# Are all concerted electrophilic addition reactions of alkenes syn?

The Organic Chemistry book I am using (8th edition Carey, et al.) doesn't say it straight out, but I am wondering if it's a universal rule that all concerted electrophilic additions of alkenes are syn.

In every concerted example I have seen so far in the textbook, the bonds that are being made wind up on the same side, but I don't know if it's a rule. For example: epoxidation, transition metal catalyzed hydrogenation, and hydroboration are all concerted and all syn.

Most reactants are not that large. Typically, there will be a single attacking atom (e.g. halogens, meta-chloroperbenzoic acid), two atoms bonded together directly (e.g. borane) or two atoms that are still in very close proximity (e.g. $\ce{OsO4}$ — two bonds — or a diene for a Diels-Alder reaction — three bonds). None of these even have the chance to reach across, especially if you consider that the largest of these, the diene, is in fact a large π system and inherently restricted to being planar (within very small possible deviations).