Lecture Outline 8. Ozonolysis

Name ______________________________________ Section 11.13 covers addition of HX to conjugated dienes. There's really nothing new here, so we're not going to cover this in lecture. The example in the book is a good one — the products of addition of 1 equivalent of HCl to isoprene (2-methyl-1,3-butadiene) can be predicted based on what we already know about carbocation stabilities and resonance. Of the 4 possible cations considered (2 localized, 2 delocalized), the reaction goes via the one most stable cation only. 1 With these ideas in mind, write the mechanism of the reaction below and draw the two products (ignore stereochem). Please do this carefully, or the next two questions won't make sense. HCl (1 equiv) 2 As the text also points out, the product ratio depends on the reaction conditions. At low temperature, e.g., –80°C, the reaction is under kinetic control, meaning that the major product is the one that's formed faster. The chloride nucleophile adds faster to the C of the carbocation that has more + charge. Which product predominates under these conditions? 3 At higher temp (e.g., 45°C, presumably in the proper solvent), the more stable product predominates. This is usually not the one that's formed faster at low temp. Furthermore, if the two products are isolated, then separately exposed to the high-temp reaction conditions, the same mixture of the two products is formed as obtained from the higher-temperature HCl addition. This must mean that the products find a way to equilibrate under these conditions, so their ratio depends only on their relative free energies. The reaction is thus said to be under thermodynamic control. (a) Which product predominates under these conditions? (b) How do you think they equilibrate? What's the mechanistic pathway? Lecture outline And now for something completely different... 8. Ozonolysis C O3 C (CH3)2S C O + O ozone mechanism: C C (add the charges, lone pairs, and curvy arrows!) O C O C O O O C O O O O C this part is just an add'n to the CC !-bond O C O Me2S O O O C examples — 1. O3 2. Me2S 1. O3 2. Me2S an "ozonide" C 9. Carbene additions Ph H A carbene is an extremely reactive species with a neutral, divalent C, i.e., C C Cl H A carbenoid is a species that reacts like a carbene think of this as a "masked carbene". – X! H C Carbenoids are generally less reactive than carbenes C Metal !+ H X H H Metal Creation of carbenes and carbenoids — Carbenes can be made by (1) photolysis of diazomethane — h! H2CN2 + H2C: N2 or (2) by base-promoted α-elimination of H and halogen — (this typically works only for formation of dihalocarbenes — why?) Cl2C H + Cl2C: Cl2C HO Cl Cl Carbenoids have both an electronegative halogen and an electropositive metal attached to the same C — e.g., the carbenoid in the box below is known as the "Simmons-Smith reagent". I H2C I Zn (Cu) I H2C I H2C: ZnI ZnI Reactions of carbenes and carbenoids — Carbenes and carbenoids add to π-bonds to make cyclopropanes, e.g., H2C: + Most carbene additions are stereospecifically syn — HCCl3 HO– H2CI2 Zn (Cu) Reactions of alkynes — 1. Addition of HX — Markovnikov regiochem; non-stereospecific R C Cl HCl CH R C CH2 HCl Cl R C CH3 Cl H Cl Cl R C CH2 a vinylic cation — unstable (comparable to 1° alkyl cation in stability) 2. Addition of X2 — anti stereochem (usually) R C CH Br2 Br H C R C Br Br2 Br R Br C CH Br Br 3. Hydration — catalyzed by acid and Hg2+ — overall Markovnikov add'n of 2 Hs and O to triple bond R C CH + H2 O OH H+ Hg2+ R C O CH2 R C CH3 an enol — (has –OH on a double-bond C) — unstable; tautomerizes rapidly Enols are unstable and can't normally be isolated because they tautomerize rapidly. It's not immediately obvious why having an OH on a C=C would be a problem — it's really not — it's just that the carbonyl (C=O) tautomer is more stable thermodynamically and the Ea for the tautomerization is low. mechanism (without the Hg2+ catalyst for simplicity): R C CH H OH2 O O R C CH3 — Acidic soln, H+ and H2O only; NO HO– !!! — Notice also that intermediates formed under acidic conditions are generally pos(+)-charged. R H C enol CH2 That was the simplified version... now the real story — the rxn is very slow with H+ catalyst alone because formation of the unstable vinylic cation is difficult. The Hg2+ catalyst accelerates rxn by adding to triple bond. The mercury is later replaced by H+. R R C C CH CH Hg+ OH OH2 R or Hg2+ R C CH Hg+ C CH H+ Hg2+ etc (as above) OH R C – Hg2+ CH2 OH R C CH2 Hg+ enol To predict the products of hydration of an alkyne, we can imagine that the mechanism goes via addition of an H+ to make a vinylic cation — which of the two possible vinylic cations is more stable? The positive C is the one the oxygen bonds to, and the resulting enol tautomerizes. H+, Hg2+ H2O H+, Hg2+ H2O