Superior effect of a pi-acceptor ligand (phosphine-electron-deficient olefin ligand) in the Negishi coupling involving alkylzinc reagents

Palladium-catalyzed Negishi cross-coupling involving primary and secondary alkyls, even in the presence of beta-H, can be achieved at ambient temperature using chelating ligands containing a phosphine and an electron-deficient olefin. The superior effects of the ligands were shown not only in the desired cross-coupling product yields but also in the fast reaction at mild conditions. This reaction has been also scaled up to 50 g in 0.005 mol % catalyst (20,000 TONs) at room temperature.     

"Microfluidic drifting"--implementing three-dimensional hydrodynamic focusing with a single-layer planar microfluidic device

We introduce a novel fluid manipulation technique named "microfluidic drifting" to enable three-dimensional (3D) hydrodynamic focusing with a simple single-layer planar microfluidic device.     

n + n]-Heterometallomacrocyclic complexes (n > or = 2) prepared from platinum(II)-centred ditopic 2,2':6',2'-terpyridine ligands: dimensional cataloguing by pulsed-field gradient spin-echo NMR spectroscopy

The reaction of 4'-(2-propyn-1-oxy)-2,2':6',2'-terpyridine (HC[triple bond]CCH2Oterpy) with trans-[PtI2(PR3)2] (R = Et, (n)Bu, Ph) results in the regioselective formation of the metalloditopic ligands trans-[Pt(C[triple bond]CCH2Oterpy)2(PR3)2], crystallographic data for which are presented. Each ditopic ligand reacts with FeCl(2).4H(2)O to give heterometallomacrocycles, the smallest of which is a [2 + 2] macrocycle, confirmed structurally for R = Et. The NMR spectroscopic data confirm the formation of symmetrical species, i.e. macrocyclic and not polymeric species. The distribution of products has been investigated using pulsed-field gradient spin-echo (PGSE) diffusion NMR spectroscopy, and indicates that the kinetic products from the reactions of 1, 2 or 3(L) with iron(II) are [Fe(n)L(n)](2n+) with n = 2, 3 or 4. For L = 1 and 2, these mixtures of products convert in solution to the thermodynamically favoured [Fe(2)L(2)](4+).     

Synthesis of beta-C-galacto-pyranosides with fluorine on the pseudoanomeric substituent

[reaction: see text] beta-C-galacto-Pyranosides with CHF and CF2 substitutes for the glycosidic oxygen were prepared through a four-step sequence starting from a central 1-thio-1,2-O-isopropylidene acetal alcohol and different alpha-fluoro- and alpha,alpha-difluoro acids. The key step in the synthesis is the oxocarbenium cyclization of an intermediate enol ether-thioacetal to a C1-substituted glycal.     

Rapid three-step cleavage of RNA and DNA model systems promoted by a dinuclear Cu(II) complex in methanol. energetic origins of the catalytic efficacy

A dinuclear Cu(II) complex of 1,3-bis-N(1)-(1,5,9-triazacyclododecyl)propane with an associated methoxide (2-Cu(II)(2):(-OCH(3))) was prepared, and its kinetics of reaction with an RNA model (2-hydroxypropyl-p-nitrophenyl phosphate (1, HPNPP)) and two DNA models (methyl p-nitrophenyl phosphate (3) and iso-butyl p-chlorophenyl phosphate (4)) were studied in methanol solution at (s)(s)pH 7.2 +/- 0.2. X-ray diffraction structures of 2-Cu(II)(2):(-OH)(H(2)O)(CF(3)SO(3)-)(3):0.5CH(3)CH(2)OCH(2)CH(3) and 2-Cu(II)(2):(-OH)((C(6)H(5)CH(2)O)(2)PO(2)-)(CF(3)SO(3)-)2 show the mode of coordination of the bridging -OH and H(2)O between the two Cu(II) ions in the first complex and bridging -OH and phosphate groups in the second. The kinetic studies with 1 and 3 reveal some common preliminary steps prior to the chemical one of the catalyzed formation of p-nitrophenol. With 3, and also with the far less reactive substrate (4), two relatively fast events are cleanly observed via stopped-flow kinetics. The first of these is interpreted as a binding step which is linearly dependent on [catalyst] while the second is a unimolecular step independent of [catalyst] proposed to be a rearrangement that forms a doubly Cu(II)-coordinated phosphate. The catalysis of the cleavage of 1 and 3 is very strong, the first-order rate constants for formation of p-nitrophenol from the complex being approximately 0.7 s(-1) and 2.4 x 10(-3) s(-1), respectively. With substrate 3, 2-Cu(II)(2):(-OCH(3)) exhibits Michaelis-Mentin kinetics with a k(cat)/K(M) value of 30 M(-1) s(-1) which is 3.8 x 10(7)-fold greater than the methoxide promoted reaction of 3 (7.9 x 10(-7) M(-1) s(-1)). A free energy calculation indicates that the binding of 2-Cu(II)(2):(-OCH(3)) to the transition states for 1 and 3 cleavage stabilizes them by -21 and -24 kcal/mol, respectively, relative to that of the methoxide promoted reactions. The results are compared with a literature example where the cleavage of 1 in water is promoted by a dinuclear Zn(II) catalyst, and the energetic origins of the exalted catalysis of the 2-Cu(II)(2) and 2-Zn(II)(2) methanol systems are discussed.     

The dinuclear Zn(II) complex catalyzed cyclization of a series of 2-hydroxypropyl aryl phosphate RNA models: progressive change in mechanism from rate-limiting P-O bond cleavage to substrate binding

A methoxide-bridged dinuclear Zn(II) complex of 1,3-[N,N'-bis(1,5,9-triazacyclododecane)]propane (1-Zn(II)2:(-OCH3)) was prepared, and its catalysis of the cyclization of a series of 2-hydroxypropyl aryl phosphates (4a-g) was investigated in methanol at pH 9.8, T = 25degreesC by stopped-flow spectrophotometry. An X-ray diffraction structure of the hydroxide analogue of 1-Zn(II)2:(-OCH3), namely 1-Zn(II)2:(-OH), reveals that each of the Zn(II) ions is coordinated by the three N's of the triazacyclododecane units and a bridging hydroxide. The cyclizations of substrates 4a-g reveal a progressive change in the observed kinetics from Michaelis-Menten saturation kinetics for the poorer substrates (4-OCH3 (4g); 4-H (4f); 3-OCH3 (4e); 4-Cl (4d); 3-NO2, (4c)) to second-order kinetics (linear in 1-Zn(II)2:(-OCH3)) for the better substrates (4-NO2,3-CH3 (4b); 4-NO2, (4a)). The data are analyzed in terms of a multistep process whereby a first formed complex rearranges to a reactive complex with a doubly activated phosphate coordinated to both metal ions. The kinetic behavior of the series is analyzed in terms of change in rate-limiting step for the catalyzed reaction whereby the rate-limiting step for the poorer substrates (4g-c) is the chemical step of cyclization of the substrate, while for the better substrates (4b,a) the rate-limiting step is binding. The catalysis of the cyclization of these substrates is extremely efficient. The kcat/KM values for the catalyzed reactions range from 2.75 x 10(5) to 2.3 x 10(4) M-1 s-1, providing an acceleration of 1 x 10(8) to 4 x 10(9) relative to the methoxide reaction (k2OCH3, which ranges from 2.6 x 10(-3) to 5.9 x 10(-6) M-1 s-1 for 4a-g). At a pH of 9.8 where the catalyst is maximally active, the acceleration for the substrates ranges from (1 - 4) x 10(12) relative to the background reaction at the same pH. Detailed energetics calculations show that the transition state for the catalyzed reaction comprising 1-Zn(II)2, methoxide, and 4 is stabilized by about -21 to -23 kcal/mol relative to the transition state for the methoxide reaction. The pronounced catalytic activity is attributed to a synergism between a positively charged catalyst that has high affinity for the substrate and for the transition state for cyclization, and a medium effect involving a reduced polarity/dielectric constant that complements a reaction where an oppositely charged reactant and catalyst experience charge dispersal in the transition state.     

Examination of the role of intestinal fatty acid-binding protein in drug absorption using a parallel artificial membrane permeability assay

Transcellular diffusion across the absorptive epithelial cells (enterocytes) of the small intestine is the main route of absorption for most orally administered drugs. The process by which lipophilic compounds transverse the aqueous environment of the cytoplasm, however, remains poorly defined. In the present study, we have identified a structurally diverse group of lipophilic drugs that display low micromolar binding affinities for a cytosolic lipid-binding protein - intestinal fatty acid-binding protein (I-FABP). Binding to I-FABP significantly enhanced the transport of lipophilic drug molecules across a model membrane, and the degree of transport enhancement was related to both drug lipophilicity and I-FABP binding affinity. These data suggest that intracellular lipid-binding proteins such as I-FABP may enhance the membrane transport of lipophilic xenobiotics and facilitate drug access to the enterocyte cytoplasm and cytoplasmic organelles.