Furthermore, a model CM reaction of allyl benzene (28) and 1,4-diacetoxy-2-butene ( ) was chosen to assess the E/Z selectivity profile of different-size NHC catalysts (Scheme 6B). The plot shown in Scheme 6B illustrates the E/Z ratio of product 30 as a function of conversion. Interestingly, the E/Z ratio profiles found for Ru16 and Ru17 show significant difference compared to the previously reported SIMes second-and third-generation catalysts Ru1, Ru2 and Ru21. All SIMes-based catalysts led to a product with lower E/Z ratios (∼3:1) at low conversion, then the (E)product amount increased slowly to 10:1. Interestingly, the small-NHC Ru16 and Ru17 offered low E/Z ratios at much higher conversion as compared to SIMes catalysts, which at the same conversion (70-75%) gave much higher amount of (E)-30. Presumably, the difference between these two catalyst classes can be rationalized on the basis of the smaller steric foot-print of Ru16 and Ru17 which translates to better inherent diastereoselectivity of these catalysts. At conversion higher than 80% the new catalysts promoted enough secondary metathesis events isomerising the product to the thermodynamically favoured E-isomer, similarly as their more bulky SIMes analogues did (Scheme 9B). Conversion calculated by GC. In parenthesis, conversion after 2 h of reaction a Data taken from Ref. [51]. In brackets conversion of reaction conducted in perfluorobenzene. c Data taken from Ref. [14]. TBS = tertbutyl(dimethyl)silyl.