or c 2 c 6 alkynyl where the u2014 nr 3 r 4 group is situated ortho to the nhc u2550 x group and r 3 and r 4 are independently h or c 1 6 alkyl which c 1 6 alkyl group is optionally substituted with 1 or 2 groups selected independently from methyl halogen methoxy and hydroxy or r 3 and r 4 together form a 5 or 6 membered ring optionally substituted with halogen methyl methoxy or hydroxy and optionally containing one or two double bonds n 1 or 2 x is o or s y is o or s q 1 or 0 r 5 is c 1 c 6 alkyl chr 6 w c 3 c 6 cycloalkyl chr 6 w ch 2 c 3 c 6 cycloalkyl ch 2 chr 6 w c 3 c 6 cycloalkyl chr 6 w c 5 c 6 cycloalkenyl ch 2 chr 6 w c 5 c 6 cycloalkenyl c 2 c 6 alkenyl c 2 c 6 alkynyl ar 2 chr 6 w ar 2 ch 2 chr 6 w ar 2 or chr 6 w ch 2 ar 2 where w 0 3 ar 2 is a 5 to 10 member mono or bicyclic aromatic group optionally containing 1 4 ring heteroatoms selected independently from n o and s r 6 is c 1 c.