Presentation on theme: "Miljanić Group Meeting / Literature University of Houston ▪ Houston, TX ▪ January 2 nd 2009 Ognjen Š. Miljanić Azaborines from Oregon."— Presentation transcript:
Miljanić Group Meeting / Literature University of Houston ▪ Houston, TX ▪ January 2 nd 2009 Ognjen Š. Miljanić Azaborines from Oregon
Three Papers from 2007 and 2008 Diversity through Isosterism: The Case of Boron-Substituted 1,2-Dihydro-1,2-azaborines Adam, J. V. Marwitz, Eric R. Abbey, Jesse T. Jenkins, Lev N. Zakharov, Shih-Yuan Liu Org. Lett. 2007, 9, 4905–4908. Crystal Clear Structural Evidence for Electron Delocalization in 1,2-Dihydro-1,2-azaborines Eric R. Abbey, Lev N. Zakharov, Shih-Yuan Liu J. Am. Chem. Soc. 2008, 130, 7250–7252. A Hybrid Organic/Inorganic Benzene Adam, J. V. Marwitz, Myrna H. Matus, Lev N. Zakharov, David A. Dixon, Shih-Yuan Liu Angew. Chem. Int. Ed. 2008, 47, ASAP.
Collaborative Research Shih-Yuan Liu Postdoc—Massachusetts Institute of Technology (2003–06, Daniel G. Nocera) PhD—Massachusetts Institute of Technology (2003, Gregory C. Fu) Diploma—Vienna University of Technology (1997) Professor at University of Oregon since Lev N. Zakharov Staff crystallographer at the University of Oregon (since 2004). David A. Dixon PhD—Harvard University (1976) BS—California Institute of Technology (1971) Professor at University of Alabama.
Oregon is Green and Pretty! ▪ Oregon is the 9 th biggest US state by area (2.5 times smaller than TX) ▪ Oregon ranks as 27 th US state by population (3.7 million, 7 times smaller than TX) ▪ Capital city is Salem, biggest city is Portland (600,000) ▪ Moderate climate: pleasant summers and rainy (but not overly cold) winters ▪ Two big universities: University of Oregon (Eugene) and Oregon State (Corvallis) ▪ Well known for beer, wine, agriculture, and tourism (skiing, hiking, camping)
Boron-Containing Heterocycles are Important Faraday, M. Philos. Trans. R. Soc. London 1825, 115, 440–466. Stock, A.; Pohland, E. Ber. Dtsch. Chem. Ges. 1926, 59, 2210–2215. ▪ Theoretically relevant in studies of aromaticity Chiavarino, B. et. al. J. Am. Chem. Soc. 1999, 121, 11204– ▪ B–N bond is isoelectronic and isosteric with C=C, important for pharmaceuticals Morin, C. Tetrahedron 1994, 50, 12521– ▪ Boron neutron-capture therapy (BNCT) for cancer Barth, R. F. et. al. Clin. Cancer Res. 2005, 11, 3987–4002. ▪ Borazine is a precursor to BN ceramics Mokhtari, M. et al. Chem. Eur. J. 1996, 2, 1269–1274. ▪ Borazine is a side-product in the use of ammonia-borane in hydrogen storage Marder, T. B. Angew. Chem. Int. Ed. 2007, 46, 8116–8118.
Alkene Metathesis is the Key Synthetic Step Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. N.; Liu, S.-Y. Org. Lett. 2007, 9, 4905–4908. Challenging aromatization, finally achieved with Pd black / cyclohexene: DDQ / pentane / 35 °C / 24 h Pd-C / pentane / 80 °C / 16 h Ru-C / cyclohexene / 80 °C / 16 h Rh-Al 2 O 3 / cyclohexene / 80 °C / 16 h Pd black / cyclohexene / 80 °C / 16 h Pd(PPh 3 ) 4 / benzene / 80 °C / 16 h 14 % 31 % 1 % 27 % 75 % (57 % isolated) 0 % ConditionsYield
B–Cl Bond Can be Nucleophilically Substituted Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. N.; Liu, S.-Y. Org. Lett. 2007, 9, 4905–4908. Li–Bu (butyllithium) Li–vinyl (vinyllithium) BrMg–Ph (phenylmagnesium bromide) Li–NMe 2 (lithium dimethylamide) K–SCH 2 Ph (potassium benzylthiolate) K–OtBu (potassium t-butoxide) 79 % 50 % 76 % 66 % 80 % 71 % NucleophileYield LiBEt 3 –H (Superhydride)92 %
Are 1,2-Azaborines Aromatic? Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252. To determine this, the authors performed direct structural comparison with: in order to evaluate the delocalization in the fully unsaturated structure: This study is analogous to the comparison of benzene with cyclohexadiene! is highly crystalline, so its analogs were used in the study.
Model Compounds Were Synthesized Divergently Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252.
Azaborine is Clearly Planar Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252.
Delocalization Supports Aromaticity N–B B–C4 C4–C3 C3–C2 C2–C1 C1–N BondLength (Å) ▪ In compounds A–D, bond between N and C is a double bond (~1.41 Å) ▪ Formal double bonds in B–D lengthen upon aromatization ▪ All formal single bonds shorten upon aromatization Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252.
Finally, the Unsubstituted Azaborine! Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem. Int. Ed. 2008, 47, ASAP.
NMR Studies Confirm Aromaticity Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem. Int. Ed. 2008, 47, ASAP. ▪ Boron S = 3/2 1:1:1:1 quartet ▪ Nitrogen S = 1 1:1:1 triplet (N–H) (B–H) NICS(0) NICS(1) RSE (kcal mol −1 ) H f (kcal mol −1 ) benzene −8.76 − −5.62 − −2.02 −3.0 −119.0 azaborine borazine
N–H Bond is Protic, B–H Bond is Not Hydridic ▪ The study nicely illustrates the thought process 1) simple synthetic development 2) fundamental structural studies 3) the “holy grail” discovery ▪ Versatility in B-substitution in boron heterocycles, especially in azaborines ▪ Robustness of alkene metathesis protocols ▪ Use of Cr(CO) 3 as a protecting group for the aromatic nuclei Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem. Int. Ed. 2008, 47, ASAP.
Notes on Preparing Effective Slides ▪ Design: be consistent, but rational ▪ You are presenting! Make sure the audience notices both you and the research Avoid using laser pointer too much—it takes the focus away from you ▪ Slide Titles should be messages, not description of what you did “Compound A is Synthesized in 8 Steps” is an informative title “Synthetic Route towards Compound A” is not ▪ Use contrast to highlight differences in importance Human brain does not give any inherent meaning to color (except RED) ▪ Maximize “data-to-ink” ratio by minimizing “slide junk” Avoid bold arrows, borders, vignettes—they use space and convey nothing! ▪ Give credit where credit is due, preferably with pictures of people Small personal details make presentation engaging (but maintain balance) ▪ Do not thank your audience If your presentation was any good, they should be thanking you