Ch 10 and 11 Chemistry of alcohols and thiols

relative rates of cyclic ether formation k3 k5 k6

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Unformatted text preview: ster than base displacement to give the diol. ! •  Relative Rates of Cyclic Ether Formation! k3 ≥ k5 > k6 > k4 ≥ k7 > k8 Where: kn = reaction rate constant! n = ring size! 10-31! Cyclic Ether Synthesis!   Intramolecular Williamson Reactions are stereospecific!! •  Since the reaction mechanism is Sn2, the only allowed geometry for nucleophilic displacement is 180o with respect to Nu- (O-) and Leaving group.! O D O C H C H D + Br H Br H H H »  Anti conformation H O C Br C H H D Br O + C D C No Reaction H H H »  Gauche Conformations 10-32! Neighboring-Group Participation!   Example of Neighboring Group Participation (i.e., a traceless intramolecular reaction)! C2H5 H2C C2H5 C2H5 S H2C H2C CH2 H2C Cl S C2H5 S Cl S CH2 CH2 episulfonium salt! CH2 OH2 H2O H2C CH2 + C2H5 S H3O OH OH2 OH2 H2C Note that: C2H5 S CH2 Cl H2C more probable than! C2H5 S CH2 Cl since: !(1) thioethers are better nucleophiles than H2O, and ! !(2) intramolecular 3-membered ring formation is kinetically ! !favored over a bimolecular reaction! 10-33! Alcohol Synthesis: M-H Reduction of Carbonyl Compounds!   General Reaction! OH O LiALH44 or LiAlH R1 R1   R H NaBH4 R Characteristics:! •  LiAlH4 is MUCH stronger as a hydride donor than NaBH4.! •  LiAlH4 and NaBH4 DO NOT REACT with (i.e. reduce) aromatic rings, alkenes, or alkynes.! •  Only LAH is sufficiently strong to reduce carboxylic acids or esters.! 10-34! Mechanism of M-H Reduction of C=O! H M H O H H H + M O H H H O O O H OH H3O 4 + H O O M O M(OH)4 O H H 10-35! Alcohol Synthesis: M-H Reduction of Carbonyl Compounds! Summary!   Aldehydes! •  (RCHO)!   Ketones! •  (RCOR’)!   Carboxylic Acids! •  (RCO2H)!   Esters! •  (RCO2R’)! LiAlH4! NaBH4! ! ! ! ! ! ! ! ! ! ! ! ! RCH2OH! RC(-OH)HR’! ! ! ! ! ! RCH2OH! ! ! ! ! RCH2OH and R’OH ! RCH2OH! RC(-OH)HR’! ! ! ! No Reaction! ! ! ! No Reaction! ! 10-36! ! Examples! OH O LiAlH4! NaBH4! O O LiAlH4! OH No RXN! OH H H O O O LiAlH4! HO OH H O OD NaBD4! D O O NaBH4! O O HO O H 10-37! Alcohol Synthesis: Vicinal Dihydroxylation of Alkenes!   General Reaction! H2O C C + KMnO4 OHOH C !   C + OsO4 OH OH OH H2O2 Characteristics:! •  syn addition of hydroxyl groups to alkenes! •  concerted addition to double bond, producing a cyclic metal ester, accounts for the observed stereochemistry! •  osmium tetroxide is faster and higher yielding, but is! •  TOXIC, VOLATILE and EXPENSIVE! •  potassium permanganate is cheaper and easier to use! 10-38! Mechanism: Dihydroxylation of Alkenes !   Osmium tetroxide! O O O C C O   + O Os HO C C O O OH H2O2 O O Os + HO Os O OH Potassium permanganate! O O O C C O O O Mn + HO Mn O H2O OH- O C OH + MnO2! C 10-39! Synthesis of Alcohols ! dil. KMnO4 H2O, OH- OH + OH OH OH Same! OH OsO4 + H2O2 OH OH OH HO HO KMnO4 + HO HO Same! OH OH OH OsO4 OH + H2O2 10-40! Biological Application of OsO4 Oxidation! ain Saw Safely | The Family Handyman http://www.familyhandyman.c...
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