{"id":12852,"date":"2014-08-12T09:41:42","date_gmt":"2014-08-12T08:41:42","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12852"},"modified":"2014-08-16T11:00:55","modified_gmt":"2014-08-16T10:00:55","slug":"an-unusual-16-shift-in-homotropylium-cation-exhibiting-zones-of-aromaticity","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12852","title":{"rendered":"An unusual [1,6] shift in homotropylium cation exhibiting zones of aromaticity."},"content":{"rendered":"
\n

One thing leads to another. Thus in the previous post<\/a>, I described a thermal pericyclic reaction that appears to exhibit two<\/strong> transition states resulting in two<\/strong> different stereochemical outcomes. I noted that another such reaction appeared to be a [1,6] carousel migration in homotropylium cation,[1]<\/a><\/span> where transition states for both retention and inversion of the configuration of the migrating group (respectively formally allowed and forbidden) were reported (scheme below). Here I explore this system further. \"homotropylium\"<\/a> Firstly, the pathway leading to inversion.[2]<\/a><\/span> The reaction path (\u03c9B97XD\/6-311G(d,p)\/SCRF=chloroform) has got a very odd (table-top mountain) shape, whereby the region of the transition state (IRC = 0.0) is very flat, and the region close to reactant and (identical) product is very steep. The gradient norm shows this best, with sharp spikes at IRC \u00b1 4.2. Something clearly is happening here to cause this behaviour. Before moving on to analyze this, I want you first to observe the methyl groups below. Note how one of them rotates at the start of the process, and the other at the end. I have elsewhere called this behaviour the methyl flag<\/a>, and it is due to stereoelectronic re-alignments of the C-H groups accompanying the changes in the conjugated array. \"htropa\"<\/a> \"htrop\"<\/a> \"htropG\"<\/a> The homotropylium cation is said to be homoaromatic, indicating that cyclic conjugation can be maintained across a ring in which the \u03c3 framework is interrupted at one point. A NICS probe placed at the ring critical point of this molecule reveals a chemical shift of -11.3 ppm[3]<\/a><\/span>, very similar to eg that obtained for benzene itself. The three highest doubly occupied NBOs (below) show two normal \u03c0-type orbitals and one rather different one that spans the homo-bond (the MOs, before you ask, are a bit of a mess, with lots of mixed contributions from other parts of the \u03c3 framework).<\/p>\n\n\n\n\n
HONBO (two)<\/td>\nHONBO-2<\/td>\n<\/tr>\n
\n

\"Click

Click for 3D<\/p><\/div><\/td>\n

\n

\"Click

Click for 3D<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

At the transition state for the [1,6] migration, the same NICS probe registers a value of +2.6 ppm[4]<\/a><\/span>, now firmly in the non-aromatic zone. So this reaction is characterised by two zones, ring-aromatic ones at the start and the end of the process, and a higher energy non-aromatic one in the middle of the reaction pathway as ~enclosed by the region of IRC \u00b1 4.2. The homo-bond in the aromatic zone starts with a length of 1.74\u00c5, reduces to 1.53\u00c5 at the transition state\u00a0and ends up as a normal aromatic bond of length 1.41\u00c5. Meanwhile, the relocated\u00a0homo-bond changes in the opposite sense, starting as a normal aromatic length of 1.41\u00c5, becoming 1.53\u00c5 at the transition state and ending as a homo-length of 1.74\u00c5. Presumably, virtually full strength homoaromaticity can be sustained for a homo-bond of 1.74\u00c5, but\u00a0as that bond mutates to a \u03c3-bond of 1.53\u00c5, the cyclic conjugation falls off the edge of the cliff, to be restored only at the end. Pericyclic reactions are themselves said to sustain aromatic transition states,[5]<\/a><\/span> and so a simplistic way of looking at this is that the “aromaticity” relocates (or morphs) from the reactant to the transition state, and then back again during the course of the migration. A reaction path from which one can indeed learn a lot.<\/p>\n

Now to the pathway in which the migrating group retains configuration.\u00a0This is no longer a single step concerted reaction,[6]<\/a><\/span> since at the half-way point we no longer have a transition state but a shallow intermediate (~IRC +2, [7]<\/a><\/span>). It (formally at least) becomes a two-step non-concerted process, and\u00a0the overall barrier is ~5 kcal\/mol lower<\/strong> than for the inversion path. The aromaticity changes in a similar manner to before (i.e. IRC ~-5).\"htrop-ra\"<\/a><\/p>\n

\"Htrop-ret\"<\/a>\"Htrop-retG\"<\/a><\/p>\n

So this emerges as not quite the example I thought it was, but nonetheless unusual with the “forbidden” pathway being concerted and the “allowed” pathway being non-concerted. Molecular dynamics on these two systems would indeed be interesting to see what proportion of the trajectories go via<\/em> each pathway.<\/p>\n

References<\/h2>\n
  1. A.M. Genaev, G.E. Sal\u2019nikov, and V.G. Shubin, \"Energy barriers to carousel rearrangements of carbocations: Quantum-chemical calculations vs. experiment\", Russian Journal of Organic Chemistry<\/i>, vol. 43, pp. 1134-1138, 2007. https:\/\/doi.org\/10.1134\/s1070428007080076<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C10H13(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1134556<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C10H13(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1135694<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"Gaussian Job Archive for C10H13(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1135695<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"The Aromaticity of Pericyclic Reaction Transition States\", Journal of Chemical Education<\/i>, vol. 84, pp. 1535, 2007. https:\/\/doi.org\/10.1021\/ed084p1535<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for C10H13(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1135668<\/a>\n\n<\/li>\n
  7. H.S. Rzepa, \"Gaussian Job Archive for C10H13(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1134559<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    One thing leads to another. Thus in the previous post, I described a thermal pericyclic reaction that appears to exhibit two transition states resulting in two different stereochemical outcomes. I noted that another such reaction appeared to be a [1,6] carousel migration in homotropylium cation, where transition states for both retention and inversion of the […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_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}},"categories":[559,1086],"tags":[237,68,1252],"ppma_author":[2661],"class_list":["post-12852","post","type-post","status-publish","format-standard","hentry","category-pericyclic","category-reaction-mechanism-2","tag-chemical-shift","tag-higher-energy","tag-sangean-table-top-portable-audio-device"],"yoast_head":"\nAn unusual [1,6] shift in homotropylium cation exhibiting zones of aromaticity. - 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=12852\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"An unusual [1,6] shift in homotropylium cation exhibiting zones of aromaticity. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"One thing leads to another. 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Stereoisomeric transition states for [1,4] pericyclic shifts.","author":"Henry Rzepa","date":"August 18, 2014","format":false,"excerpt":"This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples. Migration in homotropylium\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":12825,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12825","url_meta":{"origin":12852,"position":1},"title":"Using a polar bond to flip: on the knife-edge!","author":"Henry Rzepa","date":"August 10, 2014","format":false,"excerpt":"In my\u00a0first post on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be (scheme below) could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system. This was followed up by exploring what\u00a0happens at the half way stage, i.e. X=CH2,\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":11830,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11830","url_meta":{"origin":12852,"position":2},"title":"A simple pericyclic reaction encapsulating the four thermal selection rules.","author":"Henry Rzepa","date":"January 2, 2014","format":false,"excerpt":"As my previous post hints, I am performing my annual spring-clean of lecture notes on pericyclic reactions. Such reactions, and their stereochemistry, are described by a set of selection rules. I am always on the lookout for a simple example which can most concisely summarise these rules. The (hypothetical) one\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"12Ca","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/12Ca.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10706,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10706","url_meta":{"origin":12852,"position":3},"title":"Mechanistic arrow pushing. A proposed addition to its rules.","author":"Henry Rzepa","date":"June 12, 2013","format":false,"excerpt":"A little while ago, I set out some interpretations of how to push curly arrows. I also appreciate that some theoretically oriented colleagues regard\u00a0the technique as neither useful nor in the least rigorous,\u00a0whereas towards the other extreme\u00a0many synthetically minded chemists view the ability to push a reasonable\u00a0set of arrows for\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":"12-16","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/06\/12-16.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12895,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12895","url_meta":{"origin":12852,"position":4},"title":"Computationally directed synthesis: 2,3-dimethyl-2-butene + NO(+).","author":"Henry Rzepa","date":"September 6, 2014","format":false,"excerpt":"In the previous posts, I explored reactions which can be flipped between two potential (stereochemical) outcomes. This triggered a memory from Alex, who pointed out this article from 1999 in which the nitrosonium cation as an electrophile can have two outcomes A or B when interacting with the electron-rich 2,3-dimethyl-2-butene.\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"NOa","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/09\/NOa.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12782,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12782","url_meta":{"origin":12852,"position":5},"title":"Using a polar bond to flip the (stereochemical) outcome of a pericyclic reaction.","author":"Henry Rzepa","date":"August 4, 2014","format":false,"excerpt":"The outcome of pericyclic reactions con depend most simply on three conditions, any two of which determine the third. Whether the catalyst is \u0394 or h\u03bd (heat or light), the topology determining any stereochemistry and the participating electron count (4n+2\/4n). It is always neat to conjure up a simple switch\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":[]}],"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\/12852","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=12852"}],"version-history":[{"count":17,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12852\/revisions"}],"predecessor-version":[{"id":12878,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12852\/revisions\/12878"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=12852"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=12852"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=12852"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=12852"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}