Novel supported palladium catalysts have been developed based on chitosan as a support. These catalysts display excellent activity in the Suzuki and Heck reactions.

Folding is a ubiquitous process that nature uses to control the conformations of its molecular machines, allowing them to perform chemical and mechanical tasks. Over the years, chemists have synthesized foldamers that adopt well-defined and stable folded architectures, mimicking the control expressed by natural systems . Mechanically interlocked molecules, such as rotaxanes and catenanes, are prototypical molecular machines that enable the controlled movement and positioning of their component parts . Recently, combining the exquisite complexity of these two classes of molecules, donor-acceptor oligorotaxane foldamers have been synthesized, in which interactions between the mechanically interlocked component parts dictate the single-molecule assembly into a folded secondary structure . Here we report on the mechanochemical properties of these molecules. We use atomic force microscopy-based single-molecule force spectroscopy to mechanically unfold oligorotaxanes, made of oligomeric dumbbells incorporating 1,5-dioxynaphthalene units encircled by cyclobis(paraquat-p-phenylene) rings. Real-time capture of fluctuations between unfolded and folded states reveals that the molecules exert forces of up to 50 pN against a mechanical load of up to 150 pN, and displays transition times of less than 10 μs. While the folding is at least as fast as that observed in proteins, it is remarkably more robust, thanks to the mechanically interlocked structure. Our results show that synthetic oligorotaxanes have the potential to exceed the performance of natural folding proteins.

A kinetic study of the anionic polymerization of hexamethylcyclotrisiloxane ((CH3)2Si0)3 (D3) was carried out in toluene with the cryptate Li+ + 12111 as a counterion. The kinetic order with respect to the living end concentration was found to be equal to 1, and the propagation rate constant relative to cryptated ion pairs was determined at several temperatures between -20 and +20°C. The corresponding activation parameters were calculated : Ep = 9.8 kcal.mo1-1 andThe reaction of 1,3,5,7-tetramethyl-l,3,5,7-tetravinylcyclotetrasiloxane ((CH3)(CH2=CH)Si0)4 (D4*) was studied under the same conditions, the reactivity towards silanolate active centers was shown to be very close to that of D3. The rate constants of propagation and depropagation of D4* and of larger cycles D5* and D6* were determined from polymerization experiments on D4*. The propagation rate constants were found to be nearly the same for D4* and D5* : 1.2-1.3 l.mo1-1.s-1, D6* being the less reactivemonomer : 0.4 l.mo1-1.s-1. The differences observed in equilibrium constants of the cycles are essentially due to the rate constants of their formation which were found to be 0.42, 0.23, and 0.006 s-1 for D4*, D5* and D6*,respectively. ASo# = -20 cal.mol.-lK-l.

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