Synthesis of Alpha Oleﬁns: Catalytic Decarbonylation of Carboxylic Acids and Vegetable Oil Deodorizer Distillate (VODD)

: Decarbonylation of carboxylic acids provides an effective protocol for producing alpha oleﬁns; however, previous literature has focused on the palladium-bisphosphine catalysts and has only sporadically studied the palladium-monophosphine catalyst. To investigate the catalytic activity of the palladium-monophosphine catalyst on decarbonylation of carboxylic acids, new monophosphine ligands were synthesized (NP-1, NP-2, CP-1 and CP-2). By employing (1–3 mol%) palladium-naphthylphosphine catalysts, various carboxylic acids were converted into corresponding alpha alkenes with good yields and selectivity within a short period of time. Vegetable oil deodorizer distillate (VODD), which is a by-product from the vegetable oil reﬁnery process, was found to be rich in free fatty acids and there is great interest in turning vegetable oil deodorizer distillate into value-added compounds. It is noteworthy that our catalytic system could be applied to convert vegetable oil deodorizer distillate (VODD) into diesel-like hydrocarbons in a good yield.

Miller [23] and Kraus [26] employed very low catalyst loading (0.01 mol% PdCl 2 (PPh 3 ) 2 ) to catalyze a decarbonylation reaction at elevated temperatures (230-250 • C). Grubbs and Stoltz [28] reported another example of low catalyst loading catalysis (0.05 mol% PdCl 2 (PPh 3 ) 2 -Xantphos) with a portion-wise addition of acetic anhydride reported. Gooβen [24] and Scott [25] catalyzed a decarbonylation reaction under milder conditions, albeit by employing a higher catalyst loading (3 mol% Pd-DPEPhos) and with the use of an expensive high-boiling point solvent (DMPU). Jensen [30] reported the use of 0.5 mol% palladium-bisphosphine precatalyst to catalyze a decarbonylation reaction (Figure 1). Despite this, palladium, in conjunction with strong coordination bisphosphine ligands (particularly biaryl ether ligands, DPEPhos, or XantPhos), were proven to be an effective catalyst in decarbonylation reactions to afford alkenes with good alpha-selectivity [38]. However, application of palladium-monophosphine catalysts remains sporadically studied. Cramer et al. investigated the palladium-catalyzed decarbonylation of biomass-derived hydrocinnamic acid to styrene [31]. Recently, Jensen et al. reported the benefit of hemilabile POP-type ligand (e.g., DPEPhos) in the deoxygenation of fatty acids reaction [39]. Their proposed mechanism involved the deliberation of phosphine ligands for provision of reaction vacant site and re-coordination of phosphine ligands for stabilization of intermediates. Inspired by computational studies by Cramer et al. and Jensen et al., we developed N-P type monophosphines with quinoline-scaffold (NP-1 and NP-2) and C-P type Buchwald biarylmonophosphines [40] with naphthalene-scaffold (CP-1 and CP-2) (Scheme 1) and their corresponding palladium complexes crystalline structure are reported herein ( Figure 2). We would like to (1) investigate any beneficial effects of hemilabile ligands towards the decarbonylation of carboxylic acids and (2) examine the feasibility of palladium-monophosphine catalyzed decarbonylation reactions.

Results and Discussion
Oleic acid-a major component present in vegetable oils, such as peanut oil (up to 71.1%) and almond oil (up to 67.2%) [41]-was chosen as a model substrate to optimize reaction conditions. By screening different Pd-sources (Table 1, entries 1-7), PdCl 2 in conjunction with NP-1 gave trace amounts of the desired product (11%) ( Table 1, entry 1). Changing the PPh 2 moiety of ligand to PCy 2 showed an adverse effect on the product yield and no decarbonylation reaction occurred (Table 1, entry 2). Employing Pd(COD)Cl 2 with ligand NP-1 (30%) yielded promising results (Table 1, entry 7). Increasing the amount of acetic anhydride did not increase the product yields (Table 1, entries 7-9) and thus two equivalents of anhydride were applied. Amines are crucial for stabilizing palladium-active species and help to enhance selectivity of the reaction [25], hence different amines were tested (Table 1, entries 10-13). Three equivalents of N,N-diisopropylethylamine (DIPEA) were found to enhance the yields of alkenes significantly (Table 1, entry 13). The C-O bonds of fatty acids are not easily broken and therefore excessive acid anhydride was needed for activation [39] (Table 1, entries 13-14).
Pd ( With the optimized reaction conditions we then examined the catalytic activity of our N-P and C-P type ligands. However the N-P type ligand (NP-1) gave inferior results (47%, Table 1, entry 13) than the C-P type ligand (CP-1) (68%, Table 1, entry 15). The crystalline structure indicated that relative strong Pd-N bond (bond length = 2.06 Å) was formed. The Pd-N bond may not favour the dissociation and re-coordination of ligands to stabilize the reaction intermediates. Meanwhile, monodentate CP-1 may offer the flexibility for dissociation of ligands in order to provide vacant sites and re-coordinate to stabilize the palladium catalyst ( Figure 2).
Thus, we employed CP-1 as a ligand and examined different anhydrides ( With promising results in hand, we then compared the catalytic activity of our developed and synthesized monophosphines to other commercially available monophosphine ligands (Table 1, entries 24-38). Firstly, ligands were found to be crucial towards the decarbonylation reaction to afford desired alkenes. Then we found that Pd(COD)Cl 2 with CP-1 gave the highest catalytic activity when compared to other commercially available ligands (Table 1, entries [30][31][32][33][34][35][36][37][38]. A lesser amount of ligand would decrease product yields significantly (3 mol% = 36%; 6 mol% = 47%; 9 mol% = 70%, Table 1, entries 28-30). Furthermore, the reaction time could be shortened to six hours without diminishing product yields (18 h: 70%; 6 h: 68%, Table 1, entries 27 and 30). With the optimized reaction conditions, we examined the catalytic activities of our developed monophosphines. Naphthyl-scaffold monophosphines were found to give superior results to the quinoyl-scaffold monophosphines (Table 1, entries [25][26][27]. We further extended our substrate scopes with the optimization reaction conditions obtained (Figure 3). Odd-numbered alkenes are valuable building blocks for various fine chemicals, but they are largely inaccessible and expensive. Even-numbered longchain fatty acids could be easily accessed from vegetable oils [41]. Therefore, it is of great interest as to whether we could convert inexpensive, even-numbered, saturated fatty acids into value-added, odd-numbered alkenes. By applying our catalytic system, various even-numbered, long-chain fatty acids could be converted into odd-numbered alkenes in good yields and selectivity (Figure 3, entries 2-4). When the catalyst loading was lowered to 1 mol% Pd, even-numbered, long-chain fatty acids could still be smoothly converted into their corresponding odd-numbered alkenes in satisfactory yields (52%-63%). In addition to the saturated, long-chain fatty acids, 5-phenylvaleric acid and 5-(4-Fluorophenyl)valeric acid were also smoothly converted into corresponding alkenes (Figure 3, entries 7 and 8). Estragole (60%) (Figure 3, entry 5) and its derivatives (65%) (Figure 3, entry 6), which served as precursors for fungicide and fragrance [44], could be readily obtained via decarbonylation of 4-(4-methoxyphenyl)butyric acid and 4-(3,4-Dimethoxyphenyl)butyric acid, respectively. It is important to note that this study gives the first report of the synthesis of allylpyrene via decarbonylation of pyrenecarboxylic acid (60%) (Figure 3, entry 9). Vegetable oil deodorizer distillate (VODD), a by-product of the vegetable oil refinery process [45,46], was found to be rich in free fatty acids. Thus, there is great interest in turning vegetable oil deodorizer distillate into value-added compounds. Extensive studies were done on the heterogenous catalytic deoxygenation of biomass-derived fatty acids to produce alkanes/alkenes [47]. Studies on the homogenous catalytic decarbonylation of vegetable oil deodorizer distillate to produce biomass-derived hydrocarbons (diesellike hydrocarbons) were rarely found. Therefore, we envisaged a probe of the industrial application of our catalytic system using vegetable oil deodorizer distillate as a model compound. Food-grade canola oil deodorizer distillate was chosen for the investigation, and its free fatty acid content was found to be 50 wt% by titration [48]. To further identify the composition of fatty acids in the canola oil deodorizer distillate, canola oil deodorizer distillate was under acid-transesterification, followed by GC analysis [49]. It was found to mainly consist of C18:1 (oleic acid, see supporting information on Table S1 for the fatty acid profile).
We then applied our catalysts to catalyze industrial canola oil deodorizer distillate as a feedstock to obtain olefins. 1 wt% Pd(COD)Cl 2 with naphthalyl-scaffold monophosphine CP-1 was employed to catalyze the decarbonylation reaction. We found that our catalysts could catalyze the decarbonylation process of canola oil deodorizer distillate smoothly to afford olefins that mainly consist of C17-alkenes in good yield (70%) (see supporting information Figure S3 for the GCMS profile) in six hours (Scheme 2). Scheme 2. Pd(COD)Cl 2 catalyzed decarbonylation of canola oil deodorizer distillate using CP-1 as ligand.

General Procedures for the Optimization of Reaction Conditions
An array of Schlenk tubes were charged with a magnetic stirrer bar (4 mm × 10 mm) and were evacuated and backfilled with nitrogen (3 cycles). The Schlenk tubes were charged with Pd sources (3 mol%) and ligands (3-9 mol%), followed by the addition of 1 mL solvent by syringe, and was stirred for 1 min. The Schlenk tubes were then added with oleic acid (0.5 mmol), anhydride sources (0.5-3 mmol) and amines (0.5-1.5 mmol). This batch of Schlenk tube was resealed and magnetically stirred in a preheated 140 • C oil bath for 6-18 h. The reactions were allowed to reach room temperature. Ethyl acetate (~4 mL) and water (~2 mL) were added. Next, an internal standard (dodecane) was added to the organic layer and was subjected to GC-FID analysis to calculate the GC yield%.

General Procedures for the Pd-Catalyzed Decarbonylation of Carboxylic Acids
An array of Schlenk tubes were charged with magnetic stirrer bar (4 mm x 10 mm) and were evacuated and backfilled with nitrogen (3 cycles). The Schlenk tubes were charged with Pd(COD)Cl2 (3 mol%) and ligand CP-1 (9 mol%), followed by the addition of 1 mL DMAc by syringe and stirred for 1 min. The Schlenk tubes were then added with carboxylic acids substrates (0.5 mmol), benzoic anhydride (1 mmol) and DIPEA (1.5 mmol). This batch of Schlenk tube was resealed and magnetically stirred in a preheated 140 • C oil bath for 6 h. The reactions were allowed to reach room temperature. Ethyl acetate (~4 mL) and water (~2 mL) were added. The organic layer was concentrated under reduced pressure and was purified by flush column chromatography.

General Procedures for the Pd-Catalyzed Decarbonylation of Canola Oil Deodorizer Distillates
A Schlenk tube was charged with a magnetic stirrer bar (4 mm x 10 mm) and was evacuated and backfilled with nitrogen (3 cycles). The Schlenk tube was charged with Pd(COD)Cl2 (0.0043 g), ligand CP-1 (0.017 g) and 1 mL DMAc was added by syringe and stirred for one minute. The Schlenk tube was then added with benzoic anhydrides (0.226 g, 1.0 mmol), DIPEA (0.36 mL, 1.5 mmol) and 0.5 mL canola oil deodorizer distillate (with 42 wt% oleic acid, equivalent to 0.74 mmol oleic acid). The Schlenk tube was resealed and magnetically stirred in a preheated 140 • C oil bath for 6 h. The reactions were allowed to reach room temperature. Ethyl acetate (~4 mL) and water (~2 mL) were added. Then, an internal standard (dodecane) was added to the organic layer and was subjected to GC-FID analysis to calculate GC yield%.

General Procedures for the Synthesis of NP Ligands
2-(2-Bromophenyl)quinoline (0.849 g, 3.0 mmol) was dissolved in freshly distilled THF (20 mL) at room temperature under a nitrogen atmosphere. The solution was cooled to −78 • C in a dry ice/acetone bath. Titrated n-BuLi (3.3 mmol) was added dropwise by syringe. After the reaction mixture was stirred for 30 min at −78 • C, chlorodiarylphosphine (0.66 mL, 3.3 mmol) in THF (5 mL) was added. The reaction was allowed to warm to room temperature and stirred overnight. Solvent was removed under reduced pressure. After the solvent was removed under vacuum, the product was successively washed with cold MeOH/EtOH mixture. The product was then dried under vacuum. (See Supplementary Materials for detail characterization of ligands).

General Procedures for the Synthesis of CP Ligands
2-(2-Bromophenyl)naphthalene (0.849 g, 3.0 mmol) was dissolved in freshly distilled THF (20 mL) at room temperature under a nitrogen atmosphere. The solution was cooled to −78 • C in a dry ice/acetone bath. Titrated n-BuLi (3.3 mmol) was added dropwise by syringe. After the reaction mixture was stirred for 30 min at −78 • C, chlorodiarylphosphine (0.66 mL, 3.3 mmol) in THF (5 mL) was added. The reaction was allowed to warm to room temperature and stirred overnight. Solvent was removed under reduced pressure. After the solvent was removed under vacuum, the product was successively washed with cold MeOH/EtOH mixture. The product was then dried under vacuum. (See Supplementary Materials for detail characterization of ligands).

Conclusions
We synthesized the naphthalyl-scaffold and quinoline-scaffold of monophosphine ligands and demonstrated that (1-3 mol%) Pd(COD)Cl 2 with naphthalyl-scaffold monophosphine CP-1 could be employed to convert various carboxylic acids into alkenes in good yields (up to 80%) and excellent alpha-selectivity (up to 98%) in six hours. It is noteworthy that our catalytic system could be applied to an industrial sample; we applied our catalyst to convert canola oil deodorizer distillate smoothly in order to afford the desired alkenes in a good yield (70%).