ARTICLE pubs.acs.org/Organometallics
Aryl(trimethylsilyl)selenides as Reagents for the Synthesis of Mono- and Diselenoesters Deeb Taher†,§ and John F. Corrigan*,†,‡ † ‡
Department of Chemistry, The University of Western Ontario, London, ON, N6A 5B7, Canada Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, ON, Canada N6A 3K7
bS Supporting Information ABSTRACT: Silylated organoselenium reagents react under mild conditions with acid chlorides to provide a high yield route to aromatic selenoesters. The synthesis, structures, and spectroscopic properties of the selenoesters C6H5SeC(O)R (R = CH2CH3, 1; p-CH3C6H4, 2; p-C6Me4Br, 3; p-C6Me4C(O)SeC6H5, 4) and RC(O)SeFcSeC(O)R (Fc = Fe(C5H4)2; R = CH2CH3, 5; p-CH3C6H4, 6; p-BrC6Me4, 7) and RC(O)Se(C6H4)nSeC(O)R (n = 1, R = CH2CH3, 8a; n = 2, R = CH2CH3, 8b; n = 1, R = p-CH3C6H4, 9a; n = 2, R = p-CH3C6H4, 9b; n = 1, R = p-C6Me4Br, 10a; n = 2, R = p-C6Me4Br, 10b) and C6H5SeC(O)CH2CH2C(O)SeC6H5 11 are discussed. Although 11 can be prepared in high yield from the reaction of C6H5SeSiMe3 and ClC(O)CH2CH2C(O)Cl in tetrahydrofuran solvent, similar reactions in the absence of solvent led to the competitive formation of both 11 and (C6H5Se)3CCH2CH2C(O)OSiMe3 12. The new selenoesters have been characterized by multinuclear NMR (1H, 13C, 77Se), IR, and UVvis spectroscopies, electrospray mass spectrometry, and, for complexes 2, 4, 7, 10b, and 12, single-crystal X-ray diffraction.
’ INTRODUCTION The development of convenient and efficient methods for the synthesis of selenoesters has attracted considerable attention, due to the importance of these Se systems in organic synthesis.1z3 Acyl selenides and selenoesters can produce acyl radicals under mild conditions, which can be used in a large variety of interand intramolecular reactions.4 Acyl selenides are also useful synthetic intermediates and are employed as building blocks for the synthesis of heterocyclic compounds,5 in asymmetric aldol reactions,6 and as precursors for acyl radical chemistry.7,8 Selenoesters can also be converted to their corresponding acids,9 esters,9 amides,9a ketones,10 aldehydes,11 and alkenyl selenides.12 Methods to prepare selenoesters include the reaction of acyl halides with selenols, or alkali metal selenide salts,13,14 transition-metalcatalyzed carbonylation,15 and the Pd-catalyzed coupling of stannyl/silyl selenide and acyl halides.16 They have also been prepared by the reaction of aldehydes, acyl halides, or esters with organoselenolato reagents, such as C6H5SeTl,17 Hg(SeR)2 (R = alkyl, aryl),18 and Me2AlSeMe,9a,c as well as reactions between acyl chlorides and C6H5SeSeC6H5 mediated by indium iodide19 or RhCl(PPh3)3.20 Zhang and co-workers have also reported novel syntheses of selenoesters by using SmI2,21a Sm/TMSCl/H2O,21b TiCl4-Sm,21c Sm/CrCl3,21d and Sm/CoCl221e as reducing agents. Other methods include treating aryl selenocyanates with carboxylic acids,22a and α,β-unsaturated selenoesters can be prepared by the reaction of acylzirconocene chlorides with electrophilic selenium bromides.22b It has been demonstrated by Ogura and co-workers that the heavier congener tellurium reagent C6H5TeSiMe3 reacts directly with acyl chlorides under ambient conditions to yield a variety of r 2011 American Chemical Society
telluroesters, C6H5TeC(O)Ar, via the formation of Me3SiCl.23 Severengiz and du Mont demonstrated similar reactivity with Te(SiMe3)2.24 Aryl(trimethylsilyl)selenides offer a convenient, easily handled source of “ArSe” in the synthesis of metal selenolate complexes. 25 To date, the reaction chemistry of ArSeSiMe3 has not been similarly developed for the formation of selenoesters, although we recently reported the preparation of trans-Pd(PBu3)2(SeC(O)Et)2 from the reaction of trans-Pd(PBu3)2(SeSiMe3)2 with ClC(O)Et.26 Herein, we demonstrate the straightforward and efficient preparation of a variety of selenoesters from the reaction of acid chlorides and readily prepared aryl(trimethylsilyl)selenides. This includes examples of bis(trimethylsilylselenium) complexes of the type Me3SiSeAr-SeSiMe3 that have recently been reported (Ar = 1,4-C6H4; 4,40 -C6H4-C6H4; (C5H4)2Fe).27
’ RESULTS AND DISCUSSION We have found that a range of structurally diverse alkyl and aryl acid chlorides undergo reactions with phenylselenotrimethylsilane (eq 1), 1,10 -bis(trimethylsilylseleno)ferrocene (eq 2), phenylene-1,4-bis(trimethylsilylselenium) (eq 3), and biphenylene4,4 0 -bis(trimethylsilylselenium) (eq 4), in good yields. The results are summarized in Tables 1 and 2. Reactions of propionyl chloride proceed under mild conditions; however, reactions with the more sterically restricted (aromatic) acid chlorides are comparatively slow and need higher temperatures Received: August 17, 2011 Published: October 13, 2011 5943
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Table 1. Synthesis of Aryl- and Ferrocenylselenoestersa
a
Yields refer to those of pure isolated products characterized by IR and 1H, 13C, and 77Se NMR spectroscopies.
to proceed. PhSeSiMe3 þ RCðOÞCl f PhSeCðOÞR þ Me3 SiCl
ð1Þ
1; 10 -Feðη5 -C5 H4 SeSiMe3 Þ2 þ 2RCðOÞCl
f 1; 10 -Feðη5 -C5 H4 SeCðOÞRÞ2 þ 2Me3 SiCl
ð2Þ
1; 4-Me3 SiSe-C6 H4 -SeSiMe3 þ 2RCðOÞCl
f 1; 4-RðOÞCSe-C6 H4 -SeCðOÞR þ 2Me3 SiCl
ð3Þ
4; 40 -Me3 SiSe-ðC6 H4 Þ2 -SeSiMe3 þ 2RCðOÞCl
f 4; 40 -RðOÞCSe-ðC6 H4 Þ2 -SeCðOÞR þ 2Me3 SiCl
ð4Þ
Thus, when propionyl chloride, CH3CH2C(O)Cl, was reacted with phenylselenotrimethylsilane at 10 °C in THF for 2 h, C6H5SeC(O)CH2CH3 122 was obtained in 80% yield after workup at room temperature. Treatment of propionyl chloride
with reagents containing two SeSiMe3, namely, 1,10 -bis(trimethylsilylseleno)ferrocene, 1,4-bis(trimethylsilylseleno)phenylene, and biphenylene-4,40 -bis(trimethylsilylselenium), under reaction conditions similar to those for 1 afforded the corresponding selenoesters (5, 8a, and 8b, respectively) in good yield (Tables 1 and 2). Similar reactions with aromatic p-toluoyl chloride proceeded only at higher temperatures (40 °C over 5 h) to give the corresponding selenoesters 2, 6, 9a, and 9b in 7885% yields (entries 2, 6 in Table 1; entries 3, 4 in Table 2). Reactions with a more sterically encumbered aryl chloride also proceed without any difficulty, although more forcing conditions were required. Thus, 1,4-C6Me4BrC(O)Cl reacted cleanly with arylselenotrimethylsilanes (neat) at 85 °C to furnish the corresponding selenoesters in good yield (entries 3, 7 in Table 1; entries 6, 7 in Table 2). Furthermore, when 1,4-benzenedicarbonyldichloride2,3,5,6-tetramethyl was treated with 2 equiv of phenylselenotrimethylsilane at 85 °C for 5 h, the di(selenoester) 1,4-(C6H5SeC(O))2C6Me4 4 was generated in 87% yield. No reaction was observed between 1,4-(ClC(O))2C6Me4 and PhSeSiMe3 in solutions of tetrahydrofuran, even at higher temperatures (reflux). 5944
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Table 2. Synthesis of Aryl- and Ferrocenylselenoestersa
a
Yields refer to those of pure isolated products characterized by IR and 1H, 13C, and 77Se NMR.
Table 3. Selected Spectroscopic Data for the Selenoesters 111 compound
1
IR (cm ) νCO
77
1
Se{ H} NMR ppm
13
C (CO) ppm
1
1723
648
200.9
2
1691
634
192.7
3
1702
701
200.6
4
1707
701
200.6
5
1702
577
203.1
6
1632
549
194.2
7
1632
612
189.1
8a 8b
1719 1719
646 643
200.3 200.9
9a
1685
632
192.1
9b
1680
628
192.7
10a
1709
695
199.7
10b
1706
695
200.6
11
1700
658
198.2
Selected spectroscopic data for the selenoesters are listed in Table 3. The νCO absorption in the infrared spectra is observed between 1632 and 1723 cm1. The absorption for the aliphatic substituted carbonyl complexes appears at higher wavenumbers (17191723 cm1) than those of the aromatic derivatives (16321709 cm1).28 The carbonyl carbon chemical shifts are observed between δ 189203 in their respective 13C NMR spectra, where the signals of the aliphatic derivatives 1, 5, and 8 (δ = 200.3203.1) generally appear downfield to those observed
for the aromatic derivatives 24, 6, 7, 9, and 10 (δ = 189.1 200.6).2830 The nature of some of the structures of the selenoesters in the solid state was confirmed by X-ray crystallographic analysis for 2, 4, 7, and 10b. Crystallographic data and data collection parameters are summarized in Table 4. Complex 2 crystallizes in the monoclinic space group P2(1)/n, and the molecular structure of 2 together with a summary of relevant intermolecular bond distances and angles are provided in the caption of Figure 1. The selenoester 2 is monomeric and exists in the Z-configuration, as do the sulfur and tellurium analogues.3032 The C(1)Se(1) [1.953(5) Å] and C(12)Se(1) [1.911(6) Å] distances are typical for seleniumcarbon single bonds and the carbonyl C(1)O(1) distance [1.190(6) Å] is in the range observed for common esters and thioesters. The dihedral angle Se1C1 C2C3 is 89.8°, but the C6H5 ring is rotated 6.5° from an orthogonal angle with C12Se1C1. The two aromatic rings are nearly parallel, with an angle of 8.8° between them. The structural parameters for the centrosymmetric diselenoester C6H5SeC(O)C6Me4C(O)SeC6H5 4 (monoclinic, P2(1)/n, Figure 2) are similar to those for 3, although the two crystallographically equivalent C6H5 rings are more markedly twisted away from the central C6Me4 ring in 4, at an angle of 52.3°. Ferrocenyl selenoesters FcSeC(O)R have been previously prepared from Li[FcSe] and RC(O)Cl,33 and structural details have been reported for isomeric FcC(O)SeR.35 X-ray quality crystals of the ferrocenyl diselenoester BrC6H4C(O)SeFcSeC(O)C 6 H 4 Br 7 were obtained from concentrated solutions, and the molecular structure of 7 is illustrated in Figure 3. 5945
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Table 4. Crystallographic Data 2
4
7
10b
12
formula
C17H17BrOSe
C24H22O2Se2
C32H32Br2FeO2Se2
C34H32Br2O2Se2 3 CH2Cl2 C25H28O2Se3Si
formula weight
396.18
500.34
822.17
875.26
space group
monoclinic
monoclinic
monoclinic
triclinic
orthorhombic
crystal system
P 2(1)/n
P 2(1)/n
P 2(1)/c
P1
P 2(1) 2(1) 2(1)
625.44
Z
4
2
2
2
4
a (Å)
12.0338(8)
8.4304(3)
21.5761(8)
7.9348(6)
10.5264(4)
b (Å)
5.8357(3)
8.4811(4)
5.6000(2)
11.5667(7)
15.3887(4)
c (Å)
22.778(2)
14.8491(6)
12.6532(6)
19.5538(9)
16.0798(6) 90.0
α (°)
90
90
90
103.036(4)
β (°)
98.951(3)
100.941(2)
95.295(2)
94.354(3)
90.0
γ (°)
90
90
90
102.575(3)
90.0
volume (A3)
1580.1(2)
1042.20(7)
1522.3(1)
1691.6(2)
2604.7(2)
Fcal (mg cm1)
1.665
1.594
1.794
1.718
1.595
temperature (K)
200(2)
200(2)
200(2)
200(2)
200(2)
F(000)
784
500
808
864
1240
μ (Mo Kα, mm1)
4.898
3.564
5.539
4.737
4.301
θ min, θ max (°)
1.81, 27.53
2.59, 27.49
1.90, 27.48
1.86, 27.45
h, k, l (min; max)
15, 7, 29; 15, 7, 29 10, 11, 19; 10, 11, 19 28, 7, 16; 27, 7, 16 10, 14, 25; 9, 14, 25
13, 19, 20; 13, 19, 20
total reflns
6730
4248
6358
10 953
5949
unique reflns
3630
2390
3482
7666
4452
R(int)
0.0945
0.0302
0.0498
0.0551
0.118
data/restraints/parameter
1748/0/185
1933/0/129
2380/0/182
3946/0/396
4452/0/283
goodness of fit on F2
0.926
1.049
1.048
0.966
1.077
R1, wR2 [I g 2σ(I)]
0.0485, 0.0961
0.0345, 0.0848
0.0487, 0.1167
0.0540, 0.1063
0.0895, 0.2482
R1, wR2 (all data)
0.1507, 0.1268
0.0463, 0.0903
0.0824, 0.1290
0.1467, 0.1336
0.1217, 0.2228
max., min. peaks
0.579, 0.700
0.426, 0.764
1.118, 1.318
0.681, 0.804
2.602, 2.056
1.83, 27.49
in final Fourier map (e Å3)
Figure 1. Thermal ellipsoid plot (40% probability level) of 2 with the atom numbering scheme. Selected bond distances (Å) and angles (°): Se1C1 1.953(5), Se1C12 1.911(6), C1O1 1.190(6); C1Se1 C12 100.7(2), C(2)C1Se1 109.2(4), C2C1Se1C12 166.2.
The organometallic 7 sits about a crystallographic inversion center in the monoclinic space group P2(1)/c. The two selenoester groups are held in a trans configuration in the solid state with the two Cp rings adopting a staggered conformation. The Fe atom sits on a crystallographic inversion center with the planes of the cyclopentadienyl rings parallel and the selenium atoms lying slightly below the C5 rings, toward the iron center. The C6 and C5 rings are not coplanar (rotated 24°), and the dihedral angle between C1Se1C6 and Se1C6O1 is 10.5°. Structural information of the related di(selenoester) BrC6Me4C(O)Se(C6H4)2SeC(O)C6Me4Br 10b was also obtained by a single-crystal X-ray structure determination. Slow evaporation of the dichloromethane solutions of 10b at 25 °C led to the
Figure 2. Thermal ellipsoid plot (40% probability level) of 4 with the atom numbering scheme. The diselenoester resides about a crystallographic inversion center relating the two halves of the molecule. Selected bond distances (Å) and angles (°): Se1C1 1.944(3), Se1C7 1.913(3), C1O1 1.197(3); C1Se1C7 100.4(1), C2C1Se1 124.2(2), C2C1Se1C12 174.7, C1Se1C7C8 123.4.
formation of yellow crystals. The molecular structure of 10b and selected bond distances and angles are shown in Figure 4. The structure consists of two BrC6Me4C(O)Se centers linked by the biphenylene unit. The two phenylene rings in 10b are noncoplanar in the solid state (rotated by 22.7°), with a CipsoCipso bond length of 1.500(7) Å between the two rings.27a,c Treatment of succinyl chloride in THF solution with phenyl(trimethylsilyl)selenide (1:2) at 40 °C produced the di(selenoester) C6H5SeC(O)CH2CH2C(O)SeC6H5 11 in excellent yield (Scheme 1). The 1H NMR spectra for 11 display the expected singlet for the equivalent methylene groups at δ 3.05, and the 13C{1H} NMR spectrum for the equivalent carbonyl units gives rise to one signal at 198.2 ppm. However, when the same reaction was 5946
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Organometallics repeated in the absence of solvent at 80 °C for 5 h, 11 was isolated together with the unsymmetrical tri(selenoether) (C6H5Se)3CCH2CH2C(O)OSiMe3 12. The formation of 12 at the expense of 11 under these more forcing conditions occurs via addition of two additional PhSe to the carbonyl moiety and, presumably,
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HOSiMe3 generated from adventitious water reacting with any ClSiMe3 present. The identity of 12 was assigned via NMR data, with a singlet observed at 536.8 ppm in the 77Se{1H} NMR spectrum at a much higher field than that observed for 11 (657.9 ppm), while the 1H NMR spectrum indicated the retention of one trimethylsilyl group in the compound, with a singlet resonance present at 0.21 ppm and two resonances observed for inequivalent CH2 centers. The molecular structure of 12 was confirmed by X-ray analysis of a weakly scattering crystal and is illustrated in Figure 5 together with a summary of bond distances and angles. All SeC distances are identical at the level of the data (Se1C1 1.98(1) Å, Se2C1 1.97(1) Å, and Se3C1 1.97(1) Å). The ferrocenyl selenoesters 57 all show electrochemical responses due to the Fe(II/III) couple. The reversibility and relative oxidation potentials of the redox processes in these compounds were determined by cyclic voltammetry (CV) in THF solutions containing 0.1 M [(n-Bu)4N]ClO4 (TBAP) as the supporting electrolyte. The ferrocenyl selenoesters show one oxidation wave for each of at E1/2 = 242 mV (5), E1/2 = 236 mV (6), and at E1/2 = 275 mV (7), respectively, vs ferrocene and assigned to the Fe(II/III) redox couple. The cyclic voltammogram for these compounds showed a well-defined single electron
Figure 3. Two thermal ellipsoid plots of the molecular structure of 7 (40% probability level) with the atom numbering scheme. The molecule resides about a crystallographic inversion center. Selected bond distances (Å) and angles (°): Se1C1 1.889(4), Se1C6 1.944(4), C6O1 1.202(5); C1Se1C6 99.7(2), C2C1Se1 124.7(3), C1Se1C6C7 170.1.
Figure 4. Molecular structure of 10b with the atom numbering scheme. Thermal elipsoids are drawn at the 40% probability level. Selected bond distances (Å) and angles (°): Se1C1 1.907(5), Se1C7 1.949(6), C7C8 1.485(7), Se2C18 1.912(5), Se2C24 1.952(6), C24C25 1.493(7); C1Se1C7 99.1(2), Se1C7C8 110.2(4), C1Se1 C7C8 174.7.
Figure 5. Molecular structure of 12 (40% probability level) with the atom numbering scheme (the hydrogen atoms are omitted for clarity). Selected bond distances (Å) and angles (°): Se(1)C(1) 1.98(1), Se(2)C(1) 1.97(1), Se(3)C(1) 1.97(1); Se(2)C(1)Se(1) 104.3(5), Se(3)C(1)Se(1) 114.5(5), Se(2)C(1)Se(3) 104.7(5), Si(1)O(2)C(4)C(3) 178.3.
Scheme 1
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Scheme 2
wave that was reversible in nature with ic/ia ∼ 1, as observed for other ferrocenylselenoesters.34 The potential of the E1/2 is dependent on the electron density of the selenoester bridge on the ferrocenyl unit,35 and the E1/2 values for 57 are considerably more positive than the corresponding value for ferrocene, in keeping with the electron-withdrawing nature of the C(O)SeR groups. Within the series 57, however, E1/2 values show dependence on the electronic nature of R. The differences in the 77Se NMR chemical shifts (ppm) for 57 are also consistent with the observed changes in the values of E1/2 higher oxidation potentials correlating with an upfield chemical shift, in agreement with reports by Singh and co-workers34 and Burgess and Morley.35 The facile synthesis of 7 suggested that condensation reactions with the di(acid chloride) p-ClC(O)C6Me4C(O)Cl and Fe(η5-C5H4SeSiMe3)2 might allow for the formation of oligomeric complexes.36 Molecules containing repeating ferrocenyl units in the main chain have been the focus of intense research efforts, due, in part, to the attractiveness of incorporating this electrochemically active and stable fragment into polymer backbones.37 After heating a 1:1 molar ratio of Fe(η5-C5H4SeSiMe3)2 and p-ClC(O)C6Me4C(O)Cl at 85 °C in a vacuum-sealed glass tube, [(SeC5H4)Fe(C5H4Se)C(O)C6Me4C(O)]x 13 (x ≈ 5) is obtained as an orange solid that is reasonably soluble in organic solvents (Scheme 2). In a typical experiment, after about 18 h, the poly(ferrocenyldiselenoester) 13 was isolated in ∼40% yield after precipitation from THF solutions with hexane. The 1H, 13C, and 77Se NMR and IR spectroscopies and elemental analysis are in agreement with the proposed oligomeric structure of 13. Qualitatively, the 13C{1H} NMR spectrum of 13 is similar to that observed for monomeric 7. A signal for the SeC(O) moieties is detected as a resonance at 202.5 ppm for 13 (δ = 189.1 for 7), and an additional, weak carbonyl signal at 170.9 ppm is assigned to C6Me4C(O)Cl end groups. An
estimate of the molecular weight (Mn) of several samples was obtained from relative integration of the two carbonyl signals and indicate that 13 displays moderate degrees of oligomerization (Xn, x = 5, Mn = 2900). The chlorine content, as determined via elemental analysis, is also in agreement with this end-group analysis. 77Se{1H} NMR spectra of molecular 7 display a sharp signal at 611.7 ppm, and a somewhat broader signal is observed for oligomeric 13 at δ = 611.1 (W1/2 = 29 Hz), in addition to weaker, broadened resonances at 611.8 and 612.3 ppm. Whereas the intense signal for 13 is assigned to SeC(O)C6Me4C(O)Se units, the additional signals are assigned to (SeC(O)C6Me4C(O)Cl) centers. For comparison, 77Se{1H} NMR spectra of samples prepared with a 1:2 ratio of Fe(η5-C5H4SeSiMe3)2/ p-ClC(O)C6Me4C(O)Cl display a much stronger signal for the peak at 611.8 ppm (and a marked decrease in the intensity of the signal observed at 611.1 ppm). Similarly, 13C{1H} NMR spectra of these samples display a strong signal at δ = 170.9 ppm and a much weaker signal at δ = 202.5 ppm. The 1H NMR spectra of 13 (Supporting Information) are also in agreement with the proposed structure via integration of methyl resonances at δ = 2.20 and 2.22 ppm (C6Me4C(O)Cl) versus an intense, broad signal at δ = 2.16 ppm. Infrared spectra of 13 display a strong carbonyl stretch at 1699 cm1 assigned to SeC(O)C, and a weaker band at 1792 cm1 assigned to terminal C(O)Cl. The cyclic voltammogram of 13 (Supporting Information) consists of a single, reversible one-electron wave with E1/2 = 0.31 V (vs ferrocene), consistent with no electrochemical communication between the spatially separated Fe centers.
’ CONCLUSIONS ArSeSiMe3 react under mild conditions with acid chlorides to provide a route to selenoesters in good yields. Diselenoesters can 5948
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Organometallics be prepared from reactions of PhSeSiMe3 with di(acid chlorides) or the reaction of acid chlorides with reagents containing two SeSiMe3 groups. This reactivity led to the formation of oligomeric [(SeC5H4)Fe(C5H4Se)C(O)C6Me4C(O)]x 13 (x ≈ 5). 13 represents, to the best of our knowledge, the first reported oligoselenoester complex.
’ EXPERIMENTAL SECTION All syntheses were carried out under an atmosphere of high-purity dried nitrogen using standard double-manifold Schlenk line techniques and nitrogen-filled glove boxes. Solvents were dried and collected using an MBraun MB-SP Series solvent purification system. Chloroform-d was dried and distilled over P2O5. NMR spectra [1H (399.763 MHz), 13 C{1H} (100.522 MHz), 77Se{1H} (76.217 MHz)] were recorded on a Varian Inova 400 NMR spectrometer. The 1H and 13C NMR chemical shifts were referenced internally to SiMe4 using the residual proton and carbon signal of the deuterated solvent, respectively. 77Se{1H} spectra are reported relative to Me2Se at 0 ppm. Ultraviolet visible spectroscopy was performed on a Varian Cary 100 spectrometer. Mass spectra and exact mass determinations were performed on a Finnigan MAT 8200 instrument. Single-crystal X-ray diffraction measurements were completed on an Enraf-Nonius KappaCCD diffractometer. Molecular structures were determined via direct methods using the SHELXTL suite of crystallographic programs.41 All non-hydrogen atoms, with the exception of disordered carbon centers, were refined with anisotropic thermal parameters. Hydrogen atoms were included as riding on their respective carbon atoms. Crystals of 12 were invariably small and weakly diffracting. For 12, the refined Flack parameter was 0.00(3). Cyclic voltammetry measurements (concentration = 0.1 103 M) were performed at a scan rate of 100 mV/s using a standard three-electrode cell with a gold working electrode, a platinum flag counter electrode, and a silver wire reference electrode in tetrahydrofuran with NBu4ClO4 (0.1 M) as the supporting electrolyte. Potentials are referenced internally to ferrocene (0.00 V) added at the end of the experiments. Materials. 1,4-Dibromo-2,3,5,6-tetramethylbenzene,38 phenyl(trimethylsilyl)selenide,39 phenylene-1,4-bis(trimethylsilylselenium),27a biphenylene-4,40 -bis(trimethylsilylselenium),27a and bis(trimethylsilylseleno)ferrocene27b were synthesized according to literature procedures. p-Toluoyl chloride and propionyl chloride were used as received from the Aldrich Chemical Co. Synthesis of 1,4-C6Me4BrC(O)Cl. To a stirring solution of 1,4dibromodurene (5 g, 17.13 mmol) in diethyl ether (100 mL) at 78 °C, t BuLi (1.7 M, 200.05 mL, 64.26 mmol) was added dropwise over 10 min. The resultant white suspension was stirred at this temperature for 1.5 h, and dry CO2 was then bubbled through the reaction mixture for 30 min. Hydrochloric acid (0.5 M, 50 mL) was added, and the solution was extracted with diethyl ether (3 50 mL). The combined fractions were dried over magnesium sulfate, and the solvent was removed by rotary evaporation, yielding a colorless powder. The acid 1,4-C6Me4BrC(O)OH was suspended in thionyl chloride (100 mL) and heated to reflux overnight during which time all of the solid dissolved to yield a brown solution. SOCl2 was removed in vacuo, leaving a brown solid. Colorless crystals (3.9 g, 14 mmol, 90%) were isolated after vacuum sublimation (80 °C, 0.01 mmHg). NMR (CDCl3, δ) 1H: 2.41 (s, 6H, CH3), 2.33 (s, 6H, CH3). 13C{1H}: 171.4, 139.2, 135.2, 13.4, 128.7, 20.6, 18.1. 275.97 (M+, rel. intens. 15). Synthesis of 1,4-C6Me4(C(O)Cl)2. In a modification of the published procedure,36a tBuLi (1.7 M, 40.3 mL, 68.5 mmol) was added to a solution of 1,4-dibromodurene (5.0 g, 17.1 mmol) in diethyl ether (100 mL) at 78 °C. The resultant white suspension was stirred at this temperature for 1.5 h, and the reaction mixture was then warmed to room temperature overnight. The reaction mixture was again cooled to
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78 °C, and dry CO2 was bubbled through the reaction mixture for 30 min. Hydrochloric acid (0.5 M, 100 mL) was added, and the solution was extracted with diethyl ether (3 50 mL). The combined fractions were dried over magnesium sulfate, and the solvent was removed by rotary evaporation to yield a colorless powder (1,4-C6Me4(C(O)OH)2). The acid was suspended in thionyl chloride (100 mL) as described for the preparation of 1,4-C6Me4BrC(O)Cl (vide supra). After a similar workup, 1,4-C6Me4(C(O)Cl)2 was isolated as a colorless, air stable solid. Yield: 3.5 g (14 mmol, 87%). 1HNMR (CDCl3, δ): 2.29 (s, 6H, CH3). Preparation of 122. C6H5SeSiMe3 (258 mg, 1.12 mmol) was dissolved in tetrahydrofuran (20 mL). An equimolar amount of CH3CH2C(O)Cl (103 mg, 0.10 mL, 1.1 mmol) was added in one portion at 10 °C and stirred for 2 h. After removal of the solvent in vacuo, 1 was isolated as a yellow oil. Yield: 0.19 g (0.90 mmol, 80%). 1H NMR (CDCl3, δ): 7.547.23 (5H, C6H5), 2.74 (q, 2H, JHH = 8 Hz, CH2), 1.22 (t, 3H, JHH = 8 Hz, CH3). 13C{1H}: 200.9 (CO), 136.5 (Ar, Cipso), 135.7 (Ar), 129.2 (Ar), 128.7 (Ar), 40.9 (CH2), 9.4 (CH3). 77 Se{1H}: 648.7. IR (NaCl): νCO 1723 cm1. Synthesis of 2. To C6H5SeSiMe3 (0.38 mL, 0.35 g, 1.52 mmol) dissolved in 20 mL of tetrahydrofuran, 1 equiv of CH3C6H4C(O)Cl (0.20 mL, 0.23 g, 1.52 mmol) was added in one portion at 25 °C. After 12 h of stirring at 40 °C, all volatiles were removed in vacuo and the resulatant white solid residue was washed with 30 mL of n-pentane. After removal of the solvents, the residue was crystallized from n-pentane at 25 °C. The white precipitate was dried in vacuo. Yield: 0.35 g (1.27 mmol, 84%). Anal. Found: C, 61.04; H, 4.26; C14H12SeO (275.05). Calcd: C, 61.08; H, 4.40%. IR (NaCl): υCO 1691 cm1. 1H NMR (CDCl3, δ): 7.83 (d, 4H, JHH = 8 Hz, C6H4), 7.60 (m, 2H, C6H5), 7.43 (m, 3H, C6H5), 7.28 (d, 4H, JHH = 8 Hz, C6H4), 2.42 (s, 3H, CH3). 13 C{1H}: 192.7 (CO), 144.9 (Ar, Cipso), 136.3 (Ar), 136.0 (Ar, Cipso), 129.6 (Ar), 129.3 (Ar), 129.0 (Ar), 127.4 (Ar), 125.9 (Ar, Cipso), 21.8 (CH3). 77Se{1H}: 634. Synthesis of 3. p-C6Me4BrC(O)Cl (0.2 g, 0.73 mmol) was mixed with equimolar amounts of C6H5SeSiMe3 (0.17 g, 0.18 mL, 0.73 mmol) at 25 °C in a Schlenk tube, and the sample was placed in a preheated (85 °C) oven. After 12 h, the sample was cooled, Me3SiCl was removed in vacuo, and the residue was washed with n-pentane. Crystallization from n-pentanedichloromethane gave colorless crystals of 3 in 85% yield (0.24 g, 0.61 mmol). Anal. Found: C, 51.73; H, 3.93; C17H17SeOBr (396.18). Calcd: C, 51.54; H, 4.33%. IR (NaCl): υCO 1702 cm1. 1 H NMR (CDCl3, δ): 7.58 (m, 2H, C6H5), 7.41 (m, 3H, C6H5), 2.39 (s, 6H, CH3), 2.34 (s, 6H, CH3). 13C{1H}: 200.6 (CO), 140.8 (Ar, Cipso), 135.6 (Ar), 135.0 (Ar, Cipso), 130.7 (Ar, Cipso), 129.4 (Ar), 129.3 (Ar, Cipso), 126.9 (Ar, Cipso), 20.6 (CH3), 17.9 (CH3). 77Se{1H}: 701. Synthesis of 4. A 0.2 g portion of 1,4-C6Me4(C(O)Cl)2 (0.77 mmol) was reacted with 0.35 g (0.38 mL, 1.54 mmol) of C6H5SeSiMe3, as described for the preparation of 3. After a similar workup, 4 was isolated as colorless, air stable crystals. Yield: 0.33 g (0.70 mmol, 87%). Anal. Found: C, 58.02; H, 4.16; C24H22Se2O2 (500.09). Calcd: C, 57.59; H, 4.43%. IR (NaCl): υCO 1707 cm1. 1H NMR (CDCl3, δ): 7.59 (m, 4H, C6H5), 7.42 (m, 6H, C6H5), 2.27 (s, 12H, CH3). 13C{1H}: 200.6 (CO), 142.7 (Ar, Cipso), 135.6 (Ar), 129.4 (Ar), 129.3 (Ar), 129.1 (Ar), 126.8 (Ar, Cipso), 16.4 (CH3). 77Se{1H}: 701. Preparation of 5. To a solution of Fe(C5H4SeSiMe3)2 (150 mg, 0.386 mmol) in 20 mL of tetrahydrofuran was slowly added CH3CH2C(O)Cl (71.43.0 mg, 0.07 mL, 0.772 mmol) in 10 mL of tetrahydrofuran at 10 °C. After stirring for 2 h at this temperature, the reaction mixture was gradually warmed to 25 °C and all volatiles were removed in vacuo. The yellow residue was dissolved in 20 mL of n-pentane and filtered through a pad of dried Celite. Removal of the solvent under vacuum left 5 as a yellow oil in 70% yield (123 mg, 0.27 mmol). Exact mass Calcd. for C16H18FeO2Se2: (M+) 457.89936. Found: 457.89995. IR (NaCl): υCO 1702 cm1. UVvis: λmax = 435 nm (ε = 200 L mol1 cm1). 1H NMR (CDCl3, δ): 4.34 (br s, 4H, C5H4), 4.30 5949
dx.doi.org/10.1021/om200768m |Organometallics 2011, 30, 5943–5952
Organometallics (br s, 4H, C5H4), 2.61 (q, 4H, JHH = 8 Hz, CH2), 1.12 (t, 6H, JHH = 8 Hz, CH2). 13C{1H}: 203.1 (CO), 76.3 (C5H4), 71.7 (C5H4), 67.9 (Cp, Cipso), 40.3 (CH2), 9.2 (CH3). 77Se{1H}: 557. Synthesis of 6. Fe(C5H4SeSiMe3)2 (0.3 g, 0.77 mmol) was dissolved in tetrahydrofuran (20 mL), and 2 equiv of p-CH3C6H4COCl (0.23 g, 0.21 mL, 1.54 mmol) were added in one portion at 25 °C. After 12 h of stirring, all volatiles were removed in vacuo and the residue obtained was washed with n-pentane. Crystallization from n-pentane/ dichloromethane (8:1) gave yellow crystals of 6 in 80% yield (0.35 g, 0.60 mmol). Anal. Found: C, 54.16; H, 3.52; FeC26H22Se2O2 (580.22). Calcd: C, 53.82; H, 3.82%. IR (NaCl): υCO 1632 cm1. UVvis: λmax = 471 (ε = 300 L mol1 cm1). 1H NMR (CDCl3, δ): 7.79 (d, 4H, J = 8 Hz), 7.26 (d, 4H, J = 8 Hz), 4.45 (vt, 4H, J = 2 Hz, C5H4), 4.41 (vt, 4H, J = 2 Hz, C5H4), 2.41 (s, 6H, CH3). 13C{1H}: 194.2 (CO), 144.7 (Ar, Cipso), 135.7 (Ar, Cipso), 129.5 (C6H4), 127.3 (C6H4), 76.6 (C5H4), 71.8 (C5H4), 68.5 (Cp, Cipso), 21.7 (CH3). 77Se{1H}: 549. Synthesis of 7. Fe(C5H4SeSiMe3)2 (0.1 g, 0.26 mmol) was mixed with 2 equiv of C6Me4BrC(O)Cl (0.14 g, 0.52 mmol) as solids in a Schlenk tube. The sample was placed in a preheated (85 °C) oven, whereupon the solids melted, forming an orange-red, free-flowing liquid. After 12 h, the mixture became solid and was cooled to room temperature. Me3SiCl was removed in vacuo, and the residue was washed with n-pentane. Crystallization from n-pentanedichloromethane gave yellow crystals of 6 in 80% yield (0.17 g, 0.21 mmol). Anal. Found: C, 47.08; H, 3.48; FeC326H32Se2O2Br2 (821.83). Calcd: C, 46.75; H, 3.92%. IR (NaCl): υCO 1632 cm1. UVvis: λmax = 435 (ε = 250 L mol1 cm1). 1 H NMR (CDCl3, δ): 4.34 (bs, 4H, C5H4), 4.31 (bs, 4H, C5H4), 2.35 (s, 12H, CH3), 2.25 (s, 12H, CH3). 13C{1H}: 189.1 (CO), 134.9 (Ar, Cipso), 131.0 (Ar, Cipso), 130.5 (Ar, Cipso), 129.2 (Ar, Cipso), 76.0 (C5H4), 71.8 (C5H4), 69.8 (Cp, Cipso), 20.5 (CH3), 17.9 (CH3). 77 Se{1H}: 612. Synthesis of 8a. To 1,4-C6H4(SeSiMe3)2 (100 mg, 0.26 mmol) in 20 mL of tetrahydrofuran, 2 equiv of CH3CH2C(O)Cl (49 mg, 0.046 mL, 0.52 mmol) was added in one portion at 10 °C and warmed to room temperature. After 2 h of stirring at room temperature, the reaction mixture was filtered through a pad of Celite. All volatiles were removed under vacuum, and the residual solid was dissolved in n-pentane (10 mL) and stored at 25 °C to form pale yellow, platelike crystals of 8a. Yield: 0.063 g (0.18 mmol, 70%). Anal. Found: C, 41.66; H, 4.36; C28H22Se2O2.CH2Cl2 (348.16). Calcd: C, 41.40; H, 4.05%. IR (NaCl): υCO 1719 cm1. 1H NMR (CDCl3, δ): 7.49 (s, 4H, C6H4), 2.73 (q, 4H, JHH = 8 Hz, CH2), 1.21 (t, 6H, JHH = 8 Hz, CH3). 13C{1H}: 200.3 (CO), 136.3 (Ar), 127.4 (Ar, Cipso), 41.1 (CH2), 9.41 (CH3). 77 Se{1H}: 646. Synthesis of 8b. A 0.10 g portion of 4,40 -(C6H4SeSiMe3)2 (200 mg, 0.44 mmol) was reacted with 81 mg (0.08 mL, 0.88 mmol) of CH3CH2C(O)Cl, as described for the preparation of 8a (see above). After appropriate workup, 8b was isolated as yellow, air stable crystals in 75% yield (130 mg, 0.31 mmol). IR (NaCl): υCO 1719 cm1. 1H NMR (CDCl3, δ): 7.58 (m, 8H, C6H4), 2.75 (q, 4H, JHH = 8 Hz, CH2), 2.36 (t, 6H, JHH = 8 Hz, CH3). 13C{1H}: 200.9 (CO), 141.2 (Ar, Cipso), 140.7 (Ar, Cipso), 136.1 (Ar), 127.9 (Ar), 125.7 (Ar, Cipso), 41.0 (CH2), 9.4 (CH3). 77Se{1H}: 643. EI-MS [m/e (rel. intens.)]: 426 (M+, 90). Synthesis of 9a.40 p-CH3C6H4COCl (1.4 mL, 1.06 mmol) was added in one portion to 1,4-C6H4(SeSiMe3)2 (0.2 g, 0.53 mmol) and tetrahydrofuran (30 mL) at 25 °C. After 12 h of stirring at 40 °C, the solution was cooled and all volatiles were removed in vacuo. The colorless solid residue was washed with 20 mL of n-pentane. After removal of the solvents, the residue was crystallized from n-pentane/ dichloromethane (15:2) at 25 °C. The white precipitate was dried in vacuo. Yield: 0.20 g (0.42 mmol, 78%). Anal. Found: C, 55.82; H, 3.95; C22H18Se2O2 (472.06). Calcd: C, 55.92; H, 3.84%. IR(NaCl): υCO 1685 cm1. 1H NMR (CDCl3, δ): 7.82 (d, 4H, JHH = 8 Hz, C6H4), 7.62 (s, 4H, SeC6H4Se), 7.30 (d, 4H, JHH = 8 Hz, C6H4), 2.42 (s, 6H, CH3).
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C{1H}: 192.1 (CO), 145.3 (Ar, Cipso), 137.1 (Ar), 136.1 (Ar, Cipso), 129.9 (Ar), 127.7 (Ar), 127.5 (Ar, Cipso), 21.8 (CH3). 77Se{1H}: 632. Synthesis of 9b. A 0.3 g portion of p-CH3C6H4COCl (0.65 mmol) was reacted with 0.20 g (0.1.3 mmol) of 4,40 -(C6H4SeSiMe3)2, as described for the preparation of 9a (see above). After appropriate workup, 9b was isolated as pale yellow, air stable crystals. Yield: 0.30 g (0.55 mmol, 85%). Anal. Found: C, 61.03; H, 3.98; C28H22Se2O2 (548.09). Calcd: C, 61.30; H, 4.05%. IR(NaCl): υCO 1680 cm1. 1H NMR (CDCl3, δ): 7.84 (d, 4H, JHH = 8 Hz, C6H4), 7.66 (m, 8H, C6H4), 7.29 (d, 4H, JHH = 8 Hz, C6H4), 2.42 (s, 6H, CH3). 13C{1H}: 192.7 (CO), 145.0 (Ar, Cipso), 141.1 (Ar, Cipso), 136.7 (Ar), 135.9 (Ar, Cipso), 129.6 (Ar), 128.1 (Ar), 127.5 (Ar), 125.4 (Ar, Cipso), 21.8 (CH3). 77 Se{1H}: 628. Synthesis of 10a. A 0.144 g portion of C6Me4BrC(O)Cl (0.52 mmol) was mixed with 100 mg (0.26 mmol) of 1,4-C6H4(SeSiMe3)2 as solids at 25 °C in a Schlenk tube. The sample was placed in a preheated (85 °C) oven, whereupon the solids melted to yield a yellow, free-flowing liquid. After 12 h, the mixture became solid and was cooled to room temperature. Me3SiCl was removed in vacuo, and the residue was washed with n-pentane. Crystallization from n-pentanedichloromethane gave yellow, air stable crystals of 10a. Yield: 0.18 g (0.23 mmol, 73%). Anal. Found: C, 46.81; H, 3.91; C28H28Se2O2Br2 (714.25). Calcd: C, 47.08; H, 3.95%. IR (NaCl): νCO 1709 cm1. 1H NMR (CDCl3, δ): 7.61 (s, 4H, C6H4), 2.40 (s, 6H, CH3), 2.34 (s, 6H, CH3). 13C{1H}: 199.7 (CO), 140.6 (Ar, Cipso), 136.1 (Ar), 135.1 (Ar, Cipso), 129.4 (Ar, Cipso), 128.3 (Ar, Cipso), 20.6 (CH3), 18.0 (CH3). 77Se{1H}: 698. Synthesis of 10b. A 0.18 g portion of C6Me4BrC(O)Cl (0.65 mmol) was reacted with 0.15 g (0.325 mmol) of 4,40 -Me3SiSe(C6H4)2SeSiMe3, as described for the preparation of 10a (see above). After appropriate workup, 10b was isolated as pale yellow, air stable crystals. Yield: 0.18 g (0.23 mmol, 70%). Anal. Found: C, 51.73; H, 3.93; C28H22Se2O2Br2 (790.34). Calcd: C, 51.67; H, 4.08%. IR (NaCl): υCO 1706 cm1. 1H NMR (CDCl3, δ): 7.65 (m, 8H, C6H4), 2.40 (s, 6H, CH3), 2.36 (s, 6H, CH3). 13C{1H}: 200.6 (CO), 141.2 (Ar, Cipso), 140.7 (Ar, Cipso), 135.9 (Ar), 135.1 (Ar, Cipso), 130.8 (Ar, Cipso), 129.4 (Ar, Cipso), 128.2 (Ar), 126.4 (Ar, Cipso), 20.6 (CH3), 18.0 (CH3). 77Se{1H}: 695. Synthesis of 11 (Method 1). ClC(O)CH2CH2C(O)Cl (0.1 mL, 0.89 mmol) was added in one portion to C6H5SeSiMe3 (1.8 mmol) in tetrahydrofuran (30 mL) at 25 °C. After 12 h of stirring at 40 °C, the solution was cooled to room temperature and all volatiles were removed in vacuo. The colorless solid residue was washed with 20 mL of n-pentane, and the residue was crystallized from n-pentane/dichloromethane (ratio 30:1) at 25 °C. Yield: 0.30 g (0.76 mmol, 85%). Anal. Found: C, 48.55; H, 3.52; C16H14Se2O2 (396.2). Calcd: C, 48.50; H, 3.564%. IR(NaCl): υCO 1700 cm1. 1H NMR (CDCl3, δ): 7.51 (m, 4H, C6H4), 7.36 (m, 6H, C6H5), 3.05 (s, 4H, CH2). 13C{1H}: 198.2 (CO), 135.8 (Ar), 129.4 (Ar), 129.1 (Ar), 125.8 (Ar, Cipso), 41.8 (CH2). 77 Se{1H}: 658. Synthesis of 11 (Method 2) and 12. C6H5SeSiMe3 (1.8 mmol) was mixed with Cl(O)CCH2CH2C(O)Cl (0.1 mL, 0.89 mmol) at 25 °C in a Schlenk tube. The sample was placed in a preheated (80 °C) oven. After 5 h, the mixture became solid. Me3SiCl was removed in vacuo, and the residue was washed with n-pentane. Crystallization from n-pentanedichloromethane (30:1) gives colorless crystals of 11 (20% yield) and 12 (30% yield), which were manually separated. Anal. Found for 12: C, 48.51; H, 4.51; C25H28Se3SiO2 (625.45). Calcd: C, 48.01; H, 4.51%. IR(NaCl): νCO 1667 cm1. Data for 12 1H NMR (CDCl3, δ): 7.747.72 (m, 6H, C6H5), 7.407.30 (m, 9H, C6H5), 2.63 (m, 2H, CH2), 2.29 (m, 2H, CH2), 0.21 (s, 9H, SiMe3). 13C{1H}: 172.8 (CO), 135.9 (Ar), 129.4 (Ar), 129.3 (Ar), 125.9 (Ar, Cipso), 52.0 (CCH2CH2CO), 38.0 (CCH2CH2CO), 34.1 (CCH2CH2CO), 0.3 (SiCH3). 77Se{1H}: 537. Synthesis of 13. ClC(O)C6Me4C(O)Cl (0.13 g, 0.51 mmol) was mixed as powder with equimolar amounts of Fe(C5H4SeSiMe3)2 5950
dx.doi.org/10.1021/om200768m |Organometallics 2011, 30, 5943–5952
Organometallics (0.25 g, 0.51 mmol) in a thick-walled glass tube and flame-sealed in vacuo. The sample was placed in a preheated (85 °C) oven. After 18 h, the mixture became solid. Me3SiCl was removed under vacuum, and the residue was dissolved in a minimum amount of tetrahydrofuran. Cold hexanes were added rapidly to precipitate the oligomer as an orange solid. The hexanes-soluble fraction was removed and 13 remained (0.12 g, 40%) as a yellow glassy solid after drying. Anal. Found: C, 49.29; H, 3.80; Cl, 2.54; C122H112Fe5Se10O12Cl2 (2908.57). Calcd: C, 50.33; H, 3.88; Cl, 2.44%. IR (film on NaCl): νC(O)Cl 1792 cm1, νC(O)Se 1699 cm1. UVvis: λmax = 456 nm. 1H NMR (CDCl3, δ): 4.35 (s, C5H4, 12H), 4.30 (br s, C5H4, 18H), 2.22 (s, C6(CH3)4C(O)Cl, 6H), 2.20 (s, C6(CH3)4C(O)Cl, 6H), 2.16 (br s, C6(CH3)4, 36H). 13C{1H}: 202.6 (br s, CO), 170.9 (br s, CO), 142.9, 142.4, 140.8, 130.6, 129.4, 129.0, 128.9, 128.6, 75.9, 75.8, 71.8, 72.0, 69.9, 16.7, 16.4 (br s, C6(CH3)4, end group), 16.3 (br s, C6(CH3)4). 77Se{1H}: 612.3, 611.8, 611.1 ppm.
’ ASSOCIATED CONTENT
bS
Supporting Information. Crystallographic information files for 2, 4, 7, 10b, and 12 in CIF format and 1H NMR spectra and cyclic voltammogram of 13. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Address §
Current address: Department of Chemistry, The University of Jordan, Amman, Jordan.
’ ACKNOWLEDGMENT We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support of this work in the form of discovery and equipment grants and the Canada Foundation for Innovation and The University of Western Ontario for equipment funding. We thank Prof. Mark S. Workentin for the use of his electrochemical equipment and Prof. Derek P. Gates (UBC) and Kevin Noonan (UBC) for helpful discussions regarding this work. Daniel G. MacDonald and Michael C. Jennings are thanked for collecting the X-ray diffraction data. ’ REFERENCES (1) Fujiwara, S.-I.; Kambe., N. Top. Curr. Chem. 2005, 251, 87 and references therein. (2) For selected references, see: (a) Pfenninger, J.; Heuberger, C.; Graf., W. Helv. Chim. Acta 1980, 63, 2328. (b) Boger, D. L.; Mathvink, R. J. J. Org. Chem. 1992, 57, 1429. (c) Kozikowski, A. P.; Ames, A. Tetrahedron 1985, 41, 4821. (d) Boger, D. L.; Robarge., K. D. J. Org. Chem. 1988, 53, 3377. (e) Boger, D. L.; Mathvink, R. J. J. Org. Chem. 1989, 54, 1777. (f) Schwartz, C. E.; Curran, D. P. J. Am. Chem. Soc. 1990, 112, 9272. (3) (a) Nakayama, J.; Kitahara, T.; Sugihara, Y.; Sakamoto, A.; Ishii., A. J. Am. Chem. Soc. 2000, 122, 9120. (b) Murai, T.; Kamoto, T.; Kato, S. J. Am. Chem. Soc. 2000, 122, 9850. (c) Tani, K.; Murai, T.; Kato, S. J. Am. Chem. Soc. 2002, 124, 5960. (d) Niyomura, O.; Kato, S.; Inagaki, S. J. Am. Chem. Soc. 2000, 122, 2132. (4) (a) Evans, P. A.; Roseman, J. D. Tetrahedron Lett. 1995, 36, 31. (b) Evans, P. A.; Roseman, J. D. J. Org. Chem. 1996, 61, 2252. (c) Evans, P. A.; Murthy, V. S.; Roseman, J. D.; Rheingold, A. L. Angew. Chem., Int. Ed. 1999, 38, 3175. (d) Evans, P. A.; Manangan, T. J. J. Org. Chem. 2000, 65, 4523.
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