No abstract

A mechanistic investigation into the guanidine catalyzed reductive amination of CO 2 , using a combination of 1 H, 29 Si NMR, FT-IR, MS and GC profiling, is reported. Inexpensive and readily available N N N N-tetramethylguanidine (TMG) was found to be an equally effective catalyst compared to more elaborate cyclic guanidines. Different catalytic pathways to formamide 2, aminal 4 and N-methylamine 3, were identified. A pathway to formamide product 2 dominates at 23 °C. Increasing the reaction temperature to 60 °C enables a competitive, higher energy pathway to 4 and 3, which requires direct reduction of CO 2 with PhSiH 3 to formoxysilane E. Reduction of aminal 4, in the presence of CO 2 and the catalyst, led to formation of a 1 : 1 ratio of 2 and 3. The catalyst itself can be formylated under the reaction conditions, resulting in its deactivation. Thus, alkylated TMGs were found to be more stable and more active catalysts than TMG, leading to a successful organocatalyzed reductive functionalization of CO 2 with silane at 0.1 mol% catalyst loading (TON = 805 and TOF = 33.5 h-1).

The aim of this review is to demonstrate, using examples developed in our own laboratories, that carbon dioxide has real potential in synthetic organic process chemistry. As well as potential environmental benefits, it offers opportunities for enhanced diastereo- and enantioselectivity compared to processes in conventional solvents. Fine control of reagent and product solubilities can lead to selective product separation and novel CO2-induced reactions. Pd-mediated cross-coupling reactions in scCO2 can give increased yields and selectivities, especially in the presence of usually incompatible functionality such as amines, which are protected in situ by reaction with CO2. Useful reactions can also be carried out at subcritical pressures utilising either the Lewis acidity of CO2 or the Brønsted acidity of carbonic acid formed in aqueous solutions under an atmosphere of CO2. Finally, product processing (extraction, chromatography, crystallisation) can also be carried out using CO2 without significant amounts of conventional solvents. We believe that the principles demonstrated here can be applied to a wide variety of procedures of relevance to organic process chemistry, and that the use of scCO2 as a solvent, whilst currently in its infancy in this area, will be of increasing importance in the future.

There is much concern about the toxicological effects of synthetic hair dyes. As an alternative approach, renewable waste blackcurrant ( Ribes nigrum L.) fruit skins from the fruit pressing industry were extracted using acidified water with a solid-phase purification stage. Anthocyanin colorants were isolated in good yields (2-3% w/ w) and characterized by HPLC. Sorption of anthocyanins onto hair followed a Freundlich isotherm; anthocyanin-anthocyanin aggregation interactions enabled high buildup on the substrate. Sorption energy of cyanidin-3- O-glucoside (monosaccharide) > cyanidin-3- O-rutinoside (disaccharide), but sorption properties of different anthocyanin glucosides were very similar. Intense blue-colored dyeing on hair could be achieved with λ at 580 nm, typical of the anionic quinonoid base; it is suggested that hair provides an environment that enables the stabilization of the anionic quinonoid base on adsorption through association with cations in the hair and copigmentation effects. Dyeings were stable to multiple washes.

Microwave mediated RAFT polymerization leads to ultra-fast polymerizations, whilst keeping excellent control over molecular weights and molecular weight distributions; this is the first example of such a dramatic effect of microwaves on living radical polymerization kinetics, and it shows the potential for chemists to produce very well controlled polymers in a matter of minutes.