1 [Ni 0 L]-catalyzed cyclodimerization of butadiene: A computational study based on the generic [Ni 0 (butadiene) 2 PH 3 ] catalyst. Sven Tobisch and Tom Ziegler Martin-Luther-University of Halle-Wittenberg University of Calgary ICCC35 Heidelberg, Germany, July 23, 2002

2 General Introduction first observation by Reed ( H.B.W Reed J. Chem. Soc. 1954, 1931.) systematic exploration by Wilke et al. multistep addition-elimination mechanism (Ni 0  Ni II ) active catalyst complex  [bis(butadiene)Ni 0 L] (L = PR 3, P(OR) 3 ) products of the catalytic cyclodimerization [Ni 0 L]-catalyzed cyclodimerization of butadiene

3 Catalytic cycle of the [Ni 0 L]-catalyzed cyclodimerization of 1,3-butadiene (in essential parts according to Wilke et al.;  G. Wilke et al. J. Organomet. Chem. 1985, 279, 63.)

4 Computational Details DFT-calculations with a gradient-corrected XC-functional (BP86) A. D. Becke Phys. Rev. 1988, A38, J. P. Perdew Phys. Rev. 1986, B33, 8822; Phys. Rev. 1986, B34, All-electron basis set of triple-  quality for the valence electrons augmented with polarization functions [Ni 0 L]-catalyzed cyclodimerization of butadiene

5 Stereoisomers of the active catalyst and of octadienediyl-Ni II species

6 mechanistic details that are not yet firmly established: What is the geometric structure of the [Ni 0 (butadiene) 2 L] active catalyst and how does oxidative coupling of two butadiene moieties occur? What type of octadienediyl—Ni II species are involved in the reductive elimination steps? What role plays allylic isomerization in the course of the catalytic process? Which elementary process is rate-determining? [Ni 0 L]-catalyzed cyclodimerization of butadiene

7 A - oxidative coupling of two butadiene moieties

8 Key structures involved along the most feasible pathway via 1  2 (  2 -cis/trans-BD coupling (opposite enantiofaces)) [Ni 0 (  2 -BD) 2 L] 1 TS[1-2] [Ni II (  3,  1 (C 1 )-C 8 H 12 )L] 2  G ‡ = 13.6 kcal mol -1  G = 0.1 kcal mol -1

9 B – thermodynamic stability of different forms of the [Ni II (C 8 H 12 )L] complex  G [kcal mol -1 ]

10 C – allylic isomerization in octadienediyl—Ni II species isomerization of an  3 -allylic group via an  -C 3 intermediate J. W. Faller et al. J. Am. Chem. Soc. 1971, 93, S. Tobisch, R. Taube Organometallics 1999, 18, TS ISO [3]  G ‡ = 13.2 kcal mol -1

11 D – reductive elimination under ring closure

12 D – reductive elimination under ring closure  3,  1 (C 1 )-Ni II 2 TS[2-8]  4 -VCH-Ni 0 8 Key structures involved along the most feasible pathways VCH route  G ‡ = 25.3 kcal mol -1  G = kcal mol -1 bis(  3 )-Ni II 4 TS[4-9]  4 -DVCB-Ni 0 9 cis-1,2-DVCB route  G ‡ = 22.7 kcal mol -1  G = 3.2 kcal mol -1 cis,cis-COD route  G ‡ = 22.5 kcal mol -1  G = -0.2 kcal mol -1 TS[4-10]  4 -COD-Ni 0 10

13 [Ni 0 L]-catalyzed cyclodimerization of butadiene Conclusions The [Ni 0 (  2 -BD) 2 L] complex 1 is the active catalyst complex. Oxidative coupling most likely proceeds via coupling of the terminal non-coordinating carbons of two  2 -BD moieties. [Ni II (  3,  1 (C 1 )-C 8 H 12 )L] species are formed as initial coupling product. All octadienediyl —Ni II species are in a pre-established equi- librium, due to facile allylic isomerization. 2 and 4 are the prevailing octadienediyl—Ni II species. Bis(  1 )-octadienediyl—Ni II species play no role in the reaction course. The reductive elimination is rate-determi- ning and proceeds via direct paths. cis,cis-COD is predicted to be the predomi- nant product for the generic [Ni 0 (  2 - butadiene) 2 PH 3 ] catalyst.

14 published work “[Ni 0 L]-Catalyzed Cyclodimerization of 1,3-Butadiene: A Comprehensive Density Functional Investigation Based on the Generic [(C 4 H 6 ) 2 Ni 0 PH 3 ] Catalyst.“ S. Tobisch, T. Ziegler J. Am. Chem. Soc. 2002, 124, “[Ni 0 L]-Catalyzed Cyclodimerization of 1,3-Butadiene: A Density Functional Investigation of the Influence of Electronic and Steric Factors on the Regulation of the Selectivity.“ S. Tobisch, T. Ziegler J. Am. Chem. Soc., accepted for publication [Ni 0 L]-catalyzed cyclodimerization of butadiene Acknowledgement: Prof. T. Ziegler (University of Calgary) and members of his goup Deutsche Forschungsgemeinschaft (DFG)