{"id":5968,"date":"2011-12-18T21:17:35","date_gmt":"2011-12-18T21:17:35","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5968"},"modified":"2011-12-18T21:17:35","modified_gmt":"2011-12-18T21:17:35","slug":"quadruple-antarafacial-delight","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","title":{"rendered":"Quadruple antarafacial delight."},"content":{"rendered":"
\n

A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH)\u00a0review<\/a> as achieving the former, and here I take a (sideways) look at one of their predictions.<\/p>\n

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

Compound 286<\/strong> was imagined by them as a \u03c02<\/sub>+ \u03c02<\/sub>+ \u03c02<\/sub>+ \u03c02<\/sub> cycloaddition sustaining four antarafacial<\/em> bond formations across the four bonds labelled\u00a0a.\u00a0<\/em><\/span> To really see this is true, you have to look at a 3D model (if you can see this without a model, you must have a superb stereochemical brain) and inspect how the bonds form with respect to any of the four alkene units. What other patterns are perceptible?<\/p>\n

    \n
  1. Four electron pairs are involved in the reaction. In terms of aromaticity, this is a 4n electron process, and for an even (or zero) number of antarafacial components, this would be anti-aromatic transition state.<\/li>\n
  2. Following their rules, WH suggested that the instead, the photochemical cyclisation would be an allowed pericyclic process. Clearly, attempts were made to synthesise 286<\/strong> to test this hypothesis, but they appear to have been unsuccessful<\/a> and so the prediction remains unproven.<\/li>\n
  3. The arrow pushing shown for 286<\/strong> also reminds that it could be considered as a homo(anti)aromatic<\/em> molecule, with the circulation of arrows being reminiscent of how the two resonance forms of benzene can be interconverted. In this case, the arrows circulate to the dashed lines, where no pre-existing \u03c3-bond is already there (a homo-conjugated motif).<\/li>\n
  4. Putting all this together, I decided that the molecule on the right might be interesting in its own right. This is currently unknown (either by synthesis, or by calculation). By virtue of having an additional double bond, one converts it to a 4n+2 electron system. This now is an allowed thermal pericyclic process with the following aspects:\n
      \n
    1. All three alkenes and the butadiene motif would still form their bonds antarafacially<\/em>.<\/li>\n
    2. This could also be regarded as a quadruple homoaromatic<\/em> system<\/li>\n
    3. One can ask how synchronous the formation of the four new \u03c3-bonds might be?<\/li>\n
    4. And in this regard, six electrons have no problem moving synchronously, but what about ten?<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

      Well, here is the calculated transition state<\/a> for the reaction. It actually has C2<\/em><\/sub> symmetry and so is a chiral geometry.<\/p>\n

      \"\"

      Transition state for 10 electron circulation. Click for 3D.<\/p><\/div>Of the four forming \u03c3-bonds, the surprise is that two of them are really very short (1.751\u00c5) and these involve the butadiene, and the other two are much longer (2.142\u00c5) and involve the alkenes. The former really are short enough<\/a>\u00a0to call a M\u00f6bius homo-aromatic bond!<\/p>\n

      Sadly, as the IRC below shows<\/a>, the free energy activation barrier for the (endothermic) reaction is not thermally accessible, and so there would be little point to encourage anyone to try to synthesise this species. But it certainly shows that even hypothetical reactions like this can have interesting features worth learning about.<\/p>\n

      \"\"<\/a>

      IRC for 2+2+2+2 cycloaddition<\/p><\/div>\n\n\n\n
      –\"\"<\/a><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH)\u00a0review as achieving the former, and here I take a (sideways) look at one of their predictions. Compound 286 was imagined […]<\/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":false,"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":[2650],"ppma_author":[2661],"class_list":["post-5968","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-pericyclic"],"yoast_head":"\nQuadruple antarafacial delight. - 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=5968\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Quadruple antarafacial delight. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH)\u00a0review as achieving the former, and here I take a (sideways) look at one of their predictions. Compound 286 was imagined […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-12-18T21:17:35+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg\" \/>\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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Quadruple antarafacial delight. - 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=5968","og_locale":"en_GB","og_type":"article","og_title":"Quadruple antarafacial delight. - Henry Rzepa's Blog","og_description":"A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. 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Compound 286 was imagined […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","og_site_name":"Henry Rzepa's Blog","article_published_time":"2011-12-18T21:17:35+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"3 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Quadruple antarafacial delight.","datePublished":"2011-12-18T21:17:35+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968"},"wordCount":508,"commentCount":2,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg","keywords":["pericyclic"],"articleSection":["Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","name":"Quadruple antarafacial delight. - 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=5968#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg","datePublished":"2011-12-18T21:17:35+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=5968#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#primaryimage","url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg","contentUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg"},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"Quadruple antarafacial delight."}]},{"@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_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pDef7-1yg","jetpack-related-posts":[{"id":5927,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url_meta":{"origin":5968,"position":0},"title":"Molecular gymnastics in 2+2 cycloadditions.","author":"Henry Rzepa","date":"December 14, 2011","format":false,"excerpt":"In this earlier post, I described how the stereochemistry of \u03c02+\u03c02 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating\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\/2011\/12\/14d1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4592,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4592","url_meta":{"origin":5968,"position":1},"title":"The stereochemistry of [8+2] pericyclic cycloadditions.","author":"Henry Rzepa","date":"July 10, 2011","format":false,"excerpt":"Steve Bachrach has blogged on the reaction shown below. If it were a pericyclic cycloaddition, both new bonds would form simultaneously, as shown with the indicated arrow pushing. Ten electrons would be involved, and in theory, the transition state would have 4n+2 aromaticity. In fact\u00a0Fernandez, Sierra and Torres have reported\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\/2011\/07\/2%2B8.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5655,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655","url_meta":{"origin":5968,"position":2},"title":"A modern take on pericyclic cycloaddition. Dimerisation of cis-butene","author":"Henry Rzepa","date":"November 28, 2011","format":false,"excerpt":"The \u03c02 + \u03c02 cyclodimerisation of cis-butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions. The photochemical reaction is known to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all-cis\u2026","rel":"","context":"In \"energy\"","block_context":{"text":"energy","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=energy"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/2%2B2-exo.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6977,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6977","url_meta":{"origin":5968,"position":3},"title":"The direct approach is not always the best: ethene + dichlorocarbene","author":"Henry Rzepa","date":"June 12, 2012","format":false,"excerpt":"The reaction between a carbene and an alkene to form a cyclopropane is about as simple a reaction as one can get. But I discussed before how simple little molecules (cyclopropenyl anion) can hold surprises. So consider this reaction: The reaction is a 4-electron pericyclic process, and so is subject\u2026","rel":"","context":"In \"limbo dancer\"","block_context":{"text":"limbo dancer","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=limbo-dancer"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/06\/dcc.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1158,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1158","url_meta":{"origin":5968,"position":4},"title":"Mechanistic M\u00e9nage \u00e0 trois","author":"Henry Rzepa","date":"November 18, 2009","format":false,"excerpt":"Curly arrow pushing is one of the essential tools of a mechanistic chemist. Many a published article will speculate about the arrow pushing in a mechanism, although it is becoming increasingly common for these speculations to be backed up by quantitative quantum mechanical and dynamical calculations. These have the potential\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":"Oxygen-nitrogen exchange between three nitrosonium cations","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/11\/no-exchange.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":5968,"position":5},"title":"Another Woodward pericyclic example dissected: all is not what it seems.","author":"Henry Rzepa","date":"May 22, 2013","format":false,"excerpt":"Here is another example gleaned from that Woodward essay of 1967 (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249), where all might not be what it seems. Woodward notes that the reaction between the (highly reactive) 1 does not occur. This is attributed to it being a disallowed \u03c06 +\u2026","rel":"","context":"In \"free energy barrier\"","block_context":{"text":"free energy barrier","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=free-energy-barrier"},"img":{"alt_text":"w2+2+2","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/w2%2B2%2B2.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\/5968","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=5968"}],"version-history":[{"count":17,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5968\/revisions"}],"predecessor-version":[{"id":5990,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5968\/revisions\/5990"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5968"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5968"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5968"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=5968"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}