Computations with phosphinous acid 4a¢ disclosed O-I intermediate N¢ with a hydrogen-bonded DBU molecule (Figure , red pathway). Deprotonation proceeds simultaneously with the Michael addition via 5-membered TS2 ' (12.5 kcal/mol) to give anionic intermediate I¢, after which the reaction proceeds as above. We also investigated carbene formation (cf J in Scheme 4), and indeed found a high-energy carbene intermediate from TS2 (Figure ). Based on these calculations, the reaction proceeds via phosphinous acid 4a¢, with formation of I¢ through TS2 ' as the rate determining step The reaction through pathway VIII, with cycloaddition from N to K, was computed with a TS energy of >33 kcal/mol (Figure ). The proposed a-attack in pathway IV to give anion G could not be found, instead the internal alkene product was formed through a 4-membered ligand coupling TS (Figure ). Likewise, pathway VI via intermediate L resulted in a three-membered ligand coupling TS at 16 kcal/mol (Figure ) which is 3.5 kcal/mol higher than TS2' (Figure ). These results are in agreement with the observed complete regioselectivity for terminal over internal alkene formation. Further investigations were performed to support the formation of intermediate K. Upon shorter reaction time, the synthesis of 5a-D from 4a and 1a-D2 was accompanied by isolation of another product, which was identified as compound 6-D2 (Scheme 7). This finding was highly unexpected, as iodine(III) compounds with alkyl ligands are generally unstable. Compound 6-D2 could be converted to product 5a-D upon treatment with DBU, which supports that K is an intermediate in the reaction. Upon kinetic NMR studies of reactions of 4a with 1a, the corresponding intermediate 6-H2 could be detected in minor amounts (Figure ). The varying Z:E ratios obtained for 5a-D can be explained by different selectivity in protonation of I-D vs I-D2 from above or below, as the energy difference for synvs anti-elimination is much higher. The mechanistic differences between VBX vinylations of thiols and phosphorus nucleophiles can be rationalized by invoking the preferential formation of a 5-membered TS over a 3-membered TS, which is generally required in ligand coupling mechanisms. This is in line with our previous findings in arylation of enolates with diaryliodonium salts, where a 5-membered pathway via an I-O intermediate was preferred over a 3-membered ligand coupling TS from the corresponding I-C intermediate. Based on this conclusion, other ambident nucleophiles would also be expected to coordinate to the iodine with subsequent Michael-type attack to yield terminal alkenes, whereas mononucleophilic species should give internal vinylation products. This is in line with the C-vinylation of nitrocyclohexanone, which gave the terminal product as the major regioisomer. Strong nucleophiles could prefer I-Nu coordination followed by ligand coupling or a-attack to give the internal product, as observed in vinylation of in situ-generated sulfenate anions. The possibility to control the regioselectivity by modulating the nucleophile structure should be of great interest in organic synthesis, and our continued studies of controlling VBX reactivity will be reported in due time.