{"id":8776,"date":"2012-12-24T16:07:43","date_gmt":"2012-12-24T16:07:43","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8776"},"modified":"2017-08-18T08:45:53","modified_gmt":"2017-08-18T07:45:53","slug":"how-to-tame-an-oxidant-the-mysteries-of-tpap-tetra-n-propylammonium-perruthenate","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8776","title":{"rendered":"How to tame an oxidant: the mysteries of “tpap” (tetra-n-propylammonium perruthenate)."},"content":{"rendered":"
\n

tpap<\/strong><\/em>[1]<\/a><\/span>, as it is affectionately known<\/a>, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. Whereas ruthenium tetroxide (RuO4<\/sub>) is a voracious oxidant[2]<\/a><\/span>, its radical anion countered by a tetra-propylammonium cation is considered a more moderate animal[3]<\/a><\/span>. In this post, I want to try to use quantum mechanically derived energies as a pathfinder for exploring what might be going on (or a reality-check if you like).\u00a0<\/p>\n

 <\/p>\n

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

A basic (i.e.<\/em> simple) mechanism for oxidation of an alcohol by\u00a0RuO4<\/sub> is shown above. Here I reality-check this mechanistic pathway with the help of \u03c9B97XD\/Def2-SVPP\/SCRF=dichloromethane calculations. I should point out that since the mechanism is going to involve ion-pairs, it is particularly important to adopt a solvent=corrected model from the outset[4]<\/a><\/span>. TS1<\/a> is the transition state for addition of the alcohol to the metal, a process which involves a synchronous proton transfer for the singlet electronic state.<\/p>\n\n\n\n
\n
\"TS1.

TS1 as a singlet. Click for 3D<\/p><\/div>\n<\/td>\n

\"tpap-TS1\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Next comes TS2<\/a>, which involves a hydride abstraction with concomitant reduction of the oxidation number of Ru(VIII) to Ru(VI). It is higher in free energy than TS1 by 1.1 kcal\/mol. The barrier corresponds to \u0394G298<\/sub>\u2021<\/sup>\u00a037.1 kcal\/mol.\u00a0The process completes by low energy elimination of water (TS3<\/a>) from the Ru(VI) species to give RuO3<\/sub>, which either undertakes further oxidisation to give RuO2<\/sub>, or might instead be re-oxidized back to RuO4<\/sub>\u00a0by oxygen (or an amine N-oxide) to complete a catalytic cycle.<\/p>\n\n\n\n\n
\n
\"TS1.

TS2 as a singlet. Click for 3D<\/p><\/div>\n<\/td>\n

–\"tpap-TS2\"<\/td>\n<\/tr>\n
\n
\"TS2

TS2 as a triplet. Click for 3D<\/p><\/div>\n<\/td>\n

\"RuO4-ts2-triplet\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Right away, we have a problem;\u00a0\u0394G298<\/sub>\u2021\u00a0<\/sup>37.1 kcal\/mol is too high to be a realistic pathway, and yet\u00a0RuO4<\/sub> is a known oxidant[2]<\/a><\/span>. One way out is to see if the triplet state energy of this system might be lower. Whilst the triplet-state reactant is higher in energy (by 27.5 kcal\/mol) , TS2 <\/a>\u00a0is lower and corresponds to a reduced barrier of \u0394G298<\/sub>\u2021<\/sup>\u00a028.2 kcal\/mol. Better, but a (small?) question mark still remains, since one would really expect the barrier to be ~20 kcal\/mol or less for a “voracious oxidant”. Perhaps the incursion of triplets makes it indiscriminate? The spin density at the transition state is shown below, it extends across both oxygen, carbon and Ru.<\/p>\n

\"ts2-triplet-spin\"<\/p>\n

The tpap<\/strong><\/em> modification to this process is to use Ru(VII) in the form of a radical anion partnered with a quaternary ammonium cation. The basic reagent is therefore an ion-pair<\/a>, hence the solvation approach mentioned earlier is needed to describe the energetics of such a species. The R alkyl groups here are modelled as methyl rather than propyl.<\/p>\n

\"tpap1\"<\/p>\n

TS2<\/a> for this radical-ion-pair is shown below, and it has\u00a0\u0394G298<\/sub>\u2021<\/sup>\u00a030.8 kcal\/mol, 2.6 kcal\/mol higher than for the un-moderated reagent. In this case, the higher-spin quartet states<\/a> are higher \u00a0in energy (by at least ~7.9 kcal\/mol) and so do not participate.\u00a0<\/p>\n\n\n\n
\"tpap-ts2\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The spin density for tpap-TS2 also reveals it to be concentrated on Ru and one oxygen. Little is transferred to the ethanol, and we might infer then that this TS corresponds to transfer of two-electrons from the ethanol to the Ru-oxidant. This corresponds to Ru(VII) being reduced to Ru(V), i.e.<\/em> a 2-electron oxidation\/reduction. Unfortunately, an attempt to chart the reaction across a whole reaction coordinate (IRC) failed with SCF-convergence problems, which might suggest a change in the spin-configuration during this process. A more sophisticated multi-configurational approach might be needed to properly establish the electron dynamics of what is turning out to be a more complex reaction than first seemed.<\/p>\n

\"tpap-ts2-spin-density\"<\/p>\n

It is time to sum up what might have been learnt.<\/p>\n

    \n
  1. A reality check on the energetics of a viable-looking mechanistic route can establish whether such a mechanism does have a low enough free-energy to be viable at (in this case) room temperatures.<\/li>\n
  2. In fact, observing that our initial mechanism had too high an energy led us to discover a triplet-state path that was significantly lower in energy.\u00a0However, even this is still a bit too high.<\/li>\n
  3. The tpap-variation<\/strong><\/em> of the oxidant, which enforces a doublet-state upon the mechanism,has a barrier which appears to be similar to the triplet-state\u00a0RuO4<\/sub> mechanism. This too may be too high in energy. At least we can probably rule out a quartet-state mechanism.<\/li>\n
  4. And so it seems appropriate to end here by noting that experimentally[3]<\/a><\/span> the kinetics of tpap oxidations appear to be autocatalytic. The rate speeds up once some RuO3<\/sub> (or RuO2<\/sub>) has been formed, and this suggests that perhaps a binuclear system containing two Ru atoms is a faster oxidant than the mononuclear variety. This reminds of the mechanism for Sharpless perepoxidation, where two metal centres were needed to control the stereochemistry.<\/li>\n<\/ol>\n

    So after all of this, we have not really found an explanation of why tpap is a more selective and moderate oxidant than the rapacious RuO4<\/sub>. But perhaps this is because more complex models with more than one Ru-atom need to be constructed. This would in turn allow the oxidative hydride abstraction from the alcohol to occur in a larger (7) ring transition state, which is always the preferred geometry for such transfers. If I find such, I will report back here.<\/p>\n

    References<\/h2>\n
    1. S.V. Ley, J. Norman, W.P. Griffith, and S.P. Marsden, \"Tetrapropylammonium Perruthenate, Pr<sub>4<\/sub>N<sup>+<\/sup>RuO<sub>4<\/sub>\n <sup>-<\/sup>, TPAP: A Catalytic Oxidant for Organic Synthesis\", Synthesis<\/i>, vol. 1994, pp. 639-666, 1994. https:\/\/doi.org\/10.1055\/s-1994-25538<\/a>\n\n<\/li>\n
    2. D.G. Lee, U.A. Spitzer, J. Cleland, and M.E. Olson, \"The oxidation of cyclobutanol by ruthenium tetroxide and sodium ruthenate\", Canadian Journal of Chemistry<\/i>, vol. 54, pp. 2124-2126, 1976. https:\/\/doi.org\/10.1139\/v76-304<\/a>\n\n<\/li>\n
    3. D.G. Lee, Z. Wang, and W.D. Chandler, \"Autocatalysis during the reduction of tetra-n-propylammonium perruthenate by 2-propanol\", The Journal of Organic Chemistry<\/i>, vol. 57, pp. 3276-3277, 1992. https:\/\/doi.org\/10.1021\/jo00038a009<\/a>\n\n<\/li>\n
    4. J. Kong, P.V.R. Schleyer, and H.S. Rzepa, \"Successful Computational Modeling of Isobornyl Chloride Ion-Pair Mechanisms\", The Journal of Organic Chemistry<\/i>, vol. 75, pp. 5164-5169, 2010. https:\/\/doi.org\/10.1021\/jo100920e<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      tpap, as it is affectionately known, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. Whereas ruthenium tetroxide (RuO4) is a voracious oxidant, its radical anion countered by a tetra-propylammonium cation is considered a more moderate animal. In this post, I want to try to use quantum mechanically derived energies […]<\/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":[4],"tags":[1138,24,40,969,157,968,843,967,970,373],"ppma_author":[2661],"class_list":["post-8776","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-catalysis","tag-energy","tag-free-energy","tag-low-energy-elimination","tag-metal","tag-react-freq","tag-reaction-mechanism","tag-ruo4-ethanol","tag-triplet-state-energy","tag-tutorial-material"],"yoast_head":"\nHow to tame an oxidant: the mysteries of "tpap" (tetra-n-propylammonium perruthenate). - 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=8776\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to tame an oxidant: the mysteries of "tpap" (tetra-n-propylammonium perruthenate). - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"tpap, as it is affectionately known, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. 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An undergraduate experiment.","author":"Henry Rzepa","date":"April 15, 2014","format":false,"excerpt":"The journal of chemical education can be a fertile source of ideas for undergraduate student experiments. Take this procedure for asymmetric epoxidation of an alkene. When I first spotted it, I thought not only would it be interesting to do in the lab, but could be extended by incorporating some\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":[]},{"id":16441,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16441","url_meta":{"origin":8776,"position":1},"title":"An alternative mechanism for nucleophilic substitution at silicon using a tetra-alkyl ammonium fluoride.","author":"Henry Rzepa","date":"May 27, 2016","format":false,"excerpt":"In the previous post, I explored the mechanism for nucleophilic substitution at a silicon centre proceeding via retention of configuration involving a Berry-like pseudorotation.\u00a0Here\u00a0I probe an alternative route involving inversion of configuration at the Si centre. Both stereochemical modes are known to occur, depending on the leaving group, solvent and\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":16402,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16402","url_meta":{"origin":8776,"position":2},"title":"The mechanism of silylether deprotection using a tetra-alkyl ammonium fluoride.","author":"Henry Rzepa","date":"May 25, 2016","format":false,"excerpt":"The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre\u00a0occurs with inversion\u00a0of configuration at the carbon, the mechanism being known by\u00a0the term\u00a0SN2\u00a0(a story I have also told\u00a0in this post). 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":16942,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16942","url_meta":{"origin":8776,"position":3},"title":"\u03c3 or \u03c0 nucleophilic reactivity of imines? A mechanistic reality check using substituents.","author":"Henry Rzepa","date":"October 9, 2016","format":false,"excerpt":"Previously, a mechanistic twist to the oxidation of imines using peracid had emerged. Time to see how substituents respond to this mechanism. With\u00a0X = NO2 100% oxaziridine and no nitrone is obtained experimentally; with\u00a0X =\u00a0NMe2\u00a0, the population is inverted with nitrone as the dominant product at\u00a078%. Calculations (\u03c9B97XD\/Def2-TZVPP\/SCRF=dichloromethane), data collection\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":8588,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8588","url_meta":{"origin":8776,"position":4},"title":"Why is the Sharpless epoxidation enantioselective? Part 1: a simple model.","author":"Henry Rzepa","date":"December 9, 2012","format":false,"excerpt":"Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess,. Here is the first step in trying to explain how this magic is achieved. The scheme above shows how (achiral) prop-2-enol is converted using the asymmetric catalyst\u00a0(R,R)-diethyl tartrate \u00a0and t-butyl hydroperoxide as oxidant\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\/12\/sharpless.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":8776,"position":5},"title":"Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.","author":"Henry Rzepa","date":"August 13, 2025","format":false,"excerpt":"In the previous post, I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. 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":"","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\/8776","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=8776"}],"version-history":[{"count":52,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8776\/revisions"}],"predecessor-version":[{"id":18716,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8776\/revisions\/18716"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8776"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8776"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8776"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8776"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}