Polyhedron xxx (2014) xxx–xxx

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Polyhedron

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Palladium meditated CPhenyl–H bond activation of 2-furylimines versustert-2-furylbenzylamines

http://dx.doi.org/10.1016/j.poly.2014.03.0410277-5387/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 22 27403475.E-mail address: hongxing_wang@tju.edu.cn (H.-X. Wang).

Please cite this article in press as: Z.-X. Hu et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.03.041

Zhao-Xia Hu a, Nan Ma a, Jin-Hua Zhang a, Wen-Ping Hu b, Hong-Xing Wang a,⇑a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, Chinab Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 February 2014Accepted 22 March 2014Available online xxxx

Keywords:PalladiumCyclometalationC–H bond activationCoordinationRegioselectivity

The reactions of 2-furylimines 2a–f and Na2PdCl4 in the presence of NaOAc at 8–10 �C result in nitrogen–palladium coordinated complexes 3a–f. Reduction of 2d–f with NaBH4 followed by N-methylation leadsto the corresponding tert-2-furylbenzylamines 5a–c. Treatment of 5a–c with Na2PdCl4 at the same reac-tion condition as mentioned above affords palladacycles 6a–c where the Pd atoms connect to the phenylring rather than the furyl ring. The fact that 5a–c are more active than 2d–f in CPhenyl–H bond activationimplies that the electron density of C10 or C8 atom in former is higher than those in latter. Compounds3a–f, 5a–c, 6a–c were identified by elemental analysis, IR and NMR. In addition, the structures of 3b, 3fand 6c were also confirmed by their single crystal X-ray diffractions.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Donating group assisted C–H bond activation, e.g. cyclopallada-tion has emerged as a powerful tool in a variety of transformationsincluding C–C, C–X (X = F, Cl, Br, I, N, O, S) bond formations [1].With the rapid development of drug chemical industry, chemistsare eager to obtain some pharmaceutically active compoundswhich cannot be synthesized by convention methods. However,regioselective cyclopalladation reaction provides such an opportu-nity. Regioselective cyclopalladation has been investigated exten-sively so far for many kinds of ligands [2] particular forbenzylideneamines [3]. For an example, Martínez and co-workersreported that when benzylbenzylideneamines were treated withpalladium(II) acetate, the formation of five-membered endo palla-dacycles was dominated [3b] and the major reason was attributedto the pseudo-aromatic character of the endo palladacycles (alsocalled endo effect) [4]. The authors [3b] also found that the forma-tion of five-membered exo palladacycles only took place (A) whenendo cyclometalation required the formation of a six-memberedcompound via aliphatic C–H bond activation or when the sterichindrance affected the planarity of the imines in an importantway. Chang and co-workers [5] observed that the reactions of

bulky 2-furylimines or 2-thienylimines and Na2PdCl4 producedthe palladium coordinated complexes (B), the endo and exo palla-dacycles requiring CFuryl–H and CAliphatic–H bond activations,respectively were not isolated. Our previous studies on the cyclo-metalations of tert-ferrocenylamines revealed that the C–H bondactivation by platinum(II) always occurred at the ferrocenyl ringrather than phenyl ring [6]. Since a lot of cyclometalations are inelectrophilic substitution process, how the electron densities ofthe aromatic or aliphatic carbons affect the regioselective C–Hbond activation will become an interesting topic. In this paper,we will present here the comparative studies on the cyclopallada-tion of N-(2-furylmethylene)benzylamines and tert-N-(2-furylm-ethyl)benzylamines. In order to better elucidate theregioselectivity, the reactions of three furylimines, N-(2-furylmeth-ylene)anilines (R = 4-CH3, H, 2-Cl) and Na2PdCl4 will be also added.

CH3

H3C CH3

N

R

Pd

O O

R= H, CH3

2 N

CHX

Pd N

HC

Cl

ClX

X= N, S

A B

Cl2

Cl2

2 Z.-X. Hu et al. / Polyhedron xxx (2014) xxx–xxx

2. Results and discussion

O

N (CH2)n

Pd

ON

Pd

endo exo

RR

n=0, 1

Chart 1. The forms of palladacycles (left, from 2a–f. right, from 2d–f).

2.1. Synthesis of complexes 3a–f and palladacycles 6a–c

The reactions of furan-2-carbaldehyde 1 with anilines and ben-zylamines resulted in aldimines 2a–f in good yields (Scheme 1).Treatment of 2a–f with Na2PdCl4 at 8–10 �C in the presence ofNaOAc in methanol afforded the coordinated complexes 3a–f asyellow or orange solids. But the endo palladacycles from 2a–fand exo palladacycles from 2d–f (Chart 1) were not isolated. Later,we realized that the reaction temperature might affect the forma-tion of palladacycles, so we increased the reaction temperature to25 �C and even to 50 �C. However, the results were same asdescribed previously.

Frustrated by the results of 2 particular for 2d–f, we turn ourattention to the cyclopalladation of 5a–c (also Scheme 1). As illus-trated in Scheme 1, Ligands 5a–c were prepared from 2d–f afterreduction with NaBH4 followed by N-methylation with formalde-hyde and NaBH3CN. Furylamines 5a–c were allowed to react withNa2PdCl4 at 8–10 �C in the presence of NaOAc. After the reactionscompleted (monitored by TLC), the solvent was evaporated andthe residues were dissolved in acetone followed by addition of ace-tone solution of PPh3. Removal of acetone afforded the crude prod-ucts. After purification with chromatography, yellow solids wereisolated in moderate yields and were finally characterized as palla-dacycles 6a–c.

2.2. Characterization of 3a–f, 5a–c, and 6a–c

The structures of 3a–f, 4a–c, 5a–c and 6a–c were identified byelemental analysis, IR and NMR. In IR spectra, the C@N stretchvibration absorption of 2 appears at m � 1624 cm�1 whereas theabsorption of 3 shifts to m �1613 cm�1 indicating that theCH@N� � �Pd coordination pattern in 3 is present. The 1H NMR spec-tra of 3 clearly demonstrate that both the chemical shifts of H4 in3a–f and H5 in 3d–f locate at the downfield, and they ared � 8.50 ppm and d � 4.60 ppm, respectively. However, the IRabsorption of 6 does not show significant changes compared tothe IR spectra of 5 due to the characteristics of saturated nitrogenatoms. In 5a–c, the satellites of H5 are singlet (d � 3.70 ppm). But,they are split into a typical AB coupling system in 6a–c and shift todownfield (d � 4.35 ppm). The H4 give out a similar splitting patternas H5 demonstrated. A better explanation for this phenomenon is

O N(CH2)n

n=0, R = 4-CH3(a), H(b), 2-Cl(R = 4-CH3O(a), 4-CH3(b), H(c) n=1, R = 4-CH3O(d), 4-CH3(e)

OO

4 2

anilines or benzylam

5 R = CH3O(a), CH3(b), H(c)

..

ON

CH3

1 42 3

5

R

O

HN

R

..(1) Na2PdCl4/NaOAc, C

(2) PPh3/CH3COC

NaBH4

EtOH-H2O

1

aq. HCHONaBH3CN,

HOAcCH3CN

R

Scheme 1. Synthesis of comple

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that H4, H5 are diastereotopic because the nitrogen atom in com-pound 6 is a chiral centre. Additionally, the chemical shifts of N–CH3 in 6 move to the lower magnetic field with d � 2.90 ppm.

2.3. Crystal structures of 3b, 3f and 6c

The structures of complexes 3b, 3f and palladacycle 6cwere confirmed further by their single crystal X-ray diffractions(Figs. 1–3). The crystallographic data and refined parameters for3b, 3f and 6c are tabulated in Table 1, and their selected bondlengths and angles are listed in Table 2. X-ray diffraction studiesdemonstrates that two molecules of 3b or 3f (Figs. 1 and 2) coordi-nate to palladium(II) atom in a trans orientation. A similar transcoordination pattern between N atom and PPh3 (Fig. 3) is alsoobserved in 6c. The average N–Pd bond lengths in 3b (2.012 Å)and 3f (2.011 Å) are slight shorter than that of palladium(II) coor-dinated bulky furylimines (2.050 Å) [5]. However, the N–Pd bondlength in 6c (2.154 Å) is longer than those of 3b, 3f probablybecause of the formation of five-membered rigid palladacycle.The C–Pd bond length (2.013 Å) is longer than that of the reportedcyclopalladated amines (2.000 Å) [2a]. The N1–Pd–N2 bond anglein 3b (178.59 ) is very close to the idea 180 angle. However, theN1–Pd–N2 bond angle in 3f (173.50 ) and the N–Pd–P angle in 6c(173.75 ) are smaller than ideal 180 . In 6c, palladium(II) atom isin a slight distorted square-planar environment, bonded to Cl, P,N and CPhenyl atoms. The deviations of the atoms from the meanplane defined by the former five atoms are Pd �0.0482, Cl0.1725, P �0.1503, N �0.1852 and CPhenyl 0.2111 Å, respectively.

2.4. Regioselective cyclopalladation

As we know, the electrophilic and nucleophilic substitutionseasily take place at the a- and b-positions of the furyl ring,

c) n=0, R = 4-CH3(a), H(b), 2-Cl(c), H(f) n=1, R = 4-CH3O(d), 4-CH3(e), H(f)

3

ines

Na2PdCl4/NaOAc

8-10 oC

ON

CH3

Pd

PPh3Cl

6

8

6 R = CH3O(a), CH3(b), H(c)

1 492 3

5

7R

H3OH, 8-10 o C

H3

10

ON

(CH2)n

Pd ClCl

ON

(CH2)n

2 31 4 5

R

R810

xes 3 and palladacycles 6.

.doi.org/10.1016/j.poly.2014.03.041

Fig. 1. Crystal structure of 3b (H atoms are omitted for clarity).

Fig. 2. Crystal structure of 3f (H atoms are omitted for clarity).

Fig. 3. Crystal structure of 6c (H atoms are omitted for clarity).

Z.-X. Hu et al. / Polyhedron xxx (2014) xxx–xxx 3

respectively. This knowledge is well confirmed by the facts thatcycloplatination(II) of 3-furylimines [7] or 3-thienylimines [8]

Please cite this article in press as: Z.-X. Hu et al., Polyhedron (2014), http://dx

always occur at the a-position of the furyl ring (Chart 2). Indeed,in the reactions of 2a–f and Na2PdCl4, we have not observed theendo palladacycles (see Chart 1), demonstrating that the electro-philic cyclopalladation at C3 (b-position) of furyl ring is disfavored.Although there is another report on the cyclopalladation of fur-ancarbothioamides in which the C–H bond activation occurs atthe b-position of furyl ring [9], this example is not simply classifiedinto the category bearing the endo effect.

Since the endo products are excluded, the formation of exo pal-ladacycles from 2d–f (Chart 1) are anticipated. Our hypothesis arebased on the following factors, i.e. first, the electron density of C8or C10 atom in phenyl ring is comparatively higher than that ofC3 atom in furyl ring (Scheme 1). Second, both C8 and C10 atomsin phenyl ring are not blocked and finally, no steric hindranceaffects the planarity of the imines 2d–f. From the cyclometalationpoint of view and Martínez’ conclusion, all these factors will favorthe formation of exo palladacycles. Unfortunately, we are not suc-cessful to isolate these exo products. It is believed that complexes3d–f are so stable that they prevent the C10–H or C8–H bond acti-vation to give the exo palladacycles. The other explanation is notalso excluded, i.e. the exo palladacycles may be present in the mix-tures, but their amounts are too small to separate. Anyway, the for-mation of 3d–f demonstrates that the C10–H or C8–H bonds in 2d–fare not easily activated.

To some extent, ligands 5a–c look like 2d–f in structure, but, thenitrogen atoms in former are in the saturated state. With the expe-rience of regioselective cyclopalladation of tert-ferrocenylbenzyl-amines [6], we believe that if the cyclopalladations of 5a–c takeplace, the five-membered palladacycles via the CPhenyl–H bond acti-vation will be the most reasonable products as the CFuryl–H bondactivation is excluded due to its lower electron density of furyl b-position. As we expected, palladacycles 6a–c were successfully iso-lated (Scheme 1). However, the palladium coordinated complexessimilar to 3 have not been observed in our experiments. All theseresults are in good agreement with our earlier observation in cyclo-palladation of tert-ferrocenylbenzylamines [6]. Although 5a–c and2a–f are different, their coordination mode with palladium(II)should be same. The reason why we have not isolated the palla-dium-coordinated complexes is that they might all be transformedinto the corresponding palladacycles with the aid of NaOAc. Thesuccessful conversion of 5a–c into the palladacycles 6a–c mayimply that the electron density of C10 or C8 atom in 5a–c is higherthan those in 2d–f.

3. Conclusions

In summary, the regioselectivity of cyclometallation dependsentirely on the electron density of the ortho aromatic carbons ofthe ligands. Obviously, the ligands tert-N-(2-furylmethyl)benzyl-amines are more active in the CPhenyl–H bond activation than N-(2-furylmethylene)benzylamines.

4. Experimental

4.1. Materials and instruments

Furan-2-carbaldehyde, anilines, benzylamines, NaBH4, NaBH3

CN, NaOAc, PPh3 were obtained commercially and were used with-out further purification. Na2PdCl4 was prepared in our laboratory.All of the solvents were purified with standard methods prior touse. Melting points were obtained on a Yanaco micro melting pointapparatus and were uncorrected. Elemental analyses were mea-sured on a Carlo Erba 1106 Elemental analyzer. 1H NMR spectrawere obtained with Bruker AV-400 spectrometer (400 MHz) usingCDCl3 as the solvent and TMS as the internal standard unless they

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Table 1Crystallographic data and refined parameters for 3b, 3f and 6c.

Compd. 3b 3f 6c

Empirical formula C68H63Cl6N6O8Pd3 C24H22Cl2N2O2Pd C31H29ClNOPPdFormula weight 1624.14 547.74 604.37Wavelength(Å) 0.71073 0.71073 0.71073Crystal system, space group triclinic, P�1 monoclinic, P2(1)/n monoclinic, C2/ca (Å) 9.400(2) 9.666(2) 35.871(4)b (Å) 12.541(3) 10.026(2) 9.374(1)c (Å) 15.095(3) 23.473(5) 16.711(2)a (�) 76.37(3) 90 90b (�) 84.51(3) 92.27(3) 106.76(2)c (�) 76.93(3) 90 90V (Å3) 1682.9(6) 2273.1(8) 5380.5(2)Z 1 4 8qcalc (Mg m�3) 1.603 1.601 1.492l (mm�1) 1.090 1.075 0.873F(000) 817 1104 2464h Range for data collection 1.71�6 h 6 25.02� 1.74�6 h 6 27.87� 2.25�6 h 6 25.02�Limiting indices �9 6 h 6 11,

�11 6 k 6 14,�17 6 l 6 17

�12 6 h 6 12,�13 6 k 6 10,�30 6 l 6 30

�42 6 h 6 42,�11 6 k 6 8,�19 6 l 6 19

Reflections collected/unique 10078/5889[R(int) = 0.0326] 16691/5417[R(int) = 0.0451] 13496/4737[R(int) = 0.0274]Maximum and minimum transmission 0.9375, 0.8988 0.9583, 0.8818 0.9024, 0.8177Data/restraints/parameters 5889/129/463 5417/0/280 4737/0/326Final R1, wR2 [I > 2r(I)] 0.0453, 0.1201 0.0311, 0.0705 0.0223, 0.0548Largest difference peak and hole (e �3) 1.049, �0.801 0.691, �0.997 0.284, �0.288

Table 2Selected bond lengths and angles for 3b, 3f and 6c.

3b 3f 6c

Bond lengths (Å)Pd(1)–N(2) 2.007(3) Pd(1)–N(2) 2.0042(2) Pd(1)–C(31) 2.013(2)Pd(1)–N(1) 2.016(3) Pd(1)–N(1) 2.0179(2) Pd(1)–N(1) 2.1541(2)Pd(1)–Cl(2) 2.3024(1) Pd(1)–Cl(2) 2.3026(7) Pd(1)–P(1) 2.2613(6)Pd(1)–Cl(1) 2.3202(1) Pd(1)–Cl(1) 2.3076(7) Pd(1)–Cl(1) 2.4121(6)

Bond angles (�)N(2)–Pd(1)–N(1)

178.59(1)N(2)–Pd(1)–N(1)173.50(7)

C(31)–Pd(1)–N(1)81.86(8)

N(2)–Pd(1)–Cl(2)90.66(9)

N(2)–Pd(1)–Cl(2)90.20(6)

C(31)–Pd(1)–P(1)98.55(7)

N(1)–Pd(1)–Cl(2)89.44(9)

N(1)–Pd(1)–Cl(2)90.31(6)

N(1)–Pd(1)–P(1)173.75(5)

N(2)–Pd(1)–Cl(1)89.32(9)

N(2)–Pd(1)–Cl(1)88.55(6)

C(31)–Pd(1)–Cl(1)165.06(7)

N(1)–Pd(1)–Cl(1)90.59(9)

N(1)–Pd(1)–Cl(1)91.04(6)

N(1)–Pd(1)–Cl(1)91.06(5)

Cl(2)–Pd(1)–Cl(1)179.40(3)

Cl(2)–Pd(1)–Cl(1)178.46(2)

P(1)–Pd(1)–Cl(1)89.92(2)

C(7)–N(1)–Pd(1)122.4(2)

C(8)–N(1)–Pd(1)117.12(2)

C(1)–N(1)–Pd(1)116.1(2)

C(7)–N(1)–Pd(1)120.31(1)

C(18)–N(2)–Pd(1)126.5(2)

C(20)–N(2)–Pd(1)127.63(2)

C(12)–N(2)–Pd(1)115.6(2)

C(19)–N(2)–Pd(1)112.54(1)

OPt

N

N(CH3)2

CH3

SPt

N CH(CH3)C6H5

S(CH3)2

CH3

α

β

α

β

Chart 2. Reported cycloplatinated 3-furylimines and 3-thienylimines.

4 Z.-X. Hu et al. / Polyhedron xxx (2014) xxx–xxx

are mentioned elsewhere. IR spectra were recorded on a BIO-RAD3000 spectrophotometer, KBr pellet was used to get the IR spectra.

4.2. Synthesis of 2a–f

General method for the synthesis of 2a–f is exemplified by thesynthesis of 2b.

To a 100 mL round bottomed flask was added 25 mmol of 2-fur-aldehyde and 25 mmol aniline in 20–40 mL dry methanol. Themixture was stirred for complete dissolution at room temperatureand was monitored by TLC (ethyl acetate/petroleum ether). Thesolid was precipitated and filtered. If no solid precipitated, the mix-ture was evaporated in vacuo to afford the residue. The final pure

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aldimine 2b was obtained after the recrystallization or columnchromatography (silica gel, ethyl acetate/hexane = 1:5, v/v).

4.2.1. [a-(C4H3O)CH@NC6H4CH3-4 (2a)A light-yellow solid. Yield: 85%. M.p. 42–43 �C (Lit. [10] m.p.

41–42 �C). IR: m 3113(m), 3027(w), 2960(w), 2880(w), 1624(vs),1600(m), 1557(m), 1504(m), 1478(m), 830(s) cm�1.

4.2.2. [a-(C4H3O)CH@NC6H5 (2b)A yellow–brown solid. Yield: 70%. M.p. 57–58 �C (Lit. [11] m.p.

56–57 �C). IR: m 3113(m), 3055(w), 3021(w), 1625(vs), 1585(m),1557(m), 1472(m),766(s), 695(s) cm�1.

4.2.3. [a-(C4H3O)CH@NC6H4Cl-2 (2c)A yellow solid. Yield: 81%. M.p. 66–67 �C. Anal. Calc. for C11H8

ClNO: C, 64.25; H, 3.92; N, 6.81. Found: C, 64.11; H, 3.90; N,6.79%. IR: m 3106(w), 3062(w), 1627(vs), 1577(m), 1553(m),1462(m), 750(s) cm�1.

4.2.4. [a-(C4H3O)CH@NCH2C6H4OCH3-4 (2d) [12]A yellow liquid. Yield: 88%. Anal. Calc. for C13H13NO2: C, 72.54;

H, 6.09; N, 6.51. Found: C, 72.32; H, 6.07; N, 6.49%. IR: m 3025(w),2956(m), 2881(w), 2835(w), 1623(vs), 1583(m), 1483(m),1463(m),822(s) cm�1.

4.2.5. [a-(C4H3O)CH@NCH2C6H4CH3-4 (2e)A yellow liquid. Yield: 82%. Anal. Calc. for C13H13NO: C, 78.36; H,

6.58; N, 7.03. Found: C, 78.32; H, 6.54; N, 7.00%. IR: m 3030(w),2917(m), 2863(m), 2866(w), 1624(vs), 1564(m), 1511(m),1485(m), 1439(m), 811(s) cm�1.

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Z.-X. Hu et al. / Polyhedron xxx (2014) xxx–xxx 5

4.2.6. [a-(C4H3O)CH@NCH2C6H5 (2f) [13]An orange liquid. Yield: 80%. Anal. Calc. for C13H11NO: C, 77.81;

H, 5.99; N, 7.56. Found: C, 77.63; H, 5.97; N, 7.54%. IR: m 3030(m),2969(m), 2923(w), 2875(w), 1623(vs), 1580(s), 1484(m), 1453(m),747(s), 698(s) cm�1.

4.3. Synthesis of 3a–f

General method for the synthesis of 3a–f is exemplified by thesynthesis of 3b.

To a 50 mL round bottomed flask was added 2b (2 mmol) andsodium acetate (1.05 mmol) in methanol (20 mL). Then, with con-tinuous stirring, a methanol (17.7 mL) solution of Na2PdCl4

(1 mmol) was slowly added and completed within 30 min. Thereaction was carried out under constant stirring at 8–10 �C for20 h. The mixture was filtered and then washed with methanoland ether, respectively to afford the coordinated complex 3b.

4.3.1. Pd[Cl{a-(C4H3O)CH@NC6H4CH3-4}2 (3a)A yellow solid. Yield: 67%. M.p. >250 �C. Anal. Calc. for C24H22

Cl2N2O2Pd: C, 52.62; H, 4.05; N, 5.11. Found: C, 52.32; H, 4.03; N,5.09%. IR: m 3113(m), 3021(w), 2958(w), 2881(w), 1613(vs),1600(w), 1559(m), 1502(m), 831(s) cm�1. 1H NMR: d 8.51(s, 1H,H4), 7.32(d, 1H, H1), 7.11–7.34(t, 4H, PhH), 6.18(bs, 1H, H3),5.21(m, 1H, H2), 2.45(s, 3H, Ph–CH3) ppm.

4.3.2. Pd[Cl{a-(C4H3O)CH@NC6H5}2 (3b)An orange solid. Yield: 72%. M.p. >162 �C (dec.). Anal. Calc. for

C22H18Cl2N2O2Pd: C, 50.84; H, 3.49; N, 5.39. Found: C, 50.55; H,3.44; N, 5.37%. IR: m 3057(w), 3019(w), 1614(vs), 1581(m),1505(m), 1452(m), 767(s), 695(s)cm�1. 1H NMR: d 8.52(s, 1H,H4), 7.32(d, 1H, H1), 7.09–7.36(m, 5H, PhH), 6.21(s, 1H, H3),5.23(m, 1H, H2) ppm.

4.3.3. Pd[Cl{a-(C4H3O)CH@NC6H4Cl-2}2 (3c)A yellow solid. Yield: 80%. M.p. >230 �C (dec.). Anal. Calc. for C22

H16Cl4N2O2Pd: C, 44.89; H, 2.74; N, 4.76. Found: C, 44.61; H, 2.73;N, 4.73%. IR: m 3106(w), 3062(w), 1613(vs), 1582(m), 1501(m),1450(m), 750(s) cm�1. 1H NMR: d 8.56(s, 1H, H4), 7.52(d, 1H,H1), 7.31–7.62(m, 4H, PhH), 6.31(d, 1H, H3), 5.32(t, 1H, H2) ppm.

4.3.4. Pd[Cl{a-(C4H3O)CH@NCH2C6H4OCH3-4}2 (3d)A yellow solid. Yield: 74%. M.p. 171–172 �C. Anal. Calc. for C26

H26Cl2N2O4Pd: C, 51.38; H, 4.31; N, 4.61. Found: C, 60.55; H,4.87; N, 2.19%. IR: m 3101(w), 2952(m), 2928(w), 2834(w),1614(vs), 1602(m), 1583(m), 1512(m), 1463(m), 822(s) cm�1. 1HNMR: d 8.51(s, 1H, H4), 7.30(d, 1H, H1), 6.85–7.26(q, 4H, PhH),6.20(d, 1H, H3), 5.22(t, 1H, H2), 4.60(m, 2H, H5), 3.89(s, 3H,OCH3) ppm.

4.3.5. Pd[Cl{a-(C4H3O)CH@NCH2C6H4CH3-4}2 (3e)A yellow solid. Yield: 78%. M.p. 153–155 �C. Anal. Calc. for C26

H26Cl2N2O2Pd: C, 54.23; H, 4.55; N, 4.86. Found: C, 53.94.12; H,4.53; N, 4.85%. IR: m 3030(w), 2954(m), 2923(m), 2853(w),1615(vs), 1564(m), 1511(m), 1464(m), 1439(m), 811(s) cm�1. 1HNMR: d 8.52(s, 1H, H4), 7.29(d, 1H, H1), 6.72–7.21(q, 4H, PhH),6.21(d, 1H, H3), 5.21(t, 1H, H2), 4.61(m, 2H, H5), 2.50(s, 3H, Ph–CH3) ppm.

4.3.6. Pd[Cl{a-(C4H3O)CH@NCH2C6H5}2 (3f)A yellow solid. Yield: 76%. M.p. 148–152 �C. Anal. Calc. for C24

H22Cl2N2O2Pd: C, 52.62; H, 4.05; N, 5.11. Found: C, 52.35; H,4.02; N, 5.08%. IR: m 3035(m), 3023(m), 2905(w), 2848(w),1614(vs), 1498(m), 1468(m), 739(s), 696(s) cm�1. 1H NMR: d8.55(s, 1H, H4), 7.31(d, 1H, H1), 7.18–7.52(m, 5H, PhH), 6.23(d,1H, H3), 5.22(t, 1H, H2), 4.60(m, 2H, H5) ppm.

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4.4. Synthesis of 4a–c

General method for the synthesis of 4a–c is exemplified by thesynthesis of 4c.

To a 100 mL round bottom flask was added 15 mmol of 2f in30–40 mL dry ethanol, stirring for complete dissolution to obtaina yellow solution. Then, sodium borohydride (1.961 g, 53 mmol)was added portion wise and slowly bubbled through the reactionmixture, with magnetic stirring. After the addition was completethe reaction mixture was stirred at room temperature for 2–4 h.After completion of the reaction, 20 mL of water was added slowlyunder an ice bath, the reaction mixture was rapidly becomingorange and the sodium borohydride was continued to break downunder constant stirring. The aqueous phase was extracted threetimes with CH2Cl2 and the combined organic phase was washedwith water, saturated brine and dried (MgSO4), filtered and evapo-rated. Column chromatography gave pure 4c.

4.4.1. [a-(C4H3O)CH2NHCH2C6H4OCH3-4 (4a)An orange liquid [14]. Yield: 92%. Anal. Calc. for C13H15NO2: C,

71.87; H, 6.96; N, 6.45. Found: C, 71.87; H, 6.90; N, 6.44%. IR: m3334(w), 2920(m), 2836(w), 1612(m), 1513(m), 1489(w),1464(m), 810(s) cm�1.

4.4.2. [a-(C4H3O)CH2NHCH2C6H4CH3-4 (4b)An orange liquid. Yield: 90%. Anal. Calc. for C13H15NO: C, 77.58;

H, 7.51; N, 6.96. Found: C, 77.31; H, 7.45; N, 6.96%. IR: m 3410(w),3120(m), 2919(s), 2851(w), 1618(m), 1520(m), 1461(m), 809(s)cm�1.

4.4.3. [a-(C4H3O)CH2NHCH2C6H5 (4c)An orange liquid [15]. Yield: 89%. Anal. Calc. for C12H13NO: C,

76.98; H, 7.00; N, 7.48. Found: C, 77.00; H, 6.93; N, 7.46%. IR: m3318(w), 3079(m), 3023(w), 2959(s), 2921(m), 2859(w),2831(w), 1490(w), 1451(m), 1104(m), 1042(m), 762(s), 703(vs)cm�1.

4.5. Synthesis of 5a–c

General method for the synthesis of 5a–c is exemplified by thesynthesis of 5c.

To a stirred solution of 4c (8 mmol) in acetonitrile, sodium cya-noborohydride (1.508 g, 24 mmol) in 10 mL acetonitrile was addeda solution of formaldehyde (80 mmol) in 6.7 mL water. Then,0.80 mL of acetic acid and 16 mL of acetonitrile was added drop-wise and completed in 30 min. The mixture was stirred for 4–5 hat room temperature and added acetic acid until the solutionwas neutral. The aqueous phase was washed with potassiumhydroxide solution and extracted with dichloromethane(3 � 50 mL). The combined organic phase was washed with satu-rated brine and separated. The extracts were dried over Na2SO4

and removed to give crude products which were further purifiedby column chromatography (ethyl acetate/petroleum ether (60–90 �C, v/v = 1:3) to afford 5c.

4.5.1. [a-(C4H3O)CH2N(CH3)CH2C6H4OCH3-4 (5a)A yellow oil. Yield: 78%. Anal. Calc. for C14H17NO2: C, 72.70; H,

7.41; N, 6.06. Found: C, 72.72; H, 7.33; N, 6.05%. IR: m 3051(w),2947(w), 2903(w), 2830(m), 1602(w), 1513(m), 1447(m),1071(m), 763(s), 704(vs) cm�1. 1H NMR: d 7.39(bs, 1H, H1),6.85–7.26(t, 4H, PhH), 6.33(m, 1H, H2), 6.20(d, 1H, H3), 3.80(s,3H, OCH3), 3.56(s, 2H, H4), 3.48(s, 2H, H5), 2.22(s, 3H, N–CH3) ppm.

4.5.2. [a-(C4H3O)CH2N(CH3)CH2C6H4CH3-4 (5b)A yellow oil. Yield: 77%. Anal. Calc. for C14H17NO: C, 78.10; H,

7.96; N, 6.51. Found: C, 78.12; H, 7.89; N, 6.48%. IR: m 3045(w),

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2922(m), 2842(m), 2784(m), 1599(m), 1514(s), 1453(s), 1363(s),801(s), 731(s) cm�1. 1H NMR: d 7.37(bs, 1H, H1), 7.13–7.26(t, 4H,PhH), 6.32(m, 1H, H2), 6.18(d, 1H, H3), 3.78 (s, 2H, H4), 3.75(s,2H, H5), 2.34(s, 3H, Ph–CH3), 2.21(s, 3H, N–CH3) ppm.

4.5.3. [a-(C4H3O)CH2N(CH3)CH2C6H5 (5c)A yellow oil. Yield: 75%. Anal. Calc. for C13H15NO: C, 77.58; H,

7.51; N, 6.96. Found: C, 77.58; H, 7.45; N, 6.96%. IR: m 3058(w),2927(w), 2841 (s), 2786(m), 1600(m), 1496(m), 1453(m),1075(m), 999(m), 748(s), 695(s) cm�1. 1H NMR: d 7.35(s, 1H, H1),6.53–7.23 (m, 5H, PhH), 6.29 (s, 1H, H2), 6.12(bs, 1H, H3), 3.81(s,2H, H4), 3.77(s, 2H, H5), 2.23(s, 3H, N–CH3) ppm.

4.6. Synthesis of 6a–c

General procedure: To a stirred methanol solution (15 mL) of5a–c (1 mmol) and NaOAc (1.05 mmol) were added dropwise asolution of Na2PdCl4 (1 mmol) in 30 mL methanol. The mixturewas stirred for 20 h at 8–10 �C and then evaporated. The residuewas dissolved in acetone and a solution of acetone (PPh3, 0.78 g,3 mmol) was added. The mixture was stirred for another 1 h. Thesolvent was removed in vacuo, and the residue was purified by col-umn chromatography (silica gel, ethyl acetate/hexane = 1:3, v/v) togive 6.

4.6.1. a-[PdCl(PPh3){a-(C4H3O)CH2N(CH3)CH2C6H3OCH3-4 (6a)A yellow solid. Yield: 54%. M.p. 171–172 �C. Anal. Calc. for C32

H31ClNO2PPd: C, 60.58; H, 4.92; N, 2.21. Found: C, 60.55; H, 4.87;N, 2.19%. IR: m 3050(w), 1588(m), 1565(m), 1465(m), 1436(m),1094(m), 808(m)cm�1. 1H NMR d 7.67(bs, 1H, H1), 7.31–7.51(m,15H, PPh3), 6.95(d, 1H, H8), 6.88(s, 1H, H6), 6.81(d, 1H, H9),6.50(m, 1H, H2), 6.39(d, 1H, H3), 4.52(d, 2H, H4), 4.35(m, 2H,H5), 3.86(s, 3H, OCH3), 2.97(s, 3H, N–CH3) ppm.

4.6.2. a-[PdCl(PPh3){a-(C4H3O)CH2N(CH3)CH2C6H3CH3-4 (6b)A yellow solid. Yield: 58%. M.p. 153–155 �C. Anal. Calc. for C32

H31ClNOPPd: C, 62.15; H, 5.05; N, 2.26. Found: C, 61.91; H, 5.01;N, 2.21%. IR: m 3068(w), 2926(m), 2856(m), 1589(w), 1565(w),1459(m), 1435(m), 1095(m), 811(s) cm�1. 1H NMR: d 7.72(s, 1H,H1), 7.34–7.59(m, 15H, PPh3), 6.95(d, 1H, H8), 6.87(s, 1H, H6),6.80 (d, 1H, H9), 6.51(d, 1H, H2), 6.30(d, 1H, H3), 4.44(d, 2H, H5),4.31(q, 2H, H4), 2.93(s, 3H, N–CH3), 2.84(s, 3H, Ph–CH3) ppm.

4.6.3. a-[PdCl(PPh3){a-(C4H3O)CH2N(CH3)CH2C6H4} (6c)A yellow solid. Yield: 55%. M.p. 148–152 �C. Anal. Calc. for C31

H29ClNOPPd: C, 61.60; H, 4.84; N, 2.32. Found: C, 61.57; H, 4.82;N, 2.31%. IR: m 3067, 2922(m), 2853(m), 1641(w), 1578(w),1434(w), 738(m), 691(s)cm–1. 1H NMR: d 7.70(s, 1H, H1), 7.35–7.60(m, 15H, PPh3), 6.96(t, 1H, H7), 6.91(d, 1H, H8), 6.86(d, 1H,H6), 6.79(t, 1H, H9), 6.57(m, 1H, H2), 6.41(t, 1H, H3), 4.51(d, 2H,H4), 4.34(d, 2H, H5), 2.86(s, 3H, N–CH3) ppm.

4.7. X-ray crystallographic study of 3b, 3f and 6c

Crystals of 3b, 3f and 6c were grown at room temperature byslow evaporation of a mixture of hexane and ethyl acetate over aperiod of one week. A single crystal of these complexes wasmounted on a Bruker SMART CCD diffractometer equipped withmonochromated graphite Mo Ka (k = 0.71073 Å) radiation at ambi-ent temperature (T = 293 K) using the x–2h multi-scans technique

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for data collection. Semi-empirical absorption corrections wereapplied using the SABABS program [15]. The structure was solvedby direct methods and refined by the full-matrix least-squares pro-cedure on F2 using the SHELX suite of programs [16]. Crystallo-graphic data and refined parameters for 3b, 3f and 6c aresummarized in Table 1. Their selected bond lengths and bondangles are tabulated in Table 2.

Acknowledgements

We are grateful to Li-Ke Chemical Corporation limited (China)for the financial support for this project.

Appendix A. Supplementary data

CCDC 979319, 979320 and 979318 contain the supplementarycrystallographic data for complexes 3b, 3f and 6c, respectively.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallo-graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

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