The isolation of 4a-c was again achieved by reaction with DMAP. 4a-c•(DMAP)2 were isolated as yellow powders by precipitation in n-hexane in 50%-75% yield. Spectroscopic data are consistent with their formulation. For example, for 4c•(DMAP)2, is characterised by two sets of doublets in the high field region of the 1 H NMR spectrum at  = 0.22 (2H, 3 JHH =10.5 Hz) and  = 1.30 (2H, 3 JHH =10.5 Hz) ppm. The mutual coupling between these resonances was confirmed by a COSY NMR experiment. Clear evidence for the syn-selective double insertion is also evident from single-crystal X-ray diffraction of 4c•(DMAP)2 (Figure ). To gain a deeper understanding of the stereochemistry a series of plausible mechanisms were investigated by DFT calculations using the B3PW91 functional. Prior DFT studies have modelled the addition of ethylene to a derivative of 1b, but stereochemistry was not a consideration in this case. The reaction of 1b with cyclopropene 2a is calculated to proceed by an initial formation of a weak van der Waals complex Int-1 followed by a stereoselective syn-addition transition state TS-1 that connects directly to the product Int-2 (Figure ). Int-2 (R 1 = Me, R 2 = Ph) is a direct analogue of the spectroscopically observed intermediate 3d (R 1 = R 2 = Ph). This reaction is predicted to be exergonic by G°(298K) = -29.0 kcal mol -1 with a readily accessible activation barrier of G ‡ (298K) = 5.7 kcal mol -1 .