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. 2009 Apr 13;28(7):2038-2045.
doi: 10.1021/om800760x.

Computational Studies on the Pt(II)-Catalyzed Cycloisomerization of 1,6-dienes into Bicyclopropanes: A Mechanistic Quandary Evaluated by DFT

Franziska Bell  1 , Jason Holland, Jennifer C Green, Michel R Gagné
Affiliations

Computational Studies on the Pt(II)-Catalyzed Cycloisomerization of 1,6-dienes into Bicyclopropanes: A Mechanistic Quandary Evaluated by DFT

Franziska Bell et al. Organometallics. .

Abstract

The mechanism of the (bis(phosphanylethyl)phosphane)Pt(2+) catalyzed cyclo-isomerization reaction of 7-methyl-octa-1,6-diene to form 1-isopropylbicyclo[3.1.0]hexane was studied using computational methods. The cyclopropanation step was found to be the turnover-limiting step. The overall reaction proceeds via both a 5-exo and a 6-endo route. W conformations were shown to facilitate cyclopropanation, but do not have any influence on the rate of the 1,2-hydride shifts.

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Figures

Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3
Intermediates observed and studied for the conversion of diene to 13.
Figure 1
Figure 1
Energy pathway for the Pt catalyzed cyclopropanation reaction; red for the 6-endo pathway and blue for the 5-exo pathway. Transition states of a particular reaction are indicated by the corresponding reactant and product number, connected by an arrow. The energy zero is taken as that of 1a + the free diene.
Figure 2
Figure 2
Labeling of atoms used in (a) 6-endo and (b) 5-exo pathway
Scheme 4
Scheme 4
Conformer dependent cyclization profiles. The activation energies given are for gas phase calculations. Formal charges on the Pt moiety are not assigned.
Figure 3
Figure 3
Energy diagram illustrating the interconversion of 12 and 6, both with and without the benefit of ethylene.
Figure 4
Figure 4
Proposed associative cyclopropanation transition states involving ethene: a) 6-endo b) 5-exo.
Figure 5
Figure 5
Energies of dissociative and associative cyclopropanation transition states.
Figure 6
Figure 6
Associative TS for the conversion of 6 to 13 and the ethylene adduct of the catalyst.
Figure 7
Figure 7
Schematic diagram depicting (a) a W shaped and (b) a non-W shaped structure. The p orbital of the cationic carbon forms a stroke of the W.
Figure 8
Figure 8
Orbitals involved in a per-caudal interaction top: HOMO of 6, bottom: HOMO of 12
Figure 9
Figure 9
Schematic Diagrams of 14 and 15 with key bond lengths (Å) and angles (°).
Figure 10
Figure 10
Activation energies for the two possible 5-exo and 6-endo pathways.
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