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Letter
A Phosphonium Ylide as an Ionic Nucleophilic Catalyst for Primary Hydroxyl Group Selective Acylation of Diols Yasunori Toda, Tomoyuki Sakamoto, Yutaka Komiyama, Ayaka Kikuchi, and Hiroyuki Suga ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02281 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017
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ACS Catalysis
A Phosphonium Ylide as an Ionic Nucleophilic Catalyst for Primary Hydroxyl Group Selective Acylation of Diols Yasunori Toda,* Tomoyuki Sakamoto, Yutaka Komiyama, Ayaka Kikuchi, and Hiroyuki Suga* Department of Materials Chemistry, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ABSTRACT: Carbonyl-stabilized phosphonium ylides exhibit great utility in modern organic synthesis, and are known as an ambident nucleophile at the carbonyl oxygen atom and at the α-carbon atom. However, they have found limited use as catalysts. We focused on the inherent nucleophilicity of the oxygen atom to develop an ionic nucleophilic catalysis, and the phosphonium ylidecatalyzed primary alcohol selective acylation of mixed diols with acid anhydrides has been demonstrated. Mechanistic studies revealed the behavior of a catalyst, which would contribute to the field of ylide chemistry. KEYWORDS: phosphonium ylides, nucleophilic catalysts, organocatalysis, acylation, diols
Phosphonium (P-) ylides – the cationic site is composed of a phosphonium ion and the negatively charged atom directly attached to the phosphorus center – are a specific type of zwitterion, and are commonly used or invoked as a key intermediate in the Wittig reaction.1 Taking account of the nucleophilicity of the anionic carbon, versatile transformations have been accomplished.2-4 Despite being attractive chemical species, however, their catalytic ability has remained elusive until today presumably due to difficulty in regeneration of the Pylide.4,5 Hence the development of P-ylide catalysis is of formidable challenge and can open a new frontier for the field of ylide chemistry. Carbonyl-stabilized P-ylides, playing an important role in modern organic synthesis,2d-e,3a-e,4d-f are known as ambident nucleophiles.6 Recently, Byrne and Mayr et al. reported on the mechanism of the reaction of formyl- and acetyl-stabilized Pylides.7 On the basis of their study, O-attack by the ylides toward carbon electrophiles, e.g. benzhydrylium ions, alkyl halides, acyl chlorides, is intrinsically and kinetically favored. It has also been shown that the C-acetylated, thermodynamically-favored, product was obtained exclusively in the reaction with acetic anhydride. Therefore, it could be feasible to establish nucleophilic catalysis by the stabilized ylide if O-acylated intermediate is allowed to undergo acyl-transfer to external nucleophiles. For this inquiry, we have designed an ionic organocatalyst 1 having a novel structural motif (Figure 1). The aryl group introduced into the ylide carbon moiety is expected to prevent undesired C-acylation, because O-acylation provides not only a kinetically stable product but also a thermodynamically stable product owing to the aromatic stabilization.8 The carboxylate ion derived from acid anhydride, in turn, could be positioned proximal to the phosphonium ion, which would guide the nucleophile towards the carbonyl group of O-acylated intermediate.9 Thus, we reasoned that acylation of alcohols might be a suitable transformation for identifying a behavior of 1 as an ionic nucleophilic catalyst.10 As often encountered in organic synthesis, selective acyl protection of a primary hydroxyl group in the presence of secondary hydroxyl group(s) exhibits great utility. In addition to conventional reagent-control methods,11 a number of catalyst-
Figure 1. Working hypothesis: The design and synthesis of a phosphonium ylide as an ionic nucleophilic catalyst for acyl transfer reactions.
control methods, especially by Lewis acidic metals, have been developed to date.12 By contrast, few organic small molecules have used for this purpose,13 although 4dimethylaminopyridine (DMAP) is a very effective catalyst for alcohol acylation with acid anhydrides.9a,14 This communication describes the exploration of a phosphonium ylide catalyst that can achieve the primary alcohol selective acylation of primary-secondary diols with high conversion using commercially available acid anhydrides.
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First, we synthesized P-ylide 1a (Figure 1, G1 = H, G2 = Me) from its hydrobromic acid salt 1a·HBr.15 It can be assumed that dehydrohalogenation of phosphonium salts would provide the corresponding ylides, and treatment of 1a·HBr with sodium hydroxide in methanol and trituration with dichloromethane/hexane afforded 1a as a stable solid. The 3D-structure of 1a was confirmed by X-ray crystallographic analysis (Figure 2a). The observed P-C1 bond length [1.764(1) Å] is slightly longer than that of typical unstabilized phosphonium ylides,16a and the observed O-C2 bond length [1.284(2) Å] is similar to that of carbonylstabilized yildes.16b In addition, the length of C1-C2 bond [1.431(2) Å] is between a phenyl ring (1.40 Å) and a normal Csp2-Csp2 single bond (1.46 Å).16c These bond lengths and the torsion angle of P-C1-C2-O [0.2(2)°] suggested electron delocalization within the ylide system. Next, DFT calculations were performed at the B3LYP/6-311++G** level of theory. The optimized structure of 1a obtained is well reflected in the crystalline state, and thus the molecular electrostatic potential was also calculated as a simple indicator of the nucleophilic site of the molecule. As shown in Figure 2b, the negative potential is concentrated at the oxygen atom, implying that Oattack of 1 might predominate. The crystallographic and theoretical analyses gave impetus to the investigation of the ylide nucleophilic catalysis further. At the outset of our studies, we conducted 1H NMR experiments to ascertain whether the nucleophilic activation of acid anhydride by P-ylide 1a was possible and also whether the acylated alcohol could be obtained. The experiments were performed using a solution of 1a in CDCl3 at room temperature by adding 2.5-15 equivalents of isobutyric anhydride (Figure 3). When ylide 1a was mixed with 2.5 equivalents of acid anhydride, new signals (shown by arrows) appeared and a signal of 1a significantly shifted downfield, indicating generation of O-acylated intermediate A in equilibrium (ca. 30% conversion based on the integration of 1a).17 After the addition of excess amount of the anhydride (15 equiv), the bright yellow solution turned pale yellow and more than 90% of 1a was converted to the newly observed species. Thus we attempted high-resolution mass analysis from the NMR sample solution, and the ESI positive measurement produced a peak at m/z = 439.1832, which corresponds to a phosphonium cation of A. To our delight, treatment of this solution of A with one equivalent of 1-heptanol and triethylamine yielded isobutyrate 2 in 98% after 24 hours, whereas only half of the alcohol was acylated in the absence of 1a (Scheme 1a). It should be noted
Figure 2. (a) X-ray crystal structure of 1a (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and torsion angles (°): P–C1 1.764(1), O-C2 1.284(2); C1-C2 1.431(2); P-C1-C2-O 0.2(2). (b) Electrostatic potential map (charge) of 1a on the 0.002 au isodensity surface. The potential is calculated at the B3LYP/6311++G** level of theory and shown with identical chromatic scales (red is more negative; blue is more positive).
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Figure 3. Changes in the 1H NMR spectrum (300 MHz, CDCl3) of ylide 1a upon addition of isobutyric anhydride in the range of 6.2-8.4 ppm. From bottom to top, the spectra of 1a in the presence of 0.0 equiv (green), 2.5 equiv (blue), 5.0 equiv (purple), 10 equiv (pink), and 15 equiv (red) of acid anhydride are depicted. The signals shown by the orange arrows correspond to the aromatic protons of A.
Scheme 1. 1H NMR experiments for the development of phosphonium ylide catalysis
that the ylide regeneration could not be observed during the course of the reaction probably owing to the rapid formation of A. This was supported by the competitive reaction between ylide 1a and 1-heptanol with isobutyric anhydride shown in Scheme 1b, where acylation of 1a proceeds to generate A much faster than esterification to 2 (Scheme 1b). Following results of the feasibility study, we investigated the selectivity in the acylation of a 1:1 mixture of primary and secondary alcohols with isobutyric anhydride (Table 1). Control experiments revealed that the combined use of 1a and an auxiliary base was important to achieve better conversion with higher selectivity (Table 1, Entries 1-4). DMAP afforded a considerable amount of secondary alcohol-acylated product, while the reactivity was satisfactory (Table 1, Entries 5 and 6). Similar tendency was also observed in less polar solvents, such as hexane and toluene (Table 1, Entries 7-12). Lastly, the use of 1.5 equivalents of acid anhydride slightly improved both the yield and selectivity (Table 1, Entry 13). Moreover, in situ generated ylide can be applicable to this system, leading to a similar result when 1a·HBr was employed as a precursor of ylide 1a (Table 1, Entry 14).18 The initial scope of diols 4 is summarized in Table 2. To minimize an acyl group migration between primary and secondary hydroxyl groups, two reactions were run side-by-side under the same conditions, one of which was used for monitoring a reaction progress by 1H NMR (see Supporting Infor-
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Table 1. Optimization of phosphonium ylide-catalyzed selective acylation of 1-heptanola
(a)
1a (10 mol %) (iPrCO)2O (1.5 equiv)
HO OH
Ph
w/o Et3N toluene (0.8 M), rt, 24 h
4a
HO OC(O)iPr
Ph
+ 6a + 7a
5a: 81% 95% conv. 5a:6a:7a = 85:11:4
(b) 4a
3 (%)b
selectivity (2:3)b
entry
catalyst
Et3N
solvent
2 (%)b
1 2
1a 1a
yes no
CDCl3 CDCl3
79 45
4 5
95:5 90:10
3 4
-
yes no
CDCl3 CDCl3
39 32
5 7
89:11 82:18
5
DMAP
yes
CDCl3
89
25
77:23
6c 7
DMAP 1a
yes yes
CDCl3 hexane
74 79
20 5
79:21 94:6
8 9c
DMAP
yes yes
hexane hexane
46 71
10 24
82:18 75:25
10 11
1a -
yes yes
toluene toluene
82 38
3 3
96:4 93:7
12c
DMAP
yes
toluene
72
22
77:23
13d 14d
1a 1a·HBr
yes yes
toluene toluene
90 90
4 5
96:4 95:5
a Unless otherwise noted, all reactions were carried out using 0.20 mmol of 1-heptanol, 0.20 mmol of 1-methylheptanol, and 0.24 mmol of isobutyric anhydride. bDetermined by 1H NMR analysis. cReaction time for 15 min. d1.5 equivalents of acid anhydride were used.
Table 2. Scope of diolsa
a
Unless otherwise noted, all reactions were carried out on a 0.40 mmol scale using 4, 10 mol % of catalyst (1a, 1a·HBr, or 1c), 1.5 equiv of isobutyric anhydride, and 2.0 equiv of triethylamine in toluene (0.5 mL) at room temperature. On the basis of the reaction monitoring experiment by 1H NMR, the selectivity (5:6) was shown in parentheses. bA trace amount of 6 was contaminating. cNMR yield of 5. dCombined yield of 5 and 6 (95:5 mixture).
1a (10 mol %) Ac2O (1.5 equiv)
HO
w/o Et3N toluene (0.8 M), rt, 24 h
(c)
OH
OH
BnO BnO
O
4h OMe
AcO OAc +
Ph
AcO OH +
Ph
8a: 83%
OAc
Ph
9
10
98% conv. 8:9:10 = 86:5:9 (cf. w/ Et3N; >99% conv. 8:9:10 = 46:9:45) OH
1a (10 mol %) Ac2O (1.5 equiv)
BnO
toluene (0.8 M) rt, 24 h
BnO
OAc
O
8b: 93% OMe
Scheme 2. Phosphonium ylide-catalyzed acylation with acid anhydrides under base-free conditions
the selectivity of acylation and worked up the other reaction for the best period of isolation. The ratio of 5 and 6 at the period of quenching those reactions can be estimated from the NMR tracking (Table 2, shown in parentheses), and primary monoesters 5a-h were isolated as sole products except for the case of 5i. 1,2- and 1,3-Diols 4a-d afforded the corresponding products with high selectivity (>95:5) under either condition A or B. It should be emphasized that 1,2-diols 4a and 4b underwent highly selective acylation (e.g. 97:3 for 5a),19 since moderate selectivity (e.g. 74:26 for 5a) was observed in the reported organocatalytic system using imidazobenzothiazoles.13c Although 1,4- and 1,5-diols 4e-g were challenging, the use of catalyst 1b (Figure 1, G1 = G2 = Me) elegantly enhanced the selectivity. Carbohydrates are also a tempting target, and glucose and xylose derivatives 4h and 4i were tolerated to furnish 5h and 5i in a highly selective fashion, respectively. Furthermore, base-free conditions were examined since DMAP has been found to promote acylation of alcohols with acid anhydrides even in the absence of an auxiliary base.13f As shown in Scheme 2a, base-free acylation has been realized while the selectivity marginally dropped. Notably, primary alcohol acetylation of 1,2-diol 4a by P-ylide catalysis also proved successful, affording 8a in 83% isolated yield (Scheme 2b). In contrast, a considerable amount of 1,2-bis-acetate 10 was formed with employment of triethylamine, which presents the advantage of the reactions without bases. Conversion of 4h to mono-acetate 8b was accomplished over 90% isolated yield (Scheme 2c). In summary, we have demonstrated phosphonium ylidecatalyzed primary hydroxyl group selective acylation of diols. This methodology is applicable to mixed 1,n-diols, affording mono-acylated product in good to high yields. A novel design of carbonyl-stabilized ylide 1, tethering the ylide carbon and carbonyl α-carbon with aromatic system, prefers O-acylation owing to kinetic and thermodynamic controls. In addition, the unique behavior of 1 has been identified by mechanistic studies. Efforts are currently underway to extend phosphonium ylide catalysis.
ASSOCIATED CONTENT Supporting Information
mation for details). On the basis of the results, we determined
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Experimental procedures (PDF), spectroscopic data for all new compounds (PDF), and crystallographic data for 1a (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected] (Y.T.) *E-mail:
[email protected] (H.S.)
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was partially supported by the Japan Society for the Promotion of Science (JSPS) through a Grant-in-Aid for Research Activity Start-up (Grant No. JP15H06242). We gratefully acknowledge Dr. Kenji Yoza (Bruker AXS) for X-ray analysis. We also thank Nippon Shokubai Award in Synthetic Organic Chemistry, Japan (Y.T.).
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Lin, S.; Oliver, P. A.; Peterson, M. J. J. Org. Chem. 1993, 58, 7286– 7288. (15) We have recently reported 1a·HBr as a catalyst for carbon dioxide fixation, see: Toda, Y.; Komiyama, Y.; Kikuchi, A.; Suga, H. ACS Catal. 2016, 6, 6906–6910. A modified procedure for the preparation of 1a·HBr enables the purification without column chromatography. See Supporting Information for details. (16) (a) Gilheany, D. G. Chem. Rev. 1994, 94, 1339–1374. (b) Kayser, M. M.; Hatt, K. L.; Hooper, D. L. Can. J. Chem. 1991, 69, 1929– 1939. (c) Pauling, L. The nature of the chemical bond. 3rd ed.; Cornell University Press: Ithaca, New York, 1960. (17) Variable-Temperature 31P NMR experiments disclosed the equilibrium between ylide 1a and intermediate A, in which the intermediate can revert back to reactant 1a. The reaction of 1a and 2.5 equivalents of isobutyric anhydride in CDCl3 gave a 2:1 mixture of 1a and A at 30 °C, and the ratio changed to 4:1 at 50 °C by rising temperature. See Supporting Information for details. (18) To assess the catalyst design on the reactivity and selectivity, phosphonium salts with m-OH or p-OH, and an ammonium salt were examined, and 1a·HBr gave the best result. See Supporting Information for details. (19) DMAP-catalyzed acylation of 4a afforded 5a in 60%, 6a in 6%, and 7a in 28% yield within 15 min. Additionally, less than 60% conversion was observed in the absence of a catalyst (5a:6a = 90:10).
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