DOI: 10.1002/asia.200900238

Direct Oxidation of b-Aryl Substituted Aldehydes to a,b-UnsaturatedAldehydes Promoted by an o-Anisidine–Pd ACHTUNGTRENNUNG(OAc)2 Co-catalyst

Jie Liu,[a] Jin Zhu,[a] Hualiang Jiang,[a, c] Wei Wang,*[a, b, c] and Jian Li*[a]

Introduction

Interest in the use of a,b-unsaturated aldehydes has beengrowing tremendously in the recent past.[1–3] Enals have ar-guably become the most widely used substrates in theemerging field of organocatalysis.[4–8] During the past severaldecades, there have been certain advances in the prepara-tion of enals, such as the oxidation of allylic alcohols,[9] Pe-terson olefinations,[10] formylation,[11, 12] cross-metathesis re-actions of acrolein,[13] cross-aldol condensations,[14] Wittigolefination,[15] and oxidation–elimination of selenium alde-hydes.[16] Most of these methods either produce equimolaramounts of by-products and are thus not atom-economic, orsuffer from several side reactions under relatively strongbasic conditions, self-aldolization, and the narrow substrate

diversity. With the increasing demand for the enals, currentsynthetic methods must be reevaluated and optimized orsupplanted with new, direct, and more efficient strategies.

Saegusa oxidation is a convenient, efficient process forthe transformation of silyl enol silanes to corresponding a,b-unsaturated aldehydes and ketones (Scheme 1).[17,18] The dis-

tinguishing characteristics of the oxidation process includethe use of a catalytic amount of Pd ACHTUNGTRENNUNG(OAc)2 as the catalystand O2 as the oxidant. However, the method requires thepreformation of silyl enol ethers from corresponding alde-hydes and ketones.[17,18] Moreover, longer reaction time isgenerally required for aldehyde derived silyl enol ethersthan for ketone derivatives in the classic Saegusa oxidationreactions.[18] Therefore, the development of a new improvedprotocol for aldehyde related substrates is highly desired.

Toward this end, recently we have developed a newmethod for direct preparation of a,b-unsaturated aldehydesfrom readily available aldehydes. The process is co-catalyzed

[a] J. Liu, Dr. J. Zhu, Prof. Dr. H. Jiang, Prof. Dr. W. Wang,Prof. Dr. J. LiSchool of PharmacyEast China University of Science & Technology130 Meilong road, Shanghai 200237 (P.R. China)Fax: (+86) 21-64252584E-mail : jianli@ecust.edu.cn

[b] Prof. Dr. W. WangDepartment of Chemistry & Chemical BiologyUniversity of New MexicoMSC03 2060, Albuquerque, NM 87131-0001 (USA)Fax: (+1) 505-277-2609E-mail : wwang@unm.edu

[c] Prof. Dr. H. Jiang, Prof. Dr. W. WangDrug Discovery and Design CenterShanghai Institute of Materia MedicaChinese Academy of Sciences555 Zuchongzhi RoadShanghai 201203 (P.R. China)

Scheme 1. PdACHTUNGTRENNUNG(OAc)2-catalyzed (top) and (S)-diphenylprolinol/Pd ACHTUNGTRENNUNG(OAc)2

co-catalyzed Saegusa reaction (bottom).

Keywords: aldehydes · anisidine ·organocatalysis · palladium ·Saegusa reaction

Abstract: An o-anisidine-Pd ACHTUNGTRENNUNG(OAc)2 catalytic system for the direct co-catalytic Sae-gusa oxidation of b-aryl substituted aldehydes to a,b-unsaturated aldehydes hasbeen developed. The use of o-anisidine in place of (S)-diphenylprolinol made theprocess more simply and cost-effective. The process not only features the use ofunmodified aldehydes rather than enol silyl ethers, but also gives moderate togood yields (44–72 %).

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by (S)-diphenylprolinol/Pd ACHTUNGTRENNUNG(OAc)2 to give a,b-unsaturatedaldehydes in moderate to good yields (41–62 %) (Scheme 1,bottom).[19] In view of the facts that the chiral (S)-diphenyl-prolinol co-catalyst is expensive and not readily available,and the yields are relatively low, we recently sought asimple, cost-effective amine catalyst as a replacement to im-prove the process. Herein, we disclose a significantly im-proved Saegusa oxidation reaction. Notably, the simple o-anisidine was found to be an effective co-catalyst in thepresence of Pd ACHTUNGTRENNUNG(OAc)2 for a direct oxidation of unmodifiedaldehydes to enals without requiring the preformation ofsilyl enol ethers.

Results and Discussion

To identify a simple amine as a co-catalyst, we screened var-ious primary amines I–XIV instead of (S)-diphenylprolinol(20 mol%) (Table 1) under the optimal conditions—Pd-ACHTUNGTRENNUNG(OAc)2 (10 mol %) and O2 (1 atm) in DMSO 60 8C—we de-veloped earlier for the reaction of 3-phenylpropionaldehyde(1 a ; Table 1).[19] It appeared that most of the primaryamines displayed catalytic activity, but their catalytic activi-ties varied to some extent. Among them, p-bromoaniline(IX), p-anisidine (XII), and o-anisidine (XIII) demonstratedencouraging outcomes in high yields (53–54 %, Table 1, en-tries 9, 12, and 13).

We next probed the effect of solvents on the process withp-bromoaniline (IX), p-anisidine (XII), and o-anisidine(XIII) as organocatalysts (Table 2). Commonly used sol-vents, including DMSO, DME, DMF, THF, CH3CN, and tol-uene, were employed. It was found that DMSO still was thebest medium for the reaction (Table 1, entries 9, 12, and13).[19] We also probed the effect of acid additives on theprocess (Table 2, entries 16 and 17). Benzoic acid and aceticacid were not beneficial to the process. It was found thatunder the reaction conditions, no oxidized carboxylic acidfrom the corresponding aldehyde was observed.

Having established the optimal reaction conditions, wethen determined the scope of the direct Saegusa oxidationreaction using o-aniline as an organocatalyst (Table 3).Analysis of the results reveal that the aromatic rings of cin-namaldehydes bearing electron-neutral (Table 3, entry 1),electron-donating (Table 3, entries 2–6), or electron-with-drawing (Table 3, entries 7–12) substituents underwent reac-tion smoothly to afford the desired products in moderate to

Abstract in Chinese:

Table 1. Results of exploratory studies of primary amine/Pd ACHTUNGTRENNUNG(OAc)2 co-catalyzed Saegusa reaction of 3-phenylpropionaldehyde (1a) to cinnamal-dehyde (2a).[a]

Entry Cat Yield [%][b] Entry Cat Yield [%][b]

1 I 37 8 VIII 312 II 33 9 IX 533 III 33 10 X 94 IV 49 11 XI 455 V 40 12 XII 536 VI 45 13 XIII 547 VII 24 14 XIV 34

[a] Reaction conditions: see the Experimental Section. [b] Yields of iso-lated products.

Table 2. Solvent effect on the IX-, XII-, and XIII-catalyzed conversionof 3-phenylpropionaldehyde into cinnamaldehyde.[a]

Entry Cat Solvent Yield [%][b]

1 IX DME 232 IX DMF 313 IX THF 144 IX MeCN 135 IX toluene 266 XII DME 207 XII DMF 198 XII THF 209 XII MeCN 2110 XII toluene 1811 XIII DME 3312 XIII DMF 2113 XIII THF 1414 XIII MeCN 4015 XIII toluene 2716[c] XIII DMSO 5217[d] XIII DMSO 50

[a] Reaction conditions: unless specified, see the Experimental Section.DME =1,2-dimethoxyethane; DMF=N,N-dimethylformamide; THF=

tetrahydrofuran. [b] Yields of isolated products. [c] 10 mol % PhCO2Hadded for 16 h. [d] 10 mol % HOAc added for 16 h.

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good yields (44–68 %). These outcomes implied that elec-tronic features had marginal effect on the processes. Exami-nation of the results of the investigation also revealed thatthe steric effect (o- versus p- substituted pattern) alsoplayed a minimal role in governing the reaction efficiency.Remarkably, 3-(4-phenylphenyl)propionaldehyde and naph-thalene-1-propionaldehyde gave the corresponding a,b-un-saturated aldehyde in yields of 72 % and 62 %, respectively,which is in good agreement with our previous putativemechanism, namely, that the conjugation effect can more ef-ficiently facilitate the Saegusa oxidation reaction.[19] Thesame limitation is also realized for the primary amine/Pd-ACHTUNGTRENNUNG(OAc)2 co-catalyzed Saegusa oxidation reaction. Several ke-tones and esters cannot be converted into the correspondinga,b-unsaturated carbonyl compounds; nevertheless, silylenol ethers can finish these conversions.

A reaction mechanism of the primary amine/PdACHTUNGTRENNUNG(OAc)2

co-catalyzed Saegusa oxidation reaction is proposed on thebasis of the established Saegusa oxidation reaction[18] andour previous study (Scheme 2).[19] Complexation of enamine3 generated from the corresponding aldehyde with Pd-ACHTUNGTRENNUNG(OAc)2 leads to alkene/Pd complex 4. The intermediate sub-sequently converts into palladium adduct 5, a species similarto that in a classic Saegusa oxidation.[18] The iminium 6, gen-erated by b-hydride elimination of HPdOAc, is hydrolyzedto afford the enone and release the amino catalyst. Mean-while, Pd(0), decomposed from reductive elimination ofHPdOAc is subsequently oxidized by O2 to Pd(II) for thenext cycle reaction. It is reasonably argued that 3-aryl pro-pionaldehydes can more effectively facilitate the eliminationreaction through the conjugation effect, and accordingly inour studies it is observed that only such substrates can effec-tively engage in the process.

Conclusions

In conclusion, we have developed an improved amine/Pd-ACHTUNGTRENNUNG(OAc)2 co-catalytic system for the direct Saegusa oxidationreaction of unmodified aldehydes to a,b-unsaturated alde-hydes. The use of simple o-anisidine in place of (S)-diphe-nylprolinol made the process more simple and cost-effective.Moreover, the protocol, which is effective for aldehydes, iscomplementary to that of the classic Saegusa reaction withketone derived silyl enone ethers. The application of theprocess in developing new cascade reactions is currentlybeing pursued in our laboratory.

Experimental Section

Commercial reagents were used as received, unless otherwise stated.Qingdao Haiyang Chemical HG/T2354–92 silica gel was used for chroma-tography, and Huanghai silica gel plates with fluorescence F254 were usedfor thin-layer chromatography (TLC) analysis. 1H and 13C NMR spectrawere recorded on Bruker AMX-400, and tetramethylsilane (TMS) wasused as a reference. Mass spectra were recorded on a MAT-95 spectrom-eter. Melting points were tested on a melting point apparatus (SGW X-4)and are uncorrected. 3-Phenylpropanal (Table 3, entry 1) was purchasedfrom Alfa Chemical Reagent Company, and used without further purifi-cation. Literature procedures were adapted for the other 3-aryl propanals(Table 3, entries 2–14).[20]

General procedure for PdACHTUNGTRENNUNG(OAc)2 and o-anisidine (XIII) co-catalyzedconversion of 3-aryl propionaldehydes into cinnamaldehydes (Table 3): Amixture of 1 (0.45 mmol), o-anisidine (XIII) (0.09 mmol), Pd ACHTUNGTRENNUNG(OAc)2

(0.045 mmol), and DMSO (1 mL) was stirred for 2–21 h at 60 8C in an at-mosphere of oxygen. The crude product was purified by column chroma-tography on silica gel to give the desired product 2.

Cinnamaldehyde (2a) (Table 3, entry 1): 54% yield; oil ; 1H NMR(CDCl3, 400 MHz): d =6.73 (dd, 1 H, J= 7.6 and 16.0 Hz), 7.43–7.45 (m,4H), 7.49 (d, 1 H, J= 16.0 Hz), 7.56–7.59 (m, 2 H), 9.72 ppm (d, 1 H, J=

7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=128.5, 128.6, 129.1, 131.3, 134.0,152.8, 193.7 ppm; MS (EI) m/z 132 [M+], 131 (100 %); HRMS (EI) m/zcalcd C9H8O [M+]: 132.0575, found: 132.0573.

Table 3. Scope of o-aniline/Pd ACHTUNGTRENNUNG(OAc)2 co-catalyzed direct Saegusa reac-tions.[a]

Entry R Product t [h] Yield [%][b]

1 C6H5 2a 16 542 2-MeC6H4 2b 21 543 2-MeOC6H4 2c 19 454 3-MeC6H4 2d 16 585 4-MeC6H4 2e 2 586 4-EtC6H4 2 f 13 627 4-MeCOC6H4 2g 2 448 4-MeOCOC6H4 2h 10 479 4-CNC6H4 2 i 12 5510 4-NO2C6H4 2j 18 5611 2,4-Cl2C6H3 2k 9 5112 2-Me-4-NO2C6H3 2 l 11 6813 1-biphenyl 2m 13 7214 1-naphthyl 2n 13 62

[a] Reaction conditions: see the Experimental Section. [b] Yields of iso-lated products.

Scheme 2. Proposed reaction mechanism for amine/Pd ACHTUNGTRENNUNG(OAc)2 co-cata-lyzed conversion of 3-substitued propionaldehydes to 3-substitued acro-leins.

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2-Methylcinnamaldehyde (2b) (Table 3, entry 2): 54 % yield; oil ;1H NMR (CDCl3, 400 MHz): d=2.41 (s, 3 H), 6.73 (dd, 1H, J= 7.6 and16.0 Hz), 7.27 (m, 1 H), 7.32–7.36 (m, 1 H), 7.39–7.40 (m, 2H), 7.46 (d,1H, J =16.0 Hz), 9.71 ppm (d, 1H, J= 7.6 Hz); 13C NMR (CDCl3,400 MHz): d=21.3, 125.7, 128.5, 129.0, 129.1, 132.1, 134.0, 138.8, 153.0,193.7 ppm; MS (EI) m/z 146 [M+], 131 (100 %); HRMS (EI) m/z calcdC10H10O [M+]: 146.0732, found: 146.0733.

2-Methoxycinnamaldehyde (2c) (Table 3, entry 3): 45 % yield; m.p. 41–43 8C (lit. m.p. 41–42 8C);[21] 1H NMR (CDCl3, 400 MHz): d=3.94 (s, 3H),6.82 (dd, 1H, J=7.6 and 16.0 Hz), 7.02 (m, 2 H), 7.42–7.46 (m, 1H), 7.57(m, 1H), 7.86 (d, 1 H, J=16.0 Hz), 9.71 ppm (d, 1H, J =8.0 Hz);13C NMR (CDCl3, 400 MHz): d= 55.6, 110.3, 120.9, 123.0, 128.9, 129.1,132.7, 148.2, 158.3, 194.6 ppm; MS (EI) m/z 162 [M+], 131 (100 %);HRMS (EI) m/z calcd C10H10O2 [M+]: 162.0681, found: 162.0683.

3-Methylcinnamaldehyde (2d) (Table 3, entry 4): 58 % yield; oil ;1H NMR (CDCl3, 400 MHz): d=2.50 ACHTUNGTRENNUNG(s, 3H), 6.69 (dd, 1 H, J =7.6 and16.0 Hz), 7.25–7.29 ACHTUNGTRENNUNG(m, 2 H), 7.34 (m, 1 H), 7.61 (d, 1H), 7.79 ppm (d, 1 H,J =15.6 Hz); 13C NMR (CDCl3, 400 MHz): d=24.4 130.8, 131.3, 131.5,134.2, 135.8, 137.5, 142.7,155.0,198.4 ppm; MS (EI) m/z 146 [M+], 131-ACHTUNGTRENNUNG(100 %); HRMS (EI) m/z calcd C10H10O [M+]: 146.0732, found:146.0728.

4-Methylcinnamaldehyde (2e) (Table 3, entry 5): 58% yield; m.p. 46–48 8C (lit. m.p. 42–43 8C);[21] 1H NMR (CDCl3, 400 MHz): d =2.41 ACHTUNGTRENNUNG(s, 3 H),6.70 (dd, 1 H, J =7.6 and 16.0 Hz), 7.26 ACHTUNGTRENNUNG(m, 1 H), 7.45–7.49 (m, 3H),9.70 ppm (d, 1H, J =7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=21.6,127.7, 128.5, 129.9, 131.4, 142.0, 152.9, 193.7 ppm; MS (EI) m/z 146 [M+],131 ACHTUNGTRENNUNG(100 %); HRMS (EI) m/z calcd C10H10O [M+]: 146.0732, found:146.0735.

4-Ethylcinnamaldehyde (2f) (Table 3, entry 6): 62% yield; oil ; 1H NMR(CDCl3, 400 MHz): d=1.27(t, 3H, J =7.6 Hz), 2.71 (q, 2 H, J =7.6 Hz)6.71 (dd, 1H, J=7.6 and 16.0 Hz), 7.28 (d, 2H, J =7.2 Hz), 7.46–7.52 (m,3H), 9.71 ppm (d, 1H, J =8.0 Hz); 13C NMR (CDCl3, 400 MHz): d=15.2,28.9, 127.8, 128.7, 131.6, 148.3, 153.0, 193.8 ppm; MS (EI) m/z 160 [M+],131 ACHTUNGTRENNUNG(100 %); HRMS (EI) m/z calcd C11H12O [M+]: 160.0888, found:160.0891.

4-Acetylcinnamaldehyde (2 g) (Table 3, entry 7): 44% yield; m.p. 49–51 8C (lit. m.p. 49–50 8C);[21] 1H NMR (CDCl3, 400 MHz): d=2.65 (s, 3H),6.80 (dd, 1H, J =7.6 and 16.0 Hz), 7.52 (d, 1H, J =16.0 Hz), 7.68 (d, 2H,J =8.4 Hz), 8.03 (d, 2H, J= 8.4 Hz), 9.77 ppm (d, 1 H, J =7.6 Hz);13C NMR (CDCl3, 400 MHz): d= 26.7, 128.5, 128.9, 130.4, 138.1, 138.6,150.7, 193.3, 197.2 ppm; MS (EI) m/z 174 ([M+], 100 %); HRMS (EI)m/z calcd C11H10O2 [M+]: 174.0681, found: 174.0683.

4-Methoxycarbonylcinnamaldehyde (2h) (Table 3, entry 8): 47 % yield;m.p. 102–104 8C (lit. m.p. 190 8C);[22] 1H NMR (CDCl3, 400 MHz): d =3.96(s, 3H), 6.80 (dd, 1 H, J =8.0 and 16.0 Hz), 7.52 (d, 1H, J =16.0 Hz), 7.65(d, 2H, J= 8.4 Hz), 8.11 (d, 2H, J= 8.4 Hz), 9.76 ppm (d, 1H, J =7.6 Hz);13C NMR (CDCl3, 400 MHz): d= 52.2, 128.1, 130.1, 130.2, 132.0, 137.9,150.7, 166.1, 193.2 ppm; MS (EI) m/z 190 [M+], 131 (100 %); HRMS(EI) m/z calcd C11H10O3 [M+]: 190.0630, found: 190.0629.

4-Cyanocinnamaldehyde (2i) (Table 3, entry 9): 55 % yield; m.p. 133–135 8C (lit. m.p. 128–129 8C);[21] 1H NMR (CDCl3, 400 MHz): d=6.79 (dd,1H, J =7.6 and 16.0 Hz), 7.49 (d, 1 H, J=16.4 Hz), 7.68 (d, 2 H, J=

8.0 Hz), 7.74 (d, 2H, J= 8.0 Hz), 9.78 ppm(d, 1H, J =7.2 Hz); 13C NMR(CDCl3, 400 MHz): d=114.3, 118.1, 128.7, 131.2, 132.8, 138.2, 149.4,192.9 ppm; MS (EI) m/z 157 [M+], 156 (100 %); HRMS (EI) m/z calcdC10H7NO [M+]: 157.0528, found: 157.0522.

4-Nitrocinnamaldehyde (2j) (Table 3, entry 10): 56% yield; m.p. 140–142 8C (lit. m.p. 139–140 8C);[21] 1H NMR (CDCl3, 400 MHz): d=6.82 (dd,1H, J =7.6 and 16.0 Hz), 7.54 (d, 1 H, J=16.0 Hz), 7.75 (d, 2 H, J=

8.8 Hz), 8.31 (d, 2 H, J =8.8 Hz), 9.80 ppm (d, 1H, J=7.6 Hz); 13C NMR(CDCl3, 400 MHz): d=124.3, 129.1, 131.8, 140.0, 148.8, 192.8 ppm; MS(EI) m/z 177 [M+], 160 (100 %); HRMS (EI) m/z calcd C9H7NO3 [M+]:177.0426, found: 177.0427.

2,4-Dichlorocinnamaldehyde (2k) (Table 3, entry 11): 51 % yield; m.p.105–107 8C (lit. m.p. 106–108 8C);[23] 1H NMR (CDCl3, 400 MHz): d=6.68(dd, 1 H, J =7.6 and 16.0 Hz), 7.31 (d, 1H, J=8.4 Hz), 7.48 (s, 1H), 7.59(d, 1H, J =8.4 Hz), 7.85 (d, 1H, J =16.0 Hz), 9.76 ppm (d, 1H, J=

7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=127.9, 128.6, 130.2, 130.7, 130.8,135.6, 137.4, 148.5, 193.7 ppm; MS (EI) m/z 200 [M+], 165 (100 %);HRMS (EI) m/z calcd C9H6OCl2 [M+]: 199.9796, found: 199.9803.

2-Methyl-4-nitrocinnamaldehyde (2l) (Table 3, entry 12): 68 % yield; m.p.106–108 8C; 1H NMR (CDCl3, 400 MHz): d= 2.60 (s, 3H), 6.76(dd, 1 H,J =7.2 and 15.6 Hz), 7.72–7.78 (m, 2H), 8.11–8.14 (m, 2H,), 9.81 ppm (d,1H, J =7.6 Hz); 13C NMR (CDCl3, 400 MHz): d =19.9, 121.6, 125.7,127.7, 132.6, 139.1, 139.2, 146.8, 148.6, 192.9 ppm; MS (EI) m/z 191 [M+],176 (100 %); HRMS (EI) m/z calcd C10H9NO3 [M+]: 191.0582, found:191.0583.

4-Phenylcinnamaldehyde (2m) (Table 2, entry 13): 72% yield; m.p. 122–124 8C (lit. m.p. 121–122 8C);[21] 1H NMR (CDCl3, 400 MHz): d=6.78 (dd,1H, J=7.6 and 16.0 Hz), 7.39–7.55 (m, 4 H), 7.63–7.70 (m, 6H), 9.75 ppm(d, 1 H, J =8.0 Hz); 13C NMR (CDCl3, 400 MHz): d=127.1, 127.7, 128.1,128.5, 129.0, 133.0, 139.9, 144.0, 152.2, 193.6 ppm; MS (EI) m/z 208 ([M+

], 100 %); HRMS (EI) m/z calcd C15H12O [M+]: 208.0888, found:208.0890.

(E)-3-(naphthalen-1-yl)acrylaldehyde (2n) (Table 3, entry 14): 62% yield;m.p. 122–124 8C (lit. mp 123–124 8C);[24] 1H NMR (CDCl3, 400 MHz): d=

5.85 (dd, 1H, J =7.6 and 15.6 Hz), 6.54–6.65 (m, 3H), 6.91–7.03 (m,3H),7.32 (d, 1H),7.52 (d,1 H, J =15.6 Hz), 8.88 ppm (d,1 H, J =7.6 Hz);13C NMR (CDCl3,400 MHz): d=123.2, 125.8, 125.8, 126.6, 127.4, 129.0,130.8, 131.0, 131.2, 131.6,133.7, 149.5, 194.2 ppm; MS (EI) m/z 182 [M+],181 (100 %); HRMS (EI) m/z calcd C13H10O [M+]: 182.0732, found:182.0729.

Acknowledgements

We gratefully acknowledge the financial support from the National Natu-ral Science Foundation of China (Grant 90813005), the 863 Hi-Tech Pro-gram of China (Grant 2006AA020404), the China 111 Project (GrantB07023), and Shanghai Rising-Star Program (A type) of the ShanghaiMinistry of Science and Technology (Grant 07A14013).

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Received: July 2, 2009Revised: July 24, 2009

Published online: September 11, 2009

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