Chem 125 Lecture 68 4/15/08 Projected material This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

Pericyclic Reactions (in which transition states are “aromatic”) Cycloadditions: Diels-Alder Electrocyclic Reactions

11 33 conrotationdisrotation Möbius 22 Hückel connectivity    requires twist in 1 of 2 ways Hückel Transition State Motion  11 22 33 44 22 11 33 44 55 66 22 11 33 44 55 66  11 22 33 44  top touches top (even # of nodes) top touches bottom (odd # of nodes) ! 22 11 33 44 55 66

Sec pp (shown backwards in our text; same favored “aromatic” conrotatory transition state) CON 4e CON for 4n CON 8e DIS 6e DIS for 4n+2

22  Transition State HOMO 11  disrotation 6e Hückel bottom touches top (odd # of nodes) top touches top (even # of nodes) conrotation 4e Möbius

Molecules of C 4 H 4 will react with one another unless isolated. Is cyclobutadiene antiaromatic (4n)? Make it and see. O O O O Möbius conrotation impossible because of product strain O O + O=C=O h h Photochemistry changes electrocyclic preference to disrotation, because an electron is promoted to the next MO with opposite symmetry. Photochemistry changes preference of (reverse) cycloaddition too, allowing cyclobutane to become two double bonds.

Make one molecule per cage as guestBenzene Antiaromatic upfield shift? Most shift comes from other rings, still ~1.5 ppm above benzene Normal Making & Studying “antiaromatic” Cyclobutadiene above center of 8 benzene rings

Opening Dewar Benzene (1866)

Calculated Isomers of Benzene (2004) 84 calculated to be < 100 kcal above benzene. 6 > 100 kcal above benzene have been prepared. Dewar Benzene (1963) is 74 kcal above benzene but lasts 2 days at room temperature!

t 1/2 = 2 days (room temp) -11 kcal -75 kcal conrotatory disrotatory more strain aromatic 66 kcal/mole more exothermic, but only 8 kcal/mole “faster”

But shouldn’t “aromatic” 6-  -e transition state be good for disrotation? It is more fundamental that   LUMO doesn’t overlap  HOMOs (& vice versa).   

Making & Studying “antiaromatic” Cyclobutadiene To discuss making this “clamshell” we’ll need to learn some more reactions: Electrophilic Aromatic Substution and Carbonyl Reactions

Electrophilic Aromatic Substitution

H HH H H H D 2 SO 4 H HH H D H Sec H HH H H H D via A/D intermediate + D 2 SO 4 C6D6C6D6 etc. Observable! Or other electrophiles in place of D + e.g. NO 2 +, Br +, HOSO 2, R +, R-C=O R-C=O + HOSO 2 +

1 H vs. 1 H correlation in time 0.3 sec 40°C CACA DBDB Remember

+ + + SHMo2 (Simple H ü ckel Molecular Orbital Program) BenzenePentadienyl Cation addition converts  ring to  chain. H HH H H H D+D+ H HH H H H D + Locus of odd electron in radical, + charge (LUMO) in cation. - charge (HOMO) in anion, SOMO (nonbonding) Cf.

X NO H O2NO2N X H O2NO2N X H Substituent Effects on Rate X O2NO2N X O2NO2N XX NO (from HONO 2 /H 2 SO 4 ) X Relative Rate (overall) H[1] Cl0.03 NO 2 6  CH 3 25 OH1000 (CH 3 ) 3 N + 1   donation /  withdrawal  (or  ) e-donation eases formation of cation intermediates  (or  ) e-withdrawal retards formation of cation intermediates

End of Lecture 68 April 15, 2009