{"id":8216,"date":"2012-11-04T18:48:54","date_gmt":"2012-11-04T18:48:54","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8216"},"modified":"2012-11-05T07:46:24","modified_gmt":"2012-11-05T07:46:24","slug":"secrets-revealed-for-conjugate-addition-to-cyclohexenone-using-a-cu-alkyl-reagent","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8216","title":{"rendered":"Secrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent."},"content":{"rendered":"
\n

The text books say that cyclohexenone A<\/span><\/strong> will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl (1,2-addition<\/em>) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition<\/em> proceeds, to give the product B<\/strong><\/span> shown below. The standard explanation is that the alkyl copper is a “soft” nucleophile attacking the soft conjugate carbon, whereas the alkyl magnesium is a “hard” nucleophile attacking the hard carbonyl carbon. Is this the best explanation?\u00a0<\/p>\n

\"\"<\/p>\n

In 2007, one of those wonderfully simple experiments was done[1]<\/a><\/span>. The dimethyl lithium cuprate reagent (dissolved in THF) was injected into an NMR sample tube at -100\u00b0C containing A, and the spectrum measured immediately. The species identified as 4<\/strong> (the numbering as used in the reference) has two 1<\/sup>H methyl resonances[2]<\/a><\/span>\u00a0at ~ -0.04 to – 0.23 ppm (assigned to Me\u03b2<\/sub>) and -1.08 to -1.11ppm (assigned to Me\u03b1<\/sub>), and the copper coordinates to the alkene as a \u03c0-complex. If TMS cyanide is added, 4<\/strong> is immediately converted to complex 1<\/strong>, in which the \u03c0-complex is replaced by a simple C-Cu \u03c3-bond. Compound 4<\/strong> upon heating gives B<\/strong>, whilst 1<\/strong> gives the silyl enol ether of B<\/strong>.<\/p>\n

How does this match quantum simulation[3]<\/a><\/span>? First, the 1<\/sup>H NMR result<\/a> for 4<\/strong> (at the wB97XD\/6-31G(d,p)\/SCRF=THF level and with the lithium coordinated to an ether solvent) comes out as -1.4 ppm (Me\u03b1<\/sub>) and -0.31 ppm (Me\u03b2<\/sub>). The 13<\/sup>C is 76.4 and 60.6 ppm for the vinyl carbons (positions 3 and 4, obs) and 64.5\/56.7 (calc). These latter values are affected by spin-orbital coupling to the metal, which can shift the values by up to about 10 ppm[4]<\/a><\/span>, but the relative values are also in good agreement. So the reaction must proceed starting from this\u00a0\u03c0-copper complex.<\/p>\n

\"\"<\/p>\n

The IRC<\/a> reveals a concerted transfer of\u00a0\u00a0Me\u03b2<\/sub> to the\u00a0conjugate 4-position of B<\/strong>,\u00a0with a reasonable barrier to reaction which indicates that on warming to room temperatures, the complex 4<\/strong> will readily react. Formally at least, this corresponds to reductive elimination from the Cu(III) species to form a Cu(I) complex (in which however the metal now coordinates to the enol double bond rather than the alkene).<\/p>\n\n\n\n
\n
\"\"

IRC for methyl transfer. Click for 3D transition state.<\/p><\/div>\n<\/td>\n

\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

I will deal with the case of methyl transfer from 1<\/strong> in a later post. With 4<\/strong>, we can directly see that the origins of conjugate 1,4-addition an \u03b1,\u03b2-unsaturated ketone are that the Cu reagent forms a \u03c0-complex to the alkene, which positions one of the alkyl groups on the metal in the ideal position to attack in conjugate manner. Regarding the different behaviour of the magnesium Grignard reagent, it boils down to asking why it does NOT form a \u03c0-complex in this situation (I would note here that Mg-\u03c0-complexes are indeed known).<\/p>\n

References<\/h2>\n
  1. S.H. Bertz, S. Cope, M. Murphy, C.A. Ogle, and B.J. Taylor, \"Rapid Injection NMR in Mechanistic Organocopper Chemistry. Preparation of the Elusive Copper(III) Intermediate\", Journal of the American Chemical Society<\/i>, vol. 129, pp. 7208-7209, 2007. https:\/\/doi.org\/10.1021\/ja067533d<\/a>\n\n<\/li>\n
  2. S.H. Bertz, C.M. Carlin, D.A. Deadwyler, M.D. Murphy, C.A. Ogle, and P.H. Seagle, \"Rapid-Injection NMR Study of Iodo- and Cyano-Gilman Reagents with 2-Cyclohexenone:\u2009 Observation of \u03c0-Complexes and Their Rates of Formation\", Journal of the American Chemical Society<\/i>, vol. 124, pp. 13650-13651, 2002. https:\/\/doi.org\/10.1021\/ja027744s<\/a>\n\n<\/li>\n
  3. H. Hu, and J.P. Snyder, \"Organocuprate Conjugate Addition:\u2009 The Square-Planar \u201cCu<sup>III<\/sup>\u201d Intermediate\", Journal of the American Chemical Society<\/i>, vol. 129, pp. 7210-7211, 2007. https:\/\/doi.org\/10.1021\/ja0675346<\/a>\n\n<\/li>\n
  4. D.C. Braddock, and H.S. Rzepa, \"Structural Reassignment of Obtusallenes V, VI, and VII by GIAO-Based Density Functional Prediction\", Journal of Natural Products<\/i>, vol. 71, pp. 728-730, 2008. https:\/\/doi.org\/10.1021\/np0705918<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The text books say that cyclohexenone A will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl (1,2-addition) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition proceeds, to give the product B shown below. The standard explanation is that the alkyl copper is a “soft” […]<\/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":[],"tags":[157,843,721,373],"ppma_author":[2661],"class_list":["post-8216","post","type-post","status-publish","format-standard","hentry","tag-metal","tag-reaction-mechanism","tag-simulation","tag-tutorial-material"],"yoast_head":"\nSecrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent. - 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=8216\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Secrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The text books say that cyclohexenone A will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl (1,2-addition) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition proceeds, to give the product B shown below. The standard explanation is that the alkyl copper is a “soft” […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8216\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-11-04T18:48:54+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-11-05T07:46:24+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/Cu.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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Secrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent. - 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=8216","og_locale":"en_GB","og_type":"article","og_title":"Secrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent. - Henry Rzepa's Blog","og_description":"The text books say that cyclohexenone A will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl (1,2-addition) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition proceeds, to give the product B shown below. 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Investigating the mechanism using quantum calculations poses some interesting challenges, ones I have not previously discussed on this blog. My model will\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"SUHBEC. 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Such displacement at silicon famously proceeds by a quite different mechanism, which\u00a0I here quantify with\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":9572,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9572","url_meta":{"origin":8216,"position":2},"title":"A to-and-fro of electrons operating in s-cis esters.","author":"Henry Rzepa","date":"February 21, 2013","format":false,"excerpt":"I conclude my exploration of conformational preferences by taking a look at esters. As before, I start with a search definition, the ester being restricted to one bearing only sp3 carbon centers. The result of such a search is pretty clear-cut; they all exist in just one conformation, the s-cis,\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":"s-cis-ester-torsion-search","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/s-cis-ester-torsion-search.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10073,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10073","url_meta":{"origin":8216,"position":3},"title":"The mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope","author":"Henry Rzepa","date":"April 2, 2013","format":false,"excerpt":"My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). There is a much rarer alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope. Here, guanidine is used as a general acid\/base, which results in a reasonable activation barrier for\u2026","rel":"","context":"In \"acetic acid\"","block_context":{"text":"acetic acid","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=acetic-acid"},"img":{"alt_text":"alkylg","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/alkylg.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":11995,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11995","url_meta":{"origin":8216,"position":4},"title":"The wrong trousers: the anti-Markovnikov addition of borane to 2-methylpropene.","author":"Henry Rzepa","date":"March 2, 2014","format":false,"excerpt":"A staple of introductory undergraduate teaching in organic chemistry is Markovnikov's rule, which states: \"the addition of a protic acid HX to an alkene results in the acid hydrogen (H) becoming attached to the carbon with fewer alkyl substituents and the halide (X) group to the carbon with more alkyl\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":"Click for 3D","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/borane%2Bbutene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16361,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16361","url_meta":{"origin":8216,"position":5},"title":"What is the approach trajectory of enhanced (super?) nucleophiles towards a carbonyl group?","author":"Henry Rzepa","date":"May 11, 2016","format":false,"excerpt":"I have previously commented on the B\u00fcrgi\u2013Dunitz angle, this being the preferred approach trajectory of a nucleophile towards the electrophilic carbon of a carbonyl group. Some special types of nucleophile such as hydrazines (R2N-NR2) are supposed to have enhanced reactivity due to what might be described as\u00a0buttressing of adjacent lone\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":"","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\/8216","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=8216"}],"version-history":[{"count":23,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8216\/revisions"}],"predecessor-version":[{"id":8241,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8216\/revisions\/8241"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8216"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8216"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8216"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8216"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}