Helsinki-3 Direct Estimate of Conjugation, Hyperconjugation and Aromaticity With an Energy Decomposition Analysis Gernot Frenking Fachbereich Chemie, Philipps-Universität Marburg

Conjugation What is conjugation? “In a topological sense, the indication that each pair of multiple (double or triple) bonds in a polyunsaturated molecule is separated by one single bond.” Pure Appl. Chem. 1999, 71, IUPAC Definition :  -carotene, the red pigment in carrots and other vegetables “Conjugated compounds are usually more stable than bond-shifted isomers in which the double (triple) bonds are isolated from each other by more than on single one bond” (a) isolated double bonds (b) conjugated double bonds

Conjugation  Geometry  Reactivity  Properties Short C–C distance: Å

Hyperconjugation What is hyperconjugation? It is the interaction between orbitals having  symmetry where at least one of the  orbitals is located at an atom that does not have a multiple bond  π  (sat) π (sat)  π  (sat)  π  ←  π  (sat) π (sat)  ←  π  (sat)

Energy Decomposition Analysis (EDA)Extended Transition State Method (ETS) K. Morokuma, J. Chem. Phys. 1971, 55, 1236T. Ziegler, A. Rauk, Theor. Chim Acta 1977, 46, 1 A + B A-B  (A) +  (B) [  (A) +  (B)](r A-B )  E elstat  (A,B)   =NÂ  (A,B)  E Pauli NÂ   (A,B)  (A-B)  E Orb =  E int  E int +  E prep =  E(BDE) Three Steps:  (A-B)   (A-B) 

D. Cappel, S, Tüllmann, A. Krapp, F. Frenking, Angew. Chem. Int. Ed. 2005, 117, 3683.

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation BP86/TZ2P. Energy Values in kcal/mol

Conjugation

BP86/TZ2P. Energy Values in kcal/mol r = 0.95, SD=2.60 r = 0.97, SD=2.15

Conjugation BP86/TZ2P. Energy Values in kcal/mol r = 0.98, SD=2.92

Conjugation in Cyanoethynylethenes, CEEs, (Diederich) Conjugation in Cyanoethynylethenes, CEEs, (Diederich) r = 0.99, SD= 0.43 BP86/TZ2P. Energy Values in kcal/mol o C ipso o C meta 1 7 r = 0.98, SD= 0.62

r = 0.99, SD= 0.36 Calculated quinoid character:  r = (((c + c’)/2 – (b + b’)/2) + ((a + a’)/2 – (b + b’)/2))/2. I. Fernández, G. Frenking, Chem. Commun. 2006, 5030.

Hyperconjugation H 2 C=CH–CX 3 HC ≡ C–CX 3 BP86/TZ2P. Energy Values in kcal/mol  (H 2 C=CH)→  *(CX 3 )  *(H 2 C=CH)←  (CX 3 )  (H 2 C≡CH)→  *(CX 3 )  *(H 2 C≡CH)←  (CX 3 ) No clear correlation between hyperconjugation and C–C bond distances

Hyperconjugation BP86/TZ2P. Energy Values in kcal/mol

Hyperconjugation r = 0.97, SD= 0.29

Hyperconjugation BP86/TZ2P. Energy Values in kcal/mol X 3 C–CY 3

Hyperconjugation BP86/TZ2P. Energy Values in kcal/mol X 3 C–CY 3 (  * deleted) blue:  (X 3 C) →  *(CY 3 ) red:  * (X 3 C) ←  CY 3 ) black: total hyperconj the relative  donor strengths of two groups may be inverted depending on the acceptor moiety C–H > C–C C–C > C–H

D.L. Thorn, R. Hoffmann, Nouv. J. Chim. 1979, 3, 39.

I. Fernandez, G. Frenking, Chem. Eur. J., 2007, 13, 5873.

Conjugation and Hyperconjugation CONCLUSIONS  The calculated  E  values taken from the EDA can be used to estimate the strength of the relative contributions of  interactions that come from conjugation/hyperconjugation  The EDA method has a predictive value. The trend of the calculated  E  values is in very good agreement with NMR chemical shifts and experimentally derived Hammett constants.  The comparison of  E  values of cyclic system with a suitable reference compound makes it possible to estimate the stabilization due to aromaticity.  Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of  interactions and electrostatic forces may compensate for the hyperconjugative effect.

D. Cappel, S, Tüllmann, A. Krapp, F. Frenking, Angew. Chem. Int. Ed. 2005, 117, 3683.

Conjugation BP86/TZ2P. Energy Values in kcal/mol r = 0.95, SD=2.60 r = 0.97, SD=2.15 I. Fernández, G. Frenking, J. Org. Chem. 2006, 71, 2251.

Conjugation in Cyanoethynylethenes, CEEs, (Diederich) Conjugation in Cyanoethynylethenes, CEEs, (Diederich) r = 0.99, SD= 0.43 BP86/TZ2P. Energy Values in kcal/mol o C ipso o C meta 1 7 r = 0.98, SD= 0.62 I. Fernández, G. Frenking, Chem. Commun. 2006, 5030

I, Fernández, G. Frenking, Faraday Discuss., 2007, 135, 403

I. Fernández, G. Frenking, Chem. Eur. J., in print.