Only five of the molecules below may be prepared as the sole product of allylic halogenation of the respective alkene. Circle those five.

Transcription

1 Chem 232 D. J. Wardrop Problem Set 6 Answers Question 1. nly five of the molecules below may be prepared as the sole product of allylic halogenation of the respective alkene. Circle those five. 3 C 3 C 3 C C 3 C 3 3 C C 2 2 C CN C 3 3 C C 3 C 3 ( 3 C) 3 C C 2 3 C C 2 C 3 C 2 3 C C 3 Question 2 Draw the major organic product in the boxes provided for each reaction scheme below. If the reaction is regioselective, only draw the major product. Be sure to indicate the correct relative stereochemistry (i.e. dashes & wedges or axial & equatorial bonds) where appropriate. Ts Na TF An example of Williamson ether synthesis Ms NaN 3 TF NaCN CN N 3 CN Both reactions proceed via S N 2 mechanisms despite the sterically hindered nature of the C-atom undergoing substitution. This occurs because both leaving groups are at allylic positions and the S N 2 reaction is favored. For each of the transformations shown above, by what mechanism is product formed? (S N 1, S N 2, E1, or E2) If the solvent was changed from TF to DMF would the mechanism or rate of reaction change? The use of DMF, a polar non-protic solvent would increase the nucleophilicity of cyanide and increase the rate of S N 2 reaction.

2 Question 2 (contd.) Kt-Bu Which hydrogens are more accesible? Tf TF 1,4-Elimination product 1,2-Elimination product 25 C 1,2-Addition product 1,4-Addition product Thermodynamic addition product N CCl 3 upon ing, NBS undergoes decomposition to form 2 which mediates allylic bromination Cl -80 C Kinetic, Markovnikov addition product

3 Question 3 Draw the major Diels-Alder adducts in the boxes provided for each reaction scheme below. If the reaction is regioselective, only draw the major product. Be sure to indicate the correct relative stereochemistry (i.e. dashes & wedges or axial & equatorial bonds) where appropriate. C 2 C 2 endo-diels-alder adduct resulting from [4+2] cycloaddition δ- δ+ C 2 C 2 endo-diels-alder adduct resulting from regioselective [4+2] cycloaddition Which reaction will have the faster rate? Why? this substrate pair will react faster since there is less compression in TS C 2 C 2

4 Question 4 Devise a synthesis for each of the target molecules (products) below. Write the forward synthesis from the reactant. Include all necessary conditions above/below the reaction arrow for each transformation. Write out the product for each transformation in your synthesis separately. 2 2 x t-buk C 2 Cl 2 TF C Na + - N 2 I not isolated Na (metal) N 3 (l) Na + - N 2 I not isolated 2 Pd-CaC 3 lead acetate quinoline C 2 2 Pd-C C 2 C 2 3 then Zn (metal) C 2 C C

5 Question 4 (contd.) δ- hydroboration occurs from less hindered face t-bu δ+ Et remember endo rule Et C 2 t-bu B 3 TF then 2 2, Na Et C 2 t-bu Cl 2, hv Cl Cl chlorine attacks allylic radical from less hindered face at the more substituted position Cl Question 5 Why do you observe the relative stereochemistry in the product? Draw a reasonable arrow-pushing mechanism for each of the following transformations. N N

6 N Why do you see regioselectivity in introduction of bromine atom? 2, 2 Why do you see diastereoselectivity in the 1,4-addition product?

7 Question 6 The following product is not formed thermally, Why? FM theory?. What product would you expect instead using [4+4] cycloadditions are disallowed [4+2] cycloadditions are allowed

8 Question 7 Devise a synthesis for each of the target molecules (products) below. Write the forward synthesis from the reactant. Include all necessary conditions above/below the reaction arrow for each transformation. Write out the product for each transformation in your synthesis separately. C 2 2 C product starting materials Na, N 3 Diels-Alder cycloaddition proceeds according to the endo rule C 2 Epoxidation occurs from less hindered exo face C 2 N 3 starting materials intermediate + B 3 TF then 2 2, Na TsCl pyridine Ts NaN 3 DMF N 3