**3. Hydrogenation of C–C double and triple bonds**

The chemoselective hydrogenation of selected α,β-unsaturated carbonyl compounds was carried out in toluene in the presence of the homogeneous precatalyst *trans*-Pd(OAc)2(**L1** )2 at 303 K. In all cases the corresponding saturated carbonyl compound was observed in a range of chemoselectivity between 46 and 100%, as shown in **Table 2** [27].

The obtained chemoselectivity was found to be strongly structure dependent and with the employed catalyst in some cases even higher chemoselectivities compared to other palladiumbased catalysts were obtained [35]. The low chemoselectivity of substrate (IV) is due to the formation of the corresponding saturated alcohol, which is mainly explained by steric effects exerted by the α-methyl group of compound IV. Importantly, in all hydrogenation reactions carried out with the selected substrates (**Table 2**), the corresponding allyl alcohol was not formed.

Reaction conditions adopted: catalyst (5 × 10-2 mmol Pd), cinnamaldehyde = 10.5 mmol; toluene = 10 mL; p(H2) = 0.1 MPa; and *T* = 305 K.

a After 24 h.

b After 2 h.

c After 2 h at 0.2 MPa.

**Table 2.** Catalytic performances of *trans*-Pd(OAc)2(**L**<sup>1</sup> )2.

The main advantage of the catalytic system used is the complete solubility in the reaction medium (toluene) and the easy separation of the catalyst as solid upon addition of either MeOH or *n*-hexane to the catalytic solution, exploiting the fact that PLA is not soluble in both solvents. The occurrence of the catalytic reaction in the homogeneous phase mediated by a polymer anchored molecular Pd-based catalyst and not by Pd-NPs, was tested by performing catalytic reactions in the presence of Hg(0), which poisons heterogenous catalyst (colloidal and nanoparticle-based catalysts). As a result, the catalytic activity remained unchanged. The stability of the Pd-catalyst as Pd(II) in the presence of hydrogen was proved by X-ray photoelectron spectroscopy (XPS) of Pd3d, showing no occurred reduction of Pd(II) to Pd (0).

Pd-NPs synthesized by the MVS-technique and stabilized by differently end-functionalized PLA-stereocomplexes (i.e., **L1/2**, **L7/8**, **L9/10**, and **L11/12**) with pyridine, BipyOH, Bn, and OH-end groups, respectively (**Table 1**) were tested in the chemoselective C═C double bond cinnamaldehyde in Tetrahydrofuran (THF) [30]. As a result, the following order of decreasing catalytic activity as well as chemoselectivity was found: **L7/8** > **L1/2** >**L9/10** > **L11/12** (**Table 3**), which is in line with the decreasing Pd-NPs' dispersion on the polymer surface.


Catalytic conditions: Pd (1.2 × 10-3 mmol), cinnamaldehyde (141.2 μmol), THF (10.0 mL), p(H2) = 1 MPa, *t* = 2 h, and *T* = 335K.

a Turn-over frequency (TOF) referred to substrate accessible Pd-NPs' sites.

b Chemoselective for 3-phenylpropanal.

c Fourth recycling experiment.

**Substrate Conversion (%) Selectivity (%)**

8 New Advances in Hydrogenation Processes - Fundamentals and Applications

Reaction conditions adopted: catalyst (5 × 10-2 mmol Pd), cinnamaldehyde = 10.5 mmol; toluene = 10 mL; p(H2) = 0.1

)2.

The main advantage of the catalytic system used is the complete solubility in the reaction medium (toluene) and the easy separation of the catalyst as solid upon addition of either MeOH or *n*-hexane to the catalytic solution, exploiting the fact that PLA is not soluble in both solvents. The occurrence of the catalytic reaction in the homogeneous phase mediated by a polymer anchored molecular Pd-based catalyst and not by Pd-NPs, was tested by performing catalytic reactions in the presence of Hg(0), which poisons heterogenous catalyst (colloidal and nanoparticle-based catalysts). As a result, the catalytic activity remained unchanged. The stability of the Pd-catalyst as Pd(II) in the presence of hydrogen was proved by X-ray photoelectron spectroscopy (XPS) of Pd3d, showing no occurred reduction of Pd(II) to Pd (0).

MPa; and *T* = 305 K.

After 2 h at 0.2 MPa.

**Table 2.** Catalytic performances of *trans*-Pd(OAc)2(**L**<sup>1</sup>

a After 24 h.

b After 2 h.

c

96 94

>99 >99

24 100

13 46

dSolventless catalytic reaction (Pd = 1.4 × 10-2 mmol, cinnamaldehyde = 24.0 mmol, and *t* = 6 h).

**Table 3.** Catalytic performances of different Pd@Lx|y in the selective hydrogenation of cinnamaldehyde.

Pd@**L7/8** gave a 95% chemoselectivity at 100% conversion which is notably higher compared to other heterogeneous polymer based catalysts. The role of the polymer-anchored 2,2′-bipyridine functionality in **L7/8** is that of an efficient small Pd-NP size (i.e., 2 nm) stabilizer, which leads to the high chemoselectivity found for 3-phenylpropanale, due to the efficient C═C double bond coordination to small Pd-NPs [30]. High chemoselectivity for Pd@ **L7/8** was also found under solventless catalytic conditions (90% chemoselectivity for 3-phenylpropanal at 88% substrate conversion) conducted with a catalyst to a substrate ratio of 1:1714.

Pd-Nps generated by a stepwise synthesis which comprised the coordination of Pd(OAc)2 to end-functionalized **L3/4** and **L5/6** followed by a successive reduction with hydrogen in either THF or CH2Cl2 [29]. As a result, CH2Cl2 reveled the solvent of choice, since the PLA-stereocomplex is much better suspended compared to THF and as a consequence, the size of the obtained Pd-Nps in CH2Cl2 were of 3.11 nm in either cases. Pd@ **L3/4** and Pd@**L5/6** were employed to partially hydrogenate phenylacetylene and diphenylacetylene employing THF as the reaction medium. From a comparison of the catalytic data, shown in **Table 4**, emerges that Pd@**L5/6**, which is build up by the crystalline PLA-stereocomplex unit and amorphous (i.e., syndiotactic PLA) part, where the functional groups are located.


Catalytic conditions: Pd (8.34 × 10-4 mmol), alkyne (4.0 mmol), THF (10.0 mL), and *p* = 0.3 MPa.

a TOF referred to substrate accessible Pd sites.

b Chemoselectivity referred to the corresponding alkene.

c *T* = 298 K.

<sup>d</sup>*T* = 333 K.

**Table 4.** Catalytic performances of different Pd@Lx|y in the partial hydrogenation alkynes.

Hence the higher catalytic activity and chemoselectivity of Pd@**L5/6** compared to Pd@**L3/4** were explained in terms of a much easier access of Pd-Nps situated in an amorphous polymer phase. In addition, the easier access of the substrate to the Pd-NPs in Pd@**L5/6** removes more efficiently styrene (i.e., partial hydrogenation product of phenylacetylene) thus avoiding the styrene coordination to the Pd-Nps and hence the hydrogenation to ethylbenzene [37]. The partial hydrogenation of diphenylacetylene to *cis‐*stilbene occurred at 333K. A comparison of the performance of Pd@**L5/6** with other heterogeneous Pd catalysts, such as Pd-Nps supported onto carbon without additional organic modifiers led to much lower chemoselectivities (i.e., styrene <80% and *cis*-stilbene 85%) [38]. The heterogeneous PLA-stereocomplex-based Pd catalysts were easily recovered from the catalytic solution simple by centrifugation of the catalytic THF solution followed by decantation of the solution in air atmosphere. ICP-OES analysis of the THF solution confirmed a very low level of Pd leaching (i.e., >0.5 ppm). HRTEM images acquired of the different recovered heterogeneous catalyst, showed that Pd@**L7/8** is the most promising support for Pd-NPs, avoiding Pd-Nps' stabilization property of **L7/8** is due to interactions between the Pd-Nps' surface and the bipyridine functionality of **L7/8** as shown by IR-spectroscopy [35].
