A method of synthesizing a compound of formula I: comprising the step of reacting a moiety of formula II: with a moiety of formula III: in compressed carbon dioxide in the presence of a transition metal catalyst and a base, wherein L is a labile leaving group; RN1 is optionally substituted C5-20 aryl; RN2 is selected from optionally substituted C5-20aryl, optionally substituted C3-20 heterocyclyl, optionally substituted C3-7 alkyl, and optionally substituted sulfonyl; RN3 is selected from H and optionally substituted C1-7 alkyl, C3-20 heterocyclyl and C5-20 aryl; or RN2 and RN3 together with the nitrogen atom to which they are attached form optionally substituted nitrogen-containing C3-20 heterocylyl or C5-20 heteroaryl; and R1 R2 and R3 are independently selected from optionally substituted C1-7 alkyl, C5-20 aryl, C3-20 heterocyclyl, hydroxy, halo, amino and C1-7 alkoxy, or two of R1, R2 and R3, together with the silicon atom to which they are attached, may form a silicon containing C5-7 heterocyclyl group.