMR spectroscopic analysis of a copolymer, produced under conditions that gave low molecular weight, demonstrated that the copolymerization is initiated by norbornene insertion and that the catalyst mostly exists as a norbornene-last-inserted species under living conditions. Polymerization behavior coupled with DFT calculations suggested that the highly controlled living polymerization stems from the fact that the catalysts possess high affinity and high incorporation ability for norbornene as well as the characteristics of a living ethylene polymerization though under limited conditions (M(n) 225,000, M(w)/M(n) 1.15, 10-s polymerization, 25 degrees C). With the catalyst, unique block copolymers [i.e., poly(ethylene-co-norbornene)(1)-b-poly(ethylene-co-norbornene)(2), PE-b-poly(ethylene-co-norbornene)] were successfully synthesized from ethylene and norbornene. Transmission electron microscopy (TEM) indicated that the PE-b-poly(ethylene-co-norbornene) possesses high potential as a new material consisting of crystalline and amorphous segments which are chemically linked.

Journal Article↗
PubMed2004

Pincer complex-catalyzed allylation of aldehyde and imine substrates via nucleophilic eta1-allyl palladium intermediates.

Electrophilic allylic substitution of allylstannanes with aldehyde and imine substrates could be achieved by employment of palladium pincer complex catalysts. It was found that the catalytic activity of the pincer complexes is highly dependent on the ligand effects. The best results were obtained by employment of PCP pincer complexes with weakly coordinating counterions. In contrast to previous applications for electrophilic allylic substitutions via bisallylpalladium complexes, the presented reactions involve monoallylpalladium intermediates. Thus, employment of pincer complex catalysts extends the synthetic scope of the palladium-catalyzed allylic substitution reactions. Moreover, use of these catalysts eliminates the side reactions occurring in transformations via bisallylpalladium intermediates. The key intermediate of the electrophilic substitution reaction was observed by (1)H NMR spectroscopy. This intermediate was characterized as an eta(1)-allyl-coordinated pincer complex. Density functional theory (DFT) modeling shows that the electrophilic attack can be accomplished with a low activation barrier at the gamma-position of the eta(1)-allyl moiety. According to the DFT calculations, this reaction takes place via a six-membered cyclic transition-state (TS) structure, in which the tridentate coordination state of the pincer ligand is preserved. The stereoselectivity of the reaction could be explained on the basis of the six-membered cyclic TS model.

Journal Article↗
PubMed2005

An entry to a chiral dihydropyrazole scaffold: enantioselective [3 + 2] cycloaddition of nitrile imines.

We have developed a versatile strategy to access dihydropyrazoles in highly enantioenriched form. Dipolar cycloaddition of electron-deficient acceptors and in situ-generated nitrile imines proceeds with high regio- and enantioselectivity using 10 mol % chiral Lewis acid catalyst. A variety of dihydropyrazoles that incorporate functionality for further manipulation have been prepared.

Journal Article↗
PubMed2005

The Michael addition-elimination of ylides to alpha,beta-unsaturated imines. Highly stereoselective synthesis of vinylcyclopropanecarbaldehydes and vinylcyclopropylaziridines.

The Michael addition-elimination of ylide to alpha,beta-unsaturated imines leads to a highly diastereoselective and enantioselective synthesis of vinylcyclopropanecarbaldehydes in good to high yields for the first time. A sequential cyclopropanation-aziridination protocol for the preparation of cyclopropylaziridines is also developed with good diastereoselectivity in good to high yields.

Journal Article↗
PubMed2005

Catalytic asymmetric synthesis of piperidine derivatives through the [4 + 2] annulation of imines with allenes.

Although tertiary phosphines have emerged as remarkably versatile nucleophilic catalysts, there has been only very limited progress in achieving asymmetric catalysis with chiral phosphines. In this report, the first highly enantioselective variant of the Kwon annulation of imines with allenes is described. Thus, C2-symmetric chiral phosphepine 1 serves as an effective catalyst for this powerful process, furnishing an array of functionalized piperidine derivatives with very good stereoselectivity.

Journal Article↗