13C

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Asymmetric Synthesis and Stereochemical Assignment of C/ C Isotopomers Tomoya Miura, Takayuki Nakamuro, Yuuya Nagata, Daisuke Moriyama, Scott G. Stewart, and Masahiro Murakami J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07181 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Journal of the American Chemical Society

Asymmetric Synthesis and Stereochemical Assignment of 12 C/13C Isotopomers Tomoya Miura,* Takayuki Nakamuro,† Yuuya Nagata, Daisuke Moriyama, Scott G. Stewart,‡ and Masahiro Murakami* Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan

Supporting Information Placeholder ABSTRACT: A synthesis of chiral hydrocarbons having

C1 axis and C3 symmetry, which owe their chirality due to asymmetrical distribution of 12C/13C isotopes, is reported. Their absolute configurations assigned using the vibrational circular dichroism (VCD) technique conform with those deduced from the absolute configurations of the parent a-formyl cyclopropanes.

Isomers that are non-superimposable on their mirror images are chiral and there are a pair of enantiomers for them. Whereas the two enantiomers are identical in most physical properties, they respond oppositely to plane-polarized light, and thus, they are distinguished based on optical rotatory dispersion (ORD) and electronic circular dichroism (ECD). Assignment of absolute configurations is often made depending on these techniques.1,2 In case an enantiomer gives suitable single crystals and contains a relatively heavy atom(s), the anomalous X-ray scattering (AXRS)3,4 provides a reliable method to assign its absolute configuration. Nonetheless, it still remains challenging to assign absolute configurations of chiral hydrocarbons, because they lack a heavy atom(s) and are almost silent in ECD spectroscopy in the UV/Vis region (200– 800 nm). Therefore, chiral hydrocarbons are often called crypto-optically active compounds.5 It is even more so when the chirality of hydrocarbons is attributed to asymmetrical placement of isotopes, i.e., 1H/2H and 12C/13C. The two enantiomers, often termed isotopomers, are identical in distribution of electrons and protons, but different only in arrangement of neutrons. What is obtained by an X-ray analysis is an electron density map, and that observed by an ECD analysis is an excitation process of electrons. Therefore, it is impossible to identify each of the two isotopomers by the techniques based on AXRS or absorptions in the UV/Vis region. On the other hand, the vibrational circular dichroism (VCD)6 and Raman optical activity (ROA)7 techniques observe absorptions in the infrared region. Those absorptions are attributed to asymmetrical vibrational motions of nuclei which include

neutrons. Comparison of the observed spectra with those simulated with density functional theory (DFT) calculations provides a last measure when assigning absolute configurations to chiral molecules, even to chiral hydrocarbons in theory.8 There have been a few examples of assignment of absolute configurations to chiral hydrocarbons possessing asymmetrical arrangement of 1H/2H atoms on the basis of the VCD and ROA techniques (Figure 1).9–12 For chiral hydrocarbon molecules which owe their chirality due to asymmetrical arrangement of 12C/13C atoms, however, there is no example of assignment of absolute configurations as well as even synthesis. Such an assignment is more difficult because the difference in atomic masses for 13C/12C (1.08) is much smaller than that for 2H/1H (2.00). There are only minute differences in molecular vibrations between the two isotopomers to attenuate overall intensities. We now report the first example of synthesis of chiral hydrocarbon molecules due to asymmetrical arrangement of 12C/13C and assignment of their absolute configurations based on the VCD technique. (2H)H2C

CH(2H)2

H 3C

CH(2H)3

(R)-4-ethyl4-methyloctane

(S)-2H6-neopentane hydrogen isotope

VCD (2007)9

ROA (2007)10 VCD (2015)11

2H

H

2H

H

2 2H H

H

H H

2H 2H

H

(1R,2S)-2H6-[3]CPPC hydrogen isotope VCD (2017)12

This work H

13C

13C

H 13C

1-cis-diphenyl-

cyclopropane carbon isotope

H

H

H

13C

13C

H

H

H 13C

3-[3]CPPC

carbon isotope

Figure 1. Chiral hydrocarbons and assignment of absolute configurations. [3]CPPC = [3]cycloparaphenylenecyclopropane.

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N-Sulfonyl-1,2,3-triazoles have emerged as the latent precursors of a-imino diazo compounds. They react with a variety of organic molecules in the presence of transition-metal catalysts.13 For example, styrene undergoes an asymmetric cyclopropanation reaction when treated with N-sulfonyl-1,2,3-triazole in the presence of a chiral dirhodium(II) catalyst, Rh2[(S)-NTTL]4,14 to afford an enantioenriched a-imino cyclopropane.15 It is possible to convert it to a symmetrical hydrocarbon by removal of the aimino group through hydrolysis to an a-formyl group and a subsequent Tsuji-Wilkinson decarbonylation reaction with RhCl(PPh3)3.16 The resulting meso cis-1,2-diphenylcyclopropane has two benzylic carbons, one originating from styrene and the other originating from the triazole. There is styrene-a-13C (1, 99% 13C) available from commercial sources. This led us to envisage that the use of 1 in the asymmetric cyclopropanation reaction would lead to the production of a chiral hydrocarbon due to asymmetrical arrangement of 12C/13C. The resulting product would provide a chance to see if the VCD technique is a useful technique to assign absolute configuration to such 12 13 C/ C isotopomers. Notably, the conformational rigidity associated with the cyclopropane ring is advantageous to the VCD measurement and the theoretical simulation. Thus, 1 was treated with 1-mesyl-4-phenyl-1,2,3-triazole (2) in the presence of Rh2[(S)-NTTL]4 (0.4 mol %), leading to the formation of a-imino cyclopropane (Scheme 1). After hydrolysis of the a-imino group with basic alumina, 13 C-labelled a-formyl cyclopropane 3 was isolated in 92% yield. The enantioselectivity was determined to be 95% ee by a chiral stationary phase HPLC analysis, as with the case of non-labeled styrene.15 Identical stereochemistries should be induced with the labeled and nonlabeled a-imino cyclopropanes by the S catalyst, and thus, the absolute configuration of 3 was assigned as 1S,2R. Next, the 13C-labelled compound 3 was subjected to a decarbonylation reaction with RhCl(PPh3)3 (1.4 equiv, 135 °C, 22 h) to afford chiral hydrocarbon molecule, 13C1cis-diphenylcyclopropane (4) in 66% yield. No formation of the trans-isomer of 4 was observed, eliminating a possibility of racemization occurred during the process of decarbonylation.17 Therefore, the enantiomeric purity of 4 was assumed to be retained at 95% ee. The opposite isotopomer of 4 was synthesized using the corresponding R catalyst (see Supporting Information). 12C and 13C nuclei are the electronically same elements, having the same numbers of protons and electrons. Therefore, the 13C1-cisdiphenylcyclopropane (4) is electronically an achiral molecule with plane of symmetry. The only factor violating symmetry is the arrangement of neutrons, causing the cyclopropane 4 an isotopically chiral molecule with a C1axis. The VCD analysis was carried out upon the both isotopomers of 4 to observe mirror images within the region ranging from 1000 to 1500 cm–1, although the intensities of the peaks were weak (Figure 2A, the region of 1180–

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Scheme 1. Asymmetric Synthesis of Chiral Carbon Isotope Hydrocarbon 4 H

13C

N N + N

Ph 1

Ms 1) Rh2[(S)-NTTL]4 (0.4 mol %) CHCl3, 4 Å MS, 28 °C, 12 h

Ph 2 (1.2 equiv)

2) basic Al2O3, RT, 3 h

13C

2R 1S

3 92% (95% ee) H

H RhCl(PPh3)3 (1.4 equiv)

CHO

H

13C

1S 2R

p-xylene, 135 °C, 22 h 4 66%

1250 cm–1 was omitted due to strong absorption of CHCl3). In other regions, the peaks were even weaker. On the other hand, a VCD spectrum was simulated for the (1S,2R)-isomer of 4 with DFT calculation. A conformational analysis of 4 revealed that 4 had two conformations, where the two phenyl groups were tilted to the cyclopropane plane, either to the left or to the right (see Supporting Information).18 The difference in the calculated Gibbs free energies at 298 K for the two conformers was smaller than –4.4 × 10‒6 kcal/mol. Thus, the two conformers would be equally populated in the sample solution, and the VCD spectra calculated for the two conformers were simply averaged. The simulated spectrum in the 1000 to (A)

(B)

Figure 2. (A) The top VCD spectra (red and blue) are those of the individual enantiomers 4 in CHCl3 at a concentration of 0.39 M. Red; assigned as (1S,2R)-4. Blue; assigned as (1R,2S)-4. (B) Comparison of VCD spectra calculated for (1S,2R)-4 (black) with the experimental one (red). Calculations were performed at the ωB97XD/6-311G(d,p) level with a frequency scaling factor of 0.957.19


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1500 cm–1 region matched well with the spectrum observed for (1S,2R)-4 which was prepared using the S catalyst (Figure 2B). Thus, the stereochemical assignment based on the VCD analysis agrees with the assignment deduced based on the structure of the parent a-formyl cyclopropane 3. This confirmation demonstrated the validity of the VCD technique to assign the absolute configurations to chiral 12C/13C isotopomers. Of note was that the minute differences in molecular vibrations arising from the difference in atomic masses of 13C/12C (1.08) could be identified by the VCD analysis. Recently, we have reported the asymmetric synthesis of a hydrocarbon molecule, 2H6-[3]CPPC, which is C3-symmetric chiral molecule due to asymmetrical distribution of 1H/2H isotopes (Figure 1).12 Styryl-substituted 2H2-triazole undergoes a rhodium(II)-catalyzed asymmetric cyclotrimerization reaction. The resulting tris-aldimine is hydrolyzed to a formyl-substituted triangular macrocycle, which is subsequently decarbonylated. The absolute configuration of the arising 2H6-[3]CPPC was successfully assigned by the VCD technique. Next, we targeted hydrocarbon 13C3-[3]CPPC which was C3-symmetric chiral molecule due to asymmetrical distribution of 12C/13C isotopes, and challenged to assign the absolute configuration using the aforementioned VCD technique. Initially, 4bromobenzoic acid incorporating a 13C nuclei (6) was prepared from 1-bromo-4-iodobenzene (5) by treatment firstly with activated zinc and secondly with gaseous 13 CO2 (1 atm) (Scheme 2).20 Reduction of the acid 6 with borane tetrahydrofuran complex followed by oxidation with Dess-Martin periodinane (DMP) gave the 13C1-4bromobenzaldehyde (8). A Wittig reaction of 8 with methylenetriphenylphosphorane and subsequent Sonogashira reaction with ethynyltrimethylsilane afforded 13C1-1ethyl-4-[2-(trimethylsilyl)ethynyl]benzene (9). After removal of the trimethylsilyl group with potassium hydroxide, a Cu(I)-catalyzed cycloaddition reaction with methanesulfonyl azide (Ms–N3) gave the styryl-substituted 13 C1-triazole 10.21 The 13C1-triazole 10 was next subjected to the rhodium(II)-catalyzed asymmetric cyclotrimerization reaction catalyzed by Rh2[(S)-NTTL]4 (0.5 mol %) to give the tris-aldimine intermediate. The three imino groups were all hydrolyzed with basic alumina to furnish the 13C3-tris-aldehyde 11 in 34% isolated yield. The enantioselectivity was determined to be >99% ee by a chiral stationary phase HPLC analysis, as with the case of nonlabeled styryl-substituted triazole.12 The sequential threefold asymmetric cyclopropanation reactions causes further enantioenrichment, presumably in association with the Horeau principle,22 leading to the excellent ee of trisaldehyde 11. The stereochemistries of the 13C3-tris-aldehyde 11 should be identical with that of the corresponding non-labeled tris-aldehyde, and thus, the absolute configuration of 11 was assigned as 1S,2R. A decarbonylation reaction using RhCl(PPh3)3 (4.0 equiv, 1.3 equiv per formyl group) gave the desired C3-symmetric chiral hydrocarbon

Scheme 2. Asymmetric Synthesis of Chiral Carbon Isotope Hydrocarbon 12 13CH

13CO H 2

I Zn, LiCl THF 50 °C, 12 h

Br 5

13CO 2

BH3 THF

(1 atm)

DMF 50 °C, 66 h

THF RT, 18 h

Br 6 58% (2 steps) H

13CHO

DMP

1) n-BuLi, MePh3 THF, RT, 4 h

Br 7 91%

13C

P+Br–

2) Me3Si PdCl2(PPh3)2, CuI Br Et3N, 50 °C, 12 h 8 92%

CH2Cl2 RT, 2 h

2OH

1) KOH, THF/EtOH 0 °C, 1.5 h 2) Ms–N3, CuTC toluene, RT, 5 h SiMe3

9 53% (2 steps) H

13C 13C

CHCl3, 4 Å MS, 28 °C, 12 h N N N

2) basic Al2O3, RT, 3 h

2R 1S

OHC

13C

13C

H 11 34% (>99% ee)

Ms 10 52% (2 steps)

H

CHO

H

H 13C

RhCl(PPh3)3 (4.0 equiv) p-xylene, 135 °C, 22 h

CHO

H

1) Rh2[(S)-NTTL]4 (0.5 mol %)

1S 2R

H

13C

H 13C

3-[3]CPPC

13C

H

H 12 82%

C3-[3]CPPC 12. As with the case of a-formyl cyclopropane 3, no racemization occurred.17 Therefore, the optical purity of 12 was assumed to be >99% ee. Furthermore, the opposite isotopomer of 12 was also synthesized using the corresponding R catalyst (see Supporting Information). The two isotopomers of 12 were subjected to the VCD analysis, and their VCD spectra showed mirror images within the region ranging from 1000 to 1500 cm–1 (Figure 3A, the region of 1180–1250 cm–1 and 1500–1530 cm–1 were omitted due to strong absorption of CHCl3 and 12, respectively.). On the other hand, a VCD spectrum was simulated for the (1S,2R)-isotopomer of 12 with DFT calculation. Only a single conformation was found for 12 by a conformational analysis, probably because the benzene rings were fixed in the cramped space of the triangular scaffold (see Supporting Information).12 The simulated VCD spectrum in the 1000 to 1500 cm–1 region showed agreement with the observed VCD spectrum for the 13C3[3]CPPC 12 having 1S,2R stereochemistry, which was prepared using the S catalyst (Figure 3B). Thus, the stereochemical assignment of the isotopomer of 12 based on the VCD technique agreed with that chemically deduced from the structure of the tris-aldehyde 11. This result again demonstrated the validity of the VCD technique to

13



Present Addresses

(A)

† Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. Funding Sources

The authors declare no competing financial interest. This work was supported by MEXT [Grants-in-Aids for Scientific Research (S) (15H05756) (M.M.) and (C) (16K05694) (T.M.)], the Asahi Glass Foundation (T.M.), and the Kyoto University Foundation (S.G.S.). T.N. acknowledged JSPS fellowship for young scientists. Notes

(B)

‡ S.G.S. is on leave from School of Molecular Sciences, The University of Western Australia, Australia.

ACKNOWLEDGMENT We thank JASCO (Tokyo, Japan) for performing the VCD measurements. Computation time was provided by the Super-Computer System, Institute for Chemical Research, Kyoto Univ.

REFERENCES Figure 3. (A) The top VCD spectra (red and blue) are those of the individual enantiomers 12 in CHCl3. Red; assigned as (1S,2R)-12 (0.24 M). Blue; assigned as (1R,2S)-12 (0.29 M). (B) Comparison of VCD spectra calculated for (1S,2R)-12 (black) with the experimental one (red). Calculations were performed at the ωB97XD/6-311G(d,p) level with a frequency scaling factor of 0.957.19

assign the absolute configurations of chiral 12C/13C isotopomers. In summary, we have demonstrated a unique strategy to synthesize C1-axis and C3-symmetric chiral hydrocarbon molecules due to asymmetrical distribution of 12C/13C isotopes in cyclopropane rings.23 The VCD analysis can identify the optical properties of the two isotopomers, and thus, their absolute configurations have been successfully assigned by comparing the observed VCD spectra with those computed with DFT modeling. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization of the new compounds, and spectroscopic data (PDF)

AUTHOR INFORMATION Corresponding Author

*[email protected] *[email protected] ORCID

Tomoya Miura: 0000-0003-2493-0184 Takayuki Nakamuro: 0000-0002-0752-3475 Masahiro Murakami: 0000-0002-9913-3422

(1) Harada, N. Chiral Molecular Science: How Were the Absolute Configurations of Chiral Molecules Determined? “Experimental Results and Theories”. Chirality 2017, 29, 774-797. (2) Gargiulo, D.; Cai, G.; Ikemoto, N.; Bozhkova, N.; Odingo, J.; Berova, N.; Nakanishi, K. CD Exciton Chirality Method: New Chromophores for Primary Amino Groups. Angew. Chem. Int. Ed. 1993, 32, 888-891. (3) Bijvoet, J. M.; Peerdeman, A. F.; van Bommel, A. J. Determination of the Absolute Configuration of Optically Active Compounds by Means of X-Rays. Nature 1951, 168, 271-272. (4) For recent advances for X-ray analysis without the need for crystallization using ‘crystalline sponges’, see: Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. X-ray Analysis on the Nanogram to Microgram Scale Using Porous Complexes. Nature 2013, 495, 461-466. (5) de Meijere, A.; Khlebnikov, A. F.; Kostikov, R. R.; Kozhushkov, S. I.; Schreiner, P. R.; Wittkopp, A.; Yufit, D. S. The First Enantiomerically Pure Triangulane (M)‐Trispiro[2.0.0.2.1.1]nonane Is a σ‐ [4]Helicene. Angew. Chem. Int. Ed. 1999, 38, 3474-3477. (6) For a recent review on the VCD analysis, see: (a) Merten, C. Vibrational Optical Activity as Probe for Intermolecular Interactions. Phys. Chem. Chem. Phys. 2017, 19, 18803-18812. For an excellent example of isotopic difference spectra on the VCD analysis, see: (b) Freedman, T. B.; Cao, X.; Luz, Z.; Zimmermann, H.; Poupko, R.; Nafie, L. Isotopic Difference Spectra as an Aide in Determining Absolute Configuration Using Vibrational Optical Activity: Vibrational Circular Dichroism of 13C- and 2H-labelled Nonamethoxy Cyclotriveratrylene. Chirality 2008, 20, 673-680. For recent examples on the VCD analysis, see: (c) Taniguchi, T.; Manai, D.; Shibata, M.; Itabashi, Y.; Monde, K. Stereochemical Analysis of Glycerophospholipids by Vibrational Circular Dichroism. J. Am. Chem. Soc. 2015, 137, 12191-12194. (d) Pujari, S. A.; Besnard, C.; Bürgi, T.; Lacour, J. A Mild and Efficient CH2extrusion Reaction for the Enantiospecific Synthesis of Highly Configurationally Stable Tröger Bases. Angew. Chem. Int. Ed. 2015, 54, 75207523. (e) Merten, C.; Pollok, C. H.; Liao, S.; List, B. Stereochemical Communication within a Chiral Ion Pair Catalyst. Angew. Chem. Int. Ed. 2015, 54, 8841-8845. (f) Taniguchi, T.; Suzuki, T.; Satoh, H.; Shichibu, Y.; Konishi, K.; Monde, K. Preparation of Carbodiimides with One-Handed Axial Chirality. J. Am. Chem. Soc. 2018, 140, 1557715581.


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(7) Barron, L. D.; Zhu, F.; Hecht, L.; Tranter, G. E.; Isaacs, N. W. Raman Optical Activity: An Incisive Probe of Molecular Chirality and Biomolecular Structure. J. Mol. Struct. 2007, 834-836, 7-16. (8) For a new technique for assignment of absolute configurations by foil-induced Coulomb explosion imaging, see: Herwig, P.; Zawatzky, K.; Grieser, M.; Heber, O.; Jordon-Thaden, B.; Krantz, C.; Novotný, O.; Repnow, R.; Schurig, V.; Schwalm, D.; Vager, Z.; Wolf, A.; Trapp, O.; Kreckel, H. Imaging the Absolute Configuration of a Chiral Epoxide in the Gas Phase. Science 2013, 342, 1084-1086. (9) (a) Fujita, T.; Obata, K.; Kuwahara, S.; Miura, N.; Nakahashi, A.; Monde, K.; Decatur, J.; Harada, N. (R)-(+)-[VCD(+)945]-4-Ethyl4-methyloctane, the Simplest Chiral Saturated Hydrocarbon with a Quaternary Stereogenic Center. Tetrahedron Lett. 2007, 48, 4219-4222. (b) Kuwahara, S.; Obata, K.; Fujita, T.; Miura, N.; Nakahashi, A.; Monde, K.; Harada, N. (R)-(+)-[VCD(–)984]-4-Ethyl-4-methyloctane: A Cryptochiral Hydrocarbon with a Quaternary Chiral Center. (2) Vibrational CD Spectra of Both Enantiomers and Absolute Configurational Assignment. Eur. J. Org. Chem. 2010, 6385-6392. (10) Haesler, J.; Schindelholz, I.; Riguet, E.; Bochet, C. G.; Hug, W. Absolute Configuration of Chirally Deuterated Neopentane. Nature 2007, 446, 526-529. (11) Masarwa, A.; Gerbig, D.; Oskar, L.; Loewenstein, A.; Reisenauer, H. P.; Lesot, P.; Schreiner, P. R.; Marek, I. Synthesis and Stereochemical Assignment of Crypto‐Optically Active 2H6‐Neopentane. Angew. Chem. Int. Ed. 2015, 54, 13106-13109. (12) Miura, T.; Nakamuro, T.; Stewart, S. G.; Nagata, Y.; Murakami, M. Synthesis of C3-Symmetric Triangular Molecules. Angew. Chem. Int. Ed. 2017, 56, 3334-3338. (13) For reviews, see: (a) Davies, H. M. L.; Alford, J. S. Reactions of Metallocarbenes Derived from N-Sulfonyl-1,2,3-triazoles. Chem. Soc. Rev. 2014, 43, 5151-5162. (b) Anbarasan, P.; Yadagiri, D.; Rajasekar, S. Recent Advances in Transition-Metal-Catalyzed Denitrogenative Transformations of 1,2,3-Triazoles and Related Compounds. Synthesis 2014, 46, 3004-3023. (c) Jiang, Y.; Sun, R.; Tang, X.-Y.; Shi, M. Recent Advances in the Synthesis of Heterocycles and Related Substances Based on a-Imino Rhodium Carbene Complexes Derived from N-Sulfonyl-1,2,3-triazoles. Chem. Eur. J. 2016, 22, 17910-17924. (d) Li, Y.; Yang, H.; Zhai, H. The Expanding Utility of RhodiumIminocarbenes: Recent Advances in the Synthesis of Natural Products and Related Scaffolds. Chem. Eur. J. 2018, 24, 12757-12766. (e) Miura, T.; Murakami, M. Reactions of a-Imino Rhodium(II) Carbene Complexes Generated from N-Sulfonyl-1,2,3-triazoles. Rhodium Catalysis in Organic Synthesis (Ed: K. Tanaka), Wiley-VCH, Weinheim, 449470 (2019). (14) (S)-NTTL = N-naphthoyl-(S)-tert-leucinate. Müller, P.; Allenbach, Y.; Robert, E. Rhodium(II)-Catalyzed Olefin Cyclopropanation

with the Phenyliodonium Ylide Derived from Meldrum's Acid. Tetrahedron: Asymmetry 2003, 14, 779-785. (15) Chuprakov, S.; Kwok, S. W.; Zhang, L.; Lercher, L.; Fokin, V. V. Rhodium-Catalyzed Enantioselective Cyclopropanation of Olefins with N-Sulfonyl-1.2.3-triazoles. J. Am. Chem. Soc. 2009, 131, 1803418035. (16) Tsuji, J.; Ohno, K. Organic Syntheses by Means of Noble Metal Compounds XXI. Decarbonylation of Aldehydes Using Rhodium Complex. Tetrahedron Lett. 1965, 6, 3969-3971. (17) Walborsky, H. M.; Allen, L. E. Stereochemistry of Tris(triphenylphosphine)rhodium Chloride Decarbonylation of Aldehydes. J. Am. Chem. Soc. 1971, 93, 5465-5468. (18) For an X-ray crystal structure of cis-1,2-diphenylcyclopropane, see: Schmidbaur, H.; Bublak, W.; Schier, A.; Reber, G.; Müller, G. cis1,2-Diphenylcyclopropan: Molekülstruktur und Versuche zur Chelatkoordination von Gallium(I). Chem. Ber. 1988, 121, 1373-1375. (19) NIST Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database Number 101, Release 19, April 2018, R. D. Johnson III, Ed.; http://cccbdb.nist.gov/ Accessed May, 2019. (20) Kobayashi, K.; Kondo, Y. Transition-Metal-Free Carboxylation of Organozinc Reagents Using CO2 in DMF Solvent. Org. Lett. 2009, 11, 2035-2037. (21) Raushel, J.; Fokin, V. V. Efficient Synthesis of 1-Sulfonyl1,2,3-triazoles. Org. Lett. 2010, 12, 4952-4955. (22) For a recent review on the Horeau principle, see: Harned, A. M. From Determination of Enantiopurity to the Construction of Complex Molecules: The Horeau Principle and Its Application in Synthesis. Tetrahedron 2018, 74, 3797-3841. (23) For synthetic application and analytical study of carbon isotope molecules, see: (a) Evans, M. A.; Morken, J. P. Isotopically Chiral Probes for in Situ High-Throughput Asymmetric Reaction Analysis. J. Am. Chem. Soc. 2002, 124, 9020-9021. (b) Kawasaki, T.; Matsumura, Y.; Tsutsumi, T.; Suzuki, K.; Ito, M.; Soai, K. Asymmetric Autocatalysis Triggered by Carbon Isotope (13C/12C) Chirality. Science 2009, 324, 492-495. (c) Hawbaker, N. A.; Blackmond, D. G. Rationalization of Asymmetric Amplification via Autocatalysis Triggered by Isotopically Chiral Molecules. ACS Cent. Sci. 2018, 4, 776-780. (d) Hachtel, J. A.; Huang, J.; Popovs, I.; Jansone-Popova, S.; Keum, J. K.; Jakowski, J.; Lovejoy, T. C.; Dellby, N.; Krivanek, O. L.; Idrobo, J. C. Identification of Site-specific Isotopic Labels by Vibrational Spectroscopy in the Electron Microscope. Science 2019, 363, 525-528.



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TOC Graphic: H

H H 13C

H

H

H

H

H 13C

13C

13C

H

13C

H C1-axis chiral hydrocarbons

13C

H

H H

13C

H

H

13C

H

C3-symmetric chiral hydrocarbons

6


13C

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