{"id":7678,"date":"2012-09-15T18:28:18","date_gmt":"2012-09-15T17:28:18","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7678"},"modified":"2012-09-15T18:28:18","modified_gmt":"2012-09-15T17:28:18","slug":"frozen-semibullvalene-a-holy-grail-and-a-bis-homoaromatic-molecule","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=7678","title":{"rendered":"Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule)."},"content":{"rendered":"
\n

Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic behaviour can be frozen out at sufficiently low temperatures. But the barrier was sufficiently low that a challenge was set; could one achieve a system in which the barrier was removed entirely, to freeze out the coordinates of the molecule into a structure where the transition state (shown at the top) became instead a true minimum (bottom)? A similar challenge<\/a>\u00a0had been set for freezing out the transition state for the Sn2 reaction into a minimum, the topic also of a more recent post<\/a> here. Here I explore how close we might be to achieving inversion of the semibullvalene [3,3] sigmatropic potential.<\/p>\n

\"\"<\/p>\n

Why might such a frozen transition state be interesting? Well, all transition states for allowed thermal<\/span> pericyclic reactions can be described as aromatic. If one were able to transmogrify such a transition state into a minimum, then it too would be expected to be aromatic, but a most unusual type of aromatic. The C-C bonds which represent the breaking and forming bonds in a [3,3] sigmatropic rearrangement would in effect be two-centre 1-electron bonds, and those electrons would be part of the aromatic sextet. Such bonds are normally referred to as homoaromatic, examples of which are pretty rare<\/a>. In my previous post<\/a>, I had noted a crystal structure[1]<\/a><\/span> that apparently sustains two equal C-C bonds of length\u00a01.99\u00c5. However, a calculation at this geometry<\/a> reveals it in fact to be a transition state (above, top), with an imaginary mode of 275i<\/em> cm-1<\/sup>. So the challenge (computationally at least) is to find a system where this imaginary mode is changed to become real rather than imaginary.<\/p>\n

\"\"

CAZFUE. Click for animation of imaginary transition state mode.<\/p><\/div>\n

My effort to achieve this involved augmenting CAZFUE with a further two cyano groups. This did indeed reduce the imaginary mode<\/a> to 74i<\/em> cm-1<\/sup>; we are getting close!\u00a0<\/p>\n

\"\"

Tetracyano derivative of CAZFUE. Click for animation.<\/p><\/div>\n

The next step was to read a recent article[2]<\/a><\/span> in which replacing the key C-C bond with a C-N bond was observed to reduce the barrier for the rearrangement to ~ 4 kcal\/mol. So I immediately computed the tetra-azo system<\/a>, in which the two key C-C bonds are now replaced by N-N bonds in order to extend this effect.<\/p>\n

\"\"

Tetra-azo semibullvalene. Click for animation of key frozen mode.<\/p><\/div>\n

It was gratifying to observe that the [3,3] sigmatropic vibration, imaginary (i.e.<\/em> corresponding to a transition state) in the previous examples, became +ve<\/strong> (+238 cm-1<\/sup>) in this system. The two N-N bonds are however not completely symmetric (2.06 and 2.17\u00c5), but they are in effect essentially frozen at the half-way stage of the equilibrium.<\/p>\n

The final step in this path is to combine the two effects above, by exploring the di-cyano-diaza derivative<\/a>.<\/p>\n

\"\"

Di-cyano, diazo derivative. Click for 3D.<\/p><\/div>\n

This now has C2<\/sub> (chiral)\u00a0exact two-fold symmetry<\/a>, with C-N distances of 2.139\u00c5. The [3,3] sigmatropic vibrational mode is again real, with a value of 255 cm-1<\/sup>. A real candidate for synthesis perhaps?<\/p>\n

Finally, is it aromatic? The wavefunction for this system is stable (which means no triplet state lower in energy can be found), so it stands a good chance of being so. I will report back on this aspect in a later post.<\/a><\/p>\n

\n

Postscript:<\/strong> The above calculation for the last system was done at the B3LYP\/6-311G(d,p)\/SCRF=thf level. A similar result is obtained at e.g.<\/em> a \u00a0MP2\/6-311G(d,p)\/SCRF=thf level<\/a>; the \u00a0[3,3] vibrational mode has the real value of 318 cm-1<\/sup>.<\/p>\n

References<\/h2>\n
  1. L.M. Jackman, A. Benesi, A. Mayer, H. Quast, E.M. Peters, K. Peters, and H.G. Von Schnering, \"The Cope rearrangement of 1,5-dimethylsemibullvalene-2,6- and 3,7-dicarbonitriles in the solid state\", Journal of the American Chemical Society<\/i>, vol. 111, pp. 1512-1513, 1989. https:\/\/doi.org\/10.1021\/ja00186a064<\/a>\n\n<\/li>\n
  2. S. Zhang, J. Wei, M. Zhan, Q. Luo, C. Wang, W. Zhang, and Z. Xi, \"2,6-Diazasemibullvalenes: Synthesis, Structural Characterization, Reaction Chemistry, and Theoretical Analysis\", Journal of the American Chemical Society<\/i>, vol. 134, pp. 11964-11967, 2012. https:\/\/doi.org\/10.1021\/ja305581f<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[6],"tags":[912,147,1529,294,1530],"class_list":["post-7678","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-candidate-for-synthesis-perhaps","tag-energy","tag-pericyclic","tag-postscript","tag-reaction-mechanism"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-1ZQ","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7678","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7678"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7678\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7678"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7678"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7678"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}