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Reactions of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane and (1R,2S)-1,2-di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane with tetracyanoethylene (TCNE) in benzene: comparative studies on the products and their spectroscopic properties
Tetrahedron 62 (2006) 1520–1526
Reactions of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane and (1R,2S)-1,2-di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane with tetracyanoethylene (TCNE) in benzene: comparative studies on the products and their spectroscopic properties Shin-ichi Takekuma,a,* Kenji Takahashi,a Akio Sakaguchi,a Masato Sasaki,a Toshie Minematsub and Hideko Takekumaa a
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka-shi, Osaka 577-8502, Japan b School of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi-Osaka-shi, Osaka 577-8502, Japan Received 22 August 2005; accepted 4 November 2005 Available online 9 December 2005
Abstract—Reactions of the title meso forms, (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2-di(3-guaiazulenyl)1,2-di(2-thienyl)ethane (2), with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen give new compounds, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3) and 2,2,3,3-tetracyano-8-isopropyl6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4), respectively, in 74 and 21% isolated yields. Comparative studies on the above reactions as well as the spectroscopic properties of the unique products 3 and 4, possessing interesting molecular structures, are reported and, further, a plausible reaction pathway for the formation of these products is described. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction In the previous papers, 1–9 we reported a facile preparation and the crystal structures as well as the spectroscopic, chemical and electrochemical properties of the mono- and dicarbocations stabilized by a 3-guaiazulenyl group. During the course of our investigations, we have recently found (i) that the reactions of naturally occurring guaiazulene (A) with 2-furaldehyde (B) and thiophene-2-carbaldehyde (C) in methanol in the presence of hexafluorophosphoric acid gave (2-furyl)(3guaiazulenyl)methylium and (3-guaiazulenyl)(2-thienyl)methylium hexafluorophosphates (D and E) with the following two representative resonance forms [i.e., 3-guaiazulenylium and 2-furanylium (and 2-thienylium) structures], respectively, in 93 and 98% isolated yields (see Fig. 1),10 which upon reduction with zinc powder in acetonitrile afforded a mixture of a meso form and two enantiomeric forms of the molecular structures, 1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (F) and Keywords: Annulation; C–C bond cleavage; Charge-transfer complex; Furan; Guaiazulene; TCNE; Thiophene; X-ray crystal structure. * Corresponding author. Tel.: C81 6 6730 5880x4020; fax: C81 6 6727 4301; e-mail: [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.11.026
1,2-di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (G), respectively, (see Fig. 2),10 and have quite recently found (ii) that, from a comparative study on the oxidation potentials of the above meso forms, (1R,2S)-1,2-di(2furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (2), measured by means of the CV and DPV (Potential/V vs SCE) in CH2Cl2 containing 0.1 M [n-Bu4N]PF6 as a supporting electrolyte, it could be inferred that 1 and 2 simultaneously underwent two-electron oxidation of their 1,2di(3-guaiazulenyl) groups, respectively, at the potentials of C0.48 (Epa, irreversible) V by CV and C0.48 (Ep) V by DPV for 111 and C0.57 (Epa, irreversible) V by CV and C0.56 (Ep) V by DPV for 2,12 generating the corresponding di(cation-radical) species 12(C%) and 22(C%). Thus, 1 was more susceptible to oxidation than 2 and A [C0.69 (Epa, irreversible) V by CV and C0.65 (Ep) V by DPV],8 owing to the difference in ionization potential (corresponding to p-HOMO) based on those p-electron systems. The different oxidation potentials between 1 and 2 are obviously caused by the influence of a different heteroaromatic-ring. Therefore, our interest has been focused on a comparative study on the title chemistry, the reactions of 1 and 2 with TCNE, which serves as an electron acceptor [K0.75 (E1/2, quasi-reversible) V by CV and
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--
PF6
X
+
+
X CHO
A
H
in MeOH in the presence of 60% HPF6 under aerobic conditions
B, C
D, E 5
-- + X
--
PF6
PF6
1
--
PF6
X
X
2 1'
+
3'
H
H
H 4'
+
7'
B, D: X = O C, E: X = S D'', E''
D, E
D', E'
Figure 1. The reactions of A with B and C in CH3OH with HPF6 under aerobic conditions, affording the corresponding monocarbocation products D and E with the resonance forms of D 0 , E 0 and D 00 , E 00 , respectively.10
--
PF6
1''
7''
X
+
H
Zinc powder in acetonitrile under argon
3'' 4''
1
2
2'
X
1'
X
5' D, E
F, G D, F: X = O E, G: X = S
Figure 2. The reductions of D and E with zinc powder in acetonitrile at 25 8C under argon, affording a mixture of a meso form and two enantiomeric forms of the molecular structures F and G, respectively.10
K0.76 (Ep) V by DPV].13 In relation to our basic studies, in 1961 Hafner and Moritz reported that the reaction of guaiazulene (A) in petroleum ether with TCNE in AcOEt at K20 8C gave a 1:1 p-complex in 98% isolated yield, which was converted into 3-tricyanovinylguaiazulene (68% isolated yield) in DMF at room temperature.14 Furthermore, the addition and cycloaddition reactions of TCNE in organic chemistry15–19 including azulenes14,20–22 have been studied to a considerable extent, and a large number of those results and discussion have been well documented. Along with those investigations, we now wish to report the detailed title chemistry, affording the unique products, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene (3) from 1 and 2,2,3,3tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4) from 2, possessing interesting molecular structures, presumably via charge-transfer, C–C bond cleavage and, further, [p8Cp2] cycloaddtion reactions (see Scheme 1).
2. Results and discussion 2.1. Reactions of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2-di(3-guaiazulenyl)1,2-di(2-thienyl)ethane (2) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-4-(2-furyl)8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3) and 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4) Compound 3 was prepared using benzene as a solvent as shown in Figure 3 and Section 4.1.1, whose molecular structure was established on the basis of elemental analysis and spectroscopic data [UV–vis, IR, exact FAB-MS, 1H and 13 C NMR including 2D NMR (i.e., H–H COSY, NOESY, HMQC Z1H detected hetero nuclear multiple quantum coherence and HMBC Z1H detected hetero nuclear multiple bond connectivity)].
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X
X
X H
+
e
H
NC
CN
NC
CN
NC
CN
NC
CN
NC
CN
NC
CN
a1,2
1, 2
b1,2 X
X
O2
b1,2
H
+
– OOH +
H H
NC
CN
NC
CN
H H H
NC
CN
NC
CN
O2
d1,2
c1,2 π8 + π2
d1,2
1, 3, a1, b1, c1, d1: X = O 2, 4, a2, b2, c2, d2: X = S
3, 4
Scheme 1. A plausible reaction pathway for the formation of 3 and 4 yielded by the reactions of the title meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen. The partial structures 1, 2 and a1,2 are illustrated.
5' 1'
1''
7''
6
3''
5
X 2'
4a
4
TCNE
4''
1
6a
2 in benzene under oxygen
2' 1'
X
X
10b 1 10a
7 8 9
5' 1, 2
CN 3 2
CN CN CN
10 3, 4
1, 3: X = O 2, 4: X = S
Figure 3. The reactions of the title meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen.
Compound 3 (74% isolated yield) was blue prisms [RfZ 0.27 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 163 8C and decomp. O171 8C [determined by the thermal analysis (TGA and DTA)]. The characteristic UV–vis (CH2Cl2) absorption bands based on the azulenyl group were observed and the longest visible absorption wavelength appeared at lmax 622 nm (log 3Z2.69). The IR (KBr) spectrum showed a specific band based on the –C^N group at 2252 cmK1. The protonated molecular formula C26H21N4O ([MCH]C) was determined by the exact FAB-MS (3-nitrobenzyl alcohol matrix) spectrum. The elemental analysis confirmed the molecular formula C26H20N4O. The 500 MHz 1H NMR (C6D6) spectrum showed signals based on the structure of 8-isopropyl-6methyl-1,4-dihydrocyclohepta[c,d]azulene possessing a 2-furyl group at the C-4 position, which signals (d and J values) were carefully assigned using the H–H COSY technique and the computer simulation analysis (see Section 4.1.1). Moreover, the NOESY spectrum revealed apparent cross peaks between the following signals [i.e., –C1H2– and H-10; d 3.15 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)], and suggested that the 2-furyl group
was substituted at the C-4 (equatorial) position. The 125 MHz 13C NMR (C6D6) spectrum exhibited 25 carbon signals (d) assigned by the HMQC and HMBC techniques (see Section 4.1.1). Thus, the elemental analysis and these spectroscopic data for 3 led to a new molecular structure, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene (see Fig. 3). Similarly, as in the case of the preparation of 3, the target compound 4 was prepared using benzene as a solvent as shown in Figure 3 and Section 4.1.2, whose molecular structure was established on the basis of similar spectroscopic analyses as 3. Although this reaction afforded numerous products as compared with the reaction of 1 with TCNE (see Section 4.1.1), compound 4 could be isolated in 21% yield. The difference between the yields of the products 3 and 4 is obviously caused by the different oxidation potentials between 111 and 212 (see Section 1), whose oxidation potentials suggest that the reaction of 1 with TCNE readily gives the corresponding 1:2 chargetransfer (CT) complex a1 (see Scheme 1) as compared with the reaction of 2 with TCNE.
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Compound 4 was blue blocks [RfZ0.61 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 147 8C and decomp. O158 8C [determined by the thermal analysis (TGA and DTA)]. The characteristic UV–vis (CH2Cl2) absorption bands based on the azulenyl group, which spectral pattern coincided with that of 3, were observed and the longest visible absorption wavelength appeared at lmax 624 nm (log 3Z2.64). The IR (KBr) spectrum showed a specific band based on the –C^N group at 2252 cmK1, which wavenumber coincided with that of 3. The protonated molecular formula C26H21N4S ([MCH]C) was determined by the exact FAB-MS (3-nitrobenzyl alcohol matrix) spectrum. The 500 MHz 1H NMR (C6D6) spectrum showed signals based on the structure of 8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene possessing a 2-thienyl group at the C-4 position, which signals (d and J values) were carefully assigned using the H–H COSY technique and the computer simulation analysis (see Section 4.1.2). Similarly, as in the case of 3, the NOESY spectrum revealed apparent cross peaks between the following signals [i.e., –C1H2– and H-10; d 3.18 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)], and suggested that the 2-thienyl group was substituted at the C-4 (equatorial) position. The 125 MHz 13C NMR (C6D6) spectrum exhibited 25 carbon signals (d) assigned by the HMQC and HMBC techniques (see Section 4.1.2). Thus, these spectroscopic data for 4 led to a new molecular structure, 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (see Fig. 3).
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methyl-1,4-dihydrocyclohepta[c,d]azulene possessing a 2-furyl group at the C-4 (equatorial) position, presumed by means of the NOESY spectrum (see Sections 2.1 and 4.1.1). From a comparative study on the detailed spectroscopic properties of 3 and 4 (see Sections 4.1.1 and 4.1.2), it can be inferred that the crystal structure of 4 assumes similar conformation to that of 3. 2.3. A plausible reaction pathway for the formation of the products 3 and 4 From the molecular structures of the resulting products 3 and 4, obtained by the reactions of the meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen, a plausible reaction pathway for the formation of compounds 3 and 4 can be inferred as illustrated in Scheme 1; namely, (i) the reactions of 1 and 2 with TCNE gradually give the corresponding 1:2 charge-transfer (CT) complexes a1,2, respectively, whose formation was supported by the timedependent visible spectra of the reaction solutions as shown in Figures 5 and 6. The characteristic CT band based on the formation of a1, generated from the reaction of 1 with TCNE, appeared at lmax 518 nm (the time to reach
2.2. X-ray crystal structure of 3 Although an X-ray crystallographic analysis of 4 has not yet been achieved because of difficulty in obtaining a single crystal suitable for this purpose, the crystal structure of 3 has been determined by means of the X-ray diffraction (see Section 4.1.3). In the course of refinement, the atoms based on the 2-furyl group at the C-4 (equatorial) position and two methyl groups of the isopropyl group at the C-8 position for 3 were found to be disordered over two sites of equal occupancy. Therefore, those atoms in the occupancy ratio 50:50 were refined. The only ORTEP drawing of 3 noted the above disorder, indicating the molecular structure, 2,2,3,3tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene, illustrated in Figure 4a, is shown in Figure 4b. As the result, the crystal structure of 3 clarified the conformation of 2,2,3,3-tetracyano-8-isopropyl-6-
Figure 4. (a) The molecular structure of 3. (b) The ORTEP drawing of 3 (30% probability thermal ellipsoids).
Figure 5. The time-dependent visible spectra for the reaction of 1 (2.0 mg, 3.6 mmol) with TCNE (1.0 mg, 7.8 mmol) in benzene (10 mL) at 25 8C for 2 h under argon. Length of the cell: 0.1 cm. Interval time: 10 min. lmax 357, 374 and 400sh nm: the specific bands based on TCNE; lmax 518 nm: the specific band based on the formation of the CT complex a1 (see Scheme 1) (the time to reach maximum absorptionZ2 h); lmax 618, 680sh and 750sh nm: the specific bands based on 1.
Figure 6. The time-dependent visible spectra for the reaction of 2 (2.0 mg, 3.4 mmol) with TCNE (1.0 mg, 7.8 mmol) in benzene (10 mL) at 25 8C for 11 h under argon. Length of the cell: 0.1 cm. Interval time: 1 h. lmax 359, 375 and 400sh nm: the specific bands based on TCNE; lmax 517 nm: the specific band based on the formation of the CT complex a2 (see Scheme 1) (the time to reach maximum absorptionZ11 h); lmax 622, 680sh and 750sh nm: the specific bands based on 2.
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maximum absorption Z2 h), whose band revealed a bathochromic shift (D19 nm) in comparison with the characteristic visible band of (2-furyl)(3-guaiazulenyl)methylium hexafluorophosphate (D) (lmax 499 nm, log 3Z 4.61);10 and, similarly, the characteristic CT band based on the formation of a2, generated from the reaction of 2 with TCNE, appeared at lmax 517 nm (the time to reach maximum absorption Z11 h), whose band showed a bathochromic shift (D20 nm) in comparison with the characteristic visible band of (3-guaiazulenyl)(2-thienyl)methylium hexafluorophosphate (E) (lmax 497 nm, log 3Z 4.73).10 The time to reach maximum absorption indicates that 1 more readily forms the corresponding 1:2 CT complex a1 than 2, whose result is obviously caused by the different oxidation potentials between 111 and 212 (see Section 1); and (ii) although the CT complexes a1,2 generated were stable in benzene at 25 8C for a long time (O72 h) under argon and they did not give 3 and 4, the complexes a1,2 were gradually converted into 3 and 4 in benzene at 25 8C under oxygen. Furthermore, the reactions of 1 and 2 with a two molar amount of TCNE in toluene at K20 8C for 24 h under argon gave the corresponding 1:2 CT complexes a1,2, respectively, in 18% isolated yield from 123 and 23% isolated yield from 2,24 whose complexes were gradually converted into 3 and 4, respectively, in benzene at 25 8C under oxygen. Along with the above results, the reactions of (2-furyl)(3-guaiazulenyl)methylium and (3guaiazulenyl)(2-thienyl)methylium hexafluorophosphates (D and E) with an equimolar amount of TCNE in acetonitrile at 25 8C for 72 h under oxygen did not afford any products. From these results, it can be inferred that the CT complexes a1,2 generated are converted into 3 and 4 presumably via the 3-guaiazulenylium-ions b1,2 yielded by the C–C bond cleavage of the ethane units of a1,2, the azulenequinodimethanes d1,2 produced by the deprotonations of the C4-methyl groups of c1,2 by the attack of OK% 2 generated by the electron transfer from the TCNEK% to O2 and, further, the [p8Cp2] cycloaddition reactions of d1,2 with TCNE, as illustrated in Scheme 1. On the other hand, in 1988 one of us (S. Takekuma) reported that the autoxidation of guaiazulene (A) under several reaction conditions gave a wide variety of products possessing interesting molecular structures, respectively, through such several types of the reactions as oxidative dimerization, oxidation of side chains, azulenequinone formation, intermolecular onecarbon transfer reactions and rearrangements to naphthalenoid, benzenoid and 1H-inden-1-one derivatives, simultaneously.25 In this case, by the attack of oxygen, A was considered to initially form an electron-transfer complex (AC% OK% 2 ), which in turn leads to the formation of various products.
3. Conclusion We have reported the following four points in this paper: (i) the reactions of the title meso forms, (1R,2S)-1,2-di(2furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (2), with a two molar amount of tetracyanoethylene (TCNE) in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen gave new compounds 3 and 4, respectively, in 74 and 21% isolated yields; (ii) the detailed spectroscopic analyses of these
products led to the molecular structures, 2,2,3,3-tetracyano4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene for 3 and 2,2,3,3-tetracyano-8-isopropyl-6methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene for 4;(iii)alongwiththespectroscopicdatafor3and4,weclarified the crystal structure of 3; and, further, (iv) a plausible reaction pathway for the formation of the unique products 3 and 4, possessinginterestingmolecularstructures,wasdescribed. 4. Experimental 4.1. General Thermal (TGA/DTA) and elemental analyses were taken on a Shimadzu DTG-50H thermal analyzer and a Yanaco MT-3 CHN corder, respectively. MS spectra were taken on a JEOL The Tandem Mstation JMS-700 TKM data system. UV–vis and IR spectra were taken on a Beckman DU640 spectrophotometer and a Shimadzu FTIR-4200 Grating spectrometer, respectively. NMR spectra were recorded with a JEOL GX-500 (500 MHz for 1H and 125 MHz for 13 C) at 25 8C. The 1H NMR spectra were assigned using the computer-assisted simulation analysis (the software: gNMR developed by Adept Scientific plc) on a DELL Dimension XPS T500 personal-computer with a Pentium III processor. Cyclic and differential pulse voltammograms were measured by an ALS Model 600 electrochemical analyzer. 4.1.1. Reaction of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-4-(2-furyl)-8isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3). To a solution of TCNE (48 mg, 0.37 mmol) in benzene (10 mL) was slowly added a solution of 1 (97 mg, 0.17 mmol) [RfZ0.71 on silica-gel TLC (hexane/AcOEtZ 4:6, vol/vol)] in benzene (30 mL), turning the blue solution into a red solution. The mixture was stirred at 25 8C for 5 h under oxygen, turning the red solution into a green solution. After the reaction, the reaction solution was evaporated in vacuo, giving a green solid. The residue obtained was carefully separated by silica-gel column chromatography with hexane–ethyl acetate (3/2, vol/vol) as an eluant. The crude product 3 thus obtained was recrystallized from benzene–cyclohexane (1/5, vol/vol) (several times) to provide pure 3 as stable crystals (104 mg, 0.26 mmol, 74% yield). Compound 3: blue prisms [RfZ0.27 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 163 8C and decomp. O 171 8C [determined by thermal analysis (TGA and DTA)]; Found: C, 76.89; H, 5.15; N, 13.66%. Calcd for C26H20N4O: C, 77.21; H, 4.98; N, 13.85%; UV–vis lmax (CH2Cl2) nm (log 3), 247 (4.46), 296 (4.65), 358 (3.86), 376 (3.86), 622 (2.69), 684sh (2.58) and 754sh (2.12); IR nmax (KBr) cmK1, 2252 (–C^N); FAB-MS (3-nitrobenzyl alcohol matrix), m/z 405 ([MCH]C, 100%) and 404 (MC, 94); exact FABMS (3-nitrobenzyl alcohol matrix), found: m/z 405.1711; calcd for C26H21N4O: [MCH]C, m/z 405.1715; 500 MHz 1 H NMR (C6D6), signals based on the 1,4-dihydro-8isopropyl-6-methylcyclohepta[c,d]azulene unit at d 1.05 (6H, d, JZ6.9 Hz, (CH3)2CH-8), 2.26 (3H, s, Me-6), 2.60 (1H, sept, JZ6.9 Hz, (CH3)2CH-8), 3.15 (axial), 3.42
S.-i. Takekuma et al. / Tetrahedron 62 (2006) 1520–1526
(equatorial) (1H each, d, JZ15.2 Hz, –C1H2–), 5.11 (1H axial, br d s, pC4H–), 6.36 (1H, d, JZ10.4 Hz, H-10), 6.98 (1H, dd, JZ10.4, 2.0 Hz, H-9), 7.00 (1H, br d s, H-5) and 7.94 (1H, d, JZ2.0 Hz, H-7) and signals based on the 2-furyl group at d 5.99 (1H, dd, JZ3.5, 2.0 Hz, H-4 0 ), 6.05 (1H, ddd, JZ3.5, 0.9, 0.7 Hz, H-3 0 ) and 7.03 (1H, dd, JZ 2.0, 0.9 Hz, H-5 0 ); the NOESY spectrum showed cross peaks between the following signals [–C1H2– and H-10; d 3.15 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)]; 125 MHz 13C NMR (C6D6), signals based on the 2,2,3,3-tetracyano-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene unit at d 12.2 (Me-6), 24.1 ((CH3)2CH-8), 38.2 ((CH3)2CH-8), 44.3 (–C1H2–), 45.7 (pC4H–), 43.8, 49.6 [(NC)2C-2 or (NC)2C-3], 111.22, 111.24, 111.4, 112.2 [(NC)2C-2 or (NC)2C-3], 118.7 (C10b), 125.5 (C-6a), 126.9 (C-10), 127.4 (C-4a), 134.9 (C10a), 135.3 (C-9), 135.5 (C-7), 140.1 (C-6), 140.6 (C-5) and 144.1 (C-8) and signals based on the 2-furyl group at d 111.1 (C-4 0 ), 112.9 (C-3 0 ), 144.4 (C-5 0 ) and 148.8 (C-2 0 ). 4.1.2. Reaction of (1R,2S)-1,2-di(3-guaiazulenyl)-1,2di(2-thienyl)ethane (2) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4). To a solution of TCNE (57 mg, 0.44 mmol) in benzene (10 mL) was slowly added a solution of 2 (130 mg, 0.22 mmol) [RfZ0.70 on silica-gel TLC (hexane/AcOEtZ 4:6, vol/vol)] in benzene (20 mL), turning the blue solution into a red solution. The mixture was stirred at 25 8C for 48 h under oxygen, turning the red solution into a green solution. After the reaction, the reaction solution was evaporated in vacuo, giving a green solid. The residue obtained was carefully separated by silica-gel column chromatography with hexane–ethyl acetate (3/2, vol/vol) as an eluant. The crude product 4 thus obtained was recrystallized from dichloromethane–hexane (1/5, vol/vol) (several times) to provide pure 4 as stable crystals (40 mg, 0.10 mmol, 21% yield). Compound 4: blue blocks [RfZ0.61 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 147 8C and decomp. O158 8C [determined by thermal analysis (TGA and DTA)]; UV–vis lmax (CH2Cl2) nm (log 3), 247 (4.46), 297 (4.59), 360 (3.79), 377 (3.77), 624 (2.64), 683sh (2.55) and 760sh (2.06); IR nmax (KBr) cmK1, 2252 (–C^N); FABMS (3-nitrobenzyl alcohol matrix), m/z 421 ([MCH]C, 100%) and 420 (MC, 80); exact FAB-MS (3-nitrobenzyl alcohol matrix), found: m/z 421.1449; calcd for C26H21N4S: [MCH]C, m/z 421.1487; 500 MHz 1H NMR (C6D6), signals based on the 1,4-dihydro-8-isopropyl-6-methylcyclohepta[c,d]azulene unit at d 1.04 (6H, d, JZ6.9 Hz, (CH3)2CH-8), 2.23 (3H, s, Me-6), 2.59 (1H, sept, JZ6.9 Hz, (CH3)2CH-8), 3.18 (axial), 3.24 (equatorial) (1H each, d, JZ15.1 Hz, –C1H2–), 5.30 (1H axial, br d s, pC4H–), 6.33 (1H, d, JZ10.3 Hz, H-10), 6.96 (1H, dd, JZ10.3, 2.0 Hz, H-9), 7.17 (1H, br d s, H-5) and 7.93 (1H, d, JZ2.0 Hz, H-7) and signals based on the 2-thienyl group at d 6.65 (1H, dd, JZ5.2, 3.7 Hz, H-4 0 ), 6.80 (1H, dd, JZ5.2, 1.1 Hz, H-5 0 ) and 7.11 (1H, dd, JZ3.7, 1.1 Hz, H-3 0 ); the NOESY spectrum showed cross peaks between the following signals [–C1H2– and H-10; d 3.18 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)]; 125 MHz 13C NMR (C6D6), signals based on the 2,2,3,3-tetracyano-8-isopropyl-
1525
6-methyl-1,4-dihydrocyclohepta[c,d]azulene unit at d 12.3 (Me-6), 24.1 ((CH3)2CH-8), 38.2 ((CH3)2CH-8), 44.4 (–C1H 2–), 47.0 (pC 4H–), 44.3, 51.9 [(NC)2C-2 or (NC)2C-3], 111.3, 111.5, 111.7, 112.1 [(NC)2C-2 or (NC)2C-3], 120.8 (C-4a), 125.2 (C-6), 126.9 (C-10), 135.0 (C-10a), 135.2 (C-10b), 135.4 (C-9), 135.6 (C-7), 140.2 (C-6a), 141.0 (C-5) and 144.2 (C-8) and signals based on the 2-thienyl group at d 127.5 (C-5 0 ), 127.8 (C-4 0 ), 129.8 (C-3 0 ) and 138.6 (C-2 0 ). 4.1.3. X-ray crystal structure of 2,2,3,3-tetracyano-4(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3). A total 5443 reflections with 2qmaxZ55.08 were collected on a Rigaku AFC-5R automated four-circle diffractometer with graphite monochromated Mo Ka radi˚ , rotating anode: 50 kV, 180 mA) at ation (lZ0.71069 A 296 K. The structure was solved by direct methods (SIR92) and expanded using Fourier techniques (DIRDIF99). The atoms based on the 2-furyl group at the C-4 (equatorial) position and two methyl groups of the isopropyl group at the C-8 position for 3 were disordered over two sites of equal occupancy. Therefore, those atoms in the occupancy ratio 50:50 were refined. The non-hydrogen atoms were refined anisotropically. The final cycle of full-matrix least-squares refinement was based on F2. All calculations were performed using the Crystal Structure 3.7.0 software package. Crystallographic data have been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK and copies can be obtained on request, free of charge, by quoting the publication citation and the deposition number CCDC 248295. Crystallographic data for 3: C26H20N4O (FWZ404.47), blue prism (the crystal size, 0.80!0.30!0.30 mm3), ˚ , bZ monoclinic, P21/c (#14), aZ10.278(2) A ˚ ˚ 15.3808(19) A, cZ14.2500(19) A, bZ91.831(14)8, VZ ˚ 3, ZZ4, D calcdZ1.193 g/cm 3, m(Mo Ka) 2251.5(7) A Z0.750 cmK1, Scan widthZ(1.47C0.30 tanq)8, Scan modeZuK2q, Scan rateZ9.08/min, measured reflectionsZ5443, observed reflectionsZ5163, No of parametersZ270, R1Z0.0677, wR2Z0.2539 and Goodness of Fit Indicator Z1.027.
Acknowledgements We would like to express our considerable gratitude to Dr. Kunihisa Sugimoto (Rigaku Corporation) for his help in the X-ray crystallographic analysis of compound 3. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
References and notes 1. Takekuma, S.; Sasaki, M.; Takekuma, H.; Yamamoto, H. Chem. Lett. 1999, 999–1000. 2. Takekuma, S.; Takata, S.; Sasaki, M.; Takekuma, H. Tetrahedron Lett. 2001, 42, 5921–5924.
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3. Takekuma, S.; Tanizawa, M.; Sasaki, M.; Matsumoto, T.; Takekuma, H. Tetrahedron Lett. 2002, 43, 2073–2078. 4. Sasaki, M.; Nakamura, M.; Hannita, G.; Takekuma, H.; Minematsu, T.; Yoshihara, M.; Takekuma, S. Tetrahedron Lett. 2003, 44, 275–279. 5. Sasaki, M.; Nakamura, M.; Uriu, T.; Takekuma, H.; Minematsu, T.; Yoshihara, M.; Takekuma, S. Tetrahedron 2003, 59, 505–516. 6. Nakamura, M.; Sasaki, M.; Takekuma, H.; Minematsu, T.; Takekuma, S. Bull. Chem. Soc. Jpn. 2003, 76, 2051–2052. 7. Takekuma, S.; Sasaki, K.; Nakatsuji, M.; Sasaki, M.; Minematsu, T.; Takekuma, H. Bull. Chem. Soc. Jpn. 2004, 77, 379–380. 8. Nakatsuji, M.; Hata, Y.; Fujihara, T.; Yamamoto, K.; Sasaki, M.; Takekuma, H.; Yoshihara, M.; Minematsu, T.; Takekuma, S. Tetrahedron 2004, 60, 5983–6000. 9. Takekuma, S.; Hata, Y.; Nishimoto, T.; Nomura, E.; Sasaki, M.; Minematsu, T.; Takekuma, H. Tetrahedron 2005, 61, 6892–6907. 10. Takekuma, S.; Takahashi, K.; Sakaguchi, A.; Shibata, Y.; Sasaki, M.; Minematsu, T.; Takekuma, H. Tetrahedron 2005, 61, 10349–10362. 11. The electrochemical measurement conditions of 1 are as follows: the cyclic and differential pulse voltammograms (Potential/V vs SCE) of 1 (3.0 mg, 5.4 mmol) in 0.1 M [nBu4N]PF6, CH2Cl2 (10 mL) at a glassy carbon (ID: 3 mm) and a platinum wire served as the working and auxiliary electrodes; scan rates 100 mV sK1 at 25 8C under argon, respectively. For comparative purposes, the oxidation potential using ferrocene as a standard material showed C0.42 (Ep) V by DPV and C0.42 (E1/2, quasi-reversible) V by CV in 0.1 M [n-Bu4N]PF6, CH2Cl2 under the same electrochemical conditions as 1. 12. The cyclic and differential pulse voltammograms of 2 (3.0 mg, 5.1 mmol) were measured under the same electrochemical conditions as 1. 13. The electrochemical measurement conditions of TCNE are as follows: the cyclic and differential pulse voltammograms (Potential/V vs SCE) of TCNE (3.0 mg, 23.4 mmol) in 0.1 M [n-Bu4N]BF4, CH3CN (10 mL) at a glassy carbon (ID: 3 mm) and a platinum wire served as the working and auxiliary electrodes; scan rates 100 mV sK1 at 25 8C under argon, respectively. For comparative purposes, the oxidation potential using ferrocene as a standard material showed C0.45 (Ep) V by DPV and C0.42 (E1/2, quasi-reversible) V by CV in 0.1 M [n-Bu4N]BF4, CH3CN under the same electrochemical conditions as TCNE. 14. Hafner, K.; Moritz, K.-L. Liebigs Ann. Chem. 1961, 650, 92–97. 15. (a) Fatiadi, A. J. Synthesis 1986, 249–284. (b) Fatiadi, A. J. Synthesis 1987, 749–789.
16. Bruni, P.; Tosi, G. Gazz. Chim. Ital. 1997, 127, 435–459. 17. Webster, O. W. J. Polym. Sci., Part A: Polym.Chem. 2002, 40, 210–221. 18. Flamini, A. Curr. Org. Chem. 2003, 7, 1793–1820. 19. Clements, P.; Gream, G. E.; Kirkbride, P. K.; Pyke, S. M. Helv. Chim. Acta 2005, 88, 2003–2021. 20. Roland, J. R.; McKusick, B. C. J. Am. Chem. Soc. 1961, 83, 1652–1657. 21. Scott, L. T.; Kirms, M. A. J. Am. Chem. Soc. 1982, 104, 3530–3531. 22. Balduzzi, S.; Mueller-Bunz, H.; McGlinchey, M. J. Chem. Eur. J. 2004, 10, 5398–5405. 23. Preparation of complex a1: To a solution of TCNE (5.7 mg, 45 mmol) in toluene (1 mL) was slowly added a solution of 1 (12.4 mg, 23 mmol) in toluene (1 mL) at K20 8C under argon, and then was stirred for 2 h. After addition of hexane (8 mL) to the reaction solution, the mixture was allowed to stand at K20 8C for 24 h under argon, giving a precipitation of a dark red solid, which was centrifuged at 2.5 krpm for 1 min. The crude product a1 thus obtained was carefully washed with hexane and dried well in vacuum desiccator to provide stable a1 as a dark red powder (3.2 mg, 3.95 mmol, 18% yield). Complex a1: mp O157 8C [decomp., determined by thermal analysis (TGA and DTA)]; UV–vis lmax (benzene) nm, 295, 340, 357, 372 and 515; IR nmax (KBr) cmK1, 2206 (–C^N); FAB-MS (positive) (3-nitrobenzyl alcohol matrix), m/z 555 ([MK2(TCNE)CH]C); FAB-MS (negative) (3-nitrobenzyl alcohol matrix), m/z 128 (TCNEK); exact FAB-MS (positive) (3-nitrobenzyl alcohol matrix), found: m/z 555.3244; calcd for C40H43O2: [MK2(TCNE)CH]C, m/z 555.3263. 24. Preparation of complex a2: To a solution of TCNE (11.4 mg, 89 mmol) in toluene (2 mL) was slowly added a solution of 2 (26 mg, 44 mmol) in toluene (2 mL) at K20 8C under argon, and then was stirred for 2 h. After addition of hexane (16 mL) to the reaction solution, the mixture was allowed to stand at K20 8C for 24 h under argon, giving a precipitation of a dark red solid, which was centrifuged at 2.5 krpm for 1 min. The crude product a2 thus obtained was carefully washed with hexane and dried well in vacuum desiccator to provide stable a2 as a dark red powder (8.3 mg, 9.8 mmol, 23% yield). Complex a2: mp O120 8C [decomp., determined by thermal analysis (TGA and DTA)]; UV–vis lmax (benzene) nm, 297, 342, 358, 375 and 508; IR nmax (KBr) cmK1, 2206 (–C^N); FAB-MS (positive) (3-nitrobenzyl alcohol matrix), m/z 843 ([MCH]C) and 715 ([MKTCNECH]C); FAB-MS (negative) (3-nitrobenzyl alcohol matrix), m/z 128 (TCNEK); exact FAB-MS (positive) (3-nitrobenzyl alcohol matrix), found: m/z 715.2965; calcd for C46H43N4S2: [MKTCNECH]C, m/z 715.2929. 25. Takekuma, S.; Matsubara, Y.; Yamamoto, H.; Nozoe, T. Bull. Chem. Soc. Jpn. 1988, 61, 475–481.
Reactions of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane and (1R,2S)-1,2-di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane with tetracyanoethylene (TCNE) in benzene: comparative studies on the products and their spectroscopic properties Shin-ichi Takekuma,a,* Kenji Takahashi,a Akio Sakaguchi,a Masato Sasaki,a Toshie Minematsub and Hideko Takekumaa a
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka-shi, Osaka 577-8502, Japan b School of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi-Osaka-shi, Osaka 577-8502, Japan Received 22 August 2005; accepted 4 November 2005 Available online 9 December 2005
Abstract—Reactions of the title meso forms, (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2-di(3-guaiazulenyl)1,2-di(2-thienyl)ethane (2), with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen give new compounds, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3) and 2,2,3,3-tetracyano-8-isopropyl6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4), respectively, in 74 and 21% isolated yields. Comparative studies on the above reactions as well as the spectroscopic properties of the unique products 3 and 4, possessing interesting molecular structures, are reported and, further, a plausible reaction pathway for the formation of these products is described. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction In the previous papers, 1–9 we reported a facile preparation and the crystal structures as well as the spectroscopic, chemical and electrochemical properties of the mono- and dicarbocations stabilized by a 3-guaiazulenyl group. During the course of our investigations, we have recently found (i) that the reactions of naturally occurring guaiazulene (A) with 2-furaldehyde (B) and thiophene-2-carbaldehyde (C) in methanol in the presence of hexafluorophosphoric acid gave (2-furyl)(3guaiazulenyl)methylium and (3-guaiazulenyl)(2-thienyl)methylium hexafluorophosphates (D and E) with the following two representative resonance forms [i.e., 3-guaiazulenylium and 2-furanylium (and 2-thienylium) structures], respectively, in 93 and 98% isolated yields (see Fig. 1),10 which upon reduction with zinc powder in acetonitrile afforded a mixture of a meso form and two enantiomeric forms of the molecular structures, 1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (F) and Keywords: Annulation; C–C bond cleavage; Charge-transfer complex; Furan; Guaiazulene; TCNE; Thiophene; X-ray crystal structure. * Corresponding author. Tel.: C81 6 6730 5880x4020; fax: C81 6 6727 4301; e-mail: [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.11.026
1,2-di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (G), respectively, (see Fig. 2),10 and have quite recently found (ii) that, from a comparative study on the oxidation potentials of the above meso forms, (1R,2S)-1,2-di(2furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (2), measured by means of the CV and DPV (Potential/V vs SCE) in CH2Cl2 containing 0.1 M [n-Bu4N]PF6 as a supporting electrolyte, it could be inferred that 1 and 2 simultaneously underwent two-electron oxidation of their 1,2di(3-guaiazulenyl) groups, respectively, at the potentials of C0.48 (Epa, irreversible) V by CV and C0.48 (Ep) V by DPV for 111 and C0.57 (Epa, irreversible) V by CV and C0.56 (Ep) V by DPV for 2,12 generating the corresponding di(cation-radical) species 12(C%) and 22(C%). Thus, 1 was more susceptible to oxidation than 2 and A [C0.69 (Epa, irreversible) V by CV and C0.65 (Ep) V by DPV],8 owing to the difference in ionization potential (corresponding to p-HOMO) based on those p-electron systems. The different oxidation potentials between 1 and 2 are obviously caused by the influence of a different heteroaromatic-ring. Therefore, our interest has been focused on a comparative study on the title chemistry, the reactions of 1 and 2 with TCNE, which serves as an electron acceptor [K0.75 (E1/2, quasi-reversible) V by CV and
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1521
--
PF6
X
+
+
X CHO
A
H
in MeOH in the presence of 60% HPF6 under aerobic conditions
B, C
D, E 5
-- + X
--
PF6
PF6
1
--
PF6
X
X
2 1'
+
3'
H
H
H 4'
+
7'
B, D: X = O C, E: X = S D'', E''
D, E
D', E'
Figure 1. The reactions of A with B and C in CH3OH with HPF6 under aerobic conditions, affording the corresponding monocarbocation products D and E with the resonance forms of D 0 , E 0 and D 00 , E 00 , respectively.10
--
PF6
1''
7''
X
+
H
Zinc powder in acetonitrile under argon
3'' 4''
1
2
2'
X
1'
X
5' D, E
F, G D, F: X = O E, G: X = S
Figure 2. The reductions of D and E with zinc powder in acetonitrile at 25 8C under argon, affording a mixture of a meso form and two enantiomeric forms of the molecular structures F and G, respectively.10
K0.76 (Ep) V by DPV].13 In relation to our basic studies, in 1961 Hafner and Moritz reported that the reaction of guaiazulene (A) in petroleum ether with TCNE in AcOEt at K20 8C gave a 1:1 p-complex in 98% isolated yield, which was converted into 3-tricyanovinylguaiazulene (68% isolated yield) in DMF at room temperature.14 Furthermore, the addition and cycloaddition reactions of TCNE in organic chemistry15–19 including azulenes14,20–22 have been studied to a considerable extent, and a large number of those results and discussion have been well documented. Along with those investigations, we now wish to report the detailed title chemistry, affording the unique products, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene (3) from 1 and 2,2,3,3tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4) from 2, possessing interesting molecular structures, presumably via charge-transfer, C–C bond cleavage and, further, [p8Cp2] cycloaddtion reactions (see Scheme 1).
2. Results and discussion 2.1. Reactions of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2-di(3-guaiazulenyl)1,2-di(2-thienyl)ethane (2) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-4-(2-furyl)8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3) and 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4) Compound 3 was prepared using benzene as a solvent as shown in Figure 3 and Section 4.1.1, whose molecular structure was established on the basis of elemental analysis and spectroscopic data [UV–vis, IR, exact FAB-MS, 1H and 13 C NMR including 2D NMR (i.e., H–H COSY, NOESY, HMQC Z1H detected hetero nuclear multiple quantum coherence and HMBC Z1H detected hetero nuclear multiple bond connectivity)].
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X
X
X H
+
e
H
NC
CN
NC
CN
NC
CN
NC
CN
NC
CN
NC
CN
a1,2
1, 2
b1,2 X
X
O2
b1,2
H
+
– OOH +
H H
NC
CN
NC
CN
H H H
NC
CN
NC
CN
O2
d1,2
c1,2 π8 + π2
d1,2
1, 3, a1, b1, c1, d1: X = O 2, 4, a2, b2, c2, d2: X = S
3, 4
Scheme 1. A plausible reaction pathway for the formation of 3 and 4 yielded by the reactions of the title meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen. The partial structures 1, 2 and a1,2 are illustrated.
5' 1'
1''
7''
6
3''
5
X 2'
4a
4
TCNE
4''
1
6a
2 in benzene under oxygen
2' 1'
X
X
10b 1 10a
7 8 9
5' 1, 2
CN 3 2
CN CN CN
10 3, 4
1, 3: X = O 2, 4: X = S
Figure 3. The reactions of the title meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen.
Compound 3 (74% isolated yield) was blue prisms [RfZ 0.27 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 163 8C and decomp. O171 8C [determined by the thermal analysis (TGA and DTA)]. The characteristic UV–vis (CH2Cl2) absorption bands based on the azulenyl group were observed and the longest visible absorption wavelength appeared at lmax 622 nm (log 3Z2.69). The IR (KBr) spectrum showed a specific band based on the –C^N group at 2252 cmK1. The protonated molecular formula C26H21N4O ([MCH]C) was determined by the exact FAB-MS (3-nitrobenzyl alcohol matrix) spectrum. The elemental analysis confirmed the molecular formula C26H20N4O. The 500 MHz 1H NMR (C6D6) spectrum showed signals based on the structure of 8-isopropyl-6methyl-1,4-dihydrocyclohepta[c,d]azulene possessing a 2-furyl group at the C-4 position, which signals (d and J values) were carefully assigned using the H–H COSY technique and the computer simulation analysis (see Section 4.1.1). Moreover, the NOESY spectrum revealed apparent cross peaks between the following signals [i.e., –C1H2– and H-10; d 3.15 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)], and suggested that the 2-furyl group
was substituted at the C-4 (equatorial) position. The 125 MHz 13C NMR (C6D6) spectrum exhibited 25 carbon signals (d) assigned by the HMQC and HMBC techniques (see Section 4.1.1). Thus, the elemental analysis and these spectroscopic data for 3 led to a new molecular structure, 2,2,3,3-tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene (see Fig. 3). Similarly, as in the case of the preparation of 3, the target compound 4 was prepared using benzene as a solvent as shown in Figure 3 and Section 4.1.2, whose molecular structure was established on the basis of similar spectroscopic analyses as 3. Although this reaction afforded numerous products as compared with the reaction of 1 with TCNE (see Section 4.1.1), compound 4 could be isolated in 21% yield. The difference between the yields of the products 3 and 4 is obviously caused by the different oxidation potentials between 111 and 212 (see Section 1), whose oxidation potentials suggest that the reaction of 1 with TCNE readily gives the corresponding 1:2 chargetransfer (CT) complex a1 (see Scheme 1) as compared with the reaction of 2 with TCNE.
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Compound 4 was blue blocks [RfZ0.61 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 147 8C and decomp. O158 8C [determined by the thermal analysis (TGA and DTA)]. The characteristic UV–vis (CH2Cl2) absorption bands based on the azulenyl group, which spectral pattern coincided with that of 3, were observed and the longest visible absorption wavelength appeared at lmax 624 nm (log 3Z2.64). The IR (KBr) spectrum showed a specific band based on the –C^N group at 2252 cmK1, which wavenumber coincided with that of 3. The protonated molecular formula C26H21N4S ([MCH]C) was determined by the exact FAB-MS (3-nitrobenzyl alcohol matrix) spectrum. The 500 MHz 1H NMR (C6D6) spectrum showed signals based on the structure of 8-isopropyl-6-methyl-1,4dihydrocyclohepta[c,d]azulene possessing a 2-thienyl group at the C-4 position, which signals (d and J values) were carefully assigned using the H–H COSY technique and the computer simulation analysis (see Section 4.1.2). Similarly, as in the case of 3, the NOESY spectrum revealed apparent cross peaks between the following signals [i.e., –C1H2– and H-10; d 3.18 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)], and suggested that the 2-thienyl group was substituted at the C-4 (equatorial) position. The 125 MHz 13C NMR (C6D6) spectrum exhibited 25 carbon signals (d) assigned by the HMQC and HMBC techniques (see Section 4.1.2). Thus, these spectroscopic data for 4 led to a new molecular structure, 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (see Fig. 3).
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methyl-1,4-dihydrocyclohepta[c,d]azulene possessing a 2-furyl group at the C-4 (equatorial) position, presumed by means of the NOESY spectrum (see Sections 2.1 and 4.1.1). From a comparative study on the detailed spectroscopic properties of 3 and 4 (see Sections 4.1.1 and 4.1.2), it can be inferred that the crystal structure of 4 assumes similar conformation to that of 3. 2.3. A plausible reaction pathway for the formation of the products 3 and 4 From the molecular structures of the resulting products 3 and 4, obtained by the reactions of the meso forms 1 and 2 with a two molar amount of TCNE in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen, a plausible reaction pathway for the formation of compounds 3 and 4 can be inferred as illustrated in Scheme 1; namely, (i) the reactions of 1 and 2 with TCNE gradually give the corresponding 1:2 charge-transfer (CT) complexes a1,2, respectively, whose formation was supported by the timedependent visible spectra of the reaction solutions as shown in Figures 5 and 6. The characteristic CT band based on the formation of a1, generated from the reaction of 1 with TCNE, appeared at lmax 518 nm (the time to reach
2.2. X-ray crystal structure of 3 Although an X-ray crystallographic analysis of 4 has not yet been achieved because of difficulty in obtaining a single crystal suitable for this purpose, the crystal structure of 3 has been determined by means of the X-ray diffraction (see Section 4.1.3). In the course of refinement, the atoms based on the 2-furyl group at the C-4 (equatorial) position and two methyl groups of the isopropyl group at the C-8 position for 3 were found to be disordered over two sites of equal occupancy. Therefore, those atoms in the occupancy ratio 50:50 were refined. The only ORTEP drawing of 3 noted the above disorder, indicating the molecular structure, 2,2,3,3tetracyano-4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene, illustrated in Figure 4a, is shown in Figure 4b. As the result, the crystal structure of 3 clarified the conformation of 2,2,3,3-tetracyano-8-isopropyl-6-
Figure 4. (a) The molecular structure of 3. (b) The ORTEP drawing of 3 (30% probability thermal ellipsoids).
Figure 5. The time-dependent visible spectra for the reaction of 1 (2.0 mg, 3.6 mmol) with TCNE (1.0 mg, 7.8 mmol) in benzene (10 mL) at 25 8C for 2 h under argon. Length of the cell: 0.1 cm. Interval time: 10 min. lmax 357, 374 and 400sh nm: the specific bands based on TCNE; lmax 518 nm: the specific band based on the formation of the CT complex a1 (see Scheme 1) (the time to reach maximum absorptionZ2 h); lmax 618, 680sh and 750sh nm: the specific bands based on 1.
Figure 6. The time-dependent visible spectra for the reaction of 2 (2.0 mg, 3.4 mmol) with TCNE (1.0 mg, 7.8 mmol) in benzene (10 mL) at 25 8C for 11 h under argon. Length of the cell: 0.1 cm. Interval time: 1 h. lmax 359, 375 and 400sh nm: the specific bands based on TCNE; lmax 517 nm: the specific band based on the formation of the CT complex a2 (see Scheme 1) (the time to reach maximum absorptionZ11 h); lmax 622, 680sh and 750sh nm: the specific bands based on 2.
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maximum absorption Z2 h), whose band revealed a bathochromic shift (D19 nm) in comparison with the characteristic visible band of (2-furyl)(3-guaiazulenyl)methylium hexafluorophosphate (D) (lmax 499 nm, log 3Z 4.61);10 and, similarly, the characteristic CT band based on the formation of a2, generated from the reaction of 2 with TCNE, appeared at lmax 517 nm (the time to reach maximum absorption Z11 h), whose band showed a bathochromic shift (D20 nm) in comparison with the characteristic visible band of (3-guaiazulenyl)(2-thienyl)methylium hexafluorophosphate (E) (lmax 497 nm, log 3Z 4.73).10 The time to reach maximum absorption indicates that 1 more readily forms the corresponding 1:2 CT complex a1 than 2, whose result is obviously caused by the different oxidation potentials between 111 and 212 (see Section 1); and (ii) although the CT complexes a1,2 generated were stable in benzene at 25 8C for a long time (O72 h) under argon and they did not give 3 and 4, the complexes a1,2 were gradually converted into 3 and 4 in benzene at 25 8C under oxygen. Furthermore, the reactions of 1 and 2 with a two molar amount of TCNE in toluene at K20 8C for 24 h under argon gave the corresponding 1:2 CT complexes a1,2, respectively, in 18% isolated yield from 123 and 23% isolated yield from 2,24 whose complexes were gradually converted into 3 and 4, respectively, in benzene at 25 8C under oxygen. Along with the above results, the reactions of (2-furyl)(3-guaiazulenyl)methylium and (3guaiazulenyl)(2-thienyl)methylium hexafluorophosphates (D and E) with an equimolar amount of TCNE in acetonitrile at 25 8C for 72 h under oxygen did not afford any products. From these results, it can be inferred that the CT complexes a1,2 generated are converted into 3 and 4 presumably via the 3-guaiazulenylium-ions b1,2 yielded by the C–C bond cleavage of the ethane units of a1,2, the azulenequinodimethanes d1,2 produced by the deprotonations of the C4-methyl groups of c1,2 by the attack of OK% 2 generated by the electron transfer from the TCNEK% to O2 and, further, the [p8Cp2] cycloaddition reactions of d1,2 with TCNE, as illustrated in Scheme 1. On the other hand, in 1988 one of us (S. Takekuma) reported that the autoxidation of guaiazulene (A) under several reaction conditions gave a wide variety of products possessing interesting molecular structures, respectively, through such several types of the reactions as oxidative dimerization, oxidation of side chains, azulenequinone formation, intermolecular onecarbon transfer reactions and rearrangements to naphthalenoid, benzenoid and 1H-inden-1-one derivatives, simultaneously.25 In this case, by the attack of oxygen, A was considered to initially form an electron-transfer complex (AC% OK% 2 ), which in turn leads to the formation of various products.
3. Conclusion We have reported the following four points in this paper: (i) the reactions of the title meso forms, (1R,2S)-1,2-di(2furyl)-1,2-di(3-guaiazulenyl)ethane (1) and (1R,2S)-1,2di(3-guaiazulenyl)-1,2-di(2-thienyl)ethane (2), with a two molar amount of tetracyanoethylene (TCNE) in benzene at 25 8C for 5 h (for 1) and 48 h (for 2) under oxygen gave new compounds 3 and 4, respectively, in 74 and 21% isolated yields; (ii) the detailed spectroscopic analyses of these
products led to the molecular structures, 2,2,3,3-tetracyano4-(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene for 3 and 2,2,3,3-tetracyano-8-isopropyl-6methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene for 4;(iii)alongwiththespectroscopicdatafor3and4,weclarified the crystal structure of 3; and, further, (iv) a plausible reaction pathway for the formation of the unique products 3 and 4, possessinginterestingmolecularstructures,wasdescribed. 4. Experimental 4.1. General Thermal (TGA/DTA) and elemental analyses were taken on a Shimadzu DTG-50H thermal analyzer and a Yanaco MT-3 CHN corder, respectively. MS spectra were taken on a JEOL The Tandem Mstation JMS-700 TKM data system. UV–vis and IR spectra were taken on a Beckman DU640 spectrophotometer and a Shimadzu FTIR-4200 Grating spectrometer, respectively. NMR spectra were recorded with a JEOL GX-500 (500 MHz for 1H and 125 MHz for 13 C) at 25 8C. The 1H NMR spectra were assigned using the computer-assisted simulation analysis (the software: gNMR developed by Adept Scientific plc) on a DELL Dimension XPS T500 personal-computer with a Pentium III processor. Cyclic and differential pulse voltammograms were measured by an ALS Model 600 electrochemical analyzer. 4.1.1. Reaction of (1R,2S)-1,2-di(2-furyl)-1,2-di(3-guaiazulenyl)ethane (1) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-4-(2-furyl)-8isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3). To a solution of TCNE (48 mg, 0.37 mmol) in benzene (10 mL) was slowly added a solution of 1 (97 mg, 0.17 mmol) [RfZ0.71 on silica-gel TLC (hexane/AcOEtZ 4:6, vol/vol)] in benzene (30 mL), turning the blue solution into a red solution. The mixture was stirred at 25 8C for 5 h under oxygen, turning the red solution into a green solution. After the reaction, the reaction solution was evaporated in vacuo, giving a green solid. The residue obtained was carefully separated by silica-gel column chromatography with hexane–ethyl acetate (3/2, vol/vol) as an eluant. The crude product 3 thus obtained was recrystallized from benzene–cyclohexane (1/5, vol/vol) (several times) to provide pure 3 as stable crystals (104 mg, 0.26 mmol, 74% yield). Compound 3: blue prisms [RfZ0.27 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 163 8C and decomp. O 171 8C [determined by thermal analysis (TGA and DTA)]; Found: C, 76.89; H, 5.15; N, 13.66%. Calcd for C26H20N4O: C, 77.21; H, 4.98; N, 13.85%; UV–vis lmax (CH2Cl2) nm (log 3), 247 (4.46), 296 (4.65), 358 (3.86), 376 (3.86), 622 (2.69), 684sh (2.58) and 754sh (2.12); IR nmax (KBr) cmK1, 2252 (–C^N); FAB-MS (3-nitrobenzyl alcohol matrix), m/z 405 ([MCH]C, 100%) and 404 (MC, 94); exact FABMS (3-nitrobenzyl alcohol matrix), found: m/z 405.1711; calcd for C26H21N4O: [MCH]C, m/z 405.1715; 500 MHz 1 H NMR (C6D6), signals based on the 1,4-dihydro-8isopropyl-6-methylcyclohepta[c,d]azulene unit at d 1.05 (6H, d, JZ6.9 Hz, (CH3)2CH-8), 2.26 (3H, s, Me-6), 2.60 (1H, sept, JZ6.9 Hz, (CH3)2CH-8), 3.15 (axial), 3.42
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(equatorial) (1H each, d, JZ15.2 Hz, –C1H2–), 5.11 (1H axial, br d s, pC4H–), 6.36 (1H, d, JZ10.4 Hz, H-10), 6.98 (1H, dd, JZ10.4, 2.0 Hz, H-9), 7.00 (1H, br d s, H-5) and 7.94 (1H, d, JZ2.0 Hz, H-7) and signals based on the 2-furyl group at d 5.99 (1H, dd, JZ3.5, 2.0 Hz, H-4 0 ), 6.05 (1H, ddd, JZ3.5, 0.9, 0.7 Hz, H-3 0 ) and 7.03 (1H, dd, JZ 2.0, 0.9 Hz, H-5 0 ); the NOESY spectrum showed cross peaks between the following signals [–C1H2– and H-10; d 3.15 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)]; 125 MHz 13C NMR (C6D6), signals based on the 2,2,3,3-tetracyano-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene unit at d 12.2 (Me-6), 24.1 ((CH3)2CH-8), 38.2 ((CH3)2CH-8), 44.3 (–C1H2–), 45.7 (pC4H–), 43.8, 49.6 [(NC)2C-2 or (NC)2C-3], 111.22, 111.24, 111.4, 112.2 [(NC)2C-2 or (NC)2C-3], 118.7 (C10b), 125.5 (C-6a), 126.9 (C-10), 127.4 (C-4a), 134.9 (C10a), 135.3 (C-9), 135.5 (C-7), 140.1 (C-6), 140.6 (C-5) and 144.1 (C-8) and signals based on the 2-furyl group at d 111.1 (C-4 0 ), 112.9 (C-3 0 ), 144.4 (C-5 0 ) and 148.8 (C-2 0 ). 4.1.2. Reaction of (1R,2S)-1,2-di(3-guaiazulenyl)-1,2di(2-thienyl)ethane (2) with TCNE; preparation and spectroscopic properties of 2,2,3,3-tetracyano-8-isopropyl-6-methyl-4-(2-thienyl)-1,4-dihydrocyclohepta[c,d]azulene (4). To a solution of TCNE (57 mg, 0.44 mmol) in benzene (10 mL) was slowly added a solution of 2 (130 mg, 0.22 mmol) [RfZ0.70 on silica-gel TLC (hexane/AcOEtZ 4:6, vol/vol)] in benzene (20 mL), turning the blue solution into a red solution. The mixture was stirred at 25 8C for 48 h under oxygen, turning the red solution into a green solution. After the reaction, the reaction solution was evaporated in vacuo, giving a green solid. The residue obtained was carefully separated by silica-gel column chromatography with hexane–ethyl acetate (3/2, vol/vol) as an eluant. The crude product 4 thus obtained was recrystallized from dichloromethane–hexane (1/5, vol/vol) (several times) to provide pure 4 as stable crystals (40 mg, 0.10 mmol, 21% yield). Compound 4: blue blocks [RfZ0.61 on silica-gel TLC (hexane/AcOEtZ4:6, vol/vol)], mp 147 8C and decomp. O158 8C [determined by thermal analysis (TGA and DTA)]; UV–vis lmax (CH2Cl2) nm (log 3), 247 (4.46), 297 (4.59), 360 (3.79), 377 (3.77), 624 (2.64), 683sh (2.55) and 760sh (2.06); IR nmax (KBr) cmK1, 2252 (–C^N); FABMS (3-nitrobenzyl alcohol matrix), m/z 421 ([MCH]C, 100%) and 420 (MC, 80); exact FAB-MS (3-nitrobenzyl alcohol matrix), found: m/z 421.1449; calcd for C26H21N4S: [MCH]C, m/z 421.1487; 500 MHz 1H NMR (C6D6), signals based on the 1,4-dihydro-8-isopropyl-6-methylcyclohepta[c,d]azulene unit at d 1.04 (6H, d, JZ6.9 Hz, (CH3)2CH-8), 2.23 (3H, s, Me-6), 2.59 (1H, sept, JZ6.9 Hz, (CH3)2CH-8), 3.18 (axial), 3.24 (equatorial) (1H each, d, JZ15.1 Hz, –C1H2–), 5.30 (1H axial, br d s, pC4H–), 6.33 (1H, d, JZ10.3 Hz, H-10), 6.96 (1H, dd, JZ10.3, 2.0 Hz, H-9), 7.17 (1H, br d s, H-5) and 7.93 (1H, d, JZ2.0 Hz, H-7) and signals based on the 2-thienyl group at d 6.65 (1H, dd, JZ5.2, 3.7 Hz, H-4 0 ), 6.80 (1H, dd, JZ5.2, 1.1 Hz, H-5 0 ) and 7.11 (1H, dd, JZ3.7, 1.1 Hz, H-3 0 ); the NOESY spectrum showed cross peaks between the following signals [–C1H2– and H-10; d 3.18 (axial; one of two signals based on the –C1H2–) and pC4H– (axial)]; 125 MHz 13C NMR (C6D6), signals based on the 2,2,3,3-tetracyano-8-isopropyl-
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6-methyl-1,4-dihydrocyclohepta[c,d]azulene unit at d 12.3 (Me-6), 24.1 ((CH3)2CH-8), 38.2 ((CH3)2CH-8), 44.4 (–C1H 2–), 47.0 (pC 4H–), 44.3, 51.9 [(NC)2C-2 or (NC)2C-3], 111.3, 111.5, 111.7, 112.1 [(NC)2C-2 or (NC)2C-3], 120.8 (C-4a), 125.2 (C-6), 126.9 (C-10), 135.0 (C-10a), 135.2 (C-10b), 135.4 (C-9), 135.6 (C-7), 140.2 (C-6a), 141.0 (C-5) and 144.2 (C-8) and signals based on the 2-thienyl group at d 127.5 (C-5 0 ), 127.8 (C-4 0 ), 129.8 (C-3 0 ) and 138.6 (C-2 0 ). 4.1.3. X-ray crystal structure of 2,2,3,3-tetracyano-4(2-furyl)-8-isopropyl-6-methyl-1,4-dihydrocyclohepta[c,d]azulene (3). A total 5443 reflections with 2qmaxZ55.08 were collected on a Rigaku AFC-5R automated four-circle diffractometer with graphite monochromated Mo Ka radi˚ , rotating anode: 50 kV, 180 mA) at ation (lZ0.71069 A 296 K. The structure was solved by direct methods (SIR92) and expanded using Fourier techniques (DIRDIF99). The atoms based on the 2-furyl group at the C-4 (equatorial) position and two methyl groups of the isopropyl group at the C-8 position for 3 were disordered over two sites of equal occupancy. Therefore, those atoms in the occupancy ratio 50:50 were refined. The non-hydrogen atoms were refined anisotropically. The final cycle of full-matrix least-squares refinement was based on F2. All calculations were performed using the Crystal Structure 3.7.0 software package. Crystallographic data have been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK and copies can be obtained on request, free of charge, by quoting the publication citation and the deposition number CCDC 248295. Crystallographic data for 3: C26H20N4O (FWZ404.47), blue prism (the crystal size, 0.80!0.30!0.30 mm3), ˚ , bZ monoclinic, P21/c (#14), aZ10.278(2) A ˚ ˚ 15.3808(19) A, cZ14.2500(19) A, bZ91.831(14)8, VZ ˚ 3, ZZ4, D calcdZ1.193 g/cm 3, m(Mo Ka) 2251.5(7) A Z0.750 cmK1, Scan widthZ(1.47C0.30 tanq)8, Scan modeZuK2q, Scan rateZ9.08/min, measured reflectionsZ5443, observed reflectionsZ5163, No of parametersZ270, R1Z0.0677, wR2Z0.2539 and Goodness of Fit Indicator Z1.027.
Acknowledgements We would like to express our considerable gratitude to Dr. Kunihisa Sugimoto (Rigaku Corporation) for his help in the X-ray crystallographic analysis of compound 3. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
References and notes 1. Takekuma, S.; Sasaki, M.; Takekuma, H.; Yamamoto, H. Chem. Lett. 1999, 999–1000. 2. Takekuma, S.; Takata, S.; Sasaki, M.; Takekuma, H. Tetrahedron Lett. 2001, 42, 5921–5924.
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3. Takekuma, S.; Tanizawa, M.; Sasaki, M.; Matsumoto, T.; Takekuma, H. Tetrahedron Lett. 2002, 43, 2073–2078. 4. Sasaki, M.; Nakamura, M.; Hannita, G.; Takekuma, H.; Minematsu, T.; Yoshihara, M.; Takekuma, S. Tetrahedron Lett. 2003, 44, 275–279. 5. Sasaki, M.; Nakamura, M.; Uriu, T.; Takekuma, H.; Minematsu, T.; Yoshihara, M.; Takekuma, S. Tetrahedron 2003, 59, 505–516. 6. Nakamura, M.; Sasaki, M.; Takekuma, H.; Minematsu, T.; Takekuma, S. Bull. Chem. Soc. Jpn. 2003, 76, 2051–2052. 7. Takekuma, S.; Sasaki, K.; Nakatsuji, M.; Sasaki, M.; Minematsu, T.; Takekuma, H. Bull. Chem. Soc. Jpn. 2004, 77, 379–380. 8. Nakatsuji, M.; Hata, Y.; Fujihara, T.; Yamamoto, K.; Sasaki, M.; Takekuma, H.; Yoshihara, M.; Minematsu, T.; Takekuma, S. Tetrahedron 2004, 60, 5983–6000. 9. Takekuma, S.; Hata, Y.; Nishimoto, T.; Nomura, E.; Sasaki, M.; Minematsu, T.; Takekuma, H. Tetrahedron 2005, 61, 6892–6907. 10. Takekuma, S.; Takahashi, K.; Sakaguchi, A.; Shibata, Y.; Sasaki, M.; Minematsu, T.; Takekuma, H. Tetrahedron 2005, 61, 10349–10362. 11. The electrochemical measurement conditions of 1 are as follows: the cyclic and differential pulse voltammograms (Potential/V vs SCE) of 1 (3.0 mg, 5.4 mmol) in 0.1 M [nBu4N]PF6, CH2Cl2 (10 mL) at a glassy carbon (ID: 3 mm) and a platinum wire served as the working and auxiliary electrodes; scan rates 100 mV sK1 at 25 8C under argon, respectively. For comparative purposes, the oxidation potential using ferrocene as a standard material showed C0.42 (Ep) V by DPV and C0.42 (E1/2, quasi-reversible) V by CV in 0.1 M [n-Bu4N]PF6, CH2Cl2 under the same electrochemical conditions as 1. 12. The cyclic and differential pulse voltammograms of 2 (3.0 mg, 5.1 mmol) were measured under the same electrochemical conditions as 1. 13. The electrochemical measurement conditions of TCNE are as follows: the cyclic and differential pulse voltammograms (Potential/V vs SCE) of TCNE (3.0 mg, 23.4 mmol) in 0.1 M [n-Bu4N]BF4, CH3CN (10 mL) at a glassy carbon (ID: 3 mm) and a platinum wire served as the working and auxiliary electrodes; scan rates 100 mV sK1 at 25 8C under argon, respectively. For comparative purposes, the oxidation potential using ferrocene as a standard material showed C0.45 (Ep) V by DPV and C0.42 (E1/2, quasi-reversible) V by CV in 0.1 M [n-Bu4N]BF4, CH3CN under the same electrochemical conditions as TCNE. 14. Hafner, K.; Moritz, K.-L. Liebigs Ann. Chem. 1961, 650, 92–97. 15. (a) Fatiadi, A. J. Synthesis 1986, 249–284. (b) Fatiadi, A. J. Synthesis 1987, 749–789.
16. Bruni, P.; Tosi, G. Gazz. Chim. Ital. 1997, 127, 435–459. 17. Webster, O. W. J. Polym. Sci., Part A: Polym.Chem. 2002, 40, 210–221. 18. Flamini, A. Curr. Org. Chem. 2003, 7, 1793–1820. 19. Clements, P.; Gream, G. E.; Kirkbride, P. K.; Pyke, S. M. Helv. Chim. Acta 2005, 88, 2003–2021. 20. Roland, J. R.; McKusick, B. C. J. Am. Chem. Soc. 1961, 83, 1652–1657. 21. Scott, L. T.; Kirms, M. A. J. Am. Chem. Soc. 1982, 104, 3530–3531. 22. Balduzzi, S.; Mueller-Bunz, H.; McGlinchey, M. J. Chem. Eur. J. 2004, 10, 5398–5405. 23. Preparation of complex a1: To a solution of TCNE (5.7 mg, 45 mmol) in toluene (1 mL) was slowly added a solution of 1 (12.4 mg, 23 mmol) in toluene (1 mL) at K20 8C under argon, and then was stirred for 2 h. After addition of hexane (8 mL) to the reaction solution, the mixture was allowed to stand at K20 8C for 24 h under argon, giving a precipitation of a dark red solid, which was centrifuged at 2.5 krpm for 1 min. The crude product a1 thus obtained was carefully washed with hexane and dried well in vacuum desiccator to provide stable a1 as a dark red powder (3.2 mg, 3.95 mmol, 18% yield). Complex a1: mp O157 8C [decomp., determined by thermal analysis (TGA and DTA)]; UV–vis lmax (benzene) nm, 295, 340, 357, 372 and 515; IR nmax (KBr) cmK1, 2206 (–C^N); FAB-MS (positive) (3-nitrobenzyl alcohol matrix), m/z 555 ([MK2(TCNE)CH]C); FAB-MS (negative) (3-nitrobenzyl alcohol matrix), m/z 128 (TCNEK); exact FAB-MS (positive) (3-nitrobenzyl alcohol matrix), found: m/z 555.3244; calcd for C40H43O2: [MK2(TCNE)CH]C, m/z 555.3263. 24. Preparation of complex a2: To a solution of TCNE (11.4 mg, 89 mmol) in toluene (2 mL) was slowly added a solution of 2 (26 mg, 44 mmol) in toluene (2 mL) at K20 8C under argon, and then was stirred for 2 h. After addition of hexane (16 mL) to the reaction solution, the mixture was allowed to stand at K20 8C for 24 h under argon, giving a precipitation of a dark red solid, which was centrifuged at 2.5 krpm for 1 min. The crude product a2 thus obtained was carefully washed with hexane and dried well in vacuum desiccator to provide stable a2 as a dark red powder (8.3 mg, 9.8 mmol, 23% yield). Complex a2: mp O120 8C [decomp., determined by thermal analysis (TGA and DTA)]; UV–vis lmax (benzene) nm, 297, 342, 358, 375 and 508; IR nmax (KBr) cmK1, 2206 (–C^N); FAB-MS (positive) (3-nitrobenzyl alcohol matrix), m/z 843 ([MCH]C) and 715 ([MKTCNECH]C); FAB-MS (negative) (3-nitrobenzyl alcohol matrix), m/z 128 (TCNEK); exact FAB-MS (positive) (3-nitrobenzyl alcohol matrix), found: m/z 715.2965; calcd for C46H43N4S2: [MKTCNECH]C, m/z 715.2929. 25. Takekuma, S.; Matsubara, Y.; Yamamoto, H.; Nozoe, T. Bull. Chem. Soc. Jpn. 1988, 61, 475–481.