[M]-/Organo-Catalyzed Aerobic Functionalization of Alkylaryl-Siloxanes: Synthesis Carboxy-, Carbonyl- and Hydroxy-Aryl-Siloxanes

This study presents a solution to the key challenge in modern chemistry of silicones – a functionalization of organosilicon compounds. We have previously shown the potential of the aerobic oxidation for a functionalization of several model p-tolyl-siloxanes to corresponding p-carboxyphenyl-derivatives. Unfortunately, this method has several drawbacks: limited scope, time and energy consumption, as well as scaling problems. We now present their solution. Mild and highly effective liquid-phase [M]-/organo-catalyzed aerobic functionalization of alkylaryl-siloxanes was developed. Reactions proceed at room temperature and atmospheric pressure within 0.5-24 h yielding products in quantitative conversions and high yields. Components of the catalytic system are simple, cheap and commercially available: Co(OAc)2 as earth-abundant metal-catalyst, and N-hydroxyphthalimide (NHPI) as an organo-catalyst. A unique combination of solvents with hexafluoroisopropanol (HFIP) being both co-solvent and process activator (usually in small quantities, 10-25%), provided high efficiency. HFIP allows to overcome scope limitations (most substrates were not oxidized in the absence of HFIP). It also dramatically increases process efficiency minimizing time and energy consumption (reaction time is 0.5-24 h, at r.t. and 1 atm) and solving scalability problems (≥100 g). Method allows to obtain a wide range of both carboxy-, carbonyl- and hydroxy-aryl-containing silanes/siloxanes of linear, hyperbranched, cyclic and cage (polyhedral oligomeric silsesquioxanes or spherosilicates) structure. This approach is applicable to both monomeric and oligomeric and polymeric substrates with various number of functional groups.