{"id":7068,"date":"2012-07-12T16:05:06","date_gmt":"2012-07-12T15:05:06","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7068"},"modified":"2015-01-07T16:51:23","modified_gmt":"2015-01-07T16:51:23","slug":"more-joining-up-of-pieces-stereocontrol-in-the-ring-opening-of-cyclopropenes","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7068","title":{"rendered":"More joining up of pieces. Stereocontrol in the ring opening of cyclopropenes."},"content":{"rendered":"
\n

Years ago, I was travelling from Cambridge to London on a train. I found myself sitting next to a chemist, and (as chemists do), he scribbled the following on a piece of paper. When I got to work the next day Vera (my student) was unleashed on the problem, and our thoughts were published<\/a>[1]<\/a><\/span>. That was then.<\/p>\n

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

This is now. I have just finished a post on ring-opening reactions of oxirene<\/a>, a 4n electron anti-aromatic ring. I was casting around for an example of a calculation done just before the modern Internet era, and happened upon the above. Although this was a mere 20 years ago, much of the detail had faded; I had not thought much about it in the intervening years, but I did recollect that although we had attributed the high stereoselectivity shown above to a stereoelectronic orbital alignment, I was not entirely happy with the interpretation. Put simply, we had relied on a molecular orbital diagram, and this diagram (in resplendent colour in the journal, one of the few being so published at that time, and for no colour charge to boot) was just too complicated. Arguably it was the fixated complexity (I remember at the time that it looked complicated no matter what the viewing angle was) that set me on the road to the use of the Web, and ultimately here to this blog. So I thought a re-analysis using modern algorithms and presentation might help simplify. The newly recalculated transition state (\u03c9B97XD\/6-311G(d,p)[2]<\/a><\/span> looks like:<\/p>\n

\"\"

Transition state for ring opening of a cyclopropene. Click for 3D.<\/p><\/div>\n

    \n
  1. The reaction is a 4n (n=1) electron electrocyclic ring opening and so according to the rules, should proceed with the formation\/cleavage of an antarafacial<\/em> bond. You might think that there are not quite enough substituents to reveal this stereochemistry, but there are if the carbene lone pair is included. So how to add the lone pair?<\/li>\n
  2. Well, its coordinates can be computed using the ELF (electron localisation function). The relevant lone pair is ringed in red below. Using (old technology, i.e.<\/em> a static figure) you may choose to believe me when I argue that this lone pair is above the plane of the forming ring from the perspective shown, whilst the terminus of the bond it forms is to the bottom. This defines an antarafacial<\/em> component. Well, I might have carefully manipulated the viewing angle to show this. Now, in 2012 rather than 1992, you can load the 3D coordinates by clicking below, and check for yourself!
    \n
    \"\"

    Lone pair centroid for the transition state. Click for 3D<\/p><\/div>\n<\/li>\n

  3. What about the stereo-control? Take a look at the angle between the axis of the C-Cl bond (atoms ringed in blue) and the centroid of the carbene lone pair (red). It is about 162\u00b0, or almost anti-periplanar. A magic orientation in organic chemistry. Time to attack the orbitals again. Our published diagram looked as below. It shows the HOMO aligning with the LUMO+2 (if your eyes are not distracted by all the other detail).\"\"
    But we can now simplify such a complex molecular orbital by using instead a localized version, an NBO. A little explanation is needed. The NBO orbital shown with red\/blue<\/strong> phases is antibonding for the C-Cl bond. That with orange\/purple<\/strong> is the carbene lone pair. Where orange overlaps with red, we have a positive overlap that stabilises the system. The NBO E2 perturbation energy is around 4.6 kcal\/mol. Although this may seem small, it is actually quite large for a through-space interaction of this type. It is this stabilisation (amounting to ~ 1.6 kcal\/mol in free energy) that accounts for the high selectivity for the stereoisomer shown above.
    \n
    \"\"

    NBO for transition state. Click for 3D.<\/p><\/div>\n<\/li>\n<\/ol>\n

    Well, I think that the passage of 20 years has enabled us to tidy up the origins of the stereoelectronic effect responsible for controlling this reaction, and to produce clearer diagrams which the reader can interactively explore for themselves. It did take 20 years to join things up though!<\/p>\n

    \"\"<\/a>[3]<\/a><\/span><\/p>\n

    References<\/h2>\n
    1. M.S. Baird, J.R. Al Dulayymi, H.S. Rzepa, and V. Thoss, \"An unusual example of stereoelectronic control in the ring opening of 3,3-disubstituted 1,2-dichlorocyclopropenes\", Journal of the Chemical Society, Chemical Communications<\/i>, pp. 1323, 1992. https:\/\/doi.org\/10.1039\/c39920001323<\/a>\n\n<\/li>\n
    2. H.S. Rzepa, \"C 5 H 5 Cl 3\", 2012. https:\/\/doi.org\/10.14469\/ch\/14180<\/a>\n\n<\/li>\n
    3. H.S. Rzepa, \"Gaussian Job Archive for C5H5Cl3\", 2015. https:\/\/doi.org\/10.6084\/m9.figshare.1285422<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      Years ago, I was travelling from Cambridge to London on a train. I found myself sitting next to a chemist, and (as chemists do), he scribbled the following on a piece of paper. When I got to work the next day Vera (my student) was unleashed on the problem, and our thoughts were published. That […]<\/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":[4,1086],"tags":[144,224,17,40,2651,862,860,2650,1305,861,843,894],"ppma_author":[2661],"class_list":["post-7068","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-cambridge","tag-chemist","tag-conformational-analysis","tag-free-energy","tag-historical","tag-internet-era","tag-london","tag-pericyclic","tag-perturbation-energy","tag-re-analysis-using-modern-algorithms","tag-reaction-mechanism","tag-skolnik"],"yoast_head":"\nMore joining up of pieces. Stereocontrol in the ring opening of cyclopropenes. - 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=7068\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"More joining up of pieces. Stereocontrol in the ring opening of cyclopropenes. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Years ago, I was travelling from Cambridge to London on a train. I found myself sitting next to a chemist, and (as chemists do), he scribbled the following on a piece of paper. When I got to work the next day Vera (my student) was unleashed on the problem, and our thoughts were published. 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A colleague (thanks Alan!) has recollected a very similar example dating from 1965 in which a related molecule known as a diazepinone\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":14575,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14575","url_meta":{"origin":7068,"position":1},"title":"Isoelectronic games: the CO analogue of diazirines as an intriguing species?","author":"Henry Rzepa","date":"September 24, 2015","format":false,"excerpt":"How does an anaesthetic work? Surprisingly, it is only recently that the possible binding sites of the anaesthetic propofol (2,6-di-isopropylphenol) have been identified using a technique known as photoaffinity labelling. A propofol analogue was constructed by replacing one of\u00a0the isopropyl groups with a trifluoromethyl diazirine group (R=CF3, X=Y=N below). Upon\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":3746,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3746","url_meta":{"origin":7068,"position":2},"title":"The Cyclol Hypothesis for protein structure: castles in the air.","author":"Henry Rzepa","date":"April 4, 2011","format":false,"excerpt":"Most scientific theories emerge slowly, over decades, but others emerge fully formed virtually overnight as it were (think Einstein in 1905). A third category is the supernova type, burning brightly for a short while, but then vanishing (almost) without trace shortly thereafter. The structure of DNA (of which I have\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\/04\/cyclol.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":7068,"position":3},"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":14601,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14601","url_meta":{"origin":7068,"position":4},"title":"Yes, no, yes. Computational mechanistic exploration of (nickel-catalysed) cyclopropanation using tetramethylammonium triflate.","author":"Henry Rzepa","date":"October 1, 2015","format":false,"excerpt":"A fascinating re-examination has appeared of a reaction first published in 1960 by Wittig and then repudiated by him in 1964 since it could not be replicated by a later student. According to the new work, the secret to a successful replication\u00a0seems to be\u00a0the presence of traces of a nickel\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":6543,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6543","url_meta":{"origin":7068,"position":5},"title":"The Dieneone-phenol controversies.","author":"Henry Rzepa","date":"April 30, 2012","format":false,"excerpt":"During the 1960s, a holy grail of synthetic chemists was to devise an efficient route to steroids. R. B. Woodward was one the chemists who undertook this challenge, starting from compounds known as dienones (e.g. 1) and their mysterious conversion to phenols (e.g. 2 or 3) under acidic conditions. This\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/04\/patha.svg","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","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\/7068","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=7068"}],"version-history":[{"count":31,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7068\/revisions"}],"predecessor-version":[{"id":13242,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7068\/revisions\/13242"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7068"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7068"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7068"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=7068"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}