{"id":16441,"date":"2016-05-27T15:53:08","date_gmt":"2016-05-27T14:53:08","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16441"},"modified":"2016-05-27T15:53:08","modified_gmt":"2016-05-27T14:53:08","slug":"an-alternative-mechanism-for-nucleophilic-substitution-at-silicon-using-a-tetra-alkyl-ammonium-fluoride","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2016\/05\/27\/an-alternative-mechanism-for-nucleophilic-substitution-at-silicon-using-a-tetra-alkyl-ammonium-fluoride\/","title":{"rendered":"An alternative mechanism for nucleophilic substitution at silicon using a tetra-alkyl ammonium fluoride."},"content":{"rendered":"

In the previous post, I explored the mechanism for nucleophilic substitution at a silicon centre proceeding via<\/em> 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 other factors.[cite]10.1016\/S0040-4020(01)89077-3[\/cite],[cite]10.1021\/ja01006a024[\/cite],[cite]10.1021\/ja00784a081[\/cite]<\/p>\n

\"\"<\/p>\n

This alternative involves attack by F–<\/sup> along the axial trajectory of the trigonal bipyramidal Si centre, with the OR group occupying the other axial position (TS1<\/strong>).\u00a0In order to prepare the OR group for elimination with inversion of stereochemistry, the ion-pair complex has to reorganise (a process replacing the previous Berry pseudorotation necessary with for stereochemical retention) via TS2.<\/strong> And finally the OR is eliminated in TS3<\/strong>. The energetics of this pathway (\u03c9B97XD\/6-31+G(d) or Def2-TZVPPD\/SCRF=thf) are shown below, with the inversion pathway coming out lower in energy than the previously reported retention pathway.\u00a0<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
System<\/th>\nRelative free energy<\/th>\nDataDOI<\/th>\n<\/tr>\n
Inversion mechanism<\/th>\n<\/tr>\n
Reactants<\/td>\n0.0<\/td>\n[cite]10.14469\/hpc\/565[\/cite]<\/td>\n<\/tr>\n
\n

TS1<\/p>\n<\/td>\n

4.9 (4.1)*<\/td>\n[cite]10.14469\/hpc\/570[\/cite]<\/td>\n<\/tr>\n
TS2<\/td>\n3.1<\/td>\n[cite]10.14469\/ch\/195052[\/cite]<\/td>\n<\/tr>\n
TS3<\/td>\n0.0 (-0.8)*<\/td>\n[cite]10.14469\/hpc\/567[\/cite]<\/td>\n<\/tr>\n
Retention mechanism<\/th>\n<\/tr>\n
TS1<\/td>\n7.9 (8.3)*<\/td>\n[cite]10.14469\/hpc\/554[\/cite]<\/td>\n<\/tr>\n
TS2<\/td>\n9.2 (8.7)*<\/td>\n[cite]10.14469\/hpc\/577[\/cite]<\/td>\n<\/tr>\n
TS3<\/td>\n5.2 (4.9)*<\/td>\n[cite]10.14469\/hpc\/539[\/cite]<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

* Values in parentheses are computed for the\u00a0Def2-TZVPPD basis set.<\/p>\n

The key new finding for the inversion mechanism is the ion-pair isomerisation (TS2), which is animated below. Transition states which involve no rearrangement at a bond (either formation\/cleavage or rotation) are quite rare, and it is nice to show one here.<\/p>\n

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

So the nucleophilic displacement reaction at 4-substituted silicon centres is really quite different from carbon.Two distinct associative\/elimination mechanisms proceeding through 5-coordinate silicon seem possible. For the specific case of tetra-alkyl\u00a0ammonium fluoride as nucleophile and an enolate anion as the leaving group, it appears that an inversion <\/span><\/strong>mechanism is favoured, and one gets strong indications of this from crystal structures of such 5-coordinate species. It might be nice to repeat this study with a reaction which is known to strongly favour retention of configuration.<\/p>\n","protected":false},"excerpt":{"rendered":"

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 other factors.[cite]10.1016\/S0040-4020(01)89077-3[\/cite],[cite]10.1021\/ja01006a024[\/cite],[cite]10.1021\/ja00784a081[\/cite] This alternative involves […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13],"tags":[329,902,1038,1432,1775,2005,2228,2258,2350,2557],"class_list":["post-16441","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2","tag-brook-rearrangement","tag-energy","tag-free-energy","tag-leaving-group","tag-nucleophilic-substitution","tag-pseudorotation","tag-si-centre","tag-sni","tag-substitution-reactions","tag-walden-inversion"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/16441","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=16441"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/16441\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=16441"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=16441"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=16441"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}