The hydroformylation of olefins, which was discovered by Roelen as early as 1938 in the course of his investigation on cobalt-catalyzed Fischer-Tropsch synthesis, has developed into one of the most widely applied homogeneously catalyzed processes in industry [1-8]. More importantly, the hydroformylation of olefins yields a mixture of aldehydes which can be further converted to carboxylic acids, alcohols or amines. These products are considered as important versatile intermediates that can be further transformed into a wide variety of high-performance chemicals, such as pharmaceuticals, agrochemicals, perfumes, and fine chemicals [9-15].

Dicyclopentadiene (DCPD) is one of the most important components of the C5 fraction in the cracking steam from naphtha and gas oils. It can be hydroformylated to tricyclodecanemonoaldehyde (TCDMA) or tricycledecanedialdehyde (TCDDA) (Scheme 1), which offer a broad range of applications [5, 11, 16]. However, the main difficulties are related to the low selectivity of TCDDA. There are two unsaturated double bonds (on positions 3, 4 and 8, 9) in DCPD and the norbornenyl moiety is therefore more reactive than the cyclopentenyl part [17, 18]. Firstly, hydroformylation might occur on the 8, 9 double bond to generate monoaldehydes (TCDMA). Subsequently, the hydroformylation may take place at the cyclopentenyl moiety of TCDMA to form dialdehydes (TCDDA) [19]. The formation of TCDMA proceeds readily; however, the conversion of TCDMA to TCDDA is challenging [11, 17-20].

Over the last few decades, many groups had been involved in the development of new catalytic systems for the hydroformylation of DCPD with high conversion and good selectivity [11, 17, 21-23]. When a non-ligand modified Rh catalyst was used in DCPD hydroformylation, the reaction conditions are often difficult. For instance, In 2007, Papp et al. [22, 23] described that more than 90% yield of TCDDA was obtained via the hydroformylation of DCPD in the presence of a non-ligand modified Rh catalyst HRh(CO)₄. However, the reaction conditions were quite harsh with a syngas pressure as high as 35 MPa in at least two reaction zones, and a required temperature in the range of 80 to 130 ℃ in the first reaction zone, which was adjusted from 120 to150 ℃ in the subsequent zone. In 2011, Pi et al. [11] reported that the water-soluble rhodium complex RhCl(CO)(TPPTS)₂ catalyzed the hydroformylation of DCPD in an aqueous/organic two-phase system containing cationic surfactants C₁₆H₃₃N(CH₃)₂C_nH_2n+1Br (n = 1, 8, 12, 16) that were used to accelerate the reaction. Garlaschell et al. [17] used cobalt-rhodium catalytic systems that were promoted by triphenylphosphine (PPh₃) in the hydroformylation of DCPD with 94.5% selectivity of TCDDA under relatively mild conditions (4.0 MPa, 110 ℃). However, this catalytic system required a large amount of bimetallic catalysts.

Thus, it is desirable to identify new ligands with rhodium complexes that lead to higher efficiency and better selectivity in the hydroformylation of DCPD under mild reaction conditions. In this context, P-donor ligands (phosphine, phosphites, phosphoroamidites) have garnered much attention in recent decades. Among the aforementioned ligands, phosphites are extremely attractive because they can be simply prepared from readily accessible precursors. In addition, phosphites exhibit high resistance to oxidative destruction because of the absence of P-C bonds [24-32]. However, the phosphite ligands in the Rh-catalyzed hydroformylation of DCPD have been rarely studied. We have recently developed the synthesis of a new class of helical C₃-symmetric monophosphite ligands, whose metal complexes show high activity and regioselectivity in the hydroformylation of styrene and other catalytic reactions [33]. Based on the synthesis and successfully application of phosphite, we considered that the ligands (L1-L4) (Fig. 1) containing different ester substituents at the 2'-binaphthyl position (OCOMe, OCOPh, OCOAdamantyl and OCOPhCl), possess obvious potential for the Rh-catalyzed hydroformylation of olefins. In addition, although the phosphites with P-O bonds are sensitive to water, bulky phosphite ligands exhibit good stability and more resistant to hydrolysis in the case of Rh-catalyzed hydroformylation [5, 34]. Moreover, bulky phosphite ligands have performed much better than PPh₃ in the enhancement of Rh-catalyzed hydroformylation reactions due to steric and electronic effects [34-36]. In the present work, we report on the highly efficient Rh-catalyzed hydroformylation of DCPD under mild conditions, wherein the rhodium complexes were prepared in situ from the ligands L1-L4.

All experiments were performed in a nitrogen environment using standard Schlenk techniques. NMR spectra were recorded using Bulker 300 or 400 MHz spectrometers. ¹H and ¹³C NMR spectra were reported with tetramethylsilane (TMS) as an internal standard. ³¹P NMR spectra were reported with 85% (volume fraction) of H₃PO₄ as an external reference. The coupling constants (J) were reported in Hertz (Hz). Spin multiplicities were given as s (singlet), d (doublet), t (triplet) and m (multiplet). High resolution mass spectra (HRMS) were recorded using a Bulker microTOF-QII mass instrument. The melting points of the solid samples were determined with an X-4 digital melting point apparatus with an attached microscope. Optical rotations were measured on a Perkin-Elmer 241 MC polarimeter at 20 ℃. Gas chromatography analysis was performed using a HP-Agilent 6890 chromatographer, equipped with a flame ionization detector (FID) and with a SE-54 column. GC-MS was performed on an Agilent 5975C with a Triple-Axis detector. Reactions were monitored using thin layer chromatography (TLC, silica gel GF254 plates). Column chromatography separations were conducted on silica gel (200-300 mesh). NEt₃, THF, Et₂O and toluene were distilled with Na and benzophenone as an indicator, and CH₂Cl₂ was dried over CaH₂ before use. All the other chemicals were obtained commercially and used without further purification.

A 25-mL stainless steel autoclave was charged with DCPD (0.13 mL), toluene (2.5 mL), ligand (0.0038 mmol), and Rh(acac)(CO)₂ (0.0038 mmol) under a nitrogen atmosphere. The autoclave was pressurized with CO and H₂ (4 MPa, 1:1). The reaction mixture was stirred with a magnetic stirrer at the reaction temperature. After a prescribed reaction time, the autoclave was quickly cooled with water to room temperature and the residue gas was slowly released. The conversion of DCPD, the selectivity of aldehydes, and the molar ratio of TCDMA/TCDDA were determined by GC analysis with a SE-54 column (30 m × 0.25 mm).

The phosphite ligands L1-OCOMe, L2-OCOPh, L3-OCOAd and L4-OCOPhCl (Fig. 1) were synthesized from the corresponding mono-protected H₈-binaphthol derivatives and PCl₃ based on our recently published procedure [33], The acquired spectroscopic data (¹H, ¹³C, ³¹P NMR and HRMS-ESI) were in agreement with previously reported results.

In order to evaluate the effects of the ligand's structure on the activity and selectivity of the rhodium catalytic system, a set of four tris-H₈-binaphthyl monophosphites containing different ester (-OCOR) appendices, L1-OCOMe, L2-OCOPh, L3-OCOAd and L4-OCOPhCl, were evaluated as ligands.

In a typical reaction, the autoclave was charged with the appropriate amount of phosphite ligand and [Rh(acac)(CO)₂] (in a 1:1 ratio) with toluene as the solvent. The reactor was then pressurized with an equimolar mixture of CO/H₂ (4 MPa), and the reaction was conducted at 110 ℃ under magnetic stirring. The progress of the reaction was monitored via GC analysis (Table 1).

As clearly shown in Table 1, distinct profiles were obtained for each different Rh(Ⅰ)/monophosphite catalytic system. The conversion of DCPD is nearly 100% for all the ligands of L1-L4, but the selectivity of TCDDA was very different with the other ligands. The monodentate phosphites L1-L4 showed better performance compared to PPh₃ in terms of selectivity towards TCDDA (Table 1, entries 1-5) for the ligands PPh₃ and L1-L4. In addition, the P atom in L1-L4 is connected with a backbone via the O atom, while the P atom of PPh₃ is connected through a C atom. Van der Slot et al. [37] proved that the P-O bond could result in a lower electronic density for phosphorus compared to the P-C bond. The electron-deficient ligand with π-accepting-ability is good, which results in a suitable lower electron density around the rhodium metal and this, in turn, weakens the metal-carbonyl bond. Hence, the P atom connected with the backbone via the O atom is favorable for the dissociation of CO and thereby accelerates the catalysis reaction. The performance comparison between the rhodium catalyst modified by ligands L3 and L4 also confirmed that good π-accepting-ability of the ligand favored catalysis (Table 1, entries 3 and 4). Using the catalyst prepared in situ from Rh(acac)(CO)₂ and ligand L4, the hydroformylation of DCPD with different solvents was investigated, and a profound solvent effect on the reaction was observed (Table 1, entries 5-8). It is noteworthy that in the non-coordinating and relatively non-polar solvents of toluene and hexane, TCDDA was obtained at yields of 89.8% and 90.1%, respectively (Table 1, entries 5 and 6). The selectivity of TCDDA was 80.6% in toluene but only 49.4% in hexane. When the reactions were performed using the weakly coordinating solvent CH₂Cl₂, the yield and selectivity of TCDDA were 92.3% and 84.2%, respectively (Table 1, entry 7). In contrast, 91.2% yield and 88.8% selectivity of TCDDA were obtained in the highly polar and coordinating solvent THF (Table 1, entry 8). Therefore, THF was found to be the preferred solvent in terms of yield and selectivity in the hydroformylation of DCPD.

Since the hydroformylation reaction is highly dependent on the reaction conditions, we set out to determine the optimal conditions for the rhodium catalyst system with ligand L4. Representative results for temperature, P/Rh molar ratio, initial syngas pressure and reaction time are shown in Table 2. In the absence of the monophosphite ligand, the non-modified rhodium catalyst yielded only 32.1% of TCDDA, while TCDMA was formed at 67.9% (Table 1, entry 1). It can be seen that the selectivity to TCDDA increased as the P/Rh molar ratio was varied from 0.5 to 1. A further increase of the P/Rh molar ratio caused a reduction in the selectivity to TCDDA. The reaction temperature has a dramatic effect on the hydroformylation of DCPD. In the range of 90-120 ℃ (Table 2, entries 3-7) a good yield of 92.8% aldehydes was achieved, and an excellent selectivity of 91.1% was obtained at 120 ℃ (Table 2, entry 5). The total pressure of the syngas was also a key element in the hydroformylation of DCPD. The selectivity to TCDDA greatly increased in the pressure range from 2 to 6 MPa (Table 2, entries 5, 8 and 9). When the pressure was increased from 6 to 8 MPa, a similar increase was observed in the selectivity, which afforded in the corresponding adduct TCDDA with 98.7% and 98.9% selectivity, respectively (Table 2, entries 9 and 10). When the reaction time for hydroformylation was reduced from 5 h to 3 h at 120 ℃ and 6 MPa, the selectivity to TCDDA was slightly reduced from 98.7% to 94.8% (Table 2, entries 9 and 11). It was found that the selectivity to TCDDA decreased dramatically to 28.3% when the reaction time was further shortened to 1 h, but increased the selectivity to TCDMA from 1.3% to 71.7% (Table 2, entries 9 and 12).

Next, we studied the influence of different rhodium precursors on the Rh-catalyzed hydroformylation of DCPD. In this case, rhodium complexes in different oxidation states were examined (Table 3). Among the tested catalytic precursors, almost all Rh(Ⅰ) precursors such as Rh(acac)(CO)₂, Rh(cod)₂BF₄, and [Rh(cod)Cl]₂ afforded better yields of the aldehydes and a higher selectivity to the desired product TCDDA (Table 3, entries 1, 3 and 4). In particular, the yield of aldehyde and the selectivity of TCDDA exhibited maximum values of 95.4% and 98.7% respectively, when Rh(acac)(CO)₂ was used as the catalyst precursors. On the contrary, the conversion, the yield and the selectivity of TCDDA sharply decreased to 17.5%, and 7.5% and 16.9% respectively, when Rh(Ⅲ) precursor RhCl₃·3H₂O was applied to the reaction (Table 3, entry 2). Some of the DCPD may be converted to the hydrogenation products dihydrodicyclopentadiene (3, 4- or 8, 9-dihydrodicyclopentadiene) or tetrahydrodicyclopentadiene or their mixture, because of the competition between hydroformylation and hydrogenation.

The data in Table 4 show the influence of different molar ratios of substrate to catalyst (S/C) on the hydroformylation of DCPD. The substrate can be converted completely (Table 4, entries 1-5). Under otherwise identical conditions, the increase of the S/C ratio from 250 to 2000 resulted in a considerable decrease of selectivity towards TCDDA from 98.7% to 84.5%, but the turnover number (TON) of TCDDA increases from 224 to 1580 (Table 4, entries 1 and 4), whereas a further increase of the S/C ratio to 4000 induced a significant decrease in both the selectivity and TON of TCDDA (Table 4, entries 4 and 5). This result may be due to the mutation of the increase in the concentration of DCPD, which causes a dramatic drop in the TON of TCDDA and further leads to the hydrogenation of a portion of DCPD. However, when the reaction time was increased from 5 to 20 h, the selectivity towards TCDDA increased to 90.5%, and the TON of TCDDA was up to 3286 (Table 4, entry 6). Therefore, increasing the reaction time could effectively increase the selectivity of TCDDA and TON in the case of high S/C ratio. In other words, when L4 was used as a ligand, the rhodium catalyst showed high catalytic activity and good stability in the hydroformylation of DCPD to produce TCDDA.

The [Rh(acac)(CO)₂/monophosphate] complexes were accessed by solution ³¹P NMR spectroscopy. In a typical experiment the complexes were generated in situ by the reaction of the rhodium precursor Rh(acac)(CO)₂ and tris-H₈-binaphthyl monophosphite in CDCl₃ under N₂ atmosphere. The ³¹P NMR spectra of the resulting solutions were acquired while the mixture was stirred at room temperature for 5-30 min. Using equimolar amounts of [Rh(acac)(CO)₂] and monophosphite ligands L1-OCOMe, L2-OCOPh, L3-OCOAd or L4-OCOPhCl in CDCl₃, the spectra showed the formation of a specie assigned to the P-Rh coordination mode (³¹P doublet in the range 114-116 ppm with J (¹⁰³Rh-³¹P) = 289-295 Hz) (Fig. 2). The ³¹P NMR spectrum obtained for the Rh(Ⅰ) complexation with L2-OCOPh evidenced the formation of species (Fig. 2(A)). In order to evaluate the relative stability of the [Rh(acac)(CO)₂ /monophosphate] complexes, the ligand L4-OCOPhCl which was used as an internal standard was added to a CDCl₃ solution of [Rh(acac)(CO)(L2-OCOPh)]. After 50 min, considerable ligand exchange resulted in the formation of a ca. 1:0.8 mixture of [Rh(acac)(CO)(L2-OCOPh)]/[Rh(acac)(CO)(L4-OCOPhCl)], in addition to the non-coordinated ligands L2-OCOPh and L4-OCOPhCl, as demonstrated by ³¹P NMR spectroscopy (Fig. 2(B)). This suggests that [Rh(acac)(CO)(L4-OCOPhCl)] is more stable than [Rh(acac)(CO)(L2-OCOPh)].

From these studies, we considered that the lower activity and selectivity obtained with Rh(Ⅰ)/L2-OCOPh catalyst might be due to the exchange of L2-OCOPh by CO ligands in [RhH(CO)_4-nL_n] type complexes, leading to different Rh(Ⅰ) catalytic species in solution, under hydroformylation conditions. To gain some insight into the catalytic species, we used L4-OCOPhCl as a model monophosphite ligand to perform a deeper inspection into the possibility of the formation of Rh(Ⅰ)/phosphite species. Different amounts of monophosphite ligands were added to Rh(acac)(CO)₂ dissolved in CDCl₃ under a N₂ atmosphere, and the mixture was stirred at room temperature for 5-30 min. As shown in Fig. 3, the ³¹P NMR spectra of the resulting solutions were registered, and the spectra revealed that in all cases, a doublet peak in the range δ = 114-116 ppm with J (¹⁰³Rh, ³¹P) = 295 Hz corresponding to a single phosphorus species was observed. When a twofold excess amount of ligand was used, the ³¹P NMR spectra revealed a 1:1 ratio between the same signal and a singlet at δ = 132 ppm, which is typical of the non-coordinated phosphite ligand (Fig. 3(3)). These results indicate that only one bulky ligand can coordinate to the rhodium precursor.

In conclusion, the catalytic systems which were readily constituted from a catalytic precursor Rh(acac)(CO)₂ and tris-H₈-binaphthyl monophosphite ligands L1-L4, were successfully developed for the Rh-catalyzed hydroformylation of DCPD to TCDDA. The results demonstrate that high selectivity of TCDDA could be obtained using ligand L4 under relatively mild reaction conditions (120 ℃, 6 MPa). Moreover, such catalytic systems can be tuned through the structural modulation of the ester substituents at the 2'-H₈-binaphthyl positions of the bulky phosphites.