# How to classify the hydride shift in the following diazacyclopenta[cd]pentalene system?

How is this hydride shift reaction type called?

I think it's not one of following types:

• electrocyclic ring opening
• sigmatropic rearrangement
• ene reaction
• cheleotropic reaction

Therefore I'm thinking what else can been the name of the observed reaction.

The ring opening reaction with hydride shift has been taken from Heterocycles 2004, 64 (1), 447–461 (DOI: 10.3987/COM-04-S(P)48).

• The back reaction looks like an "ene" reaction to me. The forward reaction would then be a "retro-ene" reaction. – ron Aug 3 '15 at 14:32
• I also wrote "ene" in the exam. The reason why I'm unsure now is the existing C(H)-N bond: I thought a ene-reaction happens with two unconnected species as in en.wikipedia.org/wiki/Ene_reaction – laminin Aug 3 '15 at 14:57
• No, the pieces can be connected. See this pdf for some "connected" hetero-ene examples. – ron Aug 3 '15 at 15:12

It took me a while to see what was actually happening. Initially, I was focused on a [1,5] suprafacial sigmatropic proton shift followed by a diaza-electrocyclic ring opening reaction. However, after drawing the corresponding orbitals I realised that I would have a eight-electron antiaromatic π system, in which a proton shift would need to occur antarafacially — not possible.[1]

It still is clear that it is some kind of pericyclic reaction, and going through the list we can exclude everything except for a (retro-) group transfer reaction for which the (retro-) ene reaction is the most well-known example. (We are transforming one σ bond into one π bond which excludes all the others.)

It took me a while to actually see the retro-ene system in the substrate, but finally I did, and I took the liberty of highlighting it in red in the scheme below. Note that you have to ignore the carbon-nitrogen bond which belongs to the lowest five-membered ring from the ene-system. It is not taking part, if anything it is scaffolding.

Scheme 1: Retro-ene reaction of the title compound with the pericyclic six-membered ring system highlighted. Dashed bonds represent those that are broken/formed within the reaction.

Counting electrons, I realised that my initial assumption of a proton shift was wrong and that it is indeed a hydride shift.

Notes:

[1] [1,n]-rearrangements are classified as suprafacial or antarafacial depending on whether the group is on the same side or the other side of a π system after migration. Technically, there is a further difference between retention and inversion, but hydrogen cannot rearrange under inversion so that is moot. The general rule for proton transfers is: If the number of bonds is $4n + 1$, the mechanism is suprafacial, for $4n + 3$ it is antarafacial.