{"id":3186,"date":"2010-12-27T21:24:29","date_gmt":"2010-12-27T20:24:29","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3186"},"modified":"2010-12-27T21:24:29","modified_gmt":"2010-12-27T20:24:29","slug":"do-marauding-electrons-go-in-packs","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3186","title":{"rendered":"Do marauding electrons go in packs?"},"content":{"rendered":"
\n

Is there a preferred pack size for electrons on the move? Or put less flamboyantly, is there an optimum, and a maximum number of arrows (electron pairs) that one might push in revealing the mechanism of a concerted reaction? A sort of village-instinct for electrons. Consider the following (known, DOI: 10.1016\/S0040-4039(00)98289-3<\/a>) reaction<\/p>\n

\"\"<\/a>

A double 3,3 sigmatropic reaction<\/p><\/div>\n

It is a double [3,3] sigmatropic rearrangement, each component involving two packs of three arrows (six electrons) each. Do these packs move together, or do they prefer to move one at a time? If the former, then we come up against another interesting question. How many of say the six bonding electrons in a triple bond can simultaneously participate in a reaction? Many a tutor of the arrow pushing exercise might say the limit is always two (I have even seen this written into a set of rules for arrow pushing). But could it be four, as shown in the above example? Or perchance even six as discussed in this post<\/a>? OK, the question is rather loaded for this example, since being a pericyclic reaction it has a built in (thermal) preference for packs of 4n+2 electrons, which enable the transition state for the process to be considered as aromatic. So in a sense it boils down to whether two aromatic packs would have any advantage in both marauding concurrently.<\/p>\n

A connection can also be made to recent work by Rainer Herges (DOI:\u00a010.1021\/jo801390x<\/a>) in which he suggests that pericyclic reactions involving four electrons at an alkyne can indeed be concerted, giving them the specific name of coarctate reactions (ones which exhibit one or more coarctate atoms at each of which two bonds are made and two bonds are broken). And one further connection is to Clar islands<\/a>, in which the tendency of electrons in polycyclic aromatic hydrocarbons (PAH) to form packs of six is discussed (these of course are not marauding).<\/p>\n

So onwards to a computational exploration (B3LYP\/6-311G(d,p), 10042\/to-6354<\/a>). Two geometries can be located, with respectively C2<\/sub> and Ci<\/sub> symmetries. They differ in the orientation of the two chair-like rings, and the former (shown below) is the more (cis<\/em>) stable. Two negative force constants are calculated (shown below) which indicate that the two [3,3] sigmatropic rearrangements like to go consecutively and not concurrently.<\/p>\n\n\n\n
\"\"

Geometry of the asynchronous pathway. Click for 3D<\/p><\/div><\/td>\n

\"\"

Geometry of the synchronous pathway. Click for 3D<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The \u0394G\u2020 is calculated as high as 58.3 kcal\/mol, which given that the reaction does proceed experimentally at ~410\u00b0C suggests this cannot be the true mechanism (discussion of which I leave to a future post). The electrons therefore move in smaller packs<\/a>, but certainly not 12!<\/p>\n

\"\"<\/a>

Hunting in packs of six, not twelve.<\/p><\/div>\n

In conclusion, one might make the analogy of electrons as medieval travellers, preferring to travel in small groups, and probably invariably stopping off a in a coaching inn for a rest whenever they see one close to their route (or perhaps even sometimes well off the route). I suspect (but cannot prove) that there are few types of bond forming or bond breaking reactions which involve more than five arrows (ten electrons) participating in a single concerted step. More, and I think the travelling electrons are likely to find a resting place \u00a0on the journey (which, if not a closed shell species, may well be a biradical, or in suitable solvent, zwitterion).<\/p>\n

So that is the challenge posed here; to find an example of a reaction involving six or more arrows which theory appears to show is concerted and where new bonds are formed, or old ones broken (this last clause excludes artificial reactions such as bond shifting in higher annulenes).<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Is there a preferred pack size for electrons on the move? Or put less flamboyantly, is there an optimum, and a maximum number of arrows (electron pairs) that one might push in revealing the mechanism of a concerted reaction? A sort of village-instinct for electrons. Consider the following (known, DOI: 10.1016\/S0040-4039(00)98289-3) reaction It is a […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[54,330,373],"ppma_author":[2661],"class_list":["post-3186","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-rainer-herges","tag-tutor","tag-tutorial-material"],"yoast_head":"\nDo marauding electrons go in packs? - 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=3186\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Do marauding electrons go in packs? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Is there a preferred pack size for electrons on the move? 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So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons? 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":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/55.svg","width":350,"height":200},"classes":[]},{"id":8961,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8961","url_meta":{"origin":3186,"position":1},"title":"The mechanism of the Benzidine rearrangement.","author":"Henry Rzepa","date":"January 6, 2013","format":false,"excerpt":"The benzidine rearrangement is claimed to be an example of the quite rare\u00a0[5,5] sigmatropic migration, which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route. The topic has been reviewed here.\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":"NCI surface. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/benzidinenci.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6102,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6102","url_meta":{"origin":3186,"position":2},"title":"Secrets of a university tutor: dissection of a reaction mechanism.","author":"Henry Rzepa","date":"January 25, 2012","format":false,"excerpt":"Its a bit like a jigsaw puzzle in reverse, finding out to disassemble a chemical reaction into the pieces it is made from, and learning the rules that such reaction jigsaws follow. The following takes about 45-50 minutes to follow through with a group of students. The problem is initially\u2026","rel":"","context":"In \"chemical reaction\"","block_context":{"text":"chemical reaction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=chemical-reaction"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/01\/pericyclic-tutorial.svg","width":350,"height":200},"classes":[]},{"id":7678,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7678","url_meta":{"origin":3186,"position":3},"title":"Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).","author":"Henry Rzepa","date":"September 15, 2012","format":false,"excerpt":"Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium\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\/CAZFUE1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":22445,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22445","url_meta":{"origin":3186,"position":4},"title":"Curly arrows in the 21st Century. Proton-coupled electron transfers.","author":"Henry Rzepa","date":"June 10, 2020","format":false,"excerpt":"One of the most fascinating and important articles dealing with curly arrows I have seen is that by Klein and Knizia on the topic of C-H bond activations using an iron catalyst. These are so-called high spin systems with unpaired electrons and the mechanism of C-H activation involves both double\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/06\/TS.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":10706,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10706","url_meta":{"origin":3186,"position":5},"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":[]}],"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\/3186","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=3186"}],"version-history":[{"count":0,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3186\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3186"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3186"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3186"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=3186"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}