{"id":11421,"date":"2013-10-20T15:57:09","date_gmt":"2013-10-20T14:57:09","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11421"},"modified":"2013-10-20T17:06:04","modified_gmt":"2013-10-20T16:06:04","slug":"six-vs-ten-aromatic-electrons","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11421","title":{"rendered":"Six vs ten aromatic electrons?"},"content":{"rendered":"
\n

Homoaromaticity is a special case of\u00a0aromaticity\u00a0in which\u00a0\u03c0-conjugation\u00a0is interrupted by a single sp3<\/sup>\u00a0hybridized\u00a0carbon atom (it is sometimes referred to as a suspended \u03c0-bond with no underlying \u03c3-foundation).\u00a0But consider the carbene shown below. This example comes from a recently published article[1]<\/a><\/span> which was highlighted on Steve Bachrach’s blog<\/a>. Here aromaticity has resulted from a slightly different phenomenon, whereby a\u00a04\u03c0-electron planar (and hence nominally anti-aromatic) molecule is elevated to aromatic peerage by promoting the two carbene \u03c3-electrons to have \u03c0-status.\u00a0<\/p>\n

\"B10\"<\/a><\/p>\n

Normally, such\u00a0electrons in a \u03c3 framework are considered to be more stable than in a p-\u03c0-framework,\u2021<\/sup> because the s-character of the former does not have a node at the nucleus and hence the electrons are bound more strongly. In this case however, the transformation from anti-aromaticity to aromaticity provides more than enough stabilisation through resonance for the\u00a0\u03c3-to-\u03c0 promotion to occur.<\/p>\n

So here I ask a different question to that posed on the aforementioned blog or article; could you achieve the same result using ten<\/strong> electrons rather than six? On the left of the diagram above for the 10-case, we have a planar 8-ring with 8 p-\u03c0-electrons and two carbene electrons. Promoting the latter would produce a 10\u03c0-aromatic ring (for another example of such, see here<\/a>). Indeed so![2]<\/a><\/span> This 10\u03c0-system has five occupied \u03c0-MOs,\u00a0a low energy delocalised \u03c3-MO of interest (below; is it\u00a0\u03c3-aromatic?) and a LUMO corresponding to the vacated carbene orbital.<\/p>\n

\"Click

Click for 3D<\/p><\/div>\n

ELF analysis (below) also reveals no carbene lone pair (basins 20 and 21 have 0.44e each, and 18\/19 2.70 each, giving that central carbon 6.28e, resembling the vinyl carbocation shown above in resonance).\u00a0<\/p>\n

\"Click

Click for 3D<\/p><\/div>\n

Not all is rosy however. This species is in fact not a stable minimum, but a transition state interconverting two buckled (and hence non-aromatic) conformations. It seems that the additional angular ring strain induced by an 8-membered ring has pushed it over the edge from aromatic royalty to non-aromatic commoness (remember this<\/a>?). But that apart, we can see that 10 electrons can behave similarly to six in inducing a two-electron promotion from a carbene lone pair. Not quite homoaromaticity; \u00a0I think it should be given its own named aromatic type! Any suggestions?\u00a0<\/p>\n

\n

\u2021<\/sup>That is certainly largely true of carbon. But much less true for the more electropositive boron. Thus it is not unusual to find such promotions occurring for planar boron frameworks.<\/p>\n

References<\/h2>\n
  1. B. Chen, A.Y. Rogachev, D.A. Hrovat, R. Hoffmann, and W.T. Borden, \"How to Make the \u03c3<sup>0<\/sup>\u03c0<sup>2<\/sup> Singlet the Ground State of Carbenes\", Journal of the American Chemical Society<\/i>, vol. 135, pp. 13954-13964, 2013. https:\/\/doi.org\/10.1021\/ja407116e<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Homoaromaticity is a special case of\u00a0aromaticity\u00a0in which\u00a0\u03c0-conjugation\u00a0is interrupted by a single sp3\u00a0hybridized\u00a0carbon atom (it is sometimes referred to as a suspended \u03c0-bond with no underlying \u03c3-foundation).\u00a0But consider the carbene shown below. This example comes from a recently published article which was highlighted on Steve Bachrach’s blog. Here aromaticity has resulted from a slightly different phenomenon, […]<\/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,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","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":[4],"tags":[578,20],"ppma_author":[2661],"class_list":["post-11421","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-low-energy","tag-steve-bachrach"],"yoast_head":"\nSix vs ten aromatic electrons? - 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=11421\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Six vs ten aromatic electrons? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Homoaromaticity is a special case of\u00a0aromaticity\u00a0in which\u00a0\u03c0-conjugation\u00a0is interrupted by a single sp3\u00a0hybridized\u00a0carbon atom (it is sometimes referred to as a suspended \u03c0-bond with no underlying \u03c3-foundation).\u00a0But consider the carbene shown below. 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