Organic silyl diene reductants (Mashima's reagent)

Reducing Agents
1

Organic silyl diene reductants have gained traction as soluble, salt-free reducing agents in synthetic chemistry, particularly within transition-metal and main-group disciplines. They were developed by Tsurugi and Mashima in 2014 as a method of selectively forming “naked” low-valent metals over their salt-contacted counterparts.[1]
The reductants primarily feature 1,4-dihydropyrazine cores which aromatize upon reaction with the substrate, releasing two organosilyl groups (which abstract halides from substrate) and two electrons. The reagents can be easily synthesized in multigram scales by subjecting the parent aromatic compounds to reduction with alkali metal in the presence of excess Me3SiCl under exclusion of air/moisture.

Procedure:[2] In the glovebox, a 150mL round-bottom flask was charged with 2,3,5,6-tetramethyl-1,4-dihydropyrazine (10mmol) and 50mL of THF. Under stirring, Me3SiCl (30mmol), and potassium were added to the solution. The flask was transferred to a swivel-frit setup connected to a double-manifold vacuum line, and the mixture was stirred under argon overnight. Insoluble materials were then filtered off, solvent removed in vacuo, and the residue purified by sublimation under reduced pressure (70degC, 0.01Torr). Product was obtained in ca. 80% yield as an air/moisture sensitive, colorless solid.

Notes:

  • Reference 2 shows literature data for synthesizing other silyl diene derivatives.
  • Methyl substitution around the 1,4-dihydropyrazine core prevents coordination of the resulting aromatic compounds to transition-metal substrates after reduction has been effected.
  • Some examples where these reducing agents have been used are:
  1. Reduction of Ti(IV) to Ti(III) in Cp2TiCl2 to [Cp2TiCl]2.[1]
  2. Ti(IV) to Ti(II) in Cp2TiCl2 to Cp2Ti(CO)2.[1]
  3. Ta(V) to Ta(IV) in Cp*TaCl4 to [Cp *TaCl3]2.[1]
  4. Formation of an isolable (diphospha)aminyl radical.[3]
  5. Reductive coupling of 2ArBiBr2 and 2ArSbBr2 to yield dibismuthenes and distibenes, respectively.[4]
  6. Reduction of late transition-metal halides to metal(0) particles (metal = Fe, Co, Ni, Cu, Pd, Pt) for use in catalysis.[5]
  7. Generation of a transient phosphinidene from PCl3.[6]
  8. Isolation of N-heterocyclic tetrylenes.[7]
  9. Organic transformations and materials chemistry.[8]
  • For stronger reducing power, the trimethylgermyl diene analog can be synthesized, which has been shown to reduce Ni(II) into nickel metal films.[9]

[1] a) Saito, T.; Nishiyama, H.; Tanahashi, H.; Kawakita, K.; Tsurugi, H.; Mashima, K. J. Am. Chem. Soc. 2014, 136, 13, 5161-5170. b) Tsurugi, H.; Mashima, K. Acc. Chem. Res. 2019, 52, 769-779.
[2] Kaim, W. J. Am. Chem. Soc. 1983, 105, 707-713.
[3] LaPierre, E. A.; Watanabe, L. K.; Patrick, B. O.; Rawson, J. M.; Tuononen, H. M.; Manners, I. J. Am. Chem. Soc. 2023, 145, 16, 9223-9232.
[4] Kumar Majhi, P.; Ikeda, H.; Sasamori, T.; Tsurugi, H.; Mashima, K.; Tokitoh, N. Organometallics 2017, 36, 7, 1224-1226.
[5] Yurino, T.; Ueda, Y.; Shimizu, Y.; Tanaka, S.; Nishiyama, H.; Tsurugi, H.; Sato, K.; Mashima, K. Angew. Chem. Int. Ed. 2015, 54, 48, 14437-14441.
[6] Frenette, B. L.; Trach, J.; Ferguson, M. J.; Rivard, E. Angew. Chem. Int. Ed. 2023, 62, 10, e202218587.
[7] Sahoo, P.; Majumdar, M. Reductively disilylated N-heterocycles as versatile organosilicon reagents. Dalton Trans. 2022, 51, 1281-1296.
[8] Raut, R. K.; Amin, S. F.; Sahoo, P.; Kumar, V. Majumdar, M. Inorganics, 2018, 6, 69–74.
[9] Vihervaara, A.; Hatanpää, T.; Mizohata, K.; Chundak, M.; Popov, G.; Ritala, M. Dalton Trans. 2022, 51, 10898-10908.