Isomerization of Internal Alkynes to Iridium(III) Allene Complexes via C–H Bond Activation: Expanded Substrate Scope, and Progress towards a Catalytic Methodology

The synthesis of a series of allene complexes (POCOP)Ir(η2-RC==CR’) 1b–4b (POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) via isomerization of internal alkynes is reported. We have demonstrated that the application of this methodology is viable for the isomerization of a wide variety of alkyne substrates. Deuterium labeling experiments support our proposed mechanism. The structures of the allene complexes 1b–4b were determined using spectroscopic data analysis. Additionally, the solid-state molecular structure of complex 2b was determined using single crystal X-ray diffraction studies and it confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, we report a preliminary step towards developing a catalytic methodology; the allene may be liberated from the metal center by exposure of the complex to an atmosphere of carbon monoxide.


Table of Contents
G e n e r a l r e m a r k s S2 Typical procedure for in-situ preparation of η 2 -allene complexes S2 Spectroscopic Characterization of 1b S 2 Spectroscopic Characterization of 1b-d5 S3 Spectroscopic Characterization of 2b S 3 Spectroscopic Characterization of 3b S 4 Spectroscopic Characterization of 4b S 5 C r o s s o v e r E x p e r i m e n t s S 6 CO Liberation of allene from iridium complex S7 K i n e t i c s o f a l l e n e c o m p l e x f o r m a t i o n S 9 Plot of rate of consumption of 1a at 328 K trial 1 S10 Plot of rate of consumption of 1a at 328 K trial 2 S10 Plot of rate of consumption of 1a at 338 K trial 1 S11 Plot of rate of consumption of 1a at 338 K trial 2 S11 Plot of rate of consumption of 1a at 348 K trial 1 S12 Plot of rate of consumption of 1a at 348 K trial 2 S12 Eyring Plot for conversion of 1a → 1b S13 Plot of rate of consumption of 1a-d5 at 328 K trial 1 S14 Plot of rate of consumption of 1a-d5 at 328 K trial 2 S14 Plot of rate of consumption of 1a-d5 at 338K trial 1 S15 Plot of rate of consumption of 1a-d5 at 338K trial 2 S15 Plot of rate of consumption of 1a-d5 at 348K trial 1 S16 Plot of rate of consumption of 1a-d5 at 348K trial 2 S16 Eyring Plot for conversion of 1a-d5 → 1b-d5 S16 K i n e t i c I s o t o p e E f f e c t S 1 7 Plot of rate of consumption of 2a at 328 K trial 1 S17 Plot of rate of consumption of 2a at 328 K trial 2 S17 Plot of rate of consumption of 2a at 338K trial 1 S18 Plot of rate of consumption of 2a at 338K trial 2 S18 Plot of rate of consumption of 2a at 348K trial 1 S19 Plot of rate of consumption of 2a at 348K trial 2 S19 Eyring Plot for conversion of 2a → 2b S20 Plot of rate of consumption of 4a at 328 K trial 1 S20 Plot of rate of consumption of 4a at 328 K trial 2 S21 Plot of rate of consumption of 4a at 338K trial 1 S21 Plot of rate of consumption of 4a at 338K trial 2 S22 Plot of rate of consumption of 4a at 348K trial 1 S22 Plot of rate of consumption of 4a at 348K trial 2 S23 Eyring Plot for conversion of 4a → 4b S23 X-ray analysis of 2b S24 S2 General Remarks. Chemicals and solvents were purchased from commercial suppliers and were used as received except as follows: TBE and 1-Phenyl-1-butyne were degassed. Reactions were carried out under argon gas in a J-Young nmr tube. Chemical shifts are given in ppm. Unless otherwise noted, the NMR spectra were recorded in C6D6. Coupling constants (J) are given in Hertz (Hz). The terms m, s, d, t, q, quint., sext., vt, represent multiplet, singlet, doublet, triplet, quartet, quintet, sextet, virtual triplet respectively. The term br means that the signal is broad. X-ray diffraction experiments were performed at the Chemistry Department X-ray Diffraction Facility of the Texas Tech University.
Typical Procedure for Preparation of η 2 -allene Complexes (1b-4b): In a J-Young NMR tube, (POCOP)IrH2 (12 mg, 20 µmol) was dissolved in C6D6 and ca. 1.5 eq. tert-butylethylene added via microsyringe. The removal of H2 with concomitant formation of 1 eq. of tert-butylethane is easily monitored using both 1 H and 31 P { 1 H} NMR spectroscopy. Addition of 1 eq. of alkyne affords exclusively the η 2 -adducts of alkynes 1a-5a. The solutions of η 2 -alkyne complexes, prepared as above, were warmed to ca. 75 °C for 5-10 h to reach full conversion to the allene complex. Definitive assignment of diastereomers is hampered by closely overlapping peaks in the 1 H NMR spectra and tentative assignments are made on the basis of previously reported NMR data of related allene complexes and 2-D NMR experiments.

S4
Spectroscopic Characterization of 3b:    Figure S5. Stacked 1 H NMR of 2b and isolated allene from 2b after CO treatment.

NMR Kinetics of Allene Complex Formation:
The rate of conversion of 1a to 1b was determined by monitoring the disappearance of the singlet of 1a in the 31 P { 1 H} NMR spectrum. The disappearance of 1b (10 µmol) in a C6D6 solution (0.6 mL) was measured at 348, 338 and 328 K over at least 3 half-lives to allow accurate determination of rate constants. A plot of ln [1at]/[1bi] vs time was essentially linear, the average (kobs = 4.173 × 10 −4 (348 K); 1.169 × 10 −4 (338 K); 3.718 × 10 −5 (328 K)). By plotting the change in reaction rate vs. temperature an Eyring Plot could be constructed (ln(k/T) vs. 1/T). The rates of isomerization, and activation parameters, for 1a-d5, 2a and 4a are qualitatively similar to those of 1a. Representative plots are given below; in addition the corresponding thermodynamic parameters are calculated.

General Data Collection
Data was collected on a Bruker PLATFORM three circle diffractometer equipped with an APEX II CCD detector and operated at 1500 W (50 kV, 30 mA) to generate (graphite monochromated) Mo Kα radiation (λ = 0.71073 Å). Crystals were transferred from the vial and placed on a glass slide in Paratone ® N oil. Two microscopes, a Motic SMZ-140 and an AmScope XY-PRT polarizing microscope, were used to identify a suitable specimen for X-ray diffraction from a representative sample of the material. The crystal and a small amount of the oil were collected on a MῑTiGen cryoloop and transferred to the instrument where it was placed under a cold nitrogen stream (Oxford) maintained at 100 K throughout the duration of the experiment. The sample was optically centered with the aid of a video camera to insure that no translations were observed as the crystal was rotated through all positions.
A unit cell collection was then carried out. After it was determined that the unit cell was not present in the CCDC database a sphere of data was collected. Omega scans were carried out with a 20 s/frame exposure time and a rotation of 0.33° per frame. After data collection, the crystal was measured for size, morphology, and color.