Alkenes are important as biologically relevant functionalities and as lucrative and extremely versatile building blocks in drug discovery. Many regio-and stereoselective strategies to access alkenes have been developed over the past decades and generally involve transition metal-catalyzed cross coupling, C-H activation or metathesis. With the increased focus on sustainable chemistry, the development of transition metal-free, regio-and stereoselective alkenylations that proceed under mild conditions is highly important. Vinylbenziodoxolones (VBX) have recently been reported as novel hypervalent iodine(III) reagents sharing the same benziodoxolone core as the popular ethynylbenziodoxolones (EBX) and Togni reagents. VBX compounds have already been employed as electrophilic vinylation reagents under transition metal-catalyzed, photocatalytic and metal-free conditions with a variety of nucleophiles (Scheme 1a). VBX shows interesting reactivity under mild conditions without need for excess reagents, and tolerates a variety of functional groups. Core-substituted Me2-VBX was reported to give improved results in some cases. 12 In parallel, the chemistry of b-heteroatom-substituted VBX reagents and their benziodoxole counterparts has also been developed. Methodology for transition metal-free vinylations from our lab highlights the intriguing reactivity of VBX reagents 1. The efficient, room temperature S-vinylation of thiols 2 resulted in formation of internal alkenes 3 with retention of the VBX alkene configuration (Scheme 1b). To the contrary, metal-free P-vinylation of phosphine oxides 4 or H-phosphinates delivered terminal alkenes 5 with complete regioselectivity (Scheme 1b). The latter regiochemistry has only been observed under transition metal-free conditions with VBX. Literature reports of reactions with vinyl(aryl)iodonium salts, which are related iodine(III) reagents, indicate that various mechanisms can be operative under metal-free conditions. Vinylic SN1 or SN2 or reactions via alkylidene carbene intermediates are common, whereas Michael additions are less likely. Vinylidene carbene formation has also been proposed in reactions with alkynyliodonium salts and EBX. Mechanistic studies of the latter revealed that thiol alkynylation can proceed via ligand coupling or Michael-type addition depending on the electronics of the reagent. To increase the utility of VBX as an efficient vinylating agent, tools to understand and predict the principles for product regiochemistry are important. Based on literature precedent, several possible mechanisms could be proposed to explain the observed regiochemistry (Scheme 1c). In this work, we have used NMR studies, deuterium-labelling and extensive DFT calculations of the S-and P-vinylations to unravel the mechanisms of VBX vinylations under transition metal-free conditions. Two mechanistic pathways are presented together with tools to predict the regiochemistry with other nucleophile classes.