Article pubs.acs.org/Organometallics
Directed Lithiation of Pentadienylsilanes Benjamin M. Day, Joseph J. W. McDouall, Jonathan Clayden, and Richard A. Layfield* School of Chemistry, The University of Manchester, Manchester, M13 9PL, U.K. S Supporting Information *
ABSTRACT: Deprotonation of dimethyl(pyrrolidylmethyl)silylpentadiene (5) and bis(2-methoxyethyl)aminomethyl(dimethyl)silylpentadiene (7) with nBuLi/tmeda and n BuLi, respectively, results in their corresponding lithium complexes, 6 and 8. The lithium cation in 6 is coordinated by an η1-pentadienyl ligand via the α-carbon and by the pendant pyrrolidyl group. The lithium cation in 8 is η2 coordinated by the pentadienyl α- and β-carbons and by the bis(methoxyethyl)amino group. The structure of 6 is retained in benzene solution, but in thf the tmeda coligand is displaced by the solvent. A 3.5:1 mixture of the W- and S-conformations of the pentadienyl carbons was observed for 8 in benzene. DFT calculations of NBO charges for the pentadienyl carbons in 6 and 8 show that lithium polarizes the electron density toward the α-carbon, although a series of electrophile quenching reactions with 6 show that regioselectivity does not depend on the electronic structure of the pentadienyl carbanion.
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INTRODUCTION The activation of C−H bonds by organolithium and other strongly basic reagents is one of the most diverse methods available for the functionalization of organic compounds.1 A key challenge for lithiation chemistry is regioselectivity, primarily because the high reactivity of organolithium reagents can result in indiscriminate deprotonation at multiple sites within the same molecule.2 Pioneering work by Gilman3 and by Wittig4 demonstrated that Lewis basic substituents in aromatic compounds can direct lithiation selectively toward the ortho positions relative to the substituent, and hence ortholithiation has become a particularly reliable method for the regioselective substitution of aromatic C−H bonds.5 In the context of C−H functionalization, allylsilanes bear some similarities to aromatic molecules owing to the presence of more than one reactive site, i.e., α or γ relative to silicon,6 and the broad synthetic applications of allylsilanes render the issue of regioselective functionalization of lithiated allylsilanes one of considerable importance.7 Chan et al. have demonstrated that allylsilanes with aminoalkyl groups built into the silyl substituent can be lithiated with sBuLi and subsequently functionalized in a selective manner.8 Electrophilic quenching of the Lewis-base-functionalized allyl-lithium compounds by alkyl halides showed good selectivity for the α-position with small alkyl halides (e.g., methyl iodide), but bulkier electrophiles (e.g., isopropyl iodide) reacted preferentially at the γposition. Aldehydes and ketones react with the aminoalkylfunctionalized allyl-lithium selectively at the γ-position. A related study by Strohmann et al. demonstrated that the regiochemistry of functionalization is also influenced by the ability of the electrophile to coordinate to lithium.9 In all of the previous studies, the polarity of the solvent, typically an ether or toluene, was found to influence the regioselectivity. The complicated structures and reactivity of ostensibly simple unsaturated metal-carbanion complexes such as lithium allylsilanes suggest that the analogous properties of more © XXXX American Chemical Society
elaborate carbanions would be even more interesting to study and to develop for synthetic applications. Our attention has therefore focused on alkali metal complexes of pentadienylsilane carbanions. Despite being used to good effect by coordination chemists as ligand transfer reagents for many years,10,11 alkali metal complexes of pentadienylsilanes have found very few applications in organic synthesis.12 If the latter type of reactivity could be developed in a reliable manner, new possibilities for organic synthesis may become possible. Alkali metal complexes of pentadienyl (pdl) carbanions show remarkable structural diversity, with the carbanion being able to adopt up to three structural geometries, i.e., the W-, S-, and Uconformations (Scheme 1).13−21 The different pentadienyl geometries provide a source of fascination in addition to presenting a barrier to developing synthetic applications; the complications multiply upon consideration of terminally substituted versions.12−14,19,21 Although comparatively few structural studies on alkali metal complexes of substituted pentadienyl carbanions are available, an emerging trend is that the W-pdl isomer is favored when the carbanion is complexed to lithium14,15 and that the U-pdl isomer is favored with potassium.11b In our crystallographic studies of lithiated pentadienylsilanes, we have found that the coordination mode of the carbanion varies in a manner that seems to depend on the steric influence of the silyl substituents: we observed η2- and η3-pdl bonding to the terminal carbon atoms in 1 and 2, respectively, and η1-pdl bonding to the central pentadienyl carbon in 3 (Scheme 1). To date, all crystallographic studies of potassium pentadienyls have revealed that the preferred bonding mode of the pentadienyl carbanion to potassium is η5-pdl, which is most likely due to the greater ionic Special Issue: Mike Lappert Memorial Issue Received: November 13, 2014
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expected on steric grounds. The synthesis of 5 and 7 started with the preparation of chloromethyl(dimethyl)pentadienylsilane (4), according to Scheme 2. Addition of 2.5 equivalents of pyrrolidine to 4 resulted in the formation of 5 in 86% yield; compound 7 was obtained in 57% yield by adding three equivalents of bis(2-methoxyethyl)amine to 4, in the absence of any solvent, and heating at 90 °C for 48 h. Addition of nBuLi to a solution of 5 in pentane, followed by one equivalent of tmeda, produced a yellow solution, from which yellow crystals of the lithium pentadienylsilane complex 6 formed upon standing at room temperature (Scheme 1). Addition of one equivalent of nBuLi to 7 in hexane resulted in the mixture gradually turning cloudy. Gentle heating dissolved the precipitate, and slow cooling of the resulting yellow solution led to the formation of yellow crystals of the lithiumpentadienylsilane complex 8. The molecular structures of 6 and 8 were determined by Xray crystallography (see Supporting Information for full details of the crystal data and structure refinement). In the case of 6 (Figure 1), the pentadienyl ligand adopts the W-conformation, as observed in all crystallographically authenticated pentadienyllithium complexes.14,15
Scheme 1. Different Geometries of Silyl-Substituted Pentadienyl (pdl) Carbanions and Structurally Characterized Pentadienyllithium Complexes (1−3)
radius of the metal. The ionic radius of the alkali metal is unlikely to be the sole factor that influences the structure of the carbanion and its coordination mode, since in all experimental examples studied to date the metal also bonds with coligands such as tmeda (N,N,N′,N′-tetramethylethylenediamine) (e.g., 1 in Scheme 1). Notwithstanding the diverse coordination chemistry of alkali metal pentadienyl complexes, a further complication is that the carbanions are capable of reacting with electrophiles at the α-, γ-, and ε-positions.12,13 Indeed, the possibility of electrophilic attack at more than one pentadienyl carbon, combined with the variable conformation of the pentadienyl chain, is the major obstacle that has hitherto prevented the widespread applications of these potentially valuable reagents in organic synthesis. We therefore now turn our attention to the rational synthesis of selectively α-lithiated pentadienylsilanes and their subsequent reactions with a range of electrophiles.
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Figure 1. Molecular structure of 6, with thermal ellipsoids at the 50% probability level. Hydrogen atoms are not shown.
RESULTS AND DISCUSSION The pentadienylsilanes 5 and 7 (Scheme 2) were designed such that, upon lithiation, the lithium cation should be held in close proximity to the α-carbon by the pendant Lewis base group, rather than migrating toward the ε-carbon, as might be
Notably, the pentadienyl C−C bond distances in 6 are 1.407(2), 1.375(2), 1.423(2), and 1.340(3) Å for C(1)−C(2), C(2)−C(3), C(3)−C(4), and C(4)−C(5), respectively; that is, they adopt a pattern of alternating long−short−long−short bonds, which contrasts with the analogous C−C distances in the pentadienyllithium complexes 1−3. The pentadienyl group coordinates to the lithium cation in an η1-fashion, resulting in a C(1)−Li(1) distance of 2.301(3) Å. The pyrrolidine group coordinates to lithium with an N(1)−Li(1) distance of 2.160(3) Å, and the tmeda ligand coordinates in a bidentate fashion, with N(2)−Li(1) and N(3)−Li(1) distances of 2.115(3) and 2.143(3) Å, respectively. Overall, the lithium cation occupies a distorted tetrahedral environment. The molecular structure of 8 (Figure 2) is qualitatively similar to that of 6 by virtue of the W-coordination mode of the pentadienylsilane ligand. However, the C−C distances in 8 follow a different pattern, being 1.380(4), 1.395(4), 1.404(4), and 1.345(4) Å for C(1)−C(2), C(2)−C(3), C(3)−C(4), and C(4)−C(5), respectively. As in 6, the lithium cation in 8 is positioned close to the α-carbon of the pentadienyl chain, although in 8 the coordination mode is more accurately described as η2, with Li(1)−C(1) and Li(1)−C(2) distances of 2.297(4) and 2.272(4) Å, respectively.
Scheme 2. Synthesis of Compounds 4−9
B
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the α-, β-, and γ-CH groups; the tmeda protons; the methyl and methylene groups bound to silcon; and the pyrrolidine methylene groups. The HOESY experiment indicates, first, that the three ligand groups remain coordinated to lithium in solution. Second, although the 1H NMR spectrum of 6 implies that a 2,4-pentadienyl anion exists in solution, the interaction between lithium and the γ-CH implies that η1−η3, or σ−π, allylic slippage is occurring in benzene. In the 1H NMR spectrum of 6 in thf-D8 at 298 K (Figure S12), the resonances corresponding to the pentadienyl protons occur at different chemical shifts from those in benzene-D6; however the pentadienyl coupling patterns are similar, indicating that the W-pdl conformation is retained in thf. Thus, the α-, β-, γ-, δ-, and ε-CH protons occur at 2.10, 6.33, 4.89, 6.12, 3.73 (trans to δ) and 3.38 ppm (cis to δ), with trans coupling constants in the range 3J = 10.7−16.4 Hz and a cis coupling constant of 9.3 Hz. A significant feature in the 1H NMR spectrum of 6 in thf-D8 is that the resonances due to tmeda (2.15 and 2.30 ppm) are shifted downfield relative to their positions in the benzene-D6 spectrum (1.77 and 1.88 ppm) and that they occur at chemical shifts that are essentially identical to those of free tmeda. This suggests that the thf solvent displaces tmeda from the lithium center in 6, a deduction that is supported by the absence of an NOE between lithium and tmeda in the 1H−7Li HOESY spectrum (Figure 3). The NOE between lithium and the silyl methyl/methylene substituents, the pyrrolidine methylene groups, and the α-CH group shows that coordination of the functionalized pentadienyl ligand to lithium persists when 6 is in thf solution. The change in the coordination environment of lithium is also reflected in the 7Li resonance, which is shifted upfield to −1.8 ppm (Figure S14). The 29Si chemical shift moves slightly upfield to δ(29Si) = −12.0 ppm (Figure S13). In contrast to 6, the NMR spectra of 8 in benzene-D6 indicate the presence of two pentadienyl isomers, with the 29Si and 7Li NMR spectra showing two distinct resonances at δ(29Si) = −9.8 and −10.0 ppm and δ(7Li) = 0.2 and −0.3 ppm (Figures S22, S23, respectively). Analysis of the 1H NMR spectrum of 8 (Figures S18, S19), particularly the 3J scalar coupling constants, revealed that the two isomers are W-8 and S-8 and that they occur in an approximate ratio of 3.5:1. The key to unravelling the rich coupling patterns in the 1H NMR of spectrum of the W/S-8 mixture is the pentadienyl β-proton, which engages in trans 3J coupling in W-8 but cis and trans 3J coupling in S-8 (Scheme 3).
Figure 2. Molecular structure of 8, with thermal ellipsoids at the 50% probability level. Hydrogen atoms are not shown.
The coordination environment of lithium in 8 is completed by the amino nitrogen atom, with Li(1)−N(1) being 2.197(4) Å, and the two methoxyethyl arms, which wrap around the metal to result in Li(1)−O(1) and Li(1)−O(2) bond distances of 1.989(4) and 1.974(4) Å, respectively, and a formal coordination number of 5. In the structures of 6 and 8, it is noteworthy that the Li(1)−C(1)−Si(1) bond angles are 94.43(11)° and 98.11(14)°, respectively; that is, both are significantly less than ideal tetrahedral angles. This structural feature indicates that the pendant Lewis base substituents are indeed “pulling” the lithium cation toward the α-carbon of the pentadienyl units. The 1H NMR spectrum of 6 in benzene-D6 at 298 K (Figure S7) can be readily assigned using a standard COSY experiment. Thus, the α-CH proton occurs as a doublet at δ(1H) = 2.40 ppm, with 3J = 15.3 Hz. The β- and δ-CH resonances overlap in the region δ(1H) = 6.79−9.94 ppm, and the γ-CH resonance occurs as a doublet of doublets at δ(1H) = 5.57 ppm, with 3J = 10.7 and 13.0 Hz. Both ε-CH protons occur as doublets of doublets, with the CH proton trans to δ-CH occurring at δ(1H) = 4.76 ppm (3J = 16.4 Hz) and the ε-CH proton cis to the δCH occurring at δ(1H) = 4.42 (3J = 9.9 Hz); the ε-germinal coupling constant is 2J = 2.5 Hz. A characteristic series of resonances for the methyl groups, the pyrrolidine substituent, and the tmeda ligand were also observed in the 1H NMR spectrum of 6 (see Supporting Information for full details). A single resonance is observed in the 29Si NMR spectrum at δ(29Si) = −11.4 ppm (Figure S9) and in the 7Li NMR spectrum at δ(1H) = 0.3 ppm (Figure S10). The 1H−7Li HOESY spectrum of 6 (Figure 3) shows that lithium has an NOE with
Scheme 3. W- (left) and S- (right) Conformations of the Pentadienyl Carbons in 8 in Benzene-D6
In W-8, the α-CH proton occurs as a doublet at 2.88 ppm, with 3J = 16.1 Hz to the β-CH proton, which occurs at δ(1H) = 6.83 ppm as a doublet of doublets. The coupling of β-CH to γCH, which has a chemical shift of δ(1H) = 5.98 ppm, is 3J = 12.5 Hz and is indicative of a trans stereochemistry. The δ-CH group occurs as a doublet of triplets at δ(1H) = 6.93 ppm, with 3 J = 16.4 Hz to γ-CH and trans-ε-CH and 3J = 10.1 Hz to cis-εCH. The cis- and trans-ε-CH protons occur at δ(1H) = 4.36 and 4.73 ppm, producing a germinal coupling of 2J = 2.4 Hz.
Figure 3. 1H−7Li HOESY NMR spectra of 6 in benzene-D6 (upper) and thf-D8 (lower). C
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−0.142 for 8, and −0.124 and −0.176 for 2. The NBO charges for the γ- and ε-carbons take intermediate values and are −0.369 and −0.458 for 6, −0.398 and −0.503 for 8, and −0.416 and −0.570 for 2. The variation in NBO charge for the α-carbons in the three complexes is intriguing and is consistent with the different coordination modes of the pentadienyl carbons to lithium. Thus, in 6 the NBO charge of the α-carbon is the largest in the series due to the combined effects of the silicon atom and the η1-coordination mode of the pentadienyl group. In 8, the NBO charge on the α-carbon is reduced slightly because the lithium cation is coordinated by an η2-pentadienyl ligand; that is, the polarizing effect of lithium is shared between the α- and βcarbons. The larger NBO charge for the β-carbon in 8 relative to that in 6 is also consistent with this argument. In 2, where the pentadienyl ligand coordinates to lithium in an η3 manner via the γ-, δ-, and ε-carbons, the NBO charge for the α-carbon is significantly lower than in 6 and 8; the NBO charges for the γ- and ε-carbons in 2 are, commensurately, greater than those in 6 and 8. Our combined experimental and computational data produce a picture of the electronic structure of internally solvated lithium pentadienyl complexes that is consistent with that of related allylic systems, according to the “site-specific electrostatic perturbation of conjugation” model proposed by Fraenkel et al.23−25 The structural data on complexes 6 and 8, in addition to the calculated NBO charges, suggested to us that 6 would be interesting to study in electrophilic quenching reactions. In principle, the site of lithiation in 6 and the relatively high NBO charge for the α-carbon might encourage regioselective quenching at one carbon in preference to the others. For these studies, we selected chloro(trimethyl)silane, ethyl tosylate, allyl bromide, methyl tosylate, methyl iodide, and dimethyldisulfide (Scheme 4, Table 2). Solvent polarity effects
In the 1H NMR spectrum of S-8, the chemical shifts of the α-, β-, and δ-CH groups occur at δ(1H) = 3.42 (doublet), 6.51 (doublet of doublets), and 7.25 ppm (doublet of triplets), respectively, with the ε-CH proton cis to δ-CH occurring at δ(1H) = 4.58 ppm (doublet of doublets). The γ-CH proton and the ε-CH proton trans to δ occur as overlapping resonances in the region δ(1H) = 5.03−4.94 ppm. The coupling between αand β-CH groups of 3J = 17.3 Hz reflects a trans stereochemistry, whereas the β- to γ-CH coupling constant of 3J = 8.8 Hz implies a cis stereochemistry. The δ-CH group at δ(1H) = 7.25 engages in trans couplings with 3J = 16.6 with γ-CH and εCH and a cis coupling to the second ε-CH group with 3J = 10.3 Hz. The ε-CH protons have a germinal coupling of 2J = 2.8 Hz. Having established the molecular structures of 6 and 8 in the solid state and in solution, we next studied how the lithium cation influences the electronic structure of the pentadienyl carbanion. Our initial hypothesis is that the pendant donor group positions lithium in proximity to the α-carbon, and hence lithium polarizes the formal negative charge toward that carbon in a way that could render it more electron rich than the γ- and ε-carbons. Density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level were used to determine natural bond orbital (NBO) charges for complexes 6 and 8 (see Supporting Information for full computational details). For comparative purposes, we also calculated NBO charges for our previously reported lithium pentadienylsilane complex [(pmdeta)Li{1-Me2(Me2N)SiC5H6}] (2, Scheme 1) (pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine), in which the pentadienyl carbanion coordinates to lithium in an η3-manner via the γ-, δ-, and ε-carbons and where the NMe2 group does not coordinate to lithium. The NBO charge calculations were performed on structures generated from the atomic coordinates as determined by crystallography and also on optimized structures including the effects of the solvent dielectric (toluene): the differences in the NBO charges for the two sets of calculations are generally not substantial (Table 1).
Scheme 4. Reactions of 6 and 9 with Various Electrophiles in Toluene and in thf (6) or in thf (9)a
Table 1. Calculated Natural Bond Orbital (NBO) Charges for the Pentadienylsilane Component of Compounds 2, 6, and 8 Si C(1) C(2) C(3) C(4) C(5)
2a
2b
6a
6b
8a
8b
1.710 −0.823 −0.124 −0.416 −0.176 −0.570
1.776 −0.838 −0.155 −0.412 −0.198 −0.695
1.519 −0.996 −0.104 −0.369 −0.140 −0.485
1.615 −1.070 −0.146 −0.386 −0.195 −0.498
1.535 −0.932 −0.199 −0.398 −0.142 −0.503
1.605 −0.972 −0.203 −0.406 −0.198 −0.508
a
Determined using experimental geometry. bDetermined using DFToptimized geometry.
The following discussion refers to the results obtained using the experimental geometries. A number of trends in the NBO charges emerge from the three systems studied. First, the NBO charges on C(1), i.e., in the α-position relative to silicon, range from −0.823 (2) to −0.932 (8) to −0.996 (6); hence they are considerably larger than the charges on the other pentadienyl carbons. These results can be rationalized in terms of a combination of the polarizing effect of the lithium cations and the ability of silicon to stabilize α-carbanions.22 Second, the NBO charges on the βand δ-carbons are similar and markedly lower than those on the other carbons, being −0.104 and −0.140 for 6, −0.199 and
a R3Si = SiMe2CH2(NC4H8) or SiMe3, R1 = Et or allyl, R2 = Me or SMe. Byproducts not shown.
were studied by conducting each reaction in thf and toluene. To explore the influence of the pendant Lewis base group, we performed control reactions using lithium (trimethylsilyl)pentadienide, [Li(C5H5SiMe3)] (9), in thf. In each reaction of 6, a crystalline sample of lithium pentadienylsilane was dissolved in toluene or thf and cooled to −78 °C; one stoichiometric equivalent of electrophile was added, and the D
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8, respectively, by the addition of one equivalent of nBuLi/ tmeda or nBuLi. Crystallographic analysis of the structure of 6 reveals an η1-pentadienyl coordination mode to lithium via the α-carbon, with additional coordination by the pyrrolidine group and tmeda. The pentadienyl C−C bond lengths in 6 adopt an alternating long−short−long−short pattern, which indicates that the carbanion experiences a degree of localization on the αcarbon. In the structure of 8, an η2-pentadienyl coordination mode via the α- and β-carbon atoms was observed, with the pendant donor group coordinating via an amino nitrogen atom and two methoxy groups. The C−C bond lengths adopt a pattern that is reminiscent of an allylic bonding mode. NMR spectroscopy of 6 in benzene reveals that the solid-state structure is retained in solution, but in thf the tmeda coligand is substituted by solvent molecules. The NMR spectra of 8 in benzene show that the pentadienyl group is a mixture of Wand S-conformations. DFT calculations of the NBO charges in 6, 8, and the related lithium pentadienylsilane 2, in which the metal is not bonded to a pendant Lewis base group, reveal that the α-stabilizing effect of silicon plays an important role in developing significant negative charge on the α-carbon. The calculated NBO charge is greatest for 6 because of the polarizing effect of the η1-bonding interaction between the α-carbon and lithium. A slightly reduced NBO charge was calculated for 8, which can be rationalized on the basis of the polarizing ability of lithium being distributed across the η2 interaction with the α- and βcarbons. In 3, the pentadienyl γ-, δ-, and ε-carbons coordinate in an η3 fashion to lithium, which results in these carbons developing greater NBO charges than in 6 and 8. On the basis of the structural information determined by experiment and calculation, complex 6 was selected for electrophilic quenching reactions with several different substrates, both in toluene and in thf. Comparative studies were undertaken using lithium (trimethylsilyl)pentadienide (9) in thf. The results of the reactivity studies are unambiguous: the site of lithiation has essentially no influence on the regiochemistry of the electrophilic quench. The obvious lack of regioselectivity in the reactions of pentadienylsilane carbanions is potentially a consequence of the particularly high reactivity of the lithium derivatives. Replacing or combining lithium with less polar metals such as magnesium, manganese, or zinc would therefore be an interesting line of inquiry.
Table 2. Product Distributions from the Reactions of 6 and 9 with Electrophiles in Toluene or thf 6 in toluene Me3SiCl MeI MeOTs EtOTs C3H5Br MeSSMe
6 in thf (9 in thf)
α
γ
ε
0 22 53 0 0 28
0 22 47 99 99 18
99 57 0 0 0 54
α 0 71 65 0 0 31
(0) (70) (60) (0) (0) (50)
γ 0 29 35 99 99 0
(0) (15) (40) (99) (99) (0)
ε 99 0 0 0 0 69
(99) (15) (0) (0) (0) (50)
reaction slowly warmed to room temperature. In the case of 9, lithium (trimethylsilyl)pentadienide was generated in situ and reactions were carried out in an identical manner to those of 6. Product ratios were determined by a combination of (Figures S24−S42) HSQC, HMBC, and COSY NMR spectra. In the quenching reaction of 6 with Me3SiCl, the trimethylsilyl group is transferred exclusively to the ε-position irrespective of the solvent. Exactly the same outcome was observed in the quenching of 9 with Me3SiCl in thf, indicating the position of the lithium cation relative to the pentadienyl carbons does not influence the reaction to any significant extent. The most probable explanation for the reaction taking place at the ε-carbon is that steric effects dominate, which is consistent with Chan’s study on related lithium allylsilanes.8 In toluene, the much smaller electrophile methyl iodide reacts with 6 to give a 22:22:57 mixture of α-, γ-, and εsubstituted products; in thf, a 71:29:0 mixture of the α-, γ-, and ε-substituted products was observed. The outcome of quenching 9 with methyl iodide in thf gave a 70:15:15 mixture of products. The methyl iodide quenching reactions of 6 and 9 also show that the directing effect of the pendant Lewis base group is weak. Methylation of 6 by methyl tosylate produces qualitatively similar outcomes in toluene and thf, in which no εsubstituted product is obtained, and with α:γ ratios of 53:47 and 65:35, respectively. The reaction of 9 with methyl tosylate in thf is also similar that of 6, with an α:γ ratio of 60:40. Surprisingly, the reactions of 6 and 9 with ethyl tosylate are entirely selective for the γ-substituted product irrespective of the solvent, although the 2,4- 1,4-, and 1,3-diene products were obtained as a 25:50:25 mixture. As with the ethylation reactions of 6 and 9, the allylation reactions with allyl bromide also resulted in substitution at the γ-carbon, although in these reactions only the 1,4-diene product was isolated. Finally, the reaction of 6 with dimethyldisulfide reveals a slight dependence of the regiochemistry of substitution on solvent, with the reaction in toluene giving an α:γ:ε ratio of 28:18:54 and the reaction in thf giving an α:ε ratio of 31:69. The outcome of the reaction of 9 with dimethyldisulfide in thf gave an equal mixture of α- and ε-substituted products. On the basis of our initial reactivity studies, it is clear that, despite the η1 coordination to lithium via the pentadienyl α-in 6, the coordination mode of the pentadienyl anion toward the lithium cation has essentially no bearing on the site of electrophilic quenching. Some solvent effects are apparent, but they do not produce significant changes in the reaction outcomes. These observations provide further illustrations of the complicated reactivity of the pentadienyl carbanion.
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EXPERIMENTAL SECTION
All reactions were performed under an argon atmosphere using standard Schlenk and glovebox techniques. thf and hexane were dried by refluxing over potassium metal and sodium−potassium alloy, respectively, for a minimum of 3 days, before being distilled onto freshly activated 3 Å molecular sieves and stored for 3 days. Pentane was collected from an Innovative Technologies solvent purification system and stored over freshly activated 3 Å molecular sieves for a minimum of 3 days before use. Benzene-D6 and THF-D8 were was degassed and refluxed over potassium metal for 3 days in a J. Young’s ampule and was then vacuum transferred to a clean ampule before use. 1,4-Pentadiene was purchased from Tokyo Chemical Industry UK. n Butyllithium (1.6 M in hexanes), chloro(chloromethyl)dimethylsilane, pyrrolidine, N,N,N′,N′-tetramethylethylenediamine, methyl iodide, allyl bromide, and chlorotrimethylsilane were purchased from Sigma-Aldrich. N,N,N′,N′-Tetramethylethylenediamine was distilled onto freshly activated 3 Å molecular sieves before use, while all other chemicals were used as received. NMR spectra were acquired using a Bruker Avance III spectrometer at the following frequencies:
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CONCLUSION The functionalized pentadienylsilanes 5 and 7 can be converted into their corresponding lithium pentadienyl complexes, 6 and E
DOI: 10.1021/om501144f Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
δ/ppm): 6.93−6.79 (m, 2H, overlapping β-CH and δ-CH), 5.57 (dd, 1H, 3JHH = 10.7 and 13.0 Hz, γ-CH), 4.76 (dd, 1H, 2JHH = 2.5 Hz, 3JHH = 16.4 Hz, trans-ε-CH), 4.42 (dd, 1H, 2JHH = 2.5 Hz, 3JHH = 9.9 Hz, cis-ε-CH), 2.40 (d, 1H, 3JHH = 15.3 Hz, α-CH), 2.31 (br, 4H, pyrrolidyl CH2), 1.88 (s, 12H, TMEDA CH3), 1.81 (s, 2H, SiCH2), 1.77 (br, 4H, TMEDA CH2), 1.42 (br, 4H, pyrrolidyl CH2), 0.32 (s, 6H, Si(CH3)2). 13C{1H} NMR (D6-benzene, 295.25 K, δ/ppm): 150.2 (β-CH), 143.2 (δ-CH), 100.1 (γ-CH), 89.5 (ε-CH2), 58.6 (pyrrolidyl CH2), 57.3 (TMEDA CH2), 52.9 (SiCH2), 47.3 (α-CH), 46.5 (TMEDA CH3), 24.2 (pyrrolidyl CH2), 1.1 (Si(CH3)2). 29Si{1H} NMR (benzene-D6, 298.15 K, δ/ppm): −11.4. 7Li{1H} NMR (benzene-D6, 293.95 K, δ/ppm): 0.3. 1H NMR (thf-D8, 298.15 K, δ/ppm): 6.33 (br t, 3JHH = 13.9 Hz, β-CH), 6.12 (dt, 1H, 3JHH = 10.4 and 16.3 Hz, δ-CH), 4.89 (dd, 1H, 3JHH = 10.7 and 12.8 Hz, γ-CH), 3.73 (br d, 1H, 3JHH = 16.4 Hz, trans-ε-CH), 3.38 (br d, 1H, 3JHH = 9.3 Hz, cis-ε-CH), 2.49 (br, 4H, pyrrolidyl CH2), 2.30 (s, free TMEDA CH2), 2.15 (s, free TMEDA CH3), 2.10 (br d, 1H, 3JHH = 15.4 Hz, αCH), 1.84 (br, 2H, SiCH2), 1.72 (br, pyrrolidyl CH2 overlapping with thf solvent resonance), −0.05 (s, 6H, Si(CH3)2). 13C{1H} NMR (thfD8, 298.15 K, δ/ppm): 149.6 (β-CH), 143.4 (δ-CH), 100.0 (γ-CH), 85.6 (ε-CH2), 59.1 (over lapping pyrrolidyl CH2 and free TMEDA CH2), 52.9 (SiCH2), 48.0 (α-CH), 46.4 (free TMEDA CH3), 24.7 (pyrrolidyl CH2), 0.34 (Si(CH3)2). 29Si{1H} NMR (thf-D8, 298.15 K, δ/ppm): −12.0. 7Li{1H} NMR (thf-D8, 294.25 K, δ/ppm): −1.8. Compound 7. Bis(2-methoxyethyl)amine (6.97 g, 52.4 mmol) was added to neat 4 (3.05 g, 17.5 mmol) in a J. Young’s ampule, and the resultant mixture stirred for 48 h at 90 °C. Pentane was added to the mixture, which effected precipitation of bis(2-methoxyethyl)ammonium chloride as the byproduct, which was removed by filtration. Removal of pentane under vacuum gave a pale yellow liquid, which was revealed to be a mixture of 7 and the amine starting material by 1H NMR spectroscopy. The liquid was then dissolved in petroleum ether (40 mL) and extracted with deionized water (3 × 20 mL) to remove the excess amine. The separated organic solution was then dried over MgSO4 and filtered, and removal of the solvent under vacuum left a pale yellow-orange oil. Vacuum distillation at
