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.


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If they are concerted, there is a very, very large probability of them being syn-specific. For explantion, let’s look at the π orbitals of ethene (scheme 1). (Remember that double bonds are planar and have two distinct sides when viewed across an entire molecule.)

pi and pi* orbitals of ethene
Scheme 1: π and π* orbitals of ethene, viewed from the side.

Reactants have one of two possibilities, typically. They are either electrophilic and will thus attack the highest occupied molecular orbital (HOMO). Or they are nucleophilic and will attack the lowest unoccupied molecular orbital (LUMO). It just so happens that the π bond is the weakest bond in a typical alkene and thus the bonding molecular orbital is typically the HOMO and the antibonding one is the LUMO.

To attack this orbital, you somehow need to interact with both lobes. If you do so, you will either attack syn-stereospecificly, i.e. arrive from one side due to simplicity. Or you are a very large attacking molecule and are able to grab around the double bond, interacting with one atom on one side and with the other one on the other side.

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).

Of course, that does not make it impossible. If you use a large enough π system in a Diels-Alder like reaction, that may be able to wind itself downwards like a spiral staircase. I haven’t seen that type of reaction yet, though.


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