Iridium-Catalyzed Selective C-C Bond Cleavage of Nitriles and Ketones

Abstract

Selective and catalytic C-C bond cleavage of pentanedinitriles and 5-oxohexanenitriles can be performed in the presence of iridium hydride complex IrH₅(Pi-Pr₃)₂ (1). The key steps of this reaction are α-C-H activation of the cyano or carbonyl group and subsequent β-carbon elimination.

iridium hydride complex - C-H activation - β-carbon elimination - nitrile - ketone

References

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<A NAME="RU15804ST-15">15</A>

General Procedure for the Iridium-Catalyzed C-C Bond Cleavage of 2-Methyl-2-phenyl-pentanedinitrile (5).
In a 25 mL sealed tube, iridium catalysts (0.1 mmol), 5
(1 mmol), and toluene (0.5 mL) were placed and stirred for 12 h at 150 °C under argon atmosphere. The yield of
2-phenylpropionitrile 6 was determined by NMR using dibenzyl as an internal standard.

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<A NAME="RU15804ST-17">17</A>

General Procedure for the Iridium-Catalyzed C-C Bond Cleavage of Nitriles and Ketones.
In a 25mL of sealed tube, IrH₅ (Pi-Pr₃)₂ (1, 0.2 mmol), nitriles or ketones (1 mmol), and toluene (0.5 mL) were placed and stirred for 12 h at 150 °C under argon atmosphere. The yields of products were determined by NMR using dibenzyl as an internal standard. All the products were confirmed by ¹H NMR, ¹³C NMR, and HRMS.

<A NAME="RU15804ST-18">18</A>

In the case of the reaction of 2-methyl-2- phenylhexane-dinitrile (12), the formation of 2-amino-1-cyano-3-methyl-3-phenylcyclopentene (13) was observed.¹⁹ The cyclization would proceed via nucleophilic addition of alkyliridium intermediate to the CN triple bond of nitriles (Scheme [4] ). This cyclic product unequivocally supports the occurrence of α-C-H bond activation and the formation of intermediate 8 (Scheme [4] ).

Scheme 4

<A NAME="RU15804ST-19">19</A>

In a 25mL of sealed tube, 2-methyl-2- phenylhexanedinitrile (12, 200 mg, 1.0 mmol), IrH₅ (Pi-Pr₃)₂ (1, 52 mg, 0.1 mmol), and toluene (0.5 mL) were placed and stirred for 24 h at 150 °C under argon atmosphere. After removal of the solvent, the mixture was purified by silica gel column chromatography and by Kugelrohr distillation to give 2-amino-1-cyano-3-methyl-3-phenylcyclopentene (13, 1:0.8 diastereomer mixture) as a colorless oil (47 mg, 24%). ¹H NMR (270 MHz, CDCl₃): δ = 1.54 (s, 3 H, -CH ₃), 1.95-2.15 [m, 2 H, -CH ₂-C(Me)(Ph)-], 2.48-2.53 [m, 2 H, -CH ₂-C(CN)-], 4.25 (br, 2 H, -NH ₂), 7.22-7.40 (m, 5 H, -C₆ H ₅). ¹³C NMR (68 MHz, CDCl₃): δ = 167.2 (C-NH₂), 144.1
(-C ₆H₅), 128.4 (-C ₆H₅), 126.7 (-C ₆H₅), 126.2 (-C ₆H₅), 118.8 (-CN), 74.2 (C-CN), 53.0 (C-CH₃), 41.5 [-CH₂-C(Me)(Ph)-], 28.1 [-CH₂-C(CN)-], 22.9 (-CH₃). MS (EI, 70 eV):
m/z (%) = 198, 183, 168, 166, 140, 129, 115, 105, 91, 77.

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