1: [2+2] Photocycloadditions in Organic Chemistry 1. Basics - addition of two alkenes (2 pi electrons + 2 pi electrons) to yield a four-membered ring. hv [2 + 2] - light required for the reaction; nothing happens if only heat is applied. > Since two molecules are reacting to form new chemical bonds, we need interaction between f illed and unf illed orbitals. For the thermal case (heat stimulus), the molecular orbitals of the alkene looks like this: LUMO (alkene) HOMO (alkene) > Therefore, we need the electrons in the HOMO of one alkene entering the LUMO of its reacting partner, which, in this case, is disfavored due to a antibonding interaction: This makes the addition DISFAVORED!! Antibonding Interaction Bonding interaction. > When light is used as the energy source instead of the heat, the electronic configuration of the alkene changes, as one electron f rom HOMO jumps to the LUMO: the new HOMO LUMO (alkene) light (hv) HOMO (alkene) singly-occupied molecular orbital (SOMO) (excited state) > Now, this "new HOMO", which contains a single electron, can interact with the empty LUMO of an unexcited alkene (there's plenty of this around because, af ter all, it ' s more stable than the excited state). HOMO (excited alkene) LUMO (unexcited alkene) > This type of approach, where one both orbitals of one molecule approach its reacting partner f rom the same side of the molecule, is called suprafacial . [2+2] addition reactions are always suprafacial. G G G - why is the antarafacial approach disfavored? > anatarafacial approach is when the orbitals from one molecule add to different sides of the reacting partner. "top" side "bottom" side (antarafacial approach) > anatarafacial approach is (almost) impossible for a [2+2] (if such antarafacial approach were possible, heat-induced [2+2] would be feasible. But this "criss-cross" approach is heavily disfavored since orthogonal orbitals cannot interact.) 2. Structural Relationship between the Reactant and Product - let's take a look at the dimerization of tr ans-butene: "up", "up" H3C CH3 hv "down","down" H3C CH 3 - lets look at the reaction between t rans- and cis- butenes: "up" H3C hv trans cis "down","down","down" H3C CH 3 CH 3 - when one of the two alkenes is in a ring, addition is cis. H H 3C O CH 3 H hv H O H 3C H O O CH 3 cis adduct G G G G G G G S. L. Schreiber, 1983 G 3. Examples!!! - first reported [2+2] photocycloaddition G. Ciamician, P. Silber (1908): O "Italian Sunlight" O 1 year(!) carvonecamphor carvone O O - Paterno-Buchi Reaction E. Paterno, G. Chieffi (1909): O hv O H G