{"id":10656,"date":"2013-05-29T14:23:40","date_gmt":"2013-05-29T13:23:40","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10656"},"modified":"2013-05-29T14:23:40","modified_gmt":"2013-05-29T13:23:40","slug":"mechanism-of-the-van-leusen-reaction","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10656","title":{"rendered":"Mechanism of the Van Leusen reaction."},"content":{"rendered":"
\n

This is a follow-up<\/a> to comment posted by Ryan, who asked about isocyanide\u2019s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article<\/a> (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C\u2261N group, with the isocyanide group acting as the acceptor of these electrons (in other words, the electrophile). In the form shown below, one negatively charged atom appears to be attacking another also carrying a negative charge. Surely this breaks the rules that like charges repel? So we shall investigate to see if this really happens.<\/p>\n

\"VL1\"<\/p>\n

I have extended the basic mechanistic scheme below to investigate other possibilities (with the aim of probing using the \u03c9B97XD\/6-311G(d,p)\/scrf=methanol theory to see how realistic any of these routes might be). Starting from 1<\/strong>, the product 6<\/strong> can now be formed by three routes from the common intermediate 3<\/strong> (there may be other routes of course not considered here). The relative computed free energies of these various species, and some of the transition states leading to them are listed in the table below.\"VanLeusen\"<\/p>\n

The path leading to 3<\/strong> is very low energy which may in part also be due to my using formaldehyde for expediency rather than a substituted aldehyde\u00a0(I have to confess to having taken another short-cut, which is to miss out any counter-ion to the TosMIC anion). The first step is defined by TS1<\/strong>, which forms a C-C bond, and results in the intermediate 2<\/strong>. TS2<\/strong> corresponds to O…C bond formation to yield 3<\/strong>. Getting to 3<\/strong> is thus a two-stage process, or a stepwise cycloaddition. The alternative would have been to regard\u00a0TosMIC anion as a 1,3-dipole (isoelectronic with e.g.diazomethane<\/a>) in which these two steps are conflated into a concerted cycloaddition across the carbonyl group.<\/p>\n

TS2<\/strong> is the interesting step from the point of view of the question raised above. It has a very low free-energy barrier from either 1<\/strong> or 2<\/strong>, and therefore appears very facile. The angle of approach by the oxy-anion to the triple bond (we established it as being triple in the previous post<\/a>) is 87\u00b0. This angle explains why a carbon bearing (a formal) negative charge easily accepts attack on itself by a nucleophile (i.e.<\/em> acting as an electrophile). The formal negative charge originates from an electron lone pair located in an sp-hybrid orbital lying along the axis of the C\u2261N bond. But the nucleophilic attack is occurring at\u00a087\u00b0 to this axis, putting electrons into the empty \u03c0* orbital of the C\u2261N bond. So in effect the two apparent “negative charges” in the mechanistic schemes above are orthogonal to each-other, which in a simplistic way explains why the diagram is not actually contradictory. The reaction itself is an example of Baldwin’s rules in action<\/a>; one of these is that a 5-endo-dig<\/em> cyclisation is allowed. This angle of attack and Baldwin’s rules may of course be related.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
System<\/td>\nRelative free energy<\/td>\n<\/tr>\n
1<\/td>\n0.0<\/td>\n<\/tr>\n
TS1<\/td>\n1.2<\/td>\n<\/tr>\n
2<\/td>\n-3.1<\/td>\n<\/tr>\n
TS2<\/td>\n1.7<\/td>\n<\/tr>\n
3<\/td>\n-14.4<\/td>\n<\/tr>\n
4<\/td>\n-16.7<\/td>\n<\/tr>\n
TS3<\/td>\n5.9<\/td>\n<\/tr>\n
5<\/td>\n4.0<\/td>\n<\/tr>\n
TS4<\/td>\n17.7<\/td>\n<\/tr>\n
6<\/td>\n-51.1<\/td>\n<\/tr>\n
TS5<\/td>\n-2.3<\/td>\n<\/tr>\n
7<\/td>\n-3.1<\/td>\n<\/tr>\n
TS6<\/td>\n57.0<\/td>\n<\/tr>\n
“8”<\/td>\n-37.4<\/td>\n<\/tr>\n
“9”<\/td>\n-36.2<\/td>\n<\/tr>\n
10<\/td>\n-25.3<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

After 3<\/strong>, the mechanism can channel several ways. For example, a proton transfer can precede the departure of Ts<\/strong> to give 4<\/strong> and thence 5<\/strong>. The final [1,2] shift to form the product 6<\/strong> has a relatively high barrier however. More likely is the Ts group heterolysing off\u00a03<\/strong>\u00a0via<\/em> TS5<\/strong> to form 7. <\/strong>All that is now needed is to shift a proton from 7<\/strong> to form 6<\/strong>, and this also can take several routes. One would involve base\/acid catalysed deprotonation\/reprotonation. Alternatively, the hydrogen could migrate by a series of uncatalysed [1,5]sigmatropic hydrogen shifts via<\/em> e.g. 8<\/strong>, 9<\/strong> or 10<\/strong>. In fact, the calculated geometries of both 8<\/strong> and 9<\/strong> show that the C…O bond is broken, thus forming entirely different products. Thus the most probable route is indeed a simple catalysed proton transfer from 7<\/strong>.<\/p>\n

This computational exploration of the mechanism has reinforced the accepted one, and hopefully cast some light on why an isocyanide can appear to act as an electrophile.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

This is a follow-up to comment posted by Ryan, who asked about isocyanide\u2019s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C\u2261N group, […]<\/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":[],"tags":[578,1070,843,373],"ppma_author":[2661],"class_list":["post-10656","post","type-post","status-publish","format-standard","hentry","tag-low-energy","tag-low-free-energy-barrier","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nMechanism of the Van Leusen reaction. - 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=10656\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Mechanism of the Van Leusen reaction. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"This is a follow-up to comment posted by Ryan, who asked about isocyanide\u2019s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C\u2261N group, […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10656\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-05-29T13:23:40+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/VL1.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":"Mechanism of the Van Leusen reaction. - 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=10656","og_locale":"en_GB","og_type":"article","og_title":"Mechanism of the Van Leusen reaction. - Henry Rzepa's Blog","og_description":"This is a follow-up to comment posted by Ryan, who asked about isocyanide\u2019s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. 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Paul Rablen presented the case that the amount of o (ortho) product in electrophilic substitution of a phenyl ring bearing an EWG (electron withdrawing group) is often large enough\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":[]},{"id":3576,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3576","url_meta":{"origin":10656,"position":2},"title":"The formation of cyanohydrins: re-writing the text books. ! or ?","author":"Henry Rzepa","date":"March 4, 2011","format":false,"excerpt":"Nucleophilic addition of cyanide to a ketone or aldehyde is a standard reaction for introductory organic chemistry. But is all as it seems? The reaction is often represented as below, and this seems simple enough. But attention to detail suggests that, HCN being a weak acid, there will be only\u2026","rel":"","context":"In \"acidic solutions\"","block_context":{"text":"acidic solutions","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=acidic-solutions"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/03\/cyano1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12895,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12895","url_meta":{"origin":10656,"position":3},"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":8540,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8540","url_meta":{"origin":10656,"position":4},"title":"The mechanism of the Birch reduction. Part 3: reduction of benzene","author":"Henry Rzepa","date":"December 4, 2012","format":false,"excerpt":"Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this? The mechanism, as elaborated in the previous two posts, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this\u2026","rel":"","context":"In \"Birch reduction\"","block_context":{"text":"Birch reduction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=birch-reduction"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-ip.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":26272,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26272","url_meta":{"origin":10656,"position":5},"title":"Pre-mechanism for the Swern Oxidation: formation of chlorodimethylsulfonium chloride.","author":"Henry Rzepa","date":"August 25, 2023","format":false,"excerpt":"The Swern oxidation is a class of \"activated\" dimethyl sulfoxide (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.\u2021 However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism\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":[]}],"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\/10656","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=10656"}],"version-history":[{"count":18,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10656\/revisions"}],"predecessor-version":[{"id":10677,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10656\/revisions\/10677"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10656"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10656"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10656"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10656"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}