Convenient ultrasound-mediated synthesis of 1,4-diazabutadienes under solvent-free conditions

Ultrasonics Sonochemistry 18 (2011) 466–469

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch

Convenient ultrasound-mediated synthesis of 1,4-diazabutadienes under solvent-free conditions Jing-Yu He, Hong-Xing Xin, Hong Yan *, Xiu-Qing Song, Ru-Gang Zhong College of Life Science and Bio-engineering, Beijing University of Technology, Pingleyuan Street No. 100, Chaoyang District, Beijing 100124, PR China

a r t i c l e

i n f o

Article history: Received 31 May 2010 Received in revised form 28 July 2010 Accepted 1 August 2010 Available online 7 August 2010 Keywords: 1,4-Diazabutadienes Ultrasonic irradiation Solvent-free synthesis

a b s t r a c t An ultrasound-assisted preparation of 1,4-diazabutadienes via smooth condensation of diketones with amines under solvent-free conditions is described. The generality of this method was examined by the synthesized N,N0 -diaryl- and N,N0 -dialkyl-1,4-diazabutadiene derivatives. In addition to experimental simplicity, the main advantages of the procedure are mild conditions, short reaction time (2–15 min) and high yields (71–98%). Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction 1,4-Diazabutadienes are of particular importance in fine chemical due to their applications as liquid crystalline and mesoporous material [1], especially as precursors for many transition metal complexes that have been used as catalysts in olefin polymerization, olefin epoxidation, and other useful protocols [2]. The traditional method for 1,4-diazabutadiene synthesis entails the condensation of an a-diketone and an amine in either organic solvents or in aqueous solutions [3]. However, due to the harsh reaction conditions, long reaction time, use of organic solvents and high temperature required, this classic reaction presents some disadvantages. Thus, the development of a more efficient method for the synthesis of 1,4-diazabutadiene is highly desirable. In recent decades, ultrasonic irradiation has been widely applied to various types of organic transformations in chemical literature [4]. Ultrasound has been described as a method that accelerate the chemical reactions proceed via the formation and adiabatic collapse of transient cavitation bubbles. Compared with conventional methods, many organic reactions could be carried out under ultrasound irradiation to give higher yields, shorter reaction times and milder conditions [5]. Moreover, solvent-free organic syntheses have received considerable attention because they are operationally simple, often involve nontoxic materials, and proceed in excellent yield with high selectivity [6]. In view of our general interest in the development of clean chemical processes [7], we report a novel and environmentally safe procedure for the preparation of

* Corresponding author. Tel.: +86 010 67396642; fax: +86 010 67392001. E-mail address: [email protected] (H. Yan). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.08.002

1,4-diazabutadienes without solvent under ultrasound irradiation (Scheme 1). 2. Experimental 2.1. Chemicals and apparatus The chemical used in this work were purchased from commercial sources without further purification. Melting points (uncorrected) were determined by X-5 instrument. IR spectra were performed as liquid films or KBr pellets on a Bruker VERTEX 70 spectrometer. NMR spectra were measured on a Bruker AVANCE 400 spectrometer. MS spectra were recorded on a ZAB-HS and ESQUIRE 6000 mass spectrometer. Elemental analyses were carried out on a CARLO ERBA 1106 elemental analysis instrument. Ultrasonication was performed in a GEX750-5C ultrasonic processor equipped with a 3 mm wide and 140 mm long probe that was immersed directly into the reaction mixture. The operating frequency was 24 kHz and the output power was 0–750 W through manual adjustment. The temperature was controlled by a Buchi B-491 water bath at 25 ± 1 °C. 2.2. General procedure for synthesis of 1,4-diazabutadienes A mixture of the amine (6.0 mmol) and glyoxal trimer dihydrate (1.0 mmol, 210 mg) or 2,3-butanedione (3.0 mmol, 258 mg) was irradiated under an ultrasonic processor at 25 ± 1 °C and 250 W. After the indicated time, the solid 1a–1g, 1l, 2a–2i and 2l were washed or recrystallized with ethanol. The liquid 1k, 2j and 2k were identified by 1H NMR after purification by distillation in the

467

J.-Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 466–469

R1NH2

+

R2

R2

Solvent-free

O

O

U. S.

R2 R1 N

r.t.

R2 N R1

1: R2= H 2: R2= CH3 R1= a: 4-CH3OC6H4; b: 4-CH3C6H4; c: 2-CH3C6H4; d: C6H5; e: 4-ClC6H4; f: 3,5-(CF3)2C6H3; g: 2,6-(CH3) 2C6H3; h: 2,4,6-(CH3) 3C6H2; i: 2,5-F 2C6H3; j: i-C3H7; k: n-C4H9; l: cyclohexyl. Scheme 1.

30 times reaction scale of general procedure. The identification of the new products 1i and 2i was established by their melting points, IR, 1H NMR, 13C NMR, MS and elemental analyses. Similarly, the 2h was further verified as a new product for its melting point was distinctly different from the reported data. The 1d, 1f and 2f were characterized by 1H NMR spectra because there were no their melting point reported. The remaining known compounds were identified by comparison of their melting points with those reported in the literature. 2.2.1. 1,4-Diphenyl-1,4-diazabutadiene (1d) M.p. 66.2–67.1 °C. 1H NMR (400 MHz, CDCl3): dH 6.80 (m, 4H, Ar-H), 7.05 (m, 2H, Ar-H), 7.45 (m, 4H, Ar-H), 8.40 (s, 2H, N@CH). 2.2.2. 1,4-Bis(3,5-ditrifluoromethylphenyl)-1,4-diazabutadiene (1f) M.p. 223.4–224.6 °C. 1H NMR (400 Hz, CDCl3): dH 7.28 (s, 4H, ArH), 7.67 (s, 2H, Ar-H), 8.48 (s, 2H, N@CH). 2.2.3. 1,4-Bis(2,5-difluorophenyl)diazabutadiene (1i) M.p. 100.8–101.5 °C. IR (KBr): 3033, 1663, 1510, 1479, 1375, 1112, 792 cm1. 1H NMR (400 MHz, CDCl3): dH 6.89–7.18 (m, 6H, Ar-H), 8.44 (s, 2H, N@CH). 13C NMR (100 MHz, CDCl3): dC 101.2, 104.2, 115.5, 135.4, 146.7, 149.0, 158.2, 160.6, 162.7. MS: 303 (M+Na+). Anal. Calc. for C14H8F4N2: C, 60.01; H, 2.88; N, 10.00. Found: C, 59.88; H, 2.89; N, 10.02%. 2.2.4. 1,4-Bis(n-butyl)-1,4-diazabutadiene (1k) Colorless liquid. 1H NMR (400 Hz, CDCl3): dH 0.95 (m, 6H, CH2CH2CH2CH3), 1.39–1.47 (m, 8H, CH2CH2CH2CH3), 3.75 (m, 4H, CH2CH2CH2CH3), 7.86 (s, 2H, N@CH). 2.2.5. 1,4-Bis(3,5-ditrifluoromethylphenyl)-2,3-dimethyl-1,4diazabutadiene (2f) M.p. 198.2–199.6 °C. 1H NMR (400 Hz, CDCl3): dH 2.18 (s, 6H, CH3), 7.24 (s, 4H, Ar-H), 7.66 (s, 2H, Ar-H). 2.2.6. 1,4-Bis(2,4,6-trimethylphenyl)-2,3-dimethyldiazabutadiene (2h) M.p. 187.4–188.2 °C. IR (KBr): 3315, 2979, 1696, 1647, 756 cm1. 1H NMR (400 MHz, CDCl3): dH 2.00 (s, 12H, Ar-CH3), 2.03 (s, 6H, CH3), 2.28 (s, 6H, Ar-CH3), 6.89 (d, J = 0.4 Hz, 4H, ArH). 13C NMR (100 MHz, CDCl3): dC 15.8, 17.8, 20.8, 124.6, 127.1, 128.6, 132.4, 145.9, 168.4. MS: 343 (M+Na+). Anal. Calc. for C22H28N2: C, 82.45; H, 8.81; N, 8.74. Found: C, 82.40; H, 8.84; N, 8.76%. 2.2.7. 1,4-Bis(2,5-difluorophenyl)-2,3-dimethyldiazabutadiene (2i) M.p. 116.7–118.0 °C. IR (KBr): 2988, 1665, 1500, 1465, 1382, 1112, 833 cm1. 1H NMR (400 MHz, CDCl3): dH 2.17 (d, J = 1.6 Hz, 6H, CH3), 6.60–6.65 (m, 2H, Ar-H), 6.77–6.82 (m, 2H, Ar-H), 7.06–

7.12 (m, 2H, Ar-H). 13C NMR (100 MHz, CDCl3): dC 16.2, 108.2, 111.4, 116.9, 138.9, 146.0, 148.4, 157.5, 159.9, 171.0. MS: 331 (M+Na+). Anal. Calc. for C16H12F4N2: C, 62.34; H, 3.92; N, 9.09. Found: C, 62.27; H, 3.93; N, 9.07%. 2.2.8. 1,4-Bis(i-propyl)-2,3-dimethyl-1,4-diazabutadiene (2j) Colorless liquid. 1H NMR (400 Hz, acetone-d6): dH 1.16 (d, J = 6.2 Hz, 12H, CH(CH3)2), 2.38 (s, 6H, CH3), 3.54 (m, 2H, CH(CH3)2). 2.2.9. 1,4-Bis(n-butyl)-2,3-dimethyl-1,4-diazabutadiene (2k) Colorless liquid. 1H NMR (400 Hz, CDCl3): dH 0.89–0.98 (m, 14H, CH2CH2CH2CH3), 1.90 (s, 6H, CH3), 3.19 (m, 4H, CH2CH2CH2CH3). 3. Results and discussion Firstly, using the formation of 1,4-bis(4-methoxyphenyl)diazabutadiene (1a) as a standard reaction, a mixture of p-anisidine and glyoxal trimer dihydrate was sonicated under various sets of conditions in order to obtain optimal experimental conditions at a constant room temperature of 25 ± 1 °C (Table 1). By increasing the irradiation power from 100 to 250 W, the reaction time of 1a decreased from 50 to 10 min and the yield increased from 66% to 98%. The reaction time and yield of 1a did not change from 250 to 350 W, therefore, 250 W of ultrasonic irradiation was sufficient to push the reaction forward. The best yield for 1a was obtained by ultrasonic irradiation for 10 min at room temperature and 250 W. The reason for the higher yield with higher irradiation power was that the reaction of p-anisidine and glyoxal proceeded via a phase change from solid to liquid, which is similar to the solvent-free preparation of azomethines [8a]. More importantly, the isolation of products was simplified, as by washing the products with ethanol, pure 1a was obtained directly, requiring no column chromatography. By contrast, when using the conventional method, 1,4-diazabutadienes were obtained with other materials, namely 1,2-dihydroxy-1,2-diamino compounds or tri- and tetraaminoethane derivatives that arose through the reaction of aromatic primary amines with 40% aqueous glyoxal in isopropyl alcohol [3a]. It is therefore obvious that ultrasound irradiation accelerates the condensation of diketones with anilines from a possibly Table 1 The ultrasound yields of 1a under solvent-free conditions.

a

Entry

Power (W)

Time (min)

Yielda (%)

1 2 3 4 5 6

100 150 200 250 300 350

50 40 30 10 10 10

66 75 84 98 98 98

Isolated yield.

468

J.-Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 466–469 Table 2 The yields of 1,4-diazabutadienes under ultrasonic irradiation.

a

Entry

R1

R2

Product

Time (min)

Yielda (Lit.) (%)

M.p. (Lit.) (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

4-CH3OC6H4 4-CH3C6H4 2-CH3C6H4 C6H5 4-ClC6H4 3,5-(CF3)2C6H3 2,6-(CH3)2C6H3 2,4,6-(CH3)3C6H2 2,5-F2C6H3 i-C3H7 n-C4H9 Cyclohexyl 4-CH3OC6H4 4-CH3C6H4 2-CH3C6H4 C6H5 4-ClC6H4 3,5-(CF3)2C6H3 2,6-(CH3)2C6H3 2,4,6-(CH3)3C6H2 2,5-F2C6H3 i-C3H7 n-C4H9 Cyclohexyl

H H H H H H H H H H H H CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 2a 2b 2c 2d 2e 2f 2g 2h 2i 2j 2k 2l

10 10 10 10 15 15 5 2 15 5 5 10 10 10 10 10 15 15 10 15 15 5 5 10

98 89 86 74 76 75 94 93 76 84 84 87 89 82 89 84 75 71 86 88 71 85 95 86

154.2–155.3 (153–154) [3a] 131.8–132.7 (132–133) [3b] 123.2–125.0 (122–124) [3a] 66.2–67.1 (–) [3c] 107.5–108.9 (107–110) [3a] 223.4–224.6 (–) [9a] 151.4–152.6 (151) [9b] 157.4–158.6 (157–158) [10] 100.8–101.5 (–) 49.8–51.2 (48–50) [3e] Liquid (liquid) [3e] 151.4–152.2 (150) [3f] 186.3–187.6 (184) [11b] 110.2–111.6 (112) [3d] 70.8–71.6 (73) [9b] 136.2–137.0 (136–137) [12b] 172.4–173.8 (172) [9d] 198.2–199.6 (–) [13] 85.3–86.5 (88) [9b] 187.4–188.2 (53–55) [14] 116.7–118.0 (–) Liquid (liquid) [3g] Liquid (liquid) [3h] 62.3–63.5 (63) [3j]

(58) [3a] (26) [3a] (58) [3a] (–) [3c] (37) [3a] (73) [9a] (85) [9b] (80) [10] (–) (77) [3e] (34) [3e] (95) [3e] (35) [11a] (50) [3d] (54) [9b] (49) [12a] (60) [9c] (26) [13] (83) [9b] (83) [14] (–) (93) [3g] (98) [3h] (66) [3i]

All yields refer to isolated products.

simultaneous thermal activation caused by ultrasonic irradiation under solvent-free conditions [4]. This method gave 98% of 1a at room temperature after 10 min, whereas the conventional method gave only 58% after 1 h at 65 °C in methanol medium [3a]. It is possible that the microenvironment of the reaction without solvent is different from that in solution, resulting in a higher concentration of local reaction sites and improving global efficiency [8b]. It is also possible that solvent-free reactions involving macroscopic organic solids can proceed via a liquid or melt phase and that ultrasonic cavitation activates the reaction mixture by inducing high local temperatures and pressure inside the bubbles (cavities) [8a,8c]. To identify the scope and limit of this reaction, various anilines and amines were used for the synthesis of 1,4-diazabutadienes (1 and 2). Under the optimized conditions previously described, the corresponding 1,4-diazabutadienes were formed in excellent yields (71–98%) within 2–15 min at room temperature and 250 W. The results are summarized in Table 2. It seemed that there was little difference between yields of 1,4diazabutadiene (1) and 2,3-dimethyl-1,4-diazabutadiene (2). This may be attributed to the reactivity similarity of diketone carbonyls activated by ultrasonic irradiation under solvent-free conditions. In the preparation of N,N0 -diaryl-1,4-diazabutadienes by the condensation of anilines with a-diketones, the yields of 1,4-diazabutadienes correlated with the electronic effects of substituents such as –OMe, –Me, –Cl, –CF3, –F, etc. Specifically, anilines with electrondonating groups reacted with a-diketones in shorter times and higher yields as compared to anilines with electron-withdrawing groups (Entry 1 vs. Entries 5, 6 and 9, Entry 13 vs. Entries 17, 18 and 21). For example, 1h was obtained in 93% yield in the condensation of 2,4,6-trimethylaniline and glyoxal within 2 min under ultrasound irradiation, but only in 80% yield in n-propanol at 60 °C for 20 h [10]. Even more drastically, 1i was obtained in 76% yield in the condensation of 2,5-difluoroaniline and glyoxal under ultrasound irradiation in 15 min, whereas no product was produced in either methanol or isopropyl alcohol under reflux for 48 h [3a]. The obstruction caused by the lower nucleophilicity of aniline with electron-withdrawing groups can therefore be circumvented by the power of ultrasound.

As shown in Table 2, the N,N0 -diaryl-1,4-diazabutadienes were prepared in 10–15 min in favorable yields (Entries 1–6 and Entries 13–21), and N,N0 -dialkyl-1,4-diazabutadienes in 5–10 min in good yields (Entries 10–12 and Entries 22–24). This may be due to the activity differences of aliphatic amines and anilines as well as the physical states of substrate molecules. Specifically, liquids such as aliphatic amines, liquid anilines and 2,3-butanedione are highly activated by cavitation effects as compared to solid anilines (p-anisidine, p-toluidine and p-chloroaniline), which proceed via a phase conversion to liquids. In the preparation of N,N0 -dialkyl1,4-diazabutadienes by the condensations of amines with a-diketones, aliphatic primary amines such as isopropylamine, n-butylamine, cyclohexylamine and ethylenediamine exhibited high reactivity under ultrasound irradiation and solvent-free conditions. For instance, 1j (Entry 10, R1 = i-C3H7) was obtained in 84% yield in 5 min, whereas the yield was 77% when the reaction was run in an aqueous solution for 2 days [3e]. Notably, the condensation of ethylenediamine and a-diketone could not produce the anticipated heterocyclic dihydropyrazine because the reaction under the solvent-free conditions was too violent. 4. Conclusion We have reported an efficient, versatile and convenient method for the preparation of N,N0 -diaryl- and N,N0 -dialkyl-1,4-diazabutadiene derivatives by the condensation of an a-diketone and an amine. The present procedure under ultrasound irradiation without solvent at room temperature significantly improved conventional methods, giving way to obvious advantages including simple operation, high yields, short reaction times and decreased toxicity. Acknowledgements This work was financially supported by Key Projects in the National Science and Technology Pillar Program during the Eleventh Five-Year Plan Period (No. 2008ZX10001-015), National Natural Sciences Foundation (No. 20872009) and Natural Sciences Foundation of Beijing (No. 200710005002).

J.-Y. He et al. / Ultrasonics Sonochemistry 18 (2011) 466–469

References [1] (a) S. Morrone, D. Guillon, D.W. Bruce, Synthesis and liquid-crystalline properties of diazabutadiene complexes of rhenium(I), Inorg. Chem. 35 (1996) 7041–7048; (b) P. Ferreira, C.D. Nunes, P.D. Vaz, N. Bion, P. Brandao, J. Rocha, Hybrid mesoporous MCM-41 type material containing 1,4-diazobutadiene chelate ligand in the walls, Prog. Solid State Chem. 33 (2005) 163–170. [2] (a) L.K. Johnson, C.M. Killian, M. Brookhart, New Pd(II)- and Ni(II)-based catalysts for polymerization of ethylene and a-olefins, J. Am. Chem. Soc. 117 (1995) 6414–6415; (b) C.D. Nunes, M. Pillinger, A.A. Valente, J. Rocha, A.D. Lopes, I.S. Goncalves, Dioxomolybdenum(VI)-modified mesoporous MCM-41 and MCM-48 materials for the catalytic epoxidation of olefins, Eur. J. Inorg. Chem. 21 (2003) 3870– 3877. [3] (a) J.M. Kliegman, R.K. Barnes, Glyoxal derivatives. II. Reaction of glyoxal with aromatic primary amines, J. Org. Chem. 35 (1970) 3140–3143; (b) L. Fotouhi, M. Zeienali, S. Dehghanpour, D. Nematollahi, Electrochemical reduction of 1,2-di(p-tolylimino)ethane and 1,2-di(2,4dimethylphenylimino)ethane in dimethylformamide, Chin. J. Chem. 25 (2007) 1577–1580.; (c) D.M. Haddleton, Polymerization catalyst metal diimine complex for olefinically unsaturated compound, PCT Int. Appl., 1997 (WO 9747661).; (d) G. Bahr, A. Kretzer, Heavy–metal complexes of bifunctional Schiff bases. II, Z. Anorg. Allg. Chem. 267 (1951) 161–173; (e) J.M. Kliegman, R.K. Barnes, Glyoxal derivatives. I. Conjugated aliphatic diimines from glyoxal and aliphatic primary amines, Tetrahedron 26 (1970) 2555–2560; (f) L. Horner, E. Jurgens, Preparation and properties of some isonitrones (oxaziranes), Chem. Ber. 90 (1957) 2184–2189; (g) D. Armesto, P. Bosch, M.G. Gallego, J.F. Martin, M.J. Ortiz, R. Perez-Ossorio, A. Ramos, Synthesis of diimines from 1,2-dicarbonyl compounds by direct catalyzed condensation, Org. Prep. Proceed. Int. 19 (1987) 181–186; (h) D.S. Tromp, M.A. Duin, A.M. Kluwer, C.J. Elsevier, Synthesis of new (a2-N,N0 diazadiene)(g2-alkene)platinum(0) compounds, Inorg. Chim. Acta 327 (2002) 90–97.; (i) N. De Kimpe, L. D’Hondt, E. Stanoeva, Synthesis of a-diones and a-diimines by regioselective a-alkylation of 2,3-alkanediimines, Tetrahedron Lett. 32 (1991) 3879–3882; (j) A.S. Abu-Surrah, H.M. Abdel-Halim, F.M. Al-Qaisi, Trans- and cis-cobalt(III), iron(III), and chromium(III) complexes based on a- and c-diimine Schiff base ligands: synthesis and evaluation of the complexes as catalysts for oxidation of L-cysteine, Z. Anorg. Chem. 634 (2008) 956–961. [4] G. Cravotto, P. Cintas, Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications, Chem. Soc. Rev. 35 (2006) 180–196. [5] (a) A. Kamal, S.F. Adil, M. Arifuddin, Ultrasonic activated efficient method for the cleavage of epoxides with aromatic amines, Ultrason. Sonochem. 12 (2005) 429–431; (b) G. Cravotto, G. Palmisano, S. Tollari, G.M. Nano, A. Penoni, The Suzuki homocoupling reaction under high-intensity ultrasound, Ultrason. Sonochem. 12 (2005) 91–94;

[6] [7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

469

(c) M.M. Mojtahedi, M.S. Abaee, V. Hamidi, A. Zolfaghari, Ultrasoundpromoted protection of alcohols. An efficient solvent-free pathway for the preparation of silyl ethers in the presence of no additive, Ultrason. Sonochem. 14 (2007) 596–598. K. Tanaka, F. Toda, Solvent-free organic synthesis, Chem. Rev. 100 (2000) 1025–1074. (a) R.Q. Li, H. Yan, A new method for synthesizing 1,4-dihydropyridine under microwave conditions, Hecheng Huaxue 13 (2005) 597–599; (b) C.L. Ni, X.H. Song, H. Yan, X.Q. Song, R.G. Zhong, Improved synthesis of diethyl 2,6-dimethyl-4-aryl-4H-pyran-3,5-dicarboxylate under ultrasound irradiation, Ultrason. Sonochem. 17 (2010) 367–369. (a) G. Rothenberg, A.P. Downie, C.L. Raston, J.L. Scott, Understanding solid/ solid organic reactions, J. Am. Chem. Soc. 123 (2001) 8701–8708; (b) G.W.V. Cave, C.L. Raston, J.L. Scott, Recent advances in solventless organic reactions: towards benign synthesis with remarkable versatility, Chem. Commun. (2001) 2159–2169; (c) M.A.P. Martins, C.P. Frizzo, D.N. Moreira, L. Buriol, P. Machado, Solvent-free heterocyclic synthesis, Chem. Rev. 109 (2009) 4140–4182. (a) M. Arndt-Rosenau, M. Hoch, J. Sundermeyer, J. Kipke, Olefin polymerization catalysts based on transition metal–diimine complexes, European Patent, EP 1284271, 2003.; (b) H. Tom Dieck, M. Svoboda, T. Greiser, Bis(diazadiene)metal(0) complexes. IV. Nickel(0) bis(chelates) with aromatic N-substituents, Z. Naturforsch. 36B (1981) 823–832; (c) M. Helldorfer, W. Milius, H.G. Alt, The influence of halogen substituents at the ligand framework of (a-diimine)nickel(II) catalyst precursors on their behavior in ethylene oligomerization and polymerization, J. Mol. Catal. A: Chem. 197 (2003) 1–13; (d) H.T. Macholdt, H. Elias, Ligand substitution in molybdenum(0) carbonyl complexes Mo(CO)5(amine) and cis-Mo(CO)4(amine)2: kinetics and highpressure effects, Inorg. Chem. 23 (1984) 4315–4321. A.J. Arduengo III, R. Krafczyk, R. Schmutzler, H.A. Craig, J.R. Goerlich, W.J. Marshall, M. Unverzagt, Imidazolylidenes, imidazolinylidenes and imidazolidines, Tetrahedron 55 (1999) 14523–14534. (a) J. Fetter, H. Vasarhelyi, M. Kajtar-Peredy, K. Lempert, J. Tamas, G. Czira, Simple and condensed b-lactams. Part 22. An unprecedented ring transformation accompanying dephthaloylation of a 3-phthalimidoazetidin2-one, Tetrahedron 51 (1995) 4763–4778; (b) D. Vorlander, W. Zeh, H. Enderlein, sym-Bisbenzeneazoethylene, Ber. Dtsch. Chem. Ges. 60B (1927) 849–857. (a) E. Lemp, A.L. Zanocco, G. Guenther, N. Pizarro, Solvent effect on the sensitized photooxygenation of cyclic and acyclic a-diimines, Tetrahedron 62 (2006) 10734–10746; (b) L.N. Ferguson, T.C. Goodwin, Absorption spectra of azines and dianils, J. Am. Chem. Soc. 71 (1949) 633–637. L. Johansson, O.B. Ryan, M. Tilset, Hydrocarbon activation at a cationic platinum(II) diimine aqua complex under mild conditions in a hydroxylic solvent, J. Am. Chem. Soc. 121 (1999) 1974–1975. H. Tuerkmen, B. Cetinkaya, 1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): electronic effects on cross-coupling reactions and thermal decompositions, J. Organomet. Chem. 691 (2006) 3749–3759.