{"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_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},"jetpack_post_was_ever_published":false},"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. This example comes from a recently published article which was highlighted on Steve Bachrach’s blog. 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This example comes from a recently published article which was highlighted on Steve Bachrach’s blog. 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It has now been detected in the atmosphere of Titan, one of Saturn's moons and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C3H3+ also\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":"","width":0,"height":0},"classes":[]},{"id":11642,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11642","url_meta":{"origin":11421,"position":1},"title":"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene.","author":"Henry Rzepa","date":"November 13, 2013","format":false,"excerpt":"Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a \u03c0-system transformed it from a formally 4n-antiaromatic \u03c0-cycle into a 4n+2 aromatic \u03c0-cycle. 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The most ubiquitous type of carbene of recent times is the one shown below as 1, often referred to as a resonance stabilised or\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1903,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1903","url_meta":{"origin":11421,"position":3},"title":"Carbobenzene: benzene with a difference","author":"Henry Rzepa","date":"April 16, 2010","format":false,"excerpt":"Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002\/chem.200601193). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene. 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Formulating an analogous mechanism for such reaction of an alkyne\u00a0sounds straightforward, but one gradually realises that it requires raiding knowledge from several other areas of (perhaps slightly more advanced) chemistry to achieve a joined up\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/1.gif?resize=350%2C200","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","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11421","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=11421"}],"version-history":[{"count":16,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11421\/revisions"}],"predecessor-version":[{"id":11446,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11421\/revisions\/11446"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11421"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11421"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11421"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11421"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}