Activation of a Carbon–Carbon Bond in Internal Alkynes: Vinylidene Rearrangement of Disubstituted Alkynes at an Ir Complex

Published as part of the Cluster C–C Activation

Abstract

Reactions of [Cp*Ir(PPh₃)Cl₂] with various internal acyl­alkynes in the presence of NaBAr^F ₄ resulted in the selective formation of iridacycles via vinylidene rearrangement. ¹³C-labeling experiments revealed that the acyl group selectively migrates to the other acetylenic carbon atom. This trend is the same as that in the vinylidene rearrangement of internal alkynes at a group 8 metal center.

Key words

rearrangement - internal alkyne - reaction mechanism

• References and Notes

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• 14 The following procedure for the synthesis of 2a is representative. In a 20 mL Schlenk flask, a mixture of [Cp*IrCl₂(PPh₃)] (1, 29.8 mg, 45.1 μmol), NaBAr^F ₄·2H₂O (46.1 mg, 50.0 μmol) and PhC≡CCOOEt (31.9 mg, 183 μmol) in C₂H₄Cl₂ (2.5 mL) was stirred at 70 °C for 30 min. The reaction was monitored by means of ³¹P{¹H} NMR spectroscopy to confirm complete consumption of 1. The brown suspension obtained was filtered through a pad of Celite. After removing the volatiles under reduced pressure, the residue was dissolved in CH₂Cl₂ and recrystallized by slow diffusion of hexane. Compound 2a was obtained as yellow crystals in 69% yield (52.0 mg). ¹H NMR (500 MHz, CDCl₃): δ = 7.75 (s, 8 H, BAr^F ₄), 7.55–7.40 (m, 19 H, BAr^F ₄ + Ph), 7.30 (t, J = 3.7 Hz, 3 H, Ph), 6.75 (m, 2 H, Ph), 4.23 (m, 1 H, CH₂CH₃), 4.02 (m, 1 H, CH₂CH₃), 1.46 (s, 15 H, Cp*), 1.11 (t, J = 7.1 Hz, 3 H, CH₂CH₃). Selected ¹³C{¹H} NMR (126 MHz, CDCl₃): δ = 186.3 (d, ² J _PC = 14 Hz, Ir–C), 184.8 (s, C=O), 96.7 (s, Cp*), 65.4 (s, OCH₂), 13.7 (s, CH₃), 8.50 (s, Cp*). ³¹P{¹H} NMR (202 MHz, CDCl₃): δ 11.3 (s, PPh₃). IR (KBr): 1610, 1560 cm^–1. Anal. Calcd for C₇₁H₅₂BClF₂₄IrO₂P: C, 51.29; H, 3.15. Found: C, 51.23; H, 3.02.
• 15 For details of the X-ray diffraction analyses, see Supporting Information. Crystallographic data for compounds 2a,g–i have been deposited with the Cambridge Crystallographic Data Centre as CCDC 1571053, 1571054, 1571055 and 1571056, respectively. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
• 16 The vinylidene intermediate could not be observed by ³¹P{¹H} NMR analysis of this reaction. In contrast, treatment of 1 with PhC≡CPh at 70 °C in the presence of NaBAr^F ₄ gives a complex which is characterized as a vinylidene complex on the basis of the ¹³C{¹H} NMR [325.9 (d, ² J _PC = 12.0 Hz)] and a preliminary X-ray diffraction study (Figure S7). Further investigation into the vinylidene species derived from intermal alkynes at an iridium center is now under way.
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