{"id":31787,"date":"2026-08-20T14:36:11","date_gmt":"2026-08-20T13:36:11","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787"},"modified":"2026-08-20T14:36:42","modified_gmt":"2026-08-20T13:36:42","slug":"how-long-can-a-c-c-bond-get-genuine-bis-homoaromatic-molecules-or-are-they-33-sigmatropic-transition-states","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","title":{"rendered":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states."},"content":{"rendered":"
\n

I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.[1]<\/a><\/span>,[2]<\/a><\/span>,[3]<\/a><\/span>. This system can apparently exhibit very long C-C bonds in the region of 2.1\u00c5, in which form it would be called a “frozen” transition state, also referred to a bis-homoaromatic<\/em> molecule, which in this form is NOT regarded as a [3,3]sigmatropic pericyclic transition state. Time for an update I thought, starting with a crystal structure search (Figure 1) for these types of species.<\/p>\n

\"\"<\/p>\n

Figure 1<\/strong>. Search query.<\/p>\n

\"\"<\/p>\n

Figure 2<\/strong>. C-C bond lengths, showing publication date.<\/p>\n

\"\"<\/p>\n

Figure 3<\/strong>. C-C bond lengths showing measurement temperature.<\/p>\n

The bottom right region of these maps (Figures 2 and 3) contains examples for which both DIST and DIST2 are more or less equal with values of ~2.1A. Note that all but one have been measure at room temperatures, which means that the structure is measured with a lot of thermal motions for the atoms. Some molecules have been singled out from the map above and two of those have been selected for DFT calculations – ICILAI\u00a0which is characterised by dispersion attractions between ethyl groups on the bulvallene unit and CASFUE which instead has polar cyano groups.<\/p>\n\n\n\n\n\n\n\n\n
CSD Entry<\/th>\nC-C 1<\/th>\nC-C 2<\/th>\nTemp<\/th>\nDOI<\/th>\n<\/tr>\n
ICILAI<\/td>\n1.931<\/td>\n2.084<\/td>\nRT<\/td>\n[4]<\/a><\/span><\/td>\n<\/tr>\n
ICILAI01<\/td>\n1.690<\/td>\n2.325<\/td>\n140<\/td>\n[5]<\/a><\/span><\/td>\n<\/tr>\n
ICIKUB<\/td>\n1.972<\/td>\n2.017<\/td>\nRT<\/td>\n[4]<\/a><\/span><\/td>\n<\/tr>\n
CAZFUE01<\/td>\n1.990<\/td>\n1.996<\/td>\nRT<\/td>\n[6]<\/a><\/span><\/td>\n<\/tr>\n
LAGHOG03<\/td>\n1.671<\/td>\n2.206<\/td>\n15<\/td>\n[7]<\/a><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Note that ICILAI, which has more or less equal bonds when measured at RT, shows very unequal bonds at\u00a0140K. The question we are asking here is whether there are any examples where the equal and long C-C bonds genuinely persist down to low temperatures and which therefore constitute especially “long” C-C bonds as part of a bis-homoaromatic<\/em> molecule. The “bond” here would not be an abnormally long conventional \u03c3-bond but instead what is called a “suspended \u03c0-bond”\u00a0and considered a “normal” length for such an interaction.<\/p>\n\n\n\n\n\n
Entry<\/th>\n\u03bdC-C<\/sub>
\nMN15L\/Def2-TZVPP<\/th>\n		DOI MN15L<\/th>\n\u03bdC-C<\/sub>
\nr2SCAN-3c\/Def2-mTZVPPL[8]<\/a><\/span><\/th>\n		\u03bdC-C<\/sub>
\n\u03c9B97XD\/Def2-TZVPP[9]<\/a><\/span><\/th>\n		DOI \u03c9B97XD<\/th>\n<\/tr>\n
CAZFUE01<\/td>\n+214.7<\/td>\n[10]<\/a><\/span>,[11]<\/a><\/span><\/td>\n+234.1<\/td>\n-281.3<\/td>\n[12]<\/a><\/span>,[13]<\/a><\/span><\/td>\n<\/tr>\n
ICILAI<\/td>\n-38.6<\/td>\n[14]<\/a><\/span><\/td>\n-155.8<\/td>\npend<\/td>\n[15]<\/a><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

What emerges from the calculations is that the characterisation as either a [3,3]-sigmatropic pericyclic transition state or as a bis-homoaromatic ground state is very dependent on the DFT functional being used.<\/p>\n

    \n
  1. For CAZFUE the conventional \u03c9B97XD\/Def2-TZVPP method gives results belonging to the classical camp of predicting both examples above as being [3,3] sigmatropic transition states, with a characteristic strong calculated negative force constant and an IRC that shows the equal bond length position only as an energy maximum along the transition coordinate, albeit with a relatively modest barrier of ~5 kcal\/mol. Such a value is higher than the energy of thermal vibrations, and so at low temperatures the structure will be frozen to either end of the IRC and show unequal C-C bonds of 1.614\u00c5 and 2.338\u00c5, typical of those found in the table above.
    \n    \"\"<\/a>
    \nFigure 4<\/strong>. \u03c9B97XD\/Def2-TZVPP IRC for CAZFUE.<\/li>\n
  2. The MN15L DFT method (which is optimised specifically for transition metal elements) predicts a symmetrical long bond bis-homoaromatic structure\u00a0(C-C 2.135\u00c5) for CAZFUE, with a positive antisymmetric C-C stretch of +215 cm-1<\/sup>, almost equal in magnitude to \u03c9B97XD but opposite in sign for the force constant (Figure 5).
    \n\"\"
    \nFigure 5<\/strong>. MN15L normal vibrational C-C mode for CAZFUI.<\/li>\n
  3. The newer r2<\/sup>SCAN-3c\/Def2-mTZVPP functional is very similar in behaviour to MN15L.<\/li>\n
  4. With MN15L, ICILAI however weakly retains the classical transition state mode, but now the C-C antisymmetric stretch is small with a negative FC (Figure 6). One wonders whether the difference is due to the dispersion attractions vs<\/em> the electronic effect of a cyano group? The \u00a0r2<\/sup>SCAN-3c\/Def2-mTZVPP method predicts a larger -ve FC, but not as large as using \u03c9B97XD.
    \n\"\"
    \nFigure 6<\/strong>. MN15L normal vibrational C-C mode for ICILAI.<\/li>\n
  5. In contrast to both MN15L and to a lesser extent \u00a0r2<\/sup>SCAN-3c\/Def2-mTZVPP, using \u03c9B97XD for ICILAI reveals it is now strongly characterised as a [3,3] transition state.<\/li>\n<\/ol>\n

    I note from Figure 2 that many of these crystal structures are quite old – clearly this area of research is no longer highly active. But such structures are proving invaluable tests of how density functional procedures perform for these molecules. Thus r2<\/sup>SCAN-3c\/Def2-mTZVPP as a recent new method[8]<\/a><\/span> is giving significantly different results from the much older \u03c9B97XD functional. But is it correct? More accurate and reliable crystal structures might help provide an answer – perhaps obtained using quantum crystallography?<\/p>\n

    References<\/h2>\n
    1. H. Rzepa, \"Predicted properties of a candidate for a frozen semibullvalene.\", 2012. https:\/\/doi.org\/10.59350\/cq2a3-g2g35<\/a>\n\n<\/li>\n
    2. H. Rzepa, \"Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).\", 2012. https:\/\/doi.org\/10.59350\/585c3-dpy86<\/a>\n\n<\/li>\n
    3. H. Rzepa, \"The ten-electron homologue of semibullvalene.\", 2012. https:\/\/doi.org\/10.59350\/8nkna-ycy23<\/a>\n\n<\/li>\n
    4. C. Wang, J. Yuan, G. Li, Z. Wang, S. Zhang, and Z. Xi, \"Metal-Mediated Efficient Synthesis, Structural Characterization, and Skeletal Rearrangement of Octasubstituted Semibullvalenes\", Journal of the American Chemical Society<\/i>, vol. 128, pp. 4564-4565, 2006. https:\/\/doi.org\/10.1021\/ja0579208<\/a>\n\n<\/li>\n
    5. S. Zhang, M. Zhan, Q. Wang, C. Wang, W. Zhang, and Z. Xi, \"Synthesis of semibullvalene derivatives via Co\n <sub>2<\/sub>\n (CO)\n <sub>8<\/sub>\n -mediated cyclodimerization of 1,4-dilithio-1,3-butadienes\", Org. Chem. Front.<\/i>, vol. 1, pp. 130-134, 2014. https:\/\/doi.org\/10.1039\/c3qo00019b<\/a>\n\n<\/li>\n
    6. 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
    7. R.V. Williams, V.R. Gadgil, P. Luger, T. Koritsanszky, and M. Weber, \"The Search for Homoaromatic Semibullvalenes. 6.<sup>1<\/sup>X-ray Structure and Charge Density Studies of 1,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride in the Temperature Range 123\u221215 K\", The Journal of Organic Chemistry<\/i>, vol. 64, pp. 1180-1190, 1999. https:\/\/doi.org\/10.1021\/jo981703q<\/a>\n\n<\/li>\n
    8. J. Chai, and M. Head-Gordon, \"Long-range corrected hybrid density functionals with damped atom\u2013atom dispersion corrections\", Physical Chemistry Chemical Physics<\/i>, vol. 10, pp. 6615, 2008. https:\/\/doi.org\/10.1039\/b810189b<\/a>\n\n<\/li>\n
    9. H. Rzepa, \"CAZFUE01 MN15L\/Def2-TZVPP => C2 symmetry => bis(homoaromatic) ? Gas phase\", 2026. https:\/\/doi.org\/10.5281\/zenodo.22009539<\/a>\n\n<\/li>\n
    10. H. Rzepa, \"CAZFUE01 => C2 symmetry => bis(homoaromatic) ? STABLE wavefunction? Gas phase\", 2026. https:\/\/doi.org\/10.5281\/zenodo.22009550<\/a>\n\n<\/li>\n
    11. H. Rzepa, \"CAZFUE01 wB97XD\/Def2-TZVPP => C2 symmetry => bis(homoaromatic) transition state\", 2026. https:\/\/doi.org\/10.5281\/zenodo.22012011<\/a>\n\n<\/li>\n
    12. H. Rzepa, \"CAZFUE01 wB97XD\/Def2-TZVPP => C2 symmetry => bis(homoaromatic) transition state Gas phase IRC\", 2026. https:\/\/doi.org\/10.5281\/zenodo.22012031<\/a>\n\n<\/li>\n
    13. H. Rzepa, \"ICILAI MN15L \/Def2-TZVPP = TS (nu -31 cm-1). Gas phase\", 2026. https:\/\/doi.org\/10.5281\/zenodo.22009559<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.,,. This system can apparently exhibit very long C-C bonds in the region of 2.1\u00c5, in which form it would be called a “frozen” transition state, also referred to a bis-homoaromatic […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federated","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},"jetpack_post_was_ever_published":false},"categories":[1745,559,1086],"tags":[],"ppma_author":[2661],"class_list":["post-31787","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-pericyclic","category-reaction-mechanism-2"],"yoast_head":"\nHow long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states. - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.,,. This system can apparently exhibit very long C-C bonds in the region of 2.1\u00c5, in which form it would be called a “frozen” transition state, also referred to a bis-homoaromatic […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2026-08-20T13:36:11+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-08-20T13:36:42+00:00\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"5 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","og_locale":"en_GB","og_type":"article","og_title":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states. - Henry Rzepa's Blog","og_description":"I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.,,. This system can apparently exhibit very long C-C bonds in the region of 2.1\u00c5, in which form it would be called a “frozen” transition state, also referred to a bis-homoaromatic […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","og_site_name":"Henry Rzepa's Blog","article_published_time":"2026-08-20T13:36:11+00:00","article_modified_time":"2026-08-20T13:36:42+00:00","author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"5 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states.","datePublished":"2026-08-20T13:36:11+00:00","dateModified":"2026-08-20T13:36:42+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787"},"wordCount":807,"commentCount":0,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/08\/semi-bullvalene.jpg","articleSection":["crystal_structure_mining","pericyclic","reaction mechanism"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","name":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states. - Henry Rzepa's Blog","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website"},"primaryImageOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/08\/semi-bullvalene.jpg","datePublished":"2026-08-20T13:36:11+00:00","dateModified":"2026-08-20T13:36:42+00:00","author":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"breadcrumb":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#primaryimage","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/08\/semi-bullvalene.jpg","contentUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/08\/semi-bullvalene.jpg","width":1151,"height":508},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"How long can a C-C bond get? Genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states."}]},{"@type":"WebSite","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/","name":"Henry Rzepa's Blog","description":"Chemistry with a twist","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-GB"},{"@type":"Person","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281","name":"Henry Rzepa","image":{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g370be3a7397865e4fd161aefeb0a5a85","url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","contentUrl":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","caption":"Henry Rzepa"},"description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London.","sameAs":["https:\/\/orcid.org\/0000-0002-8635-8390"],"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?author=1"}]}},"jetpack_publicize_connections":[],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pDef7-8gH","jetpack-related-posts":[{"id":7678,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7678","url_meta":{"origin":31787,"position":0},"title":"Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).","author":"Henry Rzepa","date":"September 15, 2012","format":false,"excerpt":"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\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/CAZFUE1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":7721,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7721","url_meta":{"origin":31787,"position":1},"title":"Predicted properties of a candidate for a frozen semibullvalene.","author":"Henry Rzepa","date":"September 17, 2012","format":false,"excerpt":"I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. The hypothesis was that such a species would be aromatic, a bis-homoaromatic\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/dicyano-diaza-aim.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8961,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8961","url_meta":{"origin":31787,"position":2},"title":"The mechanism of the Benzidine rearrangement.","author":"Henry Rzepa","date":"January 6, 2013","format":false,"excerpt":"The benzidine rearrangement is claimed to be an example of the quite rare\u00a0[5,5] sigmatropic migration, which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route. The topic has been reviewed here.\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"NCI surface. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/benzidinenci.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10743,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10743","url_meta":{"origin":31787,"position":3},"title":"Mechanism of the Boekelheide rearrangement","author":"Henry Rzepa","date":"June 26, 2013","format":false,"excerpt":"A reader asked me about the mechanism of\u00a0the reaction of 2-picoline N-oxide with acetic anhydride to give 2-acetoxymethylpyridine (the\u00a0Boekelheide Rearrangement). He wrote \"\u00a0I don't understand why the system should prefer to go via fragmentation-recombination (... the evidence being that\u00a0oxygen labelling shows scrambling)\u00a0when there is an easy concerted pathway available (...\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"Boek1","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/06\/Boek1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":25633,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25633","url_meta":{"origin":31787,"position":4},"title":"A new type of bispericyclic reaction: Cyclopropanone + butadiene.","author":"Henry Rzepa","date":"September 30, 2022","format":false,"excerpt":"The term bispericyclic reaction was famously coined by Caramella et al in 2002 to describe the unusual features of the apparently innocuous dimerisation of cyclopentadiene. It shows features of two paths for different pericyclic reactions, comprising a 2+4 cycloaddition in the early stages, but evolving into a (degenerate) pair of\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/09\/anomalous.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":23281,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23281","url_meta":{"origin":31787,"position":5},"title":"The Stevens rearrangement: how history gives us new insights.","author":"Henry Rzepa","date":"January 29, 2021","format":false,"excerpt":"In a recent post, I told the story of how in the early 1960s, Robert Woodward had encountered an unexpected stereochemical outcome to the reaction of a hexatriene, part of his grand synthesis of vitamin B12. He had constructed a model of the reaction he wanted to undertake, perhaps with\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/01\/sden.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31787","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=31787"}],"version-history":[{"count":64,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31787\/revisions"}],"predecessor-version":[{"id":31859,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31787\/revisions\/31859"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=31787"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=31787"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=31787"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=31787"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}