{"id":13688,"date":"2015-04-01T09:45:48","date_gmt":"2015-04-01T08:45:48","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=13688"},"modified":"2015-04-10T16:52:19","modified_gmt":"2015-04-10T15:52:19","slug":"mechanism-of-the-lithal-lah-reduction-of-cinnamaldehyde","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13688","title":{"rendered":"Mechanism of the Lithal (LAH) reduction of cinnamaldehyde."},"content":{"rendered":"
\n

The reduction of cinnamaldehyde by lithium aluminium hydride (LAH) was reported in a classic series of experiments[1]<\/a><\/span>,[2]<\/a><\/span>,[3]<\/a><\/span> dating from 1947-8. The reaction was first introduced into the organic chemistry laboratories here at Imperial College decades ago, vanished for a short period, and has recently been reintroduced again.\u2021<\/sup> The experiment is really simple in concept; add LAH to cinnamaldehyde and you get just reduction of the carbonyl group; invert the order of addition and you additionally get reduction of the double bond. Here I investigate the mechanism of these reductions using computation (\u03c9B97XD\/6-311+G(d,p)\/SCRF=diethyl ether).<\/p>\n

\"LAH\"<\/a>
\nThe mechanism can be envisaged as proceeding through a 1,4-hydride attack (TS14) with a hidden<\/em> intermediate (HI14) on the reaction path, or instead finding a pathway involving either one or two consecutive 1,2-attacks; TS12-1, TS12-2 via<\/em> an explicit intermediate I12. Experiment shows that quenching with D2<\/sub>O at the end of the reduction to replace a C-Al with a C-D bond certainly seems to rule out the 1,4 route, since that would not lead to incorporation of deuterium at the benzylic position. So does the computational model reflect this reality?<\/p>\n\n\n\n\n\n\n\n\n\n\n
Species<\/th>\nRelative \u0394G, kcal\/mol<\/th>\nFAIR Data<\/a>-DOI<\/th>\n<\/tr>\n
R<\/td>\n0.0<\/td>\n[4]<\/a><\/span><\/td>\n<\/tr>\n
TS14<\/td>\n+11.7 <\/td>\n[5]<\/a><\/span><\/td>\n<\/tr>\n
P14<\/td>\n-38.8 <\/td>\n[6]<\/a><\/span><\/td>\n<\/tr>\n
TS12-1<\/td>\n+8.4 <\/td>\n[7]<\/a><\/span><\/td>\n<\/tr>\n
I12<\/td>\n-35.8 <\/td>\n[8]<\/a><\/span><\/td>\n<\/tr>\n
TS12-2<\/td>\n+6.5 (42.3)<\/td>\n[9]<\/a><\/span><\/td>\n<\/tr>\n
P12<\/td>\n-52.4 <\/td>\n[10]<\/a><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

I have chosen a model in which two dimethyl ether molecules solvate the lithium cation. The reactant itself has an interesting structure, in which two of the Al-H bonds form bridges to the Li, which ends up being five-coordinated. Further weak C-H…O=C hydrogen bonding is also observed. The NCI (non-covalent-interaction) surfaces are well worth inspecting (inspection notes: the NCI surrounding the Al has artefacts, since the value of the electron density surrounding the metal is lower than covalent density for the other elements. Click on the image below to load the 3D model).<\/p>\n

\"Click

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

TS14 retains that C-H…O=C hydrogen bond, but the double Al-H-Li bridge is lost. The 8-ring for the TS allows the hydride transfer to be approximately linear, and the Bürgi-Dunitz angle of approach of the hydride to the double bond is 107.4°. Whilst the barrier is acceptably low, the reaction reaches a cul-de-sac down this path; it has no low energy escape route.<\/p>\n

\"TS14\"

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

TS12-1 loses the C-H…O=C hydrogen bond, but being 3.3 kcal\/mol lower in free energy than TS14 fortunately provides a lower energy alternative to that cul-de-sac! The Bürgi-Dunitz angle is 112.0°.
\n\"TS12-1\"
\n\"LAH12-1\"<\/a><\/p>\n

TS12-2 is required to proceed further to the dihydrocinnamyl alcohol reduction product P12, and now we have to confront the nub of the problem. Why does this further reduction only proceed when the LAH is in excess? TS12-2 itself corresponds to an Al-H addition<\/a> across a C=C double bond.[11]<\/a><\/span>†<\/sup>, with a similar barrier to TS12-1. The answer to this conundrum is to recognise that I12 forms what is called a resting state<\/i> for the reaction, and that to proceed further the reaction has to overcome the barrier from I12 to TS12-2. That barrier is 42.3 kcal\/mol, far too high to proceed thermally. When one encounters an unreasonable<\/i> barrier, one has to look very carefully at the model one has constructed for the process.
\n

\"Click

Click for 3D<\/p><\/div>
\n\"LAH12-2a\"<\/a><\/p>\n

Clearly, the model I used here is lacking something. Since the reaction only proceeds when LAH is in excess, we can formulate the hypothesis that further LAH must be added to the model, from which a more reasonable barrier might emerge. If I find out how that can be done, I will report back here.<\/p>\n

\n

\u2021<\/sup> LAH as a reagent was originally available in powder form, which could be quite tricky to handle and could cause fires if not handled properly. The lab organiser Chris tells me it now comes in standard-sized pellets<\/a> which are far easier and safer to handle in a laboratory, allowing its re-introduction.
\n†<\/sup>Biographical note.<\/b> This footnote is added because I spent three years as a Ph.D. student<\/a> trying to construct transition state models by measuring kinetic isotope effects. My failure to do so convincingly meant I decided to spend a further three years as a Post Doc inverting the concept by learning how to model transition states using quantum mechanical computation. I first applied these skills as an independent researcher to locating the transition state for Cl-H addition (vs Al-H in this post) across a C=C double bond and computing the associated isotope effects.[12]<\/a><\/span> This article ends with the assertion that “SCF-MO calculations may provide a more rational basis for interpreting kinetic isotopes than the reverse procedure of attempting to establish a transition state model from the observed kinetic data.<\/i>” It is nice to see that posterity<\/a> has shown that this assessment was about right.<\/p>\n

References<\/h2>\n
  1. R.F. Nystrom, and W.G. Brown, \"Reduction of Organic Compounds by Lithium Aluminum Hydride. I. Aldehydes, Ketones, Esters, Acid Chlorides and Acid Anhydrides\", Journal of the American Chemical Society<\/i>, vol. 69, pp. 1197-1199, 1947. https:\/\/doi.org\/10.1021\/ja01197a060<\/a>\n\n<\/li>\n
  2. R.F. Nystrom, and W.G. Brown, \"Reduction of Organic Compounds by Lithium Aluminum Hydride. II. Carboxylic Acids\", Journal of the American Chemical Society<\/i>, vol. 69, pp. 2548-2549, 1947. https:\/\/doi.org\/10.1021\/ja01202a082<\/a>\n\n<\/li>\n
  3. F.A. Hochstein, and W.G. Brown, \"Addition of Lithium Aluminum Hydride to Double Bonds\", Journal of the American Chemical Society<\/i>, vol. 70, pp. 3484-3486, 1948. https:\/\/doi.org\/10.1021\/ja01190a082<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191154<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191148<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191152<\/a>\n\n<\/li>\n
  7. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191149<\/a>\n\n<\/li>\n
  8. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191151<\/a>\n\n<\/li>\n
  9. H.S. Rzepa, and H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191156<\/a>\n\n<\/li>\n
  10. H.S. Rzepa, \"C 13 H 24 Al 1 Li 1 O 3\", 2015. https:\/\/doi.org\/10.14469\/ch\/191155<\/a>\n\n<\/li>\n
  11. H.S. Rzepa, \"Gaussian Job Archive for C2H7Al\", 2015. https:\/\/doi.org\/10.6084\/m9.figshare.1362146<\/a>\n\n<\/li>\n
  12. H.S. Rzepa, \"MNDO SCF-MO calculations of kinetic isotope effects for dehydrochlorination reactions of chloroalkanes\", Journal of the Chemical Society, Chemical Communications<\/i>, pp. 939, 1981. https:\/\/doi.org\/10.1039\/c39810000939<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

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Here is a quantum energetic exploration of what might happen when a second LAH is added\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":13899,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13899","url_meta":{"origin":13688,"position":1},"title":"The mechanism of borohydride reductions. Part 1: ethanal.","author":"Henry Rzepa","date":"April 12, 2015","format":false,"excerpt":"Sodium borohydride is the tamer cousin of lithium aluminium hydride (LAH). It is used in aqueous solution to e.g. reduce aldehydes and ketones, but it leaves acids, amides and esters alone. Here I start an exploration of why it is such a different reducing agent. Initially, I am using Li,\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":8508,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8508","url_meta":{"origin":13688,"position":2},"title":"The mechanism of the Birch reduction. Part 2: a transition state model.","author":"Henry Rzepa","date":"December 3, 2012","format":false,"excerpt":"I promised that the follow-up to on the topic of Birch reduction would focus on the proton transfer reaction between the radical anion of anisole and a proton source, as part of analysing whether the mechanistic pathway proceeds O or M. To add some context, Hammond's postulate\u00a0\u00a0states that \"the structure\u2026","rel":"","context":"In \"Birch reduction\"","block_context":{"text":"Birch reduction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=birch-reduction"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-mts.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8398,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8398","url_meta":{"origin":13688,"position":3},"title":"Di-imide reduction with a twist: A M\u00f6bius version.","author":"Henry Rzepa","date":"November 26, 2012","format":false,"excerpt":"I was intrigued by one aspect of the calculated transition state for di-imide reduction of an alkene; the calculated NMR shieldings indicated an diatropic ring current at the centre of the ring, but very deshielded shifts for the hydrogen atoms being transferred. This indicated, like most thermal pericyclic reactions, an\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\/11\/GaussViewScreenSnapz004.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8540,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8540","url_meta":{"origin":13688,"position":4},"title":"The mechanism of the Birch reduction. Part 3: reduction of benzene","author":"Henry Rzepa","date":"December 4, 2012","format":false,"excerpt":"Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this? The mechanism, as elaborated in the previous two posts, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this\u2026","rel":"","context":"In \"Birch reduction\"","block_context":{"text":"Birch reduction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=birch-reduction"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-ip.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8776,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8776","url_meta":{"origin":13688,"position":5},"title":"How to tame an oxidant: the mysteries of “tpap” (tetra-n-propylammonium perruthenate).","author":"Henry Rzepa","date":"December 24, 2012","format":false,"excerpt":"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\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":"ts2-triplet-spin","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/ts2-triplet-spin.jpg?resize=350%2C200","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","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\/13688","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=13688"}],"version-history":[{"count":84,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/13688\/revisions"}],"predecessor-version":[{"id":13819,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/13688\/revisions\/13819"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=13688"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=13688"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=13688"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=13688"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}