{"id":29725,"date":"2025-10-21T12:39:33","date_gmt":"2025-10-21T11:39:33","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29725"},"modified":"2025-11-30T14:52:19","modified_gmt":"2025-11-30T14:52:19","slug":"the-mechanism-of-borohydride-reductions-part-2-4-t-butyl-cyclohexanone-dispersion-induced-stereochemistry","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29725","title":{"rendered":"The mechanism of borohydride reductions. Part 2: 4-t-butyl-cyclohexanone – Dispersion induced stereochemistry."},"content":{"rendered":"
\n

Part one of this topic was posted more than ten years ago.[1]<\/a><\/span> I clearly forgot about it, so belatedly, here is part 2 – dealing with the stereochemistry of the reduction of tert-butyl-cyclohexanone by borohydride in water. The known stereochemistry is nicely summarised in this article, along with an extensive \u00a0history of possible explanations of the reasons for the stereochemical preference.[2]<\/a><\/span> Put simply, the hydride nucleophile attacks the carbonyl from an axial rather than equation direction with a ratio of 10:1 (\u0394\u0394G 1.37 kcal\/mol). So does the model I previously proposed[1]<\/a><\/span> support this and give any indication of why the stereochemistry is axial?<\/p>\n

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

The calculated transition states are shown below (click on image to get interactive 3D model). Note also the unusual calculated B-H…H-O hydrogen bonded distances of ~1.8\u00c5. A search of the CSD (crystal structure database) reveals surprisingly few such examples, but one interesting one is as short as ~1.73\u00c5[3]<\/a><\/span><\/p>\n

DFT calculations were conducted using Gaussian 16 at the B3LYP\/Def2-TZVPP\/SCRF=water level[4]<\/a><\/span> for transition state location and energies (including D3+BJ dispersion) and using ORCA 6.1[5]<\/a><\/span> for calculating D4 dispersion energies.[6]<\/a><\/span><\/p>\n

Lithium Borohydride<\/strong>
\n\"\"\"\"<\/p>\n

Sodium borohydride<\/strong><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

As well as computing the free energy difference \u0394\u0394G\u2021<\/sup> between the two transition states, I also looked at the total dispersion energy contributions to these energies at the third generation D3+BJ and the fourth generation D4 levels, shown below as the difference between the axial and equatorial transition states.<\/p>\n\n\n\n\n\n
Counter ion<\/td>\n\u0394D3 dispersion<\/th>\n\u0394D4 dispersion<\/th>\n\u0394G\u2021<\/sup>axial<\/sub><\/th>\n\u0394G\u2021<\/sup>equatorial<\/sub><\/th>\n\u0394\u0394Gh<\/sub>\u2020<\/sup><\/th>\n<\/tr>\n
Lithium borohydride<\/td>\n1.15<\/td>\n0.932<\/td>\n12.13<\/td>\n12.93<\/td>\n0.80<\/td>\n<\/tr>\n
Sodium borohydride<\/td>\n1.22<\/td>\n0.745<\/td>\n13.31<\/td>\n14.16<\/td>\n0.85<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\n

\u2020<\/sup>Harmonic energies.<\/small><\/p>\n

The calculations reveal that using either Li or K as the counterion to borohydride, the axial transition state is lower in energy than the equatorial, by around 0.85 to 0.80 kcal\/mol in total free energy. These values correspond to an axial\/equatorial ratio of around \u00a03.9-4.2:1, a little bit lower than observed experimental value of 10:1.[2]<\/a><\/span>\u00a0The D3+BJ and D4 dispersion energies follow the same trend, with the suggestion that the\u00a0Li ion is slightly more stereoselective than the\u00a0Na ion, perhaps due to the more compact nature of the transition state.<\/p>\n

So we might conclude that the stereochemical preference for axial hydride delivery to tert-butyl-cyclohexanone could be explained entirely<\/b> by the differing dispersion energy contributions of the two transition states. This in one way is a deeply unsatisfactory explanation, since the dispersion energy difference is the\u00a0total sum of many individual dispersion contributions\u00a0and is largely unpredictable until calculated. Chemists like simple rules and this is not apparently amenable to such a simple rule. Indeed perhaps the rule should simply be to always compare the computed dispersion energies of two (or more) isomeric transition states before seeking other explanations!<\/p>\n

\n

A source of error in the calculated free energies could be the treatment of the vibrational modes as harmonic. A quasi-harmonic treatment is available[7]<\/a><\/span> which should give a more reliable estimate of the relative free energy. Using this approach (for settings see [8]<\/a><\/span>) ones gets the values below. Although the discrimination is slightly reduced, the overall prediction of axial hydride attack is still confirmed.<\/p>\n\n\n\n\n\n\n
system><\/th>\nhG<\/th>\nqhG<\/th>\n<\/tr>\n
LiBH4 ax<\/td>\n-884.325907<\/td>\n-884.328233 (ΔΔG -0.62)<\/td>\n<\/tr>\n
LiBH4 eq<\/td>\n-884.324625<\/td>\n-884.327246<\/td>\n<\/tr>\n
NaBH4 ax<\/td>\n-1039.096789<\/td>\n-1039.100009 (ΔΔG -0.78)<\/td>\n<\/tr>\n
NaBH4 eq<\/td>\n-1039.095445<\/td>\n-1039.098771<\/td>\n<\/tr>\n<\/table>\n

References<\/h2>\n
  1. H. Rzepa, \"Part 1: ethanal.\", 2015. https:\/\/doi.org\/10.59350\/aqrgh-jw887<\/a>\n\n<\/li>\n
  2. R. Kobeti\u0107, V. Petrovi\u0107-Perokovi\u0107, V. Klju\u010dari\u0107, B. Jur\u0161i\u0107, and D.E. Sunko, \"Selective Reduction of Cyclohexanones with NaBH<sub>4<\/sub> in \u03b2-Cyclodextrin, PEG-400, and Micelles\", Supramolecular Chemistry<\/i>, vol. 20, pp. 379-385, 2008. https:\/\/doi.org\/10.1080\/10610270701268815<\/a>\n\n<\/li>\n
  3. R. Custelcean, and J.E. Jackson, \"Topochemical Control of Covalent Bond Formation by Dihydrogen Bonding\", Journal of the American Chemical Society<\/i>, vol. 120, pp. 12935-12941, 1998. https:\/\/doi.org\/10.1021\/ja982959g<\/a>\n\n<\/li>\n
  4. H. Rzepa, \"The mechanism of borohydride reductions. Part 2: 4-t-butyl-cyclohexanone\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15559<\/a>\n\n<\/li>\n
  5. F. Neese, \"Software Update: The <scp>ORCA<\/scp> Program System\u2014Version 6.0\", WIREs Computational Molecular Science<\/i>, vol. 15, 2025. https:\/\/doi.org\/10.1002\/wcms.70019<\/a>\n\n<\/li>\n
  6. E. Caldeweyher, J. Mewes, S. Ehlert, and S. Grimme, \"Extension and evaluation of the D4 London-dispersion model for periodic systems\", Physical Chemistry Chemical Physics<\/i>, vol. 22, pp. 8499-8512, 2020. https:\/\/doi.org\/10.1039\/d0cp00502a<\/a>\n\n<\/li>\n
  7. G. Luchini, J.V. Alegre-Requena, I. Funes-Ardoiz, and R.S. Paton, \"GoodVibes: automated thermochemistry for heterogeneous computational chemistry data\", F1000Research<\/i>, vol. 9, pp. 291, 2020. https:\/\/doi.org\/10.12688\/f1000research.22758.1<\/a>\n\n<\/li>\n
  8. H. Rzepa, \"Quasi-harmonic calculations for the mechanism of borohydride reductions. Part 2: 4-t-butyl-cyclohexanone\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15565<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Part one of this topic was posted more than ten years ago. I clearly forgot about it, so belatedly, here is part 2 – dealing with the stereochemistry of the reduction of tert-butyl-cyclohexanone by borohydride in water. 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Its\u00a0isolated 3D structure is quite\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":29725,"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":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":29725,"position":2},"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":[]},{"id":16402,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16402","url_meta":{"origin":29725,"position":3},"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":15048,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15048","url_meta":{"origin":29725,"position":4},"title":"I’ve started so I’ll finish. The mechanism of diazo coupling to indoles – forty (three) years on!","author":"Henry Rzepa","date":"December 24, 2015","format":false,"excerpt":"The BBC TV quiz series Mastermind\u00a0was first broadcast in the UK in 1972,\u00a0the same time\u00a0I was starting to investigate\u00a0the mechanism of diazocoupling to substituted indoles as part of my Ph.D. researches. The BBC program became known\u00a0for the\u00a0catch phrase\u00a0I've started so I'll finish;\u00a0here I will try to follow this precept with\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":29799,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29799","url_meta":{"origin":29725,"position":5},"title":"Short B-H…H-O Interactions in crystal structures – a short DFT Exploration using B3LYP+D4 and r2scan-3c","author":"Henry Rzepa","date":"October 27, 2025","format":false,"excerpt":"In the previous post, I was commenting that the transition state for borohydride reduction of a ketone contained some close contacts between the hydrogen of the borohydride and the hydrogen of water. A systematic search of the CSD reveals a modest number of such contacts have been observed in crystal\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\/29725","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=29725"}],"version-history":[{"count":52,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/29725\/revisions"}],"predecessor-version":[{"id":29867,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/29725\/revisions\/29867"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=29725"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=29725"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=29725"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=29725"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}