BackOne way to think about concerted reactions is to imagine them as being stepwise reactions where, besides the slowest step, all others have infinitesimally small activation energies. Considering the hypothetical stepwise mechanism and actual concerted mechanism of epoxide formation, show what a reaction coordinate diagram might look like for each possibility.
(b) Concerted , actual mechanism (butterfly transition state:
How does the carbonyl in mCPBA weaken the O―O σ bond (i.e., make a better leaving group)?
Calculate the atom economy of the reaction in Figure 9.24. [Catalysts are not included in the atom economy calculation.]
Write the appropriate reagent over each arrow.
In addition to using mCPBA, epoxides can be synthesized from alkenes in the two-step process shown. Give a mechanism for each step of the process.
In Chapter 9, electron-rich alkenes were oxidized under acidic conditions with mCPBA. Conjugated alkenes can be oxidized using the same reagent, but under basic conditions. Suggest a mechanism for this reaction. [Think about what is electron-rich and what is electron-poor in the reaction. Also, identify the bonds formed and broken.]
Predict the product(s) when each of the following are reacted with mCPBA, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.
(d)
Evidence for the concertedness of epoxide formation comes from the stereospecificity of the reaction. If step 2 of a hypothetical stepwise mechanism were slow enough to make the reaction non-concerted, how would the product distribution change?
What alkene would you treat with a peroxyacid in order to obtain each of the epoxides in Problem 27?
a. 2-propyloxirane
b. cyclohexene oxide
a. What alkene is required to synthesize each of the following compounds?
b. What other epoxide is formed in each synthesis?
c. Assign an R or S configuration to each asymmetric center.
1.
a. What alkene is required to synthesize each of the following compounds?
b. What other epoxide is formed in each synthesis?
c. Assign an R or S configuration to each asymmetric center.
2.
What alkene would you treat with a peroxyacid in order to obtain each of the epoxides in Problem 27?
c. 2,2,3,3-tetramethyloxirane
d. 2,3-epoxy-2-methylpentane
Magnesium monoperoxyphthalate (MMPP) epoxidizes alkenes much like mCPBA. MMPP is more stable, however, and it may be safer to use for large-scale and industrial reactions. Propose a mechanism for the reaction of trans-2-methylhept-3-ene with MMPP, and predict the structure of the product(s).
Limonene is one of the compounds that give lemons their tangy odor. Show the structures of the products expected when limonene reacts with an excess of each of these reagents.
b. m-chloroperoxybenzoic acid
Retrosynthetic analysis is the process of working backward to develop the synthesis of a new compound. In Chapter 10, we begin developing multistep syntheses in this manner. For now, try to work backward a single step by suggesting an alkene and a reagent that would give products (a)–(i). [Your answers should not include alkenes that undergo rearrangement to give the desired products.]
(f)