{"id":7344,"date":"2012-07-23T08:51:37","date_gmt":"2012-07-23T07:51:37","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7344"},"modified":"2012-07-23T08:51:37","modified_gmt":"2012-07-23T07:51:37","slug":"the-first-curly-arrows-the-denouement","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2012\/07\/23\/the-first-curly-arrows-the-denouement\/","title":{"rendered":"The first curly arrows. The d\u00e9nouement."},"content":{"rendered":"

Recollect, Robinson was trying<\/a> to explain why the nitroso group appears to be an o\/p<\/em> director of aromatic electrophilic substitution. Using \u03c3\/\u03c0 orthogonality, I suggested that the (first ever) curly arrows as he drew them could not be the complete story, and that a transition state analysis would be needed. Here it is.\u00a0<\/p>\n

\"\"<\/p>\n

Let me set the scene on how this might be done. Although aromatic electrophilic substitutions are the grand-daddy<\/a> of all mechanisms, they present some computational challenges. An electrophile is needed, and this is normally represented by E+<\/sup>. This reacts with an aromatic ring to form (so the text books show) a charged Wheland intermediate. A second stage then takes over, whereby a base (B:) abstracts the ring proton to give BH+ <\/sup>and the substituted product. This is clearly an ionic mechanism. And if one does not forget the counter-ions in all of this (see my post on not forgetting them<\/a>!), it is an ion-pair mechanism. But in relatively non-polar media, need ion-pairs form? A little while ago, I speculated that the two stages could be conflated into one, concerted, pathway. That pathway is shown above. I decided that this was a convenient template upon which to test the directing influence of the NO group. My model is going to be E=NO, R=CF3<\/sub> (OK, largely because I already had that template to hand; I daresay E=Br might also be appropriate using e.g.<\/em> acetyl hypobromite) and conducted in dichloromethane as simulated solvent. The transition states<\/a> (\u03c9B97XD\/6-311G(d,p)CPCM=DCM) turn out as below.<\/p>\n

\"\"
Transition state for p-electrophilic substitution. Click for 3D.<\/figcaption><\/figure>\n

This is a concerted reaction (no Wheland intermediate) as the IRC shows, although the relatively long O…N=O bond suggests that it is at least partially ionic\/ion-pair like (if you are wondering if there are any examples in the literature that implicate a concerted mechanistic replacement for the Wheland intermediate, you might want to take a look at this one<\/a>.)<\/p>\n

\"\"<\/p>\n

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

The alternative transition state<\/a>, leading to m<\/em>-substitution, is calculated to be 0.7 kcal\/mol lower<\/strong><\/em> in its free energy activation barrier.<\/p>\n

\"\"
Transition state for m-substitution. Click for 3D<\/figcaption><\/figure>\n

So if the nitrosyl group itself appears to be m<\/em>-directing (a more complete investigation would test this for other electrophiles), why is the product p<\/em>-substituted? Well, I also showed that nitrosobenzenes can easily dimerise<\/a>, as shown below. This species now has a \u03c0-mesomeric<\/strong> resonance shown with red arrows below which really does promote the attachment of an electrophile in the p<\/em>-position. This is now perfectly allowed; no issues of\u00a0\u03c3\/\u03c0 orthogonality here!<\/p>\n

\"\"<\/p>\n

So the d\u00e9nouement I suggest is that the experiment on which Robinson based his famous curly arrows can in fact be re-interpreted as indicating that it is the dimer of nitrosobenzene that is involved in its electrophilic substitution, and that the monomer (as with nitrobenzene) is actually m<\/em>-directing. In effect, that dimerisation (which involves two nitrogen \u03c3-lone pairs), bifurcates one of them into a \u03c0-pair, and this pair can now safely resonate with the aromatic ring to direct electrophiles. \u00a0<\/p>\n","protected":false},"excerpt":{"rendered":"

Recollect, Robinson was trying to explain why the nitroso group appears to be an o\/p director of aromatic electrophilic substitution. Using \u03c3\/\u03c0 orthogonality, I suggested that the (first ever) curly arrows as he drew them could not be the complete story, and that a transition state analysis would be needed. Here it is.\u00a0 Let me […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,11],"tags":[1039,1188,1743,1759,1780,1781,2076,2489,1338],"class_list":["post-7344","post","type-post","status-publish","format-standard","hentry","category-curl-arrows","category-interesting-chemistry","tag-free-energy-activation-barrier","tag-historical","tag-nitrosyl","tag-non-polar-media","tag-op-director","tag-op-director-of-aromatic-electrophilic-substitution","tag-reaction-mechanism","tag-tutorial-material","tag--orthogonality"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7344","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=7344"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7344\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=7344"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=7344"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=7344"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}