Controlling the “Light” Path to Metathesis

Levin, E.; Mavila, S.; Eivgi, O.; Tzur, E.; Lemcoff, N.G. “Regioselective Chromatic Orthogonality with Light-Activated Metathesis Catalysts” Angew. Chem. Int. Ed. 2015, Advance Article. DOI: 10.1002/anie.201500740

The control of molecular bond formation is fundamental to synthetic chemists. Recently, a creative synthetic route for selective ring formation by utilizing different wavelengths to orthogonally control a reaction pathway has been disclosed. Lemcoff and co-workers employ their photoisomerizable sulfur chelated catalyst and a polysilyl protecting group to provide a sleek route to five and six membered rings (Figure 1).

Figure 1. Chromatically orthogonal reagents

fig1-chromatically-orthogonal-reagents

To begin, Lemcoff and co-workers investigated the feasibility of using their sulfur chelated catalyst in tandem with the tris(trimethylsilyl)silyl protecting group. As hoped, the latent cis-dichloro isomer of the sulfur chelated catalyst, which has been shown to isomerize to the more active trans isomer upon exposure to 350 nm light,1 could be used orthogonally with the tris(trimethylsilyl)silyl protecting group that is cleaved upon exposure to 254 nm light.2 Secondly, the authors studied the use of the bulky tris(trimethylsilyl)silyl protecting group to sterically control regioselectivity in ring closing metathesis. When submitting unprotected triene 2 to ring closing metathesis, the sulfur chelated catalyst favored the six membered ring synthesis (Figure 2). However, the use of the bulky protecting group favored ring closure to five membered ring heterocycle 3, reversing the regioselectivity of the RCM (ring closing metathesis) reaction. Finally, after some optimization to account for catalyst decomposition that can occur during irradiation, Lemcoff and co-workers succeeded in designing a one-pot procedure that yielded the heterocycles with modest yields. In summary, starting with the tris(trimethylsilyl)silyl triene 1, the authors were able to produce either the six-membered alkene 4 or the five-membered alkene 5 simply by judiciously controlling the exposure order of the two wavelengths (254 nm and 350 nm).

Figure 2. Chromatically orthogonal system.

fig2-chromatically-orthogonal-system

This recent work from the laboratory of Lemcoff is an elegant application of their photoisomerizable sulfur chelated catalyst. Leveraging the orthogonality in wavelength activation between the catalyst and protecting group delivers a great one-pot reaction pathway. Hopefully, in the future we will see more examples in which the manipulation of wavelengths can be used to control the overall molecular architecture of the final product.

1Ben-Asuly, A.; Aharoni, A.; Eiesendruck, C.E.; Vidavsky, Y.; Goldberg, I.; Straub, B.F.; Lemcoff, N.G. Organometallics 2009, 28, 4652-4655.
2Brook, M.A; Gottardo, C.; Balduzzi, S.; Mohomed, M. Tetrahedron Lett. 1997, 38, 6997-7000.

Olefin Metathesis in Self-Healing Materials

ATM self healing

Neal, J. A.; Mozhdehi, D.; Guan, Z. “Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds” J. Am. Chem. Soc. 2015, 137, 4846-4850. Self-healing materials have been an objective of extensive research … [Continue reading]

New Remarkably Efficient Ethenolysis Catalysts

ethenolysis-1

Marx, V. M.; Sullivan, A. H.; Melaimi, M.; Virgil; S. C.; Keitz, B. K.; Weinberger, D. S.; Bertrand, G. and Grubbs, R. H. “Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis” Angew. Chem. Int. Ed. … [Continue reading]

Olefin Metathesis at the 249th ACS Meeting

The 249th ACS National Meeting & Exposition is fast approaching with Denver, CO preparing to host the biannual event from March 22 – 26, 2015. The theme of this spring’s event is “Chemistry of Natural Resources” which is fitting for the … [Continue reading]

Yves Chauvin, One of Metathesis Greats, Dies at 84

Yves Chauvin, one of the “metathesis greats,” passed away on January 27, 2015 at the age of 84. Winner of the Nobel Prize in Chemistry in 2005, Chauvin was a key player in proposing the metallacyclobutane intermediate of the generally accepted … [Continue reading]

The Boomerang is Back in Olefin Metathesis

Boomerang

Bates, J. M.; Lummiss, J. A. M.; C.; Bailey; G. A., and Fogg, D. E. “Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst” ACS Catal., 2014, 4 (7), 2387-94. No consensus has been … [Continue reading]

Olefin Metathesis at the 248th ACS Meeting

The 248th American Chemical Society National Meeting and Exposition will be held in San Francisco, California from August 10-14, 2014. The theme of this fall’s meeting is “Chemistry and Global Stewardship.” As olefin metathesis has made significant … [Continue reading]

Examining Polytopal Rearrangements around Ruthenium

fig_1r

Torker, S.; Khan, R.K.M.; Hoveyda, A.M. “The Influence of Anionic Ligands on Stereoisomerism of Ru Carbenes and Their Importance to Efficiency and Selectivity of Catalytic Olefin Metathesis Reactions” J. Am. Chem. Soc. 2014, 136, 3439-3455. For … [Continue reading]