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; R.sup.N1 is optionally substituted C.sub.5-20 aryl; R.sup.N2 is selected from optionally substituted C.sub.5-20aryl, optionally substituted C.sub.3-20 heterocyclyl, optionally substituted C.sub.3-7 alkyl, and optionally substituted sulfonyl; R.sup.N3 is selected from H and optionally substituted C.sub.1-7 alkyl, C.sub.3-20 heterocyclyl and C.sub.5-20 aryl; or R.sup.N2 and R.sup.N3 together with the nitrogen atom to which they are attached form optionally substituted nitrogen-containing C.sub.3-20 heterocylyl or C.sub.5-20 heteroaryl; and R.sup.1 R.sup.2 and R.sup.3 are independently selected from optionally substituted C.sub.1-7 alkyl, C.sub.5-20 aryl, C.sub.3-20 heterocyclyl, hydroxy, halo, amino and C.sub.1-7 alkoxy, or two of R.sup.1, R.sup.2 and R.sup.3, together with the silicon atom to which they are attached, may form a silicon containing C.sub.5-7 heterocyclyl group. ##STR00001##