{"id":6977,"date":"2012-06-12T22:19:39","date_gmt":"2012-06-12T21:19:39","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=6977"},"modified":"2012-12-29T17:21:06","modified_gmt":"2012-12-29T17:21:06","slug":"the-direct-approach-is-not-always-the-best-ethene-dichlorocarbene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6977","title":{"rendered":"The direct approach is not always the best: ethene + dichlorocarbene"},"content":{"rendered":"
\n

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<\/a> how simple little molecules (cyclopropenyl anion) can hold surprises. So consider this reaction<\/a>:<\/p>\n

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

\"\"

Transition state for reaction between ethene and dichlorocarbene. Click for 4D.<\/p><\/div>\n

The reaction is a 4-electron pericyclic process, and so is subject to the Woodward-Hoffmann rules, which imply that such a 4n-thermal process should go with one antarafacial<\/em> component. But there is a (rarely cited or observed) alternative, as was illustrated for the \u03c02<\/sub>+\u03c02 <\/sub>cycloaddition of ethene to itself.<\/a>\u00a0There we saw the gymnastics of a limbo dancer, with one ethene sliding up to the other rather than taking a full-frontal approach. But whilst that reaction had an unrealistic activation barrier of ~50 kcal\/mol, the reaction between dichlorocarbene and an alkene is known to be a very facile one. And so the calculation shows (below). The barrier to reaction is small, and so this is an example of a low-barrier nominally forbidden reaction which nevertheless achieves a low barrier by avoiding the direct approach of the two molecules and adopting a round-about path!<\/p>\n\n\n\n\n
\"\"<\/a><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n
\"\"<\/a><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

This round-about approach is seen best in the IRC <\/a>for the addition to dicyano-ethene. Shown above is the gradient norm along the IRC.<\/p>\n

    \n
  1. From IRC -1.2 to 0.0 (the transition state) the reaction corresponds to the formation of effectively just one C-C bond (a two electron process if you like).<\/li>\n
  2. At IRC +2.0 a second distinct feature is seen in the graph, and this now corresponds to the formation of the second C-C bond, involving a sliding motion of the carbene (again, a two-electron process).<\/li>\n<\/ol>\n

    So by breaking a four-electron process into two phases, each involving just one electron pair, a lot of the forbidden Woodward-Hoffmann character seems to be avoided. Truly the direct approach not being the best!<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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 to the Woodward-Hoffmann rules, which […]<\/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":[],"tags":[853,2650,843,373],"ppma_author":[2661],"class_list":["post-6977","post","type-post","status-publish","format-standard","hentry","tag-limbo-dancer","tag-pericyclic","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nThe direct approach is not always the best: ethene + dichlorocarbene - 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=6977\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The direct approach is not always the best: ethene + dichlorocarbene - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 to the Woodward-Hoffmann rules, which […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6977\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-06-12T21:19:39+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-12-29T17:21:06+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/06\/dcc+ethene.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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The direct approach is not always the best: ethene + dichlorocarbene - 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=6977","og_locale":"en_GB","og_type":"article","og_title":"The direct approach is not always the best: ethene + dichlorocarbene - Henry Rzepa's Blog","og_description":"The reaction between a carbene and an alkene to form a cyclopropane is about as simple a reaction as one can get. 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A common question asked by\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":"","width":0,"height":0},"classes":[]},{"id":7964,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7964","url_meta":{"origin":6977,"position":3},"title":"Text-books and the bromination of ethene.","author":"Henry Rzepa","date":"October 14, 2012","format":false,"excerpt":"There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive \"reality\" revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example? Text-books will show how ethene interacts with bromine to form a cyclic bromonium cation, which with the\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\/10\/Br2b2.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":7580,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7580","url_meta":{"origin":6977,"position":4},"title":"The Sn2 reaction and the anomaly of carbon.","author":"Henry Rzepa","date":"September 6, 2012","format":false,"excerpt":"It was three years ago that I first blogged on the topic of the Sn2 reaction. Matthias Bickelhaupt had suggested that the Sn2 reaction involving displacement at a carbon atom was an anomaly; the true behaviour was in fact exhibited by the next element down in the series, silicon. The\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\/sn2-Na1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":18822,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18822","url_meta":{"origin":6977,"position":5},"title":"Hydrogen capture by boron: a crazy reaction path!","author":"Henry Rzepa","date":"September 21, 2017","format":false,"excerpt":"A recent article reports, amongst other topics, a computationally modelled reaction involving the capture of molecular hydrogen using a substituted borane (X=N, Y=C). The mechanism involves an initial equilibrium between React and Int1, followed by capture of the hydrogen by Int1 to form a 5-coordinate borane intermediate (Int2 below, as\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":"","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\/6977","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=6977"}],"version-history":[{"count":15,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6977\/revisions"}],"predecessor-version":[{"id":8843,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6977\/revisions\/8843"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6977"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6977"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6977"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=6977"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}