Leading Question
Explain the 5-exo-trig selectivity of the initial cyclization reaction that forms 7.
5-exo-trig is a classification of ring closure reactions that follow Baldwin’s Rules. The 5 stands for the number of atoms in the new ring formed, exo means the bond broken during the ring closure is outside the ring which is formed and trig means the electrophilic carbon is sp2 (trigonal).
The following orbital diagrams are drawn so that the carbon-carbon double bond is the electrophile. This means the LUMO is drawn, so there is one node between the two pi orbitals.
5-exo-trig is more favorable than 5-endo-trig because as you can see, the direction the Nitrogen would have to attack from to give the 5-endo-trig selectively would require it to be on the other side of the carbon, or else the orbitals would not line up correctly.
