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Unformatted text preview: Alkenes II 1 Copyright, Arizona State University Copyright, Arizona State University Alkenes II Introduction to Synthesis (more...) 1 Additional Reactions of Alkenes here are a series of mainly oxidation reactions that we need to complete our survey of alkene chemistry some are not "pretty", i.e. with complex mechanisms, some that are not fully known... Some of the stuff here you will just have to "know", i.e. here is some of the "bad bit" of Organic 1.1 Formation of cis-Diols (more) TWO sets of reagents will accomplish this reaction The reaction OR OH OH OsO 4 / H 2 O 2 syn-addition (same side) cold KMnO 4 /- OH/H 2 O The mechanisms O Mn O O O O O Mn O O OH HO OH + MnO 2 syn-addition!- OH O O Mn O O O O Mn O O HO HO addition/elimination mechanism- OH O O + MnO 2 (OH) 2 O O Mn O O HO OH aqueous workup radical reduction don't need to know!! although this is obviously complex, the important part is that the MnO 4 ion starts the reaction by adding to both ends of the alkene at the same side , which is why a cis-diol must be formed note that in mechanisms involving metal atoms, the metal has enough electrons and empty orbitals to give and take electrons on its own, almost at will (almost like cheating to an organic chemist!!) O Os O O O O O Os O O H 2 O 2 HO OH + OsO 4 catalyst regenerated! don't need to know..... syn-addition! H 2 O 2 Alkenes II 2 Copyright, Arizona State University why TWO reagents? KMnO 4- inexpensive, used for large scale reactions, variable yields OsO 4- expensive, extremely toxic! good yields, used in small scale syntheses This illustrates the principle that in general there will always be more than one reagent to accomplish any transformation even if we only discuss one in this course Examples H HO H OH cold KMnO 4 /- OH/H 2 O cis-diol H 3 C C C CH 3 H H OsO 4 / H 2 O 2 C C HO OH H H H 3 C CH 3 H 3 C C C H H CH 3 OsO 4 / H 2 O 2 () H 3 C C C CH 3 H HO OH H HO OH () = * * solution not optically active meso compound achiral achiral achiral solution not optically active meso compound solution not optically active racemic mix. * * * * 1.2 Formation of Epoxides and trans-Diols (more) New reagent R C O O R O O OH peroxide peroxy acid R C O OH carboxylic acid R C O O OH Cl meta-chloroperbenzoic acid (MCPBA) Mechanism C C O H O C O R C C O C O R H O + epoxide carboxylic acid concerted mechanism - all bonds made and broken at the same time no chance for bond rotation "in the middle" - stereospecific reaction! Examples MCPBA O cis-epoxide formed cis-alkene reaction is STEREOSPECIFIC Synthesis of a trans-Diol H 3 C C C CH 3 H H MCPBA C HO C OH H CH 3 H CH 3 () ANTI-addition C O C OH H CH 3 H CH 3 H H O H H O C C H H H 3 C CH 3 H O C C H H H 3 C CH 3 O H H H 3 O + O H H H backside attack!...
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This note was uploaded on 01/07/2012 for the course CHM 233 taught by Professor Skibo during the Fall '08 term at ASU.
- Fall '08