Synthesis of squaric acid derivatives by Lewis acid-catalysed reaction of its dichloride, methyl ester chloride, diethylamide chloride, and ethyl diester with unsaturated organosilanes: new method for C–C bond formation on cyclobutenedione
The squaric acid family of derivatives, e.g. dichloride 2, methyl ester chloride 3, diethyl amide chloride 4, and ethyl diester 5, reacted with a variety of unsaturated organosilanes 6 in the presence of a catalyst, typically titanium tetrachloride, at –78 to 0 °C to give addition products (e.g., 7, 9, 11) and/or substitution products (e.g., 8, 10, 12, 13) after dechlorosilylation either via 1,2- or 1,4-addition. The mode of addition depended on the nature of the acid family and on the substitution pattern of the organosilane. The former product predominated in the reaction of the chlorides 2–4 with all silyl ketene ketals used, and with some allylsilanes and silyl enol ethers unless the reactive site (γ to a silyl group) of these silanes was more crowded, whereas the diester 5 gave rise to the latter product 13 irrespective of the substitution pattern. In some cases, catalyst and reaction temperature also affected the mode of addition. The reactivity was found to be in the order 2 > 3 > 4, being reflected in the reaction temperature.