Reports: B4
44029-B4 Dynamic Processes in [4n] Annulenes
Our research in the past year has been devoted to (1) completing our computational study exploring the [12]annulene radical anion hypersurface; (2) initiating and completing a project on the dehydro[12]annulene hypersurface; (3) designing annulenes that might undergo new forms of pi-bond shifting: two-twist bond shifting and non-degenerate bond shifting; (4) designing an aromatic [16]annulene with Möbius topology.
The Radical Anion of [12]Annulene.
The synthesis of 1,5- and 1,7-di-trans-[12]annulene radical anions were reported by Stevenson et al. and characterized by ESR spectroscopy (Accounts Chemical Research, 2007, 40, 703). Using a variety of computational methods (DFT and couple cluster methods) two undergraduate students (Miles Braten and Gertrude Gutierrez) generated results on geometries, energies and hyperfine coupling constants for a series of C12H12 radical anion isomers. We showed that the radical anion species presented by Stevenson is in fact a bicyclic radical anion; further, none of the structures studied could account for the observed ESR spectrum. In sum, our interpretation of the experimental ESR results contradicts prior published reports regarding the synthesis and characterization of di-trans-[12]annulene radical anions and has now been published in the Journal of Organic Chemistry (2008, 73, 8745).
Dehydro[12]Annulene.
The best characterized medium-sized dehydroannulenes have typically been those with two or more acetylenic units. In the case of dehydro[12]annulene, for example, analogues with two or three acetylene units have been characterized, but those with only one acetylene have proven more elusive. Recently, however, Stevenson et al. presented evidence for three different isomers of monodehydro[12]annulene, demonstrating that these might possibly have been made (Angew. Chem., Int. Ed., 2008, 47, 8714). Although some B3LYP/6-31G* computed structures were presented by Stevenson, the relative energies of the different isomers were not reported, and no dynamic processes were investigated. Undergraduate students Vivian Huynh and Talyor Wood computationally not only explored the deydro[12]annulene hypersurface (DFT and couple cluster methods), but also computed the 1H NMR of the C12H10 isomers that they located on the PES. Furthermore, barriers for interconversion of some of these isomers were computed. Their combined results do not support the prior reports that dehydro C12H10 annulene was in fact synthesized.
New Modes of Bond Shifting in Annulenes.
Our earlier work on pi-bond shifting in annulenes focused on both planar and Mobius-type systems. More recently we have focused our attention on assessing the feasibility of two-twist bond shifting (bond shifting through a figure eight topology, formally Hückel) and non-degenerate planar bond shifting. Automerization of mono-trans-[10]annulene via a two-twist mechanism had previously been shown to have a higher energy of activation (ca. 16 kcal/mol) than through the more traditional planar bond shifting mechanism (11 kcal/mol). However, undergraduates Phuong Ngyuen and Alyse Novoa, showed (using DFT and couple cluster methods), that appropriate substituents can significantly lower the barrier of two twist bond shifting. In particular, strategically placing an ethylene bridge across mono-trans-[10]annulene leads to a barrier of only 7.5 kcal/mol (CCSDT). Even more drastic was the finding that a di-benzo-fused analog could reduce this barrier to ca. 3 kcal/mol! Phuong presented this work at the 2009 ACS meeting in Salt Lake City, UT.
To test our hypothesis that non-degenerate planar BSing might be a viable mechanism for facile thermal configuration change in annulenes we focused on studying the isomerization of both cis- and trans-dihydrooctalene to the known tetra-trans-[14]annulene (CTCTCTT). Working with undergraduates Tobie Okoronkwo and Phuong Nguyen our computational results (couple cluster and DFT methods) show that once dihydrooctalene ring opens, a series of conformational and configurational steps are necessary to access the CTCTCTT configuration of [14]annulene. The lowest reaction path located was ring opening of trans-dihydrooctalene to a non-planar Huckel [14]annulene with two trans double bonds (CCCTCCT). Non-degenerate bond-shifting through this species followed by conformational steps leads to the known [14]annulene. Furthermore, our results suggest that the PES of [14]annulene is replete with Mobius minima, some of which are much lower in energy relative to some of their Hückel counterparts. Tobie Okornokwo presented her results at the Spring 2009 ACS meeting in Salt Lake City, UT.
Möbius, Aromatic, Substituted [16]Annulene Minima.
Our past work on Möbius aromaticity in annulenes and particularly our involvement with assessing the aromaticity of a benzo-fused [16]annulene with Möbius topology, synthesized by the Herges group (Nature 2007), motivated us to explore the possibility of designing a truly aromatic [16]annulene system. We chose to study alkyl substituents (e.g., methyls) to avoid the Clar effect, which is present for benzo-fused systems. This work is still in the exploratory phase. But already, using DFT (B3LYP/6-31G*), we have located an isomer that shows bond alternation en par with the synthesized benzo-fused [16]annulene of the Herges group.
