{"id":7389,"date":"2012-07-29T15:43:24","date_gmt":"2012-07-29T14:43:24","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7389"},"modified":"2012-12-31T11:53:00","modified_gmt":"2012-12-31T11:53:00","slug":"cyclopentadiene-a-hydrocarbon-at-the-crossroads-of","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389","title":{"rendered":"Cyclopentadiene: a hydrocarbon at the crossroads of …"},"content":{"rendered":"
\n

organic chemistry. It does not look like much, but this small little molecule brought us ferrocene, fluxional NMR, aromatic anions and valley-ridge inflexion points. You might not have heard of this last one, but in fact I mentioned<\/a> the phenomenon in my post on nitrosobenzene. As for being at a crossroads, more like a Y-junction. Let me explain why.<\/p>\n

\"\"<\/p>\n

Cyclopentadiene is made by thermal cracking of its dimer, and on standing it slowly reverts to this species. At its simplest, this dimerisation can be described as a \u03c0<\/sub>2s<\/sub> + \u03c0<\/sub>4s<\/sub>\u00a0pericyclic cycloaddition, one of the monomers being the \u03c0<\/sub>2s <\/sub>and the other the \u03c0<\/sub>4s<\/sub>. Two new bonds are formed; one of these is shown in black, but the other can be either the one in red (which makes the \u03c0<\/sub>4s<\/sub> the monomer on the right) or the one in blue (in which case the \u03c0<\/sub>4s<\/sub> comes from the molecule on the left). How do these two partners decide which role each is to play? Well, the short answer is that, initially at least, they do not! The reaction proceeds very asynchronously, forming at first only the black bond. Eventually, they cannot take the suspense any longer, and when the point indicated with a green dot is reached, they finally have to take a decision. Up to the green dot, the potential energy surface has followed along a valley ridge, and the green decision point is known as the bifurcation point; one with an equal probability of the reaction giving either the top dimer or the bottom dimer.<\/p>\n

\"\"<\/p>\n

If you are sharp-eyed you may notice a methyl group has been added to one of the monomers; this was done to balance the decision very slightly in favour of one route down from the green point over the other. Otherwise, the IRC pathway often just stops at the green point, unable to decide which way to take.<\/p>\n

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

You can see this oddity reflected in the gradient norm of the IRC, which at IRC -1.5 suddenly acquires a new feature, the formation of the second bond. The lesson here is to remember that bonds do not have to form at the same time, they can instead follow, one after the other.<\/p>\n

The two different dimers that result from the bifurcation are not in fact identical, they are mirror images (diastereomers because of the methyl group) of each other. They can in turn be inter-converted by a Cope rearrangement, a [3,3] sigmatropic reaction. The transition state for this process is none other than the green point reached earlier. It is indeed a transition state at a crossroads, connecting two quite different reactions, the Diels-Alder cycloaddition and the [3,3] Cope enantiomerisation of the dimer product. Such a reaction has been christened a bispericyclic reaction<\/a>, one truly at a Y-junction.<\/p>\n

Who would have thought that such an un-pretentious\u00a0molecule could teach us so much. You can see this and many other examples of pericylic reactions in my course on the topic, available on an iPad by clicking here<\/a>.<\/p>\n

Postscript:<\/strong> I have managed to run a full IRC<\/a> on the system without the methyl perturbation.<\/p>\n

\"\"<\/p>\n

The bifurcation point (green dot) is clearly seen in the following two plots at a value of \u00a0IRC +1.0<\/p>\n\n\n\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

organic chemistry. It does not look like much, but this small little molecule brought us ferrocene, fluxional NMR, aromatic anions and valley-ridge inflexion points. You might not have heard of this last one, but in fact I mentioned the phenomenon in my post on nitrosobenzene. As for being at a crossroads, more like a Y-junction. […]<\/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":"","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":[],"tags":[900,328,2650,317,142,843],"ppma_author":[2661],"class_list":["post-7389","post","type-post","status-publish","format-standard","hentry","tag-dimer-product","tag-ipad","tag-pericyclic","tag-postscript","tag-potential-energy-surface","tag-reaction-mechanism"],"yoast_head":"\nCyclopentadiene: a hydrocarbon at the crossroads of ... - 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=7389\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Cyclopentadiene: a hydrocarbon at the crossroads of ... - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"organic chemistry. It does not look like much, but this small little molecule brought us ferrocene, fluxional NMR, aromatic anions and valley-ridge inflexion points. You might not have heard of this last one, but in fact I mentioned the phenomenon in my post on nitrosobenzene. As for being at a crossroads, more like a Y-junction. […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-07-29T14:43:24+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-12-31T11:53:00+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.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":"Cyclopentadiene: a hydrocarbon at the crossroads of ... - 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=7389","og_locale":"en_GB","og_type":"article","og_title":"Cyclopentadiene: a hydrocarbon at the crossroads of ... - Henry Rzepa's Blog","og_description":"organic chemistry. It does not look like much, but this small little molecule brought us ferrocene, fluxional NMR, aromatic anions and valley-ridge inflexion points. You might not have heard of this last one, but in fact I mentioned the phenomenon in my post on nitrosobenzene. As for being at a crossroads, more like a Y-junction. […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389","og_site_name":"Henry Rzepa's Blog","article_published_time":"2012-07-29T14:43:24+00:00","article_modified_time":"2012-12-31T11:53:00+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.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=7389#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Cyclopentadiene: a hydrocarbon at the crossroads of …","datePublished":"2012-07-29T14:43:24+00:00","dateModified":"2012-12-31T11:53:00+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389"},"wordCount":540,"commentCount":6,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.svg","keywords":["dimer product","iPad","pericyclic","Postscript","potential energy surface","Reaction Mechanism"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389","name":"Cyclopentadiene: a hydrocarbon at the crossroads of ... - 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=7389#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.svg","datePublished":"2012-07-29T14:43:24+00:00","dateModified":"2012-12-31T11:53:00+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=7389#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389#primaryimage","url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.svg","contentUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/cyclopentadiene.svg"},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7389#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"Cyclopentadiene: a hydrocarbon at the crossroads of …"}]},{"@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\/pDef7-1Vb","jetpack-related-posts":[{"id":4592,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4592","url_meta":{"origin":7389,"position":0},"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":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":7389,"position":1},"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":[]},{"id":9,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9","url_meta":{"origin":7389,"position":2},"title":"A Disrotatory 4n+2 electron anti-aromatic M\u00f6bius transition state for a thermal electrocyclic reaction.","author":"Henry Rzepa","date":"April 2, 2009","format":false,"excerpt":"Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron M\u00f6bius-aromatic character. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a H\u00fcckel-aromatic 4n+2\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"Electrocylization of [14] annulene","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/p322.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5087,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5087","url_meta":{"origin":7389,"position":3},"title":"cis-Butene: a reaction coordinate dissected and methyl flags.","author":"Henry Rzepa","date":"October 12, 2011","format":false,"excerpt":"In two previous posts, I have looked at why cis-butene adopts conformation (a) rather than (b). I suggested it boiled down to electronic interactions between the methyl groups and the central alkene resulting in the formation of a H...H \"topological\" bond, rather than attraction between the H...H region to form\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/10\/cis-butene2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5927,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url_meta":{"origin":7389,"position":4},"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":9296,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9296","url_meta":{"origin":7389,"position":5},"title":"Secrets of a university tutor: unravelling a mechanism using spectroscopy.","author":"Henry Rzepa","date":"January 31, 2013","format":false,"excerpt":"It is always rewarding when one comes across a problem in chemistry that can be solved using a continuous stream of rules and logical inferences from them. The example below is one I have been using as a tutor in organic chemistry for a few years now, and I share\u2026","rel":"","context":"In \"final product\"","block_context":{"text":"final product","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=final-product"},"img":{"alt_text":"","src":"","width":0,"height":0},"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."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7389","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=7389"}],"version-history":[{"count":22,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7389\/revisions"}],"predecessor-version":[{"id":7399,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7389\/revisions\/7399"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7389"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7389"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7389"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=7389"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}