C-H Activation and C-C coupling of arenes by cationic Pt(II) complexes

The synthesis and characterization of cationic platinum complexes of the type [(R(2)PC(2)H(4)PR(2))PtMe(OEt(2))]BAr(F) (R = Cy, Et) are reported. These electrophilic platinum cations are found to react quantitatively with arenes (benzene, toluene) at room temperature by undergoing intermolecular C-H activation with concomitant C-C coupling to generate complexes of the type [[Pt(R(2)PC(2)H(4)PR(2))](2)(mu-eta(3):eta(3)-biaryl)][BAr(F)](2). The dianionic biaryl ligands in these compounds exhibit a rare mu-eta(3):eta(3)-bis-allyl bonding mode and can be removed from the complex with stoichiometric oxidants to generate the free biaryl and [(R(2)PC(2)H(4)PR(2))Pt(mu-X)](2)[BAr(F)](2) (R = Cy, Et; X = Cl, I). The cationic platinum complexes [(R(2)PC(2)H(4)PR(2))PtMe(OEt(2))]BAr(F) (R = Cy, Et) are also quite reactive with water, forming the bridging hydroxide complexes [(R(2)PC(2)H(4)PR(2))Pt(mu-OH)](2)[BAr(F)](2) (R = Cy, Et). A possible mechanism is proposed for the C-C coupling reaction based upon the structures of these bridging biphenyl complexes, which provides a new perspective for the related palladium-catalyzed oxidative coupling of arenes to form biaryls.     

Imposing curvature on a polyarene by intramolecular palladium-catalyzed arylation reactions: A simple synthesis of Dibenzo

Palladium catalysis has been found to offer an effective solution-phase alternative to gas-phase flash vacuum pyrolysis as a method for converting the planar ring system of 7, 10-di(2-bromophenyl)fluoranthene (4) to that of the C(28)H(14) bowl-shaped fullerene fragment dibenzo[a,g]corannulene (5).     

The effect of thermal processing on PVC—VI. The role of hydrogen chloride

The reaction between cumene hydroperoxide (CHP) and anhydrous hydrogen chloride under various conditions of temperature and concentration has been studied in an inert solvent (chlorobenzene) and in an oxidisable medium (cumene). The kinetics were first order with respect to each reactant; the energy of activation of the overall second-order reaction was 53.4 kJ mol^?1 K^?1. Competing ionic and free radical mechanisms were found to operate, the latter predominating at relatively high HCl concentrations, leading to rapid pro-oxidation in cumene. The effect of typical organo-tin PVC stabilizers on the pro-oxidant process were examined. It was found that dibutyltin maleate neutralized the HCl thus eliminating the pro-oxidant effect when used in stoichiometric proportions, but had little other effect. Dioctyltin bis(isol-octylthioglycollate) on the other hand not only neutralized the HCl stoichiometrically but gave additional oxidative stabilization over a wide concentration range.     

Mechanisms of antioxidant action: The effect of thermal processing on the antioxidant activity of sulphur compounds

The nickel dithiophosphates and their corresponding disulphides are shown to be more effective uv stabilisers for LDPE than the closely related nickel xanthates and disulphides (dixanthogens). In general, the nickel complexes in both series are more effective uv stabilisers than the disulphides.Evidence is presented to show that the better performance of the dithiophosphates than the xanthates is associated with their higher solubility in the polymer. For the same reason, nickel dithiolates containing long alkyl substituents are more effective uv stabilisers than those containing shorter alkyl substituents.Thermal processing adversely affects the photo-stabilising performance of nickel dithiolates whereas the performance of the less efficient disulphides is enhanced under the same conditions. In the former case, the uv stability of the polymer is shown to be directly proportional to the amount of the nickel complex left after processing. In the case of the disulphides, on the other hand, the evidence suggests that increasing the severity of their processing increases the extent of oxidation to products which are more effective uv stabilisers than the parent disulphides.     

Recoil spectrometry: Ion accelerator based elemental characterisation of surface layers

Recoil Spectrometry covers a group of techniques that are very similar to the well known Rutherford backscattering Spectrometry technique, but with the important difference that one measures the recoiling target atom rather than the projectile ion. This makes it possible to determine both the identity of the recoil and its depth of origin from its energy and velocity, using a suitable detector system. The incident ion is typically high-energy (30–100MeV)³⁵C1,⁸¹Br or¹²⁷I. Low concentrations of light elements such as C, O and N can be profiled in a heavy matrix such as Fe or GaAs. Here we present an overview of mass and energy dispersive recoil Spectrometry and illustrate its successful use in some typical applications.     

The uniqueness of the adjoint operation I

Let H be a complex, infinite-dimensional Hilbert space. Let B(H) denote the set of bounded linear operators on H. This paper contains a nonlinear characterization of the adjoint operation on B(H). The statement of this result is:THEOREM:Let h: B(H) B(H)be a function such that h(I)0.Then h(ST)=h(T)h(S)and h(S)S0for all elements Sand Tof B(H)if and only if h(S)=S^* for all S B(H).     

Direct dynamics trajectory study of the reaction of formaldehyde cation with D2: vibrational and zero-point energy effects on quasiclassical trajectories

Quasiclassical, direct dynamics trajectories have been used to study the reaction of formaldehyde cation with molecular hydrogen, simulating the conditions in an experimental study of H2CO+ vibrational effects on this reaction. Effects of five different H2CO+ modes were probed, and we also examined different approaches to treating zero-point energy in quasiclassical trajectories. The calculated absolute cross-sections are in excellent agreement with experiments, and the results provide insight into the reaction mechanism, product scattering behavior, and energy disposal, and how they vary with impact parameter and reactant state. The reaction is sharply orientation-dependent, even at high collision energies, and both trajectories and experiment find that H2CO+ vibration inhibits reaction. On the other hand, the trajectories do not reproduce the anomalously strong effect of nu2(+) (the CO stretch). The origin of the discrepancy and approaches for minimizing such problems in quasiclassical trajectories are discussed.     

Palladium-catalyzed regioselective arylation of imidazo[1,2-a]pyrimidine

Imidazo[1,2-a]pyrimidine can be arylated at the 3-position with aryl bromides in the presence of base and a catalytic amount of palladium. This provides an efficient one-step synthesis of 3-arylimidazo[1,2-a]pyrimidines from the unsubstituted heterocycle. [reaction: see text]     

Studies in bicyclo[3.1.0]hexane methanolysis. Ring opening of activated cyclopropanes under acidic and basic conditions

A bicyclo[3.1.0]hexane, with one cyclopropane carbon flanked by a ketone and an ester or an aldehyde, undergoes methanolysis with cleavage of one of the two activated cyclopropane bonds, depending on the reaction conditions. Acidic conditions yield primarily or exclusively a 4-methoxycyclohexane, while basic conditions yield a 3-methoxymethylcyclopentanone.     

Lewis base activation of lewis acids. Addition of silyl ketene acetals to aldehydes

The weak Lewis acid silicon tetrachloride can be activated by catalytic amounts of the chiral bisphosphoramide (R,R)-3 to form a highly reactive, chiral trichlorosilyl cation which is an extremely effective promoter of aldol addition reactions between aldehydes and silyl ketene acetals. The tert-butyldimethylsilyl ketene acetal of methyl acetate adds nearly instantaneously to aromatic and olefinic aldehydes as well as aliphatic aldehydes (albeit more slowly) with excellent enantioselectivity. The homologous tert-butyldimethylsilyl ketene acetal of tert-butyl propanoate adds with nearly exclusive anti diastereoselectivity to a similar range of aldehydes also with excellent enantioselectivity. The origin of the slower reaction rate with aliphatic aldehydes is revealed to be the formation of chlorosilyl ether adducts.