Activity of Palladium(II) Complexes with N-Heterocyclic Carbene Ligands in Solvent-Free Telomerization of 1,3-Butadiene with Methanol

This paper describes an investigation of the catalytic activity of a series of N-heterocyclic palladium(II) complexes with a cinnamyl auxiliary ligand in telomerization of butadiene with methanol. The catalytic activity of the cinnamyl N-heterocyclic complexes was compared with that of PEPPSI type complexes.

Solvent-free telomerization of 1,3-butadiene is of increasing interest, not only in view of the abundant opportunities for its industrial implementation, but also because this conversion-in full compliance with the green chemistry principles-involves all atoms of the reactants. The telomerization of 1,3-butadiene with methanol and water has long been applied in industrial synthesis of compounds of key importance: 1-octanol [24] and 1-octene [25].
Dow Chemical proposed an advanced effective method for 1-octene production based on palladiumcatalyzed telomerization of 1,3-butadiene with methanol [26,27]. The 1-methoxy-2,7-octadiene prepared by the telomerization was fully hydrogenated into 1-methoxyoctane with a yield of above 99%. This 1-methoxy-octane was then subjected to high-temperature cracking to produce, after distillation, 1-octene and methanol in up to 97% yield. The hydrogenation and cracking processes were carried out in the presence of conventional heterogeneous catalysts and exhibited high selectivity.
N-Heterocyclic carbene (NHC) complexes of Pd that contained carbene ligands with various structures have been extensively studied as catalysts for telomerization of butadienes with methanol [11,17,[28][29][30][31]. This type of Pd(0) derivatives was shown to exhibit high productivity (indicated by turnover numbers of up to 200 000) and selectivity (up to 98%). Based on the relevant DFT quantum-chemical simulations (B3LYP functional, LANL2DZ basis), Huo et al. [32] proposed a catalyzed telomerization mechanism that consisted of the following steps: reduction of Pd 2+ to Pd 0 followed by coordination of two diene molecules; formation of a new C-C bond in the dimer; protonation; and nucleophile attack at two potential positions to produce corresponding dimers (Scheme 2).
However, the published studies on the structural effects of NHC rings and leaving groups on the catalytic activity of the complexes are too limited. To optimize the catalytic system for telomerization, the correlation between the catalytic activity and the catalyst structure needs to be determined. 1236 TOPCHIY et al.

Synthesis of compounds.
To investigate the structural effects of the Pd-NHC complexes on the performance of The purpose of the present study was to compare, in terms of catalytic activity, palladium(II) complexes that contained identical auxiliary ligands and different N-heterocyclic carbene ligands. In particular, fivemembered (saturated and unsaturated) and six-membered (saturated) carbene ligands were tested. ACTIVITY OF PALLADIUM(II) COMPLEXES 1,3-butadiene telomerization, a series of complexes were prepared (Scheme 3).
The complexes differed in the size of the carbene ligand ring (five-membered rings in 1, 2, and 3 vs. sixmembered rings in 4), in the structure of the carbene ring (1 and 2), in substituents adjacent to the nitrogen atom (1a, 2a, and 4a vs. 1b, 2b, and 4b), and in the structure of leaving groups (1b vs. 3).
Palladium(II) complexes with cinnamyl substituents (1, 2) were produced in high yields from corresponding silver-carbene complexes (NHC)AgCl via ligand exchange with (Pd(cinn)Cl) 2 . An IMesPdCl 2 -3chloropyridine complex (3) was obtained by the reaction between in situ generated free carbenes and PdCl 2 in the presence of 3-chloropyridine. NHC complexes with extended rings were synthesized by in situ generation of free carbenes from corresponding salts followed by their treatment with (Pd(cinn)Cl) 2 (Scheme 4).
Investigation of catalytic activity. None of the complexes synthesized in this work have previously been investigated in telomerization of butadiene with methanol. Relevant prior research is limited to complexes of unsaturated five-membered N-heterocyclic carbene ligands with palladium(0) [16,31]. Moreover, the indisputably high activity of the catalytic systems proposed in these studies forces researchers to use very low catalyst loads. Bearing in mind a great number of potential impacts (e.g., the purity of the reactants or gases), such loads may make it difficult to reproduce test results. For our study, we purposely chose a catalytic system that possessed lower activity, thus allowing us to work with lab-scale catalyst loads and safely compare the findings.
The investigation of medium-activity catalytic systems rather than highly active ones enabled us to observe subtle effects of ligand structure on catalytic activity. Our findings can further be used to develop highly active catalytic systems based on Pd(0) and 1,1,3,3-tetramethyl-1,3-divinyl disiloxane (Dvtms).
Unlike complexes of palladium(0), Pd(II)-NHC complexes are air-stable and retain their compositions and properties after years of storage. Furthermore, they allow reproducible results to be obtained because their activity is relatively independent of butadiene purity.
In particular, IPr-based palladium carbene complexes, though comparable with IMes-palladium complexes in terms of telomerization activity, are inferior in their selectivity expressed as an n/iso ratio (98 : 2 for IMes vs. 92 : 8 for IPr).
Given the lack of published data on 1,3-butadiene telomerization in the presence of (NHC)Pd(II)(η 3cinnamyl)Cl complexes with saturated carbene ligands or with extended-ring ligands, we tested the synthesized complexes in solvent-free telomerization of 1,3-butadiene with methanol ( Table 1). The reaction was carried out in a metal autoclave at 60°C; the product yields were evaluated using gas chromatography and 1 H NMR.
The table clearly shows that the palladium complexes with six-membered carbene ligands, namely 6-Dipp and 6-Mes (lines 6 and 7) provided markedly lower yields of all reaction products than the Pd complexes with fivemembered carbene ligands (lines 1-5). These data suggest that a longer carbene ring-and, hence, a higher steric load on the catalytic site-impairs the catalytic activity of the Pd complex in the telomerization of 1,3-butadiene with methanol.
The data also show that a more sterically loaded substituent adjacent to the nitrogen atom not only reduces the total product yield but also increases the fraction of 3-methoxyocta-1,7-diene 6 (lines 1 vs. 2).
We further investigated the effects of the saturation/ unsaturation of the ligand's carbene ring on the catalyst In addition, we assessed the structural effects of the leaving group on the activity of Pd-NHC complexes in 1,3-butadiene telomerization, i.e. on the product yields and on the linear (5) to isomeric (6) telomer ratio. The Pd cinnamyl carbene complex exhibited higher activity compared to the 3-chloropyridine complex (lines 2 vs. 5).
Thus, we synthesized fairly intricate N-heterocyclic palladium complexes and investigated the structural effects of various carbene ligand types on the catalytic activity of the complex in telomerization of 1,3-butadiene with methanol. In particular, saturated and unsaturated five-membered ligands and extended-ring ligands with different steric substituents were tested.
The spatial structure of the complex-specifically the length and geometry of the rings and the spatial structure of the substituents in the carbene ring-was demonstrated to make a major contribution both to the catalytic activity and selectivity in 1,3-butadiene telomerization. The highest catalytic performance was achieved by the palladium complexes with five-membered N-heterocyclic carbene ligands (i.e. those containing less sterically loaded substituents adjacent to the nitrogen atoms), with π-cinnamyl as an auxiliary ligand. This finding is important for developing a coherent understanding of the relationship between the catalytic properties of the complex and its structure. Relying on the data obtained in our study, it would be reasonable to focus further research in the 1,3-butadiene telomerization area on the synthesis and investigation of palladium complexes with N-heterocyclic carbene ligands that contain smaller