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ChemComm
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COMMUNICATION
Journal Name
DOI: 10.1039/C9CC09267F
H. Kawaguchi and T. Matsuo, J. Am. Chem. Soc., 2003, 125, 14254-
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J. D. Guo, X. Deng, C. Y. Song, Y. Lu, S. L. Qu, Y. F. Dang and Z. X.
Wang, Chem. Sci., 2017, 8, 2413-2425.
Of note, Cp*2Ti is observed to undergo reversible intramolecular C-H
addition, tautomerizing between Cp*2TiII and Cp*(η6-C5Me4CH2)TiIV-H
in solution, though pathways other than RE can be invoked to explain the
reformation of Cp*2TiII. See reference 29.
studying the reactivity of 1 towards ring expanded thiophenes, e.g.
benzothiophene, and other heteroatom donors.
Chim. Acta, 1999, 288, 35-39.
7
8
9
In conclusion, we have shown the masked Ti(II) synthon, 1, is
capable of effecting the reductive ring-opening of thiophene to give
the thiotitanocycle complex 2. This OA of thiophene is the first for
an early-metal complex, and DFT calculations suggest the ring-
opening mechanism initiates via η3-coordination of thiophene to an
unmasked Ti(II) centre and subsequent back-donation into the
thiophene π*-orbitals.
Remarkably, 2 is photosensitive and
10 A. I. Nguyen, R. A. Zarkesh, D. C. Lacy, M. K. Thorson and A. F.
Heyduk, Chem. Sci., 2011, 2, 166-169.
11 S. P. Heins, P. T. Wolczanski, T. R. Cundari and S. N. MacMillan,
Chem. Sci. , 2017, 8, 3410-3418.
12 R. F. Munha, R. A. Zarkesh and A. F. Heyduk, Dalton Trans. , 2013, 42,
3751-3766.
13 (a) K. I. Gell and J. Schwartz, J. Am. Chem. Soc., 1981, 103, 2687-2695;
(b) H.-C. Chiu and I. A. Tonks, Angew. Chem. Int. Ed., 2018, 57, 6090-
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15 Z. W. Davis-Gilbert, X. Wen, J. D. Goodpaster and I. A. Tonks, J. Am.
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16 J. R. Aguilar-Calderon, J. Murillo, A. Gomez-Torres, C. Saucedo, A.
Jordan, A. Metta-Magana, M. Pink and S. Fortier, Organometallics,
2019, ASAP, doi.org/10.1021/acs.organomet.9b00637.
17 W. D. Jones, R. M. Chin, T. W. Crane and D. M. Baruch,
Organometallics, 1994, 13, 4448-4452.
18 The related titanium and zirconium ring-opened thiophene derivatives
Cp2Ti(κ2-SC6H4-2-Ph)4 and Cp2Zr[κ2-S(CH)3C(SiMe3)] are known, with
the former synthesized through salt metathesis using a lithium
arylsulphide and the latter formed through migratory ring enlargement
and dyotropic rearrangement. See reference 30.
reductively eliminates thiophene to regenerate 1. This process is an
exceedingly rare example of a fully reversible, non-substrate induced
OA/RE mechanism mediated by an early-metal. Capitalizing on the
activation of thiophene, pressuring solutions of 1 with H2 (150 psi) at
80 °C leads to HDS concomitant with the formation of butane. Thus,
these results show that base metals, such as titanium, have the
potential to act as precious metal surrogates. As the reversible redox
cycling of Vaska’s complex provided the foundation for modern
precious metal catalysis, our system may provide a unique platform
for the development of new early-metal redox catalysts – a major
advantage given the abundance and low-toxicity of titanium. Yet,
much remains to be learned on how to best access and optimize the
ligand environments and electronic structure of the early metal
complexes to harness such reactivity. In this regard, our studies of 1
are ongoing.
Conflicts of interest
The authors declare no competing financial interests.
Acknowledgement
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Soc., 2002, 124, 4182-4183.
21 R. J. Angelici, Organometallics, 2001, 20, 1259-1275.
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We are grateful to the Welch Foundation (AH-1922-20170325; S.F)
and the ACS PRF (57132-DNI3; S.F) for financial support of this
work. S.F. is an Alfred P. Sloan Foundation research fellow and is
thankful for their support. C.S. is supported by a RISE Graduate
Fellowship (NIH-5R25GM069621-16).
We also wish to
acknowledge the NSF-MRI program (CHE-1827875) for providing
funding for the purchase of an X-ray diffractometer. We are grateful
to Dr. Carl Dirk and Mr. Hamed Tavakoli for assistance with spectral
measurements.
Notes and references
24 The HOMO-1 of 2 is a guanidinate NCN-based π-orbital, and the
corresponding excitations at 402 and 370 nm to LUMO+1 and LUMO+2
are inconsequential to the observed thiophene RE chemistry.
25 J. D. Guo, Y. Lu, R. H. Zhao, Z. Y. Liu, W. Menberu and Z. X. Wang,
Chem. Eur. J., 2018, 24, 7010-7025.
1
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