Problem Set 7 - Key. Due Tuesday, November 15, 2016

Transcription

1 Problem Set 7 Key Due Tuesday, ovember 15, a. Suggest a plausible arrowpushing mechanism for the following transformation. (n) 4 is a noncoordinating cation. Et 4 Et Et : Et :Et Et a b. For reactions involving metal alkoxides, avoid the following types of elementary reaction steps for metaloxygen bonds. a ' a (use the oxygen lone pairs, not the a sigma bond) no a : ' S 2 ' a (don't break the a bond as you attack; break the a bond in a second step ) no a S 1 a (ionization of neutral species to give two ions is energetically costly; there's nothing you can do with that you can't do with a) Suggest a plausible arrowpushing mechanism for the following transformation. a a Et Et

2 here is the most plausible mechanism following Chem 201 rules Et a : a Et : a : B Et B a : Et Et a not good; costs immense energy to separate a and no : : a Et a Et :Et Et a not so good; draw a C= resonance structure to see why Et a : a Et a :Et Et a c. For reactions involving alkali organometallics, avoid the following type of elementary reaction step for metalcarbon bonds. C C: Suggest a plausible arrowpushing mechanism for the following transformation. Et 2 equiv. Et

3 plausible Et : Et : Et Et : equally plausible Et : Et : Et : Et Et : : 2. Suggest a plausible arrowpushing mechanism for the following transformation. In this mechanism, do not abbreviate the potassium alkoxide base as (B: ); instead clearly show any steps that lead to formation or cleavage of bonds to potassium. 1 nex C C K t C C : cat. tk DMS 2 C, 15 h nex C C K 91% after aq. workup K t C C t K C C K. Suggest a plausible arrowpushing mechanism for the following transformation. 2 ote the very low temperature used for these reactions. t Br i) n TF, 100 C ii) C 100 C, 5 min t 75% after workup

4 t Br Br 4. Suggest a plausible arrowpushing mechanism for the following transformation without using an S 2 displacement. Assume an aqueous workup. 2.1 equiv. Bn Cl C2 TF 70 C MgBr Bn C2 60% Bn Cl :B C 2 Bn C 2 Bn C 2 Cl Mg Bn B C 2 workup Br 5. Suggest a plausible arrowpushing mechanism for the following transformation. 4 Si is quite acidic with a pk a of Si 4 2 Et Si Si 91% 2 C (o S 2 reactions) : Si Si Si Ȯ. : Si Si Si Si Si Si Ȯ. : Si 6. Suggest a plausible arrowpushing mechanism for the following transformation. 6 Et Sn TF, 78 C i) n Et ii) 2 C=CC Et Et 72%

5 Et Sn Et Et Sn Et Et Et Et Et Additional Problems for Your Amusement and Enjoyment 7. Suggest a plausible arrowpushing mechanism for the following transformation M aq. a 80 C, 2.5 h 80% C 2 a I think opening of the lactone would precede formation of the quinone methide, but I would accept the opposite order. a. a : B a a B a 2 C a 2 C quinone methide intermediate : B a 2 C : a 2 C sticking to arrow rule C 2 a 2 C B 8. The FelkinAnh analysis is used for acyclic carbonyls, not for cyclic ketones. To predict the stereochemistry of addition to cyclic ketones it often helps to assess the conformation of the substrates and the steric interactions that affect the nucleophile trajectory. Based on those considerations, predict the stereochemistry of the s. a. 8

6 ab 4 : u TF 25 C, 18 h = b. 9 Al() 75% = t t u 9. Suggest a plausible arrowpushing mechanism for the following transformation. 10 a 2 C, 2 h C 2 66% a.ọ a B ȮḢ epimerizes C Formation of SiSi can be used to drive a wide range of reactions. Suggest a plausible arrowpushing mechanism for the following transformation. 11 Si Si cat. SiTf C 2 Cl 2 20 C, h 84% SiSi

7 emember that silicon does not undergo concerted S 2 substitution; it involves two steps. Ọ. Si Tf Si Tf Si Si Si u: Si Si Si u : Si Si Si Si Si Si. Si Si u: 11. Suggest a plausible arrowpushing mechanism for the following transformation. 12 sealed tube 2 Et 78 C, 12 h There is some variability in order, but the key steps are elimination of the carboxylate and conjugate addition of the amine.

8 2 B: : B B: : or via iminium ion B 2 B B: : : or imino tautomer 12. Suggest a plausible arrowpushing mechanism for the following transformation. 1, mol % n 250 mol% n TF 78 C 25 C, 1 h 88% carbenoid eferences 1 Babler, J..; ptak, V. P.; an,. AlkoxideCatalyzed Addition of Terminal Alkynes to Ketones J. rg. Chem. 1996; 61(1); ote: C (pka = 17); CC2 (an enolizable ketone, pka = 1920); CC (pka = 25); (pka = 8); and CC2 (pka = 50) 2 William E. Parham, Lawrence D. Jones alogenmetal exchange in esters of haloaryl acids J. rg. Chem.; 1976; 41(16); Castelhano, A. L.; orne,s.; Billedeau,.; Krantz, A. Tetrahedron Lett. 1986, 27, Šefcík, J.; ankin, S. E.; Kirchner, S. J.; McCormick, A. V. Esterification, condensation, and deprotonation equilibria of trimethylsilanol J. oncryst. Solids 1999, 258, In this paper the reaction was run with sodium hydroxide.

9 5 Šefcík, J.; ankin, S. E.; Kirchner, S. J.; McCormick, A. V. Esterification, condensation, and deprotonation equilibria of trimethylsilanol J. oncryst. Solids 1999, 258, Parrain, JL.; Beaudet, I.; Cintrat, J.C.; Duchene, A.; Quintaro, J.P. ll. Soc. Chim. Fr. 1994,11, Patrik C. Eklund,* Annika I. iska, and ainer E. Sjöholm Synthesis of ()Imperanene from the atural gnan ydroxymatairesinol J. rg. Chem. 2002, 67 (21), Boger, D. L.; Mullican, M. D.; ellberg, M..; Patel, M. J. rg. Chem. 1985, 50 (11), aubenstock,. Stereoselectivities of lithium aluminum trialkoxyhydrides J. rg. Chem. 197, 8, ono, F.; Tenaglia, A. Synlett 1998, Collins, D. J.; Downes, L. M.; Jhingran, A. G.; utschmann, S. B.; Sharp, G. J. Aust. J. Chem. 1989, 42, Jeffs, P.W.; edfearn,.; Wolfram, J. J. rg. Chem. 198, 48, a) odgson, D. M.; Stent, M. A..; Wilson, F. X. Substituted Alkenediols by Alkylative Double ing pening of Dihydrofuran and Dihydropyran Epoxides rg. Lett. 2001,, ; b) Satoh, T. xiranyl Anions and Aziridinyl Anions Chem. ev. 1996, 96, Doris E; Dechoux L; Mioskowski C ecent advances in the chemistry of carbenoids derived from epoxides SynLett 1998, 4, 74.