"#$%#&'#(!)*++!,(-./01!23#&04%(5!26&67.%08#!9:.&0/.%0;/! Page 1 of 7!

Transcription

1 1. The following questions are related to electronic structure and bonding of organic compounds. Literature references for these questions include Blanca Inés, Sigrid Holle, Richard Goddard, and Manuel Alcarazo, Angew. Chem., Int. Ed., 2010, 49, , and Huaping Li, Chad Risko, Jung Hwa Seo, Casey Campbell, Guang Wu, Jean-Luc Brédas, and Guillermo C. Bazan*, J. Am. Chem. Soc., 2011, 133, a. (5 points) Consider compound 1 shown to the right. Draw a three dimensional picture of this compound that will adequately describe the bonding in this compound. If you need to label bond angles or anything else to specify the threedimensional nature of this compound, please feel free to do so. b. (5 points) Compound 1 is best characterized as either a Lewis Acid or as a Lewis base, with one of these labels being considered to be the best label. Based on your understanding of chemistry, would compound 1 best be characterized as a Lewis acid or as a Lewis base (1 point). The majority of the credit for this question (4 points) comes from your explanation of why you have chosen either Lewis acid or Lewis base as your best description. In other words, why? c. (5 points) Consider compound 2 shown to the right. Compound 2 is a generalized structure of a carbine species that with appropriately sterically hindered R 1 groups, this is stable enough to isolate as a crystalline solid and put into a bottle. 2 Draw a three dimensional picture of 2 that will adequately describe the bonding in this compound. If you need to label bond angles or anything else to specify the three-dimensional nature of this compound, please feel "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 1 of 7 1

2 free to do so. Specifically, the hybridization of the N-atoms and the carbon atom containing the carbene are of particular interest to this question. What orbitals are used for bonding and where are the electrons? d. (5 points) Carbenes come in two types, i.e., singlet carbenes and triplet carbenes. Is compound 2 a singlet carbene or a triplet carbene? Explain your reasoning for your answer. Hint: The number of electrons may be of importance in your answer. e. (5 points) If R 1 for compound 2 is a t-butyl group, mixing of the carbene with compound 1 above in toluene at -78 o C generated deep blue solutions, from which a classical Lewis adduct, i.e., 3, was isolated. Give the structure of adduct 3. f. Interestingly, if R 1 is a 2,6-di-isopropylphenyl group in compound 2, the mixing of carbene 2 with 1, no deep blue solution is obtained and the authors comment that NMR spectroscopy indicated that there was no interaction between 1 and 2 i. (5 points) If the R1 group for 2 is a t-butyl group and classical adduct 3 is formed, describe what type of NMR spectroscopic evidence might have been observed to indicate that there was an interaction between 1 and 2. ii. (5 points) Frustrated Lewis pairs have been shown to cause chemical reactions to occur, that would not occur naturally in solution. In this case, hydrogen splitting was not possible, but disulfide (RSSR) was observed. With this observation in hand, define a frustrated Lewis pair and justify your definition. g. When the R 1 group in carbene 2 is 2,6-diisopropylphenyl, 2 reacts with C 60 to form an adduct. This substituted 2 is referred to as IDipp. o-dcb is orthodichlorobenzene. the product starting materials. "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 2 of 7

3 The length of the C C -C F bond 1 connecting the two fragments [1.502(16) Å] is consistent with single-bond character. Additionally, the visible protrusion of C F argues against the presence of a hidden H atom at an adjacent position. On the basis of these data, we propose that the reaction between C 60 and IDipp provides 2. These results contrast with previously observed reactivity with smaller carbenes, i. (5 points) Propose a structure for the adduct of IDipp with C 60. ii. (5 points) Explain the statement about other carbenes yielding cyclopropane species. What differences might be observed that could be detected experimentally? What changes from a structural standpoint might be observed? 2. Ali Asadi, Dariush Ajami, and Julius Rebek, Jr.*, J. Am. Chem. Soc., 2011, 133, Reb paper. Understanding molecular behavior in small spaces is of intrinsic interest in chemistry and, because the vast majority of medicines are synthetic molecules that fit into small spaces of proteins and nucleic acids, is relevant to human health. It has emerged that severely confined molecules behave quite differently than those in dilute solution. Reversible encapsulation is one method of isolating molecules and examining them in very small spaces of the capsules. This has led to some understanding of how molecules get in and out: 1 the amplified chemical interactions, 2 the stabilization of reactive intermediates, 3 and the facilitation of unusual reaction pathways 4 in the limited spaces. We describe here an unprecedented contortion 5 of n-alkanes in the limited space of a capsule. The adaptation of guest to host appears to be driven by the proper filling of the space and its shape. 6 paper. Rebek forms a molecular capsule, which he refers to as 1.1, because two molecules of 1 come together to form a capsule, which can hold molecules inside the capsule, which he shows a picture of the molecular capsule as 1.1. a. (5 points) Rebek reports that 1.1 can encapsulate 4-methyl-N-ptolylbenzamide 5 and 4,4 -dimethyl-trans-stilbene 6. Draw the structures of these two compounds. b. (5 points) Rebek says 6 (-2.5 ppm) and for 5 (-2.5 and -2.4 ppm) confirmed the formation of the 1 Testgiver note: C C refers here to the carbon of the carbene species, while C F refers to the carbon of the fullerene species. 2 Testgiver note: The structure of the adduct words have been blanked out. "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 3 of 7

4 corresponding encapsulation complexes. confirmed the encapsulation of 5 and of 6. c. (5 points) The critical portions of 1 H NMR spectra for a homologous series of n-alkanes encapsulated in 1.1 is shown in the next figure (Labeled as Figure 2 in his paper and labeled as Figure 2 in this cumulative examination). For the purposes of this cumulative exam, the critical issue starts to emerge with C 14 H 30 and becomes magnified with C 15 H 32. Propose a reason for the difference in structures. "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 4 of 7

5 3. Mark G. Nilson and Raymond L. Funk, J. Am. Chem. Soc., 2011, 113, Funk and co-worker have completed a total synthesis of Cortistatin J from furan. Answer the following questions about this synthesis. a. (10 points) In the following Scheme, give a structure for 13 and give a mechanistic rationalization for the entire sequence from 12 to 14. "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 5 of 7

6 b. (10 points) Give a structure for 16 in the following scheme (2.5 points) and give mechanistic rationalizations for the sequence from 15 to 16 (2.5 points) and for 16 to 17 (5 points) c. (10 points) The key point of the paper is given in the next two reactions. Provide mechanistic rationalizations for the transformation from 17 to 9. d. (10 points) Propose an intermediate structure for 21 and provide a mechanistic rationalization for the transformation from 20 to 8. e. Possible Bonus Points i. (5 points) Rationalize the transformation of 8 to 7 in the following scheme. "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 6 of 7

7 ii. (5 points) This is a question that Dr. Penn might ask in a final Ph.D. defense or research proposal defense. Dr. Penn is skeptical about the stability of 7 to 6 N HCl. Discuss possible reactions of 7 react? "#$%#&'#()*++,(-./0123#&04%(526&67.%08#9:.&0/.%0;/ Page 7 of 7