{"id":8658,"date":"2012-12-17T17:39:30","date_gmt":"2012-12-17T17:39:30","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8658"},"modified":"2023-03-10T07:59:23","modified_gmt":"2023-03-10T07:59:23","slug":"why-the-sharpless-epoxidation-is-enantioselective","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658","title":{"rendered":"Why the Sharpless epoxidation is enantioselective!"},"content":{"rendered":"
\n

Part one<\/a>\u00a0on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success.\u00a0The new model is constructed using two units of Ti(Oi<\/sup>Pr)4<\/sub>, which are likely to assemble into a dimer such as that shown below (in this crystal structure, some of the i<\/sup>Pr groups are perfluorinated).<\/p>\n

\"WAWBUR.

WAWBUR. Click for 3D<\/p><\/div>\n

This allows one to construct a transition state model as follows.<\/p>\n

\"sharpless-binuclear\"<\/p>\n

    \n
  1. Two i<\/sup>PrOH molecules are displaced by diethyl tartrate for each half of the Ti2<\/sub>(Oi<\/sup>Pr)8<\/sub>,\u00a0with the two metals then becoming bridged by one oxygen from each tartrate.\u00a0<\/li>\n
  2. Two further i<\/sup>PrOH are then displaced from the second Ti<\/span> by one of the substrate (allyl alcohol<\/strong><\/span>) and one of the oxidant (t<\/em>-butyl peroxide<\/strong><\/span>).<\/li>\n
  3. The oxygen transfer now proceeds via the second (hexacoordinate) Ti<\/span>. The first Ti<\/span> also achieves hexa-coordination via<\/em>\u00a0the carbonyl oxygen<\/span><\/strong> of one of the tartrate ester groups. It is the geometric properties of such a hexa-coordinated Ti<\/span> that in part accounts for the subtle properties of this system. Put more simply, the extra crowding at the catalytic centre of the binuclear complex restricts the space available for the transition state, making it more selective for producing one enantiomer of the epoxide.<\/li>\n<\/ol>\n

    The (\u03c9B97XD\/6-311G(d,p)\/SCRF=dichloromethane) optimised geometries are shown below. The reaction centre is shown in a magenta box for the disfavoured (R) epoxide and in green for the favoured (S) epoxide (the hydrogens are not shown for clarity; if you want to see them, click on the image to get the 3D model).<\/p>\n\n\n\n
    \n
    \"\"

    (R). Click for 3D.<\/p><\/div>\n<\/td>\n

    		\n
    \"(S).

    (S). Click for 3D<\/p><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    You can see immediately that the biggest differences between the two occur in the bottom right corner. The t-butyl-O-O group folds in for (S) and this has a knock on effect on the two ester groups of the bottom right tartrate (the disposition of the tartrate on the top left is hardly changed). This folding is mediated by the hexa-coordination of the catalytic\u00a0metal centre, together with dispersion interactions occurring to the t-butyl group, and this is helped by buttressing from the second Ti centre and its substituents.<\/p>\n

    The free energy difference \u0394\u0394G298<\/sub>\u2021<\/sup> favours the (S, FAIR DOI: 10.6084\/m9.figshare.104675<\/a>) for over the (R, FAIR DOI: 10.6084\/m9.figshare.104625<\/a>) by 3.0 kcal\/mol. This free energy difference corresponds to an enantiomeric excess of >99%.\u00a0In terms of attractive dispersion forces alone, (S) is favoured over (R) by -2.6 kcal\/mol, and hence attractive dispersion seems to be the dominant term distinguishing between the two diastereomeric transition states. This aspect of non-covalent-interactions will be investigated in another post.<\/p>\n

    \"KOGYEK.\"

    KOGYEK, a Ti oligomer.<\/p><\/div>\n

    One should however finally ask if this is the best model?<\/p>\n

      \n
    1. Not all conformations have been explored in these models, although (S) was built from (R) as a template, so many features are the same. Nevertheless, further conformational exploration may be useful.<\/li>\n
    2. Alkoxytitaniums are known to also form higher oligomers, such as the one shown above.[1]<\/a><\/span>. If their concentration is significant, these too might be catalysing the reaction. Only computation would establish if they are capable of greater stereoselectivity\/faster kinetics.<\/li>\n<\/ol>\n

      So we could end up with an answer that a number of oligomeric transition states are involved. But the one presented here, if not necessarily the most accurate or “best” model, seems good enough to form a template for further exploratory computation to see if the enantioselectivity of the reaction might be improved upon further.<\/p>\n

      References<\/h2>\n
      1. V.W. Day, T.A. Eberspacher, W.G. Klemperer, C.W. Park, and F.S. Rosenberg, \"Solution structure elucidation of early transition metal polyoxoalkoxides using oxygen-17 NMR spectroscopy\", Journal of the American Chemical Society<\/i>, vol. 113, pp. 8190-8192, 1991. https:\/\/doi.org\/10.1021\/ja00021a068<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

        Part one\u00a0on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success.\u00a0The new model is constructed using two units of Ti(OiPr)4, which are likely to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_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},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[1138,963,955,240,843,957,961,962,373],"ppma_author":[2661],"class_list":["post-8658","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-catalysis","tag-catalytic-metal-centre","tag-enantioselective","tag-free-energy-difference","tag-reaction-mechanism","tag-sharpless-epoxidation","tag-t-butyl","tag-this-free-energy-difference","tag-tutorial-material"],"yoast_head":"\nWhy the Sharpless epoxidation is enantioselective! - 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=8658\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Why the Sharpless epoxidation is enantioselective! - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Part one\u00a0on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success.\u00a0The new model is constructed using two units of Ti(OiPr)4, which are likely to […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-12-17T17:39:30+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-03-10T07:59:23+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/WAWBUR.jpg\" \/>\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":"Why the Sharpless epoxidation is enantioselective! - 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=8658","og_locale":"en_GB","og_type":"article","og_title":"Why the Sharpless epoxidation is enantioselective! - Henry Rzepa's Blog","og_description":"Part one\u00a0on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success.\u00a0The new model is constructed using two units of Ti(OiPr)4, which are likely to […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658","og_site_name":"Henry Rzepa's Blog","article_published_time":"2012-12-17T17:39:30+00:00","article_modified_time":"2023-03-10T07:59:23+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/WAWBUR.jpg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"3 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Why the Sharpless epoxidation is enantioselective!","datePublished":"2012-12-17T17:39:30+00:00","dateModified":"2023-03-10T07:59:23+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658"},"wordCount":634,"commentCount":2,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/WAWBUR.jpg","keywords":["catalysis","catalytic metal centre","Enantioselective","free energy difference","Reaction Mechanism","Sharpless epoxidation","t-butyl","This free energy difference","Tutorial material"],"articleSection":["Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8658","name":"Why the Sharpless epoxidation is enantioselective! 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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":8850,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8850","url_meta":{"origin":8658,"position":1},"title":"Sharpless epoxidation, enantioselectivity and conformational analysis.","author":"Henry Rzepa","date":"January 3, 2013","format":false,"excerpt":"I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed. Amongst the conformational possibilities are the two rotations shown\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":8733,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8733","url_meta":{"origin":8658,"position":2},"title":"Non covalent interactions in the Sharpless transition state for asymmetric epoxidation.","author":"Henry Rzepa","date":"December 19, 2012","format":false,"excerpt":"The Sharpless epoxidation of an allylic alcohol had a big impact on synthetic chemistry when it was introduced in the 1980s, and led the way for the discovery (design?) of many new asymmetric catalytic systems. Each achieves its chiral magic by control of the geometry at the transition state for\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":12308,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12308","url_meta":{"origin":8658,"position":3},"title":"Enantioselective epoxidation of alkenes using the Shi Fructose-based catalyst. 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":8761,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8761","url_meta":{"origin":8658,"position":4},"title":"Vitamin B12 and the genesis of a new theory of chemistry.","author":"Henry Rzepa","date":"December 20, 2012","format":false,"excerpt":"I have written earlier about dihydrocostunolide, and how in 1963 Corey missed spotting the electronic origins of a key step in its synthesis.. A nice juxtaposition to this failed opportunity relates to Woodward's project at around the same time to synthesize vitamin B12. The step in the synthesis that caused\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":20354,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20354","url_meta":{"origin":8658,"position":5},"title":"Epoxidation of ethene: a new substituent twist.","author":"Henry Rzepa","date":"December 21, 2018","format":false,"excerpt":"Five years back,\u00a0I speculated about the mechanism of the epoxidation of ethene by a peracid, concluding that kinetic isotope effects provided interesting evidence that this mechanism is highly asynchronous and involves a so-called \"hidden intermediate\". Here I revisit this reaction in which a small change is applied to the atoms\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.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2018\/12\/imine2.gif?resize=350%2C200&ssl=1","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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8658","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=8658"}],"version-history":[{"count":66,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8658\/revisions"}],"predecessor-version":[{"id":25947,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8658\/revisions\/25947"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8658"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8658"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8658"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8658"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}