Mechanistic Investigations on Hydrogenation, Isomerization and Hydrosilylation Reactions Mediated by a Germyl‐Rhodium System

Abstract We recently disclosed a dehydrogenative double C−H bond activation reaction in the unusual pincer‐type rhodium‐germyl complex [(ArMes)2ClGeRh] (ArMes=C6H3‐2,6‐(C6H2‐2,4,6‐Me3)2). Herein we investigate the catalytic applications of this Rh/Ge system in several transformations, namely trans‐semihydrogenation of internal alkynes, trans‐isomerization of olefins and hydrosilylation of alkynes. We have compared the activity and selectivity of this catalyst against other common rhodium precursors, as well as related sterically hindered rhodium complexes, being the one with the germyl fragment superior in terms of selectivity towards E‐isomers. To increase this selectivity, a tandem catalytic protocol that incorporates the use of a heterogeneous catalyst for the trans‐semihydrogenation of internal alkynes has been devised. Kinetic mechanistic investigations provide important information regarding the individual catalytic cycles that comprise the overall trans‐semihydrogenation of internal alkynes.


General considerations
All manipulations were carried out using standard Schlenk and glove-box techniques, under an atmosphere of argon and of high purity nitrogen, respectively. All solvents were dried, stored over 4 Å molecular sieves, and degassed prior to use. N-pentane (C5H12) was distilled under nitrogen over sodium.

General Method
In the glovebox, the appropriate mass of [(Ar Mes )2ClGeRh] (1) was added as a solid to a J-Young NMR tube. Next, 0.1 mL of a freshly prepared stock-solution of hexamethylbenzene (S1, 0.298 mM) in toluene-d8 was added and the total volume was adjusted to 0.4 mL by subsequent addition of an appropriate volume of toluene-d8. Afterward, an excess of NaBAr F (20 mg) was added as a solid. The tube was sealed with a Teflon screw-cap to ensure air tightness and was vigorously shaken before being allowed to stand overnight at room-temperature. The formation of the catalyst 2 was monitored by 1 H NMR. After total conversion, 0.1 mL of a freshly prepared stock-solution of diphenylacetylene (S2, 0.298 mM) in toluene-d8 was added (Vtot = 0.5 mL). Then, the NMR tube was degassed using freezepump method prior being charged at room temperature with hydrogen or deuterium (0.5 to 1.5 bar). Finally, the J-Young tube was placed in an oil bath at 80°C (NB: no reaction occurs at roomtemperature). The extent of the reaction was monitored by 1 H NMR using relative integrations of diphenylacetylene m,4H,HPh), catalyst (δ 7.66 ppm, br.s, 4H, Hp-Fxyl), cis-stilbene (δ 6.43 ppm, s, 2H, HC=C), trans-stilbene (δ 7.33 ppm, ps.d, 1 JHH= 7.6 Hz, 4H, HPh) and diphenylethane (δ 2.71 ppm, s, 4H, HC-C) regarding the integration of the methyl protons of hexamethylbenzene (δ 2.10, s, 18H, HMe) used here as an internal standard.

Determination of the partial orders in catalyst and diphenylacetylene
The Variable Time Normalization Analysis for the catalyst (cf. Figure S16) shows two profiles corresponding to Exp 1 (depicted in blue) and Exp 2 (depicted in red) and whose shape changes with the partial order simulated. A multipoint comparison clearly shows decent overlays for an order of 1 (a=1).

Figure S16. Variable Time Normalization
Analysis for the determination the partial order in catalyst with another set of data.

S14
The Variable Time Normalization Analysis for the diphenylacetylene (cf. Figure S17) shows two profiles corresponding to Exp 1 (depicted in blue) and Exp 3 (depicted in green) and whose shape changes with the partial order simulated. A multipoint comparison shows here also a good overlay for a zero order (b=0).

Figure S17. Variable Time Normalization
Analysis for the determination the partial order in diphenylacetylene.

Determination of the partial order in hydrogen
The initial rates for Exp 1 (pH2 = 1 bar), Exp 4 (pH2 = 0.5 bar) and Exp 5 (pH2 = 1.5 bar) can be extracted from diphenylacetylene's consumption (see Figure S18). The initial concentrations of H2 were determined from Henry's law, KH = pH2 /[H2], where Henry's law constant (KH) for H2 in toluene is 24.89 MPa L/mol. The plotting of ln(v0)=f(ln[H2]) (see Figure S19) lead to a straight line with a slope of 0.95 corresponding to the partial order in hydrogen.

Isotopic Studies:
To the overall transformation can be assigned the following kinetic law with kobs the rate constant, [dpa] the concentration in diphenylacetylene, pH2 the pressure in hydrogen or deuterium, [cat] the concentration in catalyst as well as a, b and c their respective partial orders.
According to the initial rate method, the former equation can be noted for Exp 1 and Exp 2 as Similarly, Exp 6 and Exp 7 verify the following equation Since Exp 1and Exp 6 were launched in the same conditions, then the ratio of their initial rates allows to determine the kinetic isotope effect (KIE) as stated by the following equation: This relation also applies to the comparative study of the initial rates for Exp 2 and Exp 7.

Modus Operandi for catalysis by 3 generated in situ:
A J-Young NMR tube was charged with [(Ar Mes )2ClGeRh] (1) (2.5 mg, 0.003 mmol, 10 mol%). Hexamethylbenzene was introduced by addition of 0.1 mL of a freshly prepared stock-solution (S1, 0.298 mM) in toluene-d8. The total volume of the solution was adjusted to 0.4 mL by consecutive addition of 0.3 mL of toluene-d8. Afterward, NaBAr F (20 mg, 0.0226 mmol) was added at roomtemperature, the tube was sealed with a Teflon screw-cap and the reaction was allowed to reach 80°C. After 6.5 hours at this temperature, 1 H NMR monitoring showed total conversion of 2 into 3. Then, 0.1 mL of a freshly prepared stock-solution (0.298 mM) of diphenylacetylene (corresponding to 0.0298 mmol, 1 equiv.) was added. The solution was degassed by freeze-pump technic and charged at roomtemperature with H2 (1 bar). Finally, the solution was heated up to 80°C and the extent of the reaction was monitored by 1 H NMR.

Modus Operandi for catalysis from isolated 3:
The reaction was launched in the same conditions than Exp 1. In the glovebox, [(Ar Mes )2GeRhH].BAr F (3) (5.4 mg, 3.22.10 -6 mol, 0.10 equiv.) was added as a solid to a J-Young NMR tube. Next, 0.1 mL of a freshly prepared stock-solution of hexamethylbenzene (S1, 0.298 mM) in toluene-d8 was added and the total volume was adjusted to 0.43 mL by subsequent addition of an appropriate volume of toluene-d8. Then 0.11 mL (1 equiv.) of a freshly prepared stock-solution of diphenylacetylene (S2, 0.298 mM) in toluene-d8 was added (Vtot = 0.54 mL, [3]=0.00596 M). Then, the NMR tube was degassed using freeze-pump method prior being charged at room temperature with 1 bar of hydrogen. Finally, the solution was heated up to 80°C and the extent of the reaction was monitored by 1 H NMR.

H2-semihydrogenation of diphenylacetylene catalyzed by 4.
The reaction was launched in the same conditions than Exp 1. In the glovebox, [(Ar Mes )2GeRh].BAr F (4) (4.7 mg, 2.83.10 -6 mol, 0.10 equiv.) was added as a solid to a J-Young NMR tube. Next, 0.1 mL of a freshly prepared stock-solution of hexamethylbenzene (S1, 0.298 mM) in toluene-d8 was added and the total volume was adjusted to 0.385 mL by subsequent addition of an appropriate volume of toluene-d8. Then 0.095 mL (1 equiv.) of a freshly prepared stock-solution of diphenylacetylene (S2, 0.298 mM) in toluene-d8 was added (Vtot = 0.48 mL, [4]=0.00596 M). Then, the NMR tube was degassed using freeze-pump method prior being charged at room temperature with 1 bar of hydrogen. Finally, the solution was heated up to 80°C and the extent of the reaction was monitored by 1 H NMR.

General Method
In the glovebox, the appropriate mass of [(Ar Mes )2ClGeRh] (1) was added as a solid to a J-Young NMR tube. Next, 0.1 mL of a freshly prepared stock-solution of hexamethylbenzene (S1, 0.298 mM) in toluene-d8 was added and the total volume was adjusted to 0.4 mL by subsequent addition of an appropriate volume of toluene-d8. Afterward, an excess of NaBAr F was added as a solid. The tube was sealed with a Teflon screw-cap to ensure air tightness and was vigorously shaken before being allowed to stand overnight at room-temperature. The formation of 2 was monitored by 1 H NMR. After total conversion, 0.1 mL of a freshly prepared stock-solution of cis-Stilbene (S3 for Exp 8 and 10, 0.298 mM; S4 for Exp 9, 0.894 mM) was added (Vtot = 0.5 mL). Then, the extent of the trans-isomerization was monitored over time by 1 H NMR using relative integrations of the catalyst (δ 7.66 ppm, br.s, 4H, Hp-Fxyl), cis-stilbene (δ 6.43 ppm, s, 2H, HC=C), trans-stilbene (δ 7.33 ppm, ps.d, 1 JHH= 7.6 Hz, 4H, HPh) regarding the integration of the methylic protons of hexamethylbenzene (δ 2.10, s, 18H, HMe), used here as an internal standard. Table S2. Reaction conditions for Exp 8-10.

Reaction Progress Kinetic Analysis (RPKA)
To the trans-isomerization of cis-Stilbene can be assigned the following kinetic law

= [ ] [ − ]
with k the rate constant, [cat] the concentration in catalyst, [cis-Stilbene] the concentration in cis-Stilbene and a and b their respective partial orders.
Since the concentration of the catalyst over the reaction time can be considered as constant, the latter equation can be simplified as with kobs the observed rate constant.
As such, the partial order b in cis-Stilbene can be theoretically determined by simple integrations of the kinetic law at different orders (0, 1, 2, …). However, in this peculiar case, the linear regressions of the different modelizations are too similar to clearly conclude by RPKA (e.g. Figure S37 to S39). This is why the kinetic resolution of the trans-isomerization was performed by Variable Time Normalization Analysis (VTNA).

Determination of the partial orders in catalyst and cis-Stilbene:
The Variable Time Normalization Analysis for the catalyst (cf. Figure S40) shows two profiles corresponding to Exp 8 (depicted in blue) and Exp 10 (depicted in red) and whose shape changes with the partial order simulated. A multipoint comparison clearly shows a decent overlay for an order of 1 (best fit for a broken order of 1,2).

Figure S40. Variable Time Normalization
Analysis for the determination of the partial order in catalyst.
The Variable Time Normalization Analysis for the cis-Stilbene (cf. Figure S41) shows two profiles corresponding to Exp 8 (depicted in blue) and Exp 9 (depicted in green) and whose shape changes with the partial order simulated. A multipoint comparison shows here also a decent overlay for an order of 1 (best fit for a broken order of 0.8).

Figure S41
Variable Time Normalization Analysis for the determination of the partial order in cis-Stilbene.

S32
The Normalization of the time scale in catalyst and in cis-Stilbene leads to an overlay of all reaction profiles in a straight confirming partial orders of 1,2 and 0,8 in catalyst and cis-Stilbene respectively (cf. Figure S42). Figure S42. Normalization of the time scale in catalyst and in cis-Stilbene.

Trans-isomerization of cis-Stilbene:
For the sake of comparison, the isomerization of cis-Stilbene has been launched in approximated same conditions than in the isomerization of cis-Stilbene-d2. In a J-Young NMR tube was prepared a 0.9 mL solution containing cis-Stilbene (0.1 mL from a stock solution in benzene-d6 at 0.175 mM, 0.0175 mmol, 1 equiv.), hexamethylbenzene (5.0 mg, 0.031 mmol), [(Ar Mes )2ClGeRh] (1) (1.47 mg, 0.00175 mmol, 10 mol%) and NaBAr F (20 mg). The solution was let stand overnight at room-temperature prior being heated to 100°C. The extent of the trans-isomerization was here also monitored by 1 H NMR.

Activation parameters for the insertion of cis-Stilbene in the Rh-H bond of compound 3.
The trans-isomerization reaction was repeated three times in the same conditions than for Exp 8 but at different temperature: 80°C, 90°C and 100°C. Here again, the plotting of ln[cis-Stilbene] over time lead to the extraction of the rate constant at the different temperatures. The results are gathered in Table S3 below: T (C°) k (L.mol -1 .s -1 ) 80 0,00607 90 0,01662 100 0,02957  The experimental plot of the Eyring equation leads to the following linear law with a very good fit (cf.