Structure and bonding in the title complexes are studied using model compounds trans,trans-[(C6H5)(H3P)2Pt(C triple bond C)(n)Pt(PH3)2(C6H5)] (PtCxPt; x = 2n = 4-26) at the B3LYP/LACVP* level of density functional theory. Conformations in which the platinum square planes are parallel are very slightly more stable than those in which they are perpendicular (DeltaE = 0.12 kcal mol(-1) for PtC8Pt). As the carbon-chain length increases, progressively longer C triple bond C triple bonds and shorter triple bond C-C triple bond single bonds are found. Whereas the triple bonds in HCxH become longer (and the single bonds shorter) as the interior of the chain is approached, the PtC triple bond C triple bonds in PtCxPt are longer than the neighboring triple bond. Also, the Pt-C bonds are shorter at longer chain lengths, but not the H-C bonds. Accordingly, natural bond orbital charge distributions show that the platinum atoms become more positively charged, and the carbon chain more negatively charged, as the chain is lengthened. Furthermore, the negative charge is localized at the two terminal C triple bond C atoms, elongating this triple bond. Charge decomposition analyses show no significant d-pi* backbonding. The HOMOs of PtCxPt can be viewed as antibonding combinations of the highest occupied pi orbital of the sp-carbon chain and filled in-plane platinum d orbitals. The platinum character is roughly proportional to the Pt/Cx/Pt composition (e.g., x = 4, 31 %; x = 20, 6 %). The HOMO and LUMO energies monotonically decrease with chain length, the latter somewhat more rapidly so that the HOMO-LUMO gap also decreases. In contrast, the HOMO energies of HCxH increase with chain length; the origin of this dichotomy is analyzed. The electronic spectra of PtC4Pt to PtC10Pt are simulated. These consist of two pi-pi* bands that redshift with increasing chain length and are closely paralleled by real systems. A finite HOMO-LUMO gap is predicted for PtCinfinityPt. The structures of PtCxPt are not strictly linear (average bond angles 179.7 degrees -178.8 degrees ), and the carbon chains give low-frequency fundamental vibrations (x = 4, 146 cm(-1); x = 26, 4 cm(-1)). When the bond angles in PtC12Pt are constrained to 174 degrees in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol(-1). The above conclusions should extrapolate to (C triple bond C)(n) systems with other metal endgroups.

Azole represents an important structural motif commonly present in pharmaceuticals, organic dyes, and natural products. During the past decade, elaboration of azoles and particularly 1,3-azoles has largely focused on C-C functionalization. 1 Traditional transitionmetal-mediated cross-coupling approaches have been particularly effective, allowing for regioselective arylation. 2 Despite this success, the availability of the azole coupling partner as either haloazole or metalated azole remains a scope-limiting factor. Direct arylation has emerged as an alternative method that does not require prefunctionalization, 3 and a wide variety of substrates including oxazoles, 3,4 thiazoles, 3,5,6 imidazoles, 3,5,7 indoles, 8-12 pyrroles, 8 indolizines, 13 imidazo[1,2-a]pyrimidines, 6,14 and imidazo[1,2-b]-[1,2,4]triazines 15 can be arylated using Pd catalysis. Although mechanistic details are sketchy, the direct Pd-catalyzed arylation is currently believed to involve electrophilic palladation of the azole ring. 3,7,10,13 Herein we present a line of mechanistic evidence supporting the involvement of a ring-opening mechanism in the Pd-catalyzed direct arylation of benzoxazoles.Recently, we have reported that electron-poor oxazolo[4,5-b]pyridine can be arylated at room temperature, in contrast to the strenuous reaction conditions typically required by the conventional electron-richer azoles. 16 To investigate the viability of nonelectrophilic pathways in the direct arylation, we performed a set of kinetic experiments on a series of 5-substituted benzoxazoles.Hammett studies offer the opportunity to probe the electronic demand of Pd-catalyzed arylation by introducing substituents at the C-5 position of the benzoxazole ring. The studies were performed under standard conditions (2.5 mol % of Pd(OAc) 2 /PPh 3 , Cs 2 CO 3 , 100 °C, DMF) using 20 equiv of PhI.Recognizing that the concentration of Cs 2 CO 3 is quasi-constant due to its limited solubility in DMF, and assuming constant concentrations of PhI and the catalyst, one can write the pseudofirst-order approximation to the rate law as

Cycloreversion of 4-methoxystyrene from the corresponding Tp'Re(O)(diolato) complex (Tp' = hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 degrees C. Primary ((12)C/(13)C) and secondary ((1)H/(2)H) kinetic isotope effects were determined. The primary KIEs were k(12C)/k(13C) = 1.041 +/- 0.005 at the alpha position and 1.013 +/- 0.006 at the beta position. Secondary KIEs were k(H)/k(D) = 1.076 +/- 0.005 at the alpha position and 1.017 +/- 0.005 at the beta position. Computational modeling (B3LYP/LACVP+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH(2)CHPh) exhibiting dramatically different C-O bond lengths. A Hammett study on cycloreversions of substituted styrenes from a series of Tp'Re(O)(diolato) showed dichotomous behavior for electron donors and electron-withdrawing groups as substituents: rho = -0.65 for electron donors, but rho = +1.13 for electron-withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study reflects a likelihood that multiple reaction mechanisms are involved.

We present a novel solid-phase based 45 Ti radiolabeling methodology and the implementation of 45 Ti-PET in titanium-based antineoplastics using the showcase compound [ 45 Ti](salan)Ti(dipic). This development is intended to allow elucidation of the biodistribution and pharmacokinetics of promising new Ti-based therapeutics.

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