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Organometallics 2009, 28, 6480–6488 DOI: 10.1021/om900681m
Carbon-Germanium Hyperconjugation: Solid-State and Gas-Phase Investigations of (Trialkylgermyl)methyl-Substituted Pyridinium Ions Asimo Karnezis, Richard A.J. O’Hair, and Jonathan M. White* School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne, Victoria 3010, Australia Received July 31, 2009
A crystallographic and computational study on 2- and 4-((trialkylgermyl)methyl)pyridinium ions 7a-c and 8a-c provides evidence for strong hyperconjugation between the Ge-CH2 bond and the π-deficient aromatic ring. Isodesmic equations show that the trimethylgermyl substituent stabilizes the pyridinium cations by 20-26 kJ mol-1 relative to the germanium-free analogues. Natural bond orbital analysis reveals that the major contributor to this stabilization is hyperconjugation between the Ge-CH2 bond and the aromatic π-system and that the strength of this interaction is greater for the 2-substituted ions 8a compared to the 4-substituted ions 7a. Crystallographic analysis of the 2- and 4-tert-butylgermylmethyl-substituted ions 7c and 8c provides the first structural evidence for carbon-germanium hyperconjugation; thus, the Ge-CH2 bonds are significantly longer (0.03-0.04 A˚) than standard values and the CH2-Ar bond distance is shorter. Investigation of the gas-phase unimolecular chemistry of these ions, formed via electrospray ionization (ESI) and subjected to collision-induced dissociation (CID), reveals that the principal fragmentation of these ions involves cleavage of the weak CH2-GeR3 bond, giving an ionmolecule complex between R3Geþ and a pyridine-enamine, which then undergoes further reactions.
Introduction The structural effects of C-Si and more generally C-M (M = Si, Ge, Sn) hyperconjugation have been demonstrated in a number of systems, ranging from substituted benzenes with low electron demand1,2 through to tropylium cations3 and vinyl cations.4 We have found that (trialkylsilyl)methylsubstituted pyridinium cations provide a convenient system for investigating C-Si hyperconjugation in cationic systems with relatively low electron demand; these crystalline cations are relatively stable to desilylation, and structural and spectroscopic information can be obtained.5,6 For example, both the 2- and 4-substituted (trialkylsilyl)methyl-substituted N-methylpyridinium ions 1 and 2 have been investigated both in the solid state and in solution.6 In the solid state X-ray analysis reveals that C-Si hyperconjugation in 1 and 2 manifests in lengthening of the CH2-Si bond distance and shortening of the CH2-C(Ar) bond distance, reflecting contributions of the double-bond-no-bond resonance *To whom correspondence should be addressed. E-mail: whitejm@ unimelb.edu.au. (1) Lambert, J. B.; Singer, R. A. J. Am. Chem. Soc. 1992, 114, 10246. (2) Lambert, J. B.; Shawl, C. E.; Basso, E. Can. J. Chem. 2000, 78, 1441. (3) Hassall, K. S.; White, J. M. Org. Lett. 2004, 6, 1737. (4) Muller, T.; Juhasz, M.; Reed, C. A. Angew. Chem., Int. Ed. 2004, 43, 1543. (5) Happer, A.; Ng, J.; Pool, B.; White, J. J. Organomet. Chem. 2002, 659, 10. (6) Hassall, K.; Lobachevsky, S.; White, J. M. J. Org. Chem. 2005, 70, 1993. pubs.acs.org/Organometallics
Published on Web 10/29/2009
forms 10 and 20 to the ground-state structures of these ions.
These ions have also been investigated in solution using C and 29Si spectroscopy. C-Si-π hyperconjugation reveals itself as a downfield shift in the 29Si chemical shift and a decrease in the 29Si-13C coupling constant upon methylation of the precursor pyridine derivatives 3 and 4. Both effects are greater in magnitude for the 2-substituted derivatives,6 in qualitative agreement with calculations.7 13
There is experimental evidence to suggest that the C-Ge bond is a stronger σ-donor than a C-Si bond:8,9 for example, the ionization potential for Et4Ge (measured using (7) Hassall, K.; Lobachevsky, S.; Schiesser, C. H.; White, J. M. Organometallics 2007, 26, 1361. r 2009 American Chemical Society
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photoelectron spectroscopy), which at 9.3 eV is lower than that observed for Et4Si (10.6 eV). The C-Ge bond should therefore participate in stronger hyperconjugation with neighboring electron-deficient centers. Consistent with this, Lambert showed that unimolecular solvolysis of the trimethylgermyl-substituted trifluoroacetate 5 occurs at a rate which is 1 order of magnitude faster than for the silicon congener 6, which itself already solvolyses 1012 times faster than silicon-free analogues.10,11 Calculations on β-group 4 metal substituted 1°, 2° ,and 3° carbenium ions12,13 suggest the order of hyperconjugative stabilization C-Pb > C-Sn > C-Ge > C-Si. Calculations on other neutral systems also suggest the same order of donor abilities.14 Here we extend our studies to the (trialkylgermyl)methyl-substituted pyridinium ions 7 and 8, in order to investigate the structural effects associated with C-Ge hyperconjugation in these ions and to compare these with the corresponding silicon-substituted derivatives. Figure 1. B3LYP-631G** computed structures of 7a and 8a, the corresponding neutral pyridine precursors 9a and 10a, and selected bond distances (A˚). Scheme 1. Isodesmic Equations for (Trimethylsilyl)methyl- and (Trimethylgermyl)methyl-Substituted Pyridinium Cations
Results and Discussion Computational Studies. To gain some initial insight into the properties of germanium-substituted pyridinium ions, we carried out ab initio calculations on the model systems 7a and 8a. Calculations were performed at the B3LYP/6311G** level of theory15 and were corrected for zero-point energies (ZPE). Calculated structures of the N-methylpyridinium ions 7a and 8a and the corresponding neutral pyridines with relevant structural data are presented in Figure 1. (8) Schweig, A.; Weidner, U.; Manuel, G. J. Organomet. Chem. 1973, 54, 145. (9) Carlson, T. A.; McGuire, G. E.; Jonas, A. E.; Cheng, K. L.; Anderson, C. P.; Lu, C. C.; Pullen, B. P. Electron Spectrosc., Proc. Int. Conf. 1972, 207. (10) Lambert, J. B.; Wang, G.; Teramura, D. H. J. Org. Chem. 1988, 53, 5422. (11) Lambert, J. B. Acc. Chem. Res. 1999, 32, 183. (12) Fern andez, I.; Frenking, G. J. Phys. Chem. 2007, 111, 8028. (13) Nguyen, K. A.; Gordon, M. S.; Wang, G.-T.; Lambert, J. B. Organometallics 1991, 10, 2798. (14) Nyul aszi, L.; von Rague Schleyer, O. J. Am. Chem. Soc. 1999, 121 6872. (15) Frisch, M. J.; et al. Gaussian 03, Revision B.04; Gaussian, Inc., Pittsburgh, PA, 2003.
Upon conversion from the neutral pyridine derivatives to the N-methylpyridinium ions, significant lengthening of the Ge-CH2 bond and shortening of the CH 2-Ar bond occurs, consistent with strengthening of the σC-Ge-π hyperconjugation in the more π-electron-deficient ions 7a and 8a, compared with their neutral precursors 9a and 10a. Stabilization of the germanium-substituted pyridinium ions relative to the germanium-free analogue is quantified by calculating the enthalpy change in the isodesmic equations given in Scheme 1. The previously reported silicon values are included for comparison.7 In both silicon- and germanium-substituted derivatives the equation is exothermic, consistent with stabilization of the positively charged pyridinium ion by the group 14 metal substituent; the magnitude of stabilization is greater for the 4-substituted derivatives 1a and 7a compared with the 2-substituted derivatives 2a and 8a, a result we have commented on previously for the silicon series and very likely reflects some steric destabilization of the 2-substituted pyrdinium ions 2a and 8a.6 The germanium substituent is more stabilizing in both cases by ca. 4 kJ mol-1, consistent with σC-Ge-π hyperconjugation being stronger than σC-Si-π hyperconjugation. A good indicator of the strength of the
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Scheme 2. Synthesis of ((Trialkylgermyl)methyl)pyridines 7a-c and 8a-c
Table 1. NBO Interaction Energies for the σC-Ge-π Interaction in Pyridinium Ions compd 1a 2a 7a 8a
Table 2. Selected Structural Parameters for Triflate Salts 7c and 8c
NBO interaction (kJ mol-1) 43.3 52.3 50.5 61.1
7c
8c
Bond Distances (A˚) Ge-C6 Ge-C8 Ge-C9 Ge-10 CH2-C(Ar)
1.999(3) 1.942(3) 1.935(3) 1.977(3) 1.482(4)
2.009(3) 1.947(4) 1.937(4) 1.966(3) 1.473(4)
Bond Angles (deg) C8-Ge-C6 C9-Ge-C6 C10-Ge-C6 Ge-CH2-C(Ar) C8-Ge-C9 C8-Ge-C10 C9-Ge-C10
108.7(1) 106.6(1) 108.6(1) 109.2(2) 110.8(1) 110.6(1) 111.4(1)
108.9(1) 107.5(2) 109.6(1) 110.3(2) 108.9(2) 110.9(2) 111.0(2)
Dihedral Angle C10-Ge-CH2-C(Ar)
-166.4(1)
-149.0(2)
Table 3. Comparison of Relevant Structural Parameters (A˚) for Silicon-Substituted Ions 1c and 2c with Those of the GermaniumSubstituted ions 7c and 8c compd 1c
Figure 2. Thermal ellipsoid plots for pyridinium ions 7c (top) and 8c (bottom). Ellipsoids are at the 20% probability level, and counterions have been omitted for clarity.
σC-M-π interaction is provided by NBO (natural bonding orbital) analysis16,17 of the relevant interacting orbitals. Interaction energies between the σC-M bond and the π system of the pyridinium ions 1a and 2a and 7a and 8a, which were determined by a second-order perturbation analysis, are presented in Table 1. This analysis is consistent with σC-Ge-π hyperconjugation being stronger than σC-Si-π hyperconjugation and also indicates stronger hyperconjugation at the 2-position for both systems. Synthesis. The (trialkylgermyl)methyl-substituted pyridines 7a-c and 8a-c were prepared as described in Scheme 2. Lithiation of 2- and 4-picoline was followed by quenching with a trialkylgermyl halide to provide the pyridine precursors 9a-c and 10a-c; conversion to 7a-c and 8a-c was achieved by methylation with either methyl iodide or methyl triflate. (16) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO 4.0; University of Wisconsin, Madison, WI, 1996. (17) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899.
2c 7c 8c
param Si-CH2 CH2-Ar Si-CH2 CH2-Ar Ge--CH2 CH2-Ar Ge-CH2 CH2-Ar
obsd 1.916(2) 1.480(3) 1.919(2) 1.484(3) 1.999(3) 1.482(4) 2.009(3) 1.473(4)
standard18 a
1.879 1.512 1.960 1.512
Δ þ0.037 -0.032 þ0.040 -0.028 þ0.039 -0.030 þ0.049 -0.039
a From the search fragment R3Si-CH2R0 , 626 hits with R factors e5%.
Crystallization of the triflate salts was attempted by diffusion of ether into acetonitrile solutions; however, only the tert-butyldimethylgermyl-substituted salts 7c and 8c gave crystals suitable for X-ray analysis. The salts 7a,b and 8a,b underwent significant decomposition involving loss of the germanium substituent during this process, giving N-methylated 2- and 4-picolines as the major decomposition products; we have encountered similar problems with the corresponding silicon-substituted ions.6 Ortep diagrams for 7c and 8c are presented in Figure 2, while selected bond distances and angles and dihedral angles are presented in Table 2. In both structures the tert-butyl substituent is directed away from the aromatic ring, with differences in the C10-Ge-CH2-C(Ar) dihedral angle between the two structures presumably relieving steric interactions with the
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Figure 3. CID of ions (A) 7a, (B) 8a, (C) 7d, and (D) 8d. Asterisks denote the peaks subjected to CID.
o-NMe group in 8c. In both structures the CH2-Ge bond is essentially orthogonal to the aromatic ring, a conformation which is favored on steric grounds but which also maximizes hyperconjugation between the CH2-Ge bond and the lowlying π* orbital of the charged pyridinium ring. The bond distances within the tert-butyldimethylgermanium substituent in both structures are comparable with those in other structures in the Cambridge Crystallographic Database (CCD) which contain this substituent. However, the Ge-CH2 bond distances (1.999(3) A˚ for 7c and 2.009(3) A˚ for 8c) are substantially longer than the typical R3GeCH2R0 bond distance which was obtained from a survey of the CCD (1.960(2) A˚, from 30 hits with R factors e5%). This structural effect is consistent with the presence of strong σC-Ge-π hyperconjugation in both structures and suggests that double-bond-no-bond resonance structures (e.g. 70 and 80 ) make a significant contribution to the ground-state structure in both 7c and 8c. This interaction should render some double-bond character to the CH2-Ar bond; indeed, the distances in 7c and 8c, which are 1.482(4) and 1.473(4) A˚, respectively, are significantly shorter than the “typical” RCH2-Ar bond distance obtained from the CCD of 1.512 A˚ (6496 hits containing 9942 observations). It is worthwhile comparing the structures of 7c and 8c with the computed structures of 7a and 8a. Both the computed and observed structures adopt similar conformations about the CH2-Ar bond; however, the calculated Ge-CH2 bond distances are ca. 0.05-0.06 A˚ longer than the experimental values. Extra lengthening of the CH2-Ge distances is suggestive of stronger σC-Ge-π hypercongutation, a result which is expected in the gas-phase structures, which would rely more strongly upon internal stabilization of the positive charge. The calculated Ge-Me distances are also longer than the experimental values, although to a lesser extent. Given that C-Ge hyperconjugation is reported to be stronger than C-Si hyperconjugation, we were interested in establishing whether the structural effects of σC-Ge-π hyperconjugation are significantly different from those arising from σC-Si-π hyperconjugation. Comparisons between the relevant structural parameters of the silicon-substituted
ions 1c and 2c7 and the corresponding germanium-substituted ions 7c and 8c are presented in Table 3. From the data in Table 3 it is clear that the degree of lengthening of the ArCH2-GeR3 bond and shortening of the Ar-CH2GeR3 bonds in 7a and 8a compared to standard values is similar in magnitude to the corresponding structural effects in the silicon-substituted analogues 1c and 2c; thus, no conclusions can be drawn regarding the relative strengths of σC-Ge-π and σC-Si-π hyperconjugation. Clearly the differing structural effects arising from σC-Ge-π and σC-Si-π hyperconjugation are too small to be measurable by this technique. Gas-Phase Studies. The gas-phase behavior of silicon-substituted pyrdinium ions 1a and 2a has recently been reported;19 collision -induced dissociation (CID) of the parent ions results in several interesting fragmentation reactions, which are triggered by the cleavage of the weakened ArCH2-SiMe3 bond, to initially give the ion-molecule complexes (IMC) 11 and 14, respectively (Scheme 3). The IMCs 11 and 14 then follow three competing pathways: dissociation to give the free Me3Siþ cation (path a), proton transfer from the Me3Siþ cation to the enamine (path b), and hydride transfer from the enamine to the Me3Siþ cation (path c), giving the ions 12 and 13, respectively, from 11 or the ions 15 and 16 by analogous reactions from 14. We were interested in comparing the gas-phase behavior of the ions 1a and 2a with that of the corresponding germanium-substituted ions 7a and 8a. Thus, solutions of the germanium-substituted pyridine precursors 9a and 10a in acetonitrile were methylated with both CH3I and CD3I and immediately introduced into an LTQ mass spectrometer via electrospray ionization (ESI) in the positive mode (Scheme 4). Intense signals of the parent pyridinium ions 7a and 8a were observed, which were then massselected and subjected to CID and the fragment ions analyzed. Isolation and subsequent CID of N-methyl 2- and 4-((trimethylgermyl)methyl)pyridinium ions 8a and 7a (m/z 226) (18) Allen, F. H.; Bellard, S.; Brice, M. D.; Cartwright, B. A.; Doubleday, A.; Higgs, H.; Hummelink, T.; Hummelink-Peters, B. G.; Kennard, O.; et al. Acta Crystallogr., Sect. B 1979, B35, 2331. (19) Karnezis, A.; O’Hair, R. A. J.; White, J. M. Organometallics 2009, 28, 4276.
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Scheme 3. Gas-Phase Behavior of (Trimethylsilyl)methyl-Substituted Pyridinium Ions 1a and 2a19
Scheme 4. Formation of the N-Methyl and N-Methyl-d3 2- and 4-((Trialkylgermyl)methyl)pyridinium Ions Studied via ESI Mass Spectrometry
Scheme 5. Gas-Phase Behavior of (Trimethylgermyl)methyl-Substituted Pyridinium Ions 7a and 8a
and N-methyl-d3 2and 4-((trimethylgermyl)methyl)pyridines 8d and 7d (m/z 229) were conducted (Figure 3). These both show the formation of ions at m/z 108 in addition to Me3Geþ (m/z 119) for 7a and 8a and the corresponding ion at m/z 111 in addition to Me3Geþ (m/z 119) for 7d and 8d. The relative abundances of m/z 108
or m/z 111 and m/z 119 ions were approximately 20-30% and 100%, respectively. On the basis of analogy with the silicon-substituted ions 1a and 2a (Scheme 3), we propose that the ions at m/z (108/111) obtained from CID of both 7a,d and 8a,d correspond to the N-CH3 (or N-CD3) substituted 2- and 4-picolinium ions 12
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and 15, respectively arising by formal proton transfer from the trimethylgermyl cation and the enamine intermediate within the initially formed ion-molecule complexes 17 and 18 (Scheme 5). However, in contrast to the behavior displayed by the silicon-substituted ions 1a and 2a, there is no evidence for the formation of ions m/z 106 or 108 (Scheme 3, path c) from ions 7a,d or 8a,d, suggesting that the hydride
Figure 4. Free energy profile for N-methyl-2-((trimethylgermyl)methyl)pyridinium ion at the B3LYP/6-311G** level (sum of thermal and free energies).
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transfer pathway is not competitive for these germaniumsubstituted ions. The transition state for the proton transfer step (from 8a) (Figure 4), was located at the B3LYP/6-311g(d,p) level of theory and characterized by a single imaginary frequency corresponding to transfer of the proton between the two heavy atoms. The barrier for this proton transfer was 203 kJ/mol relative to the ground-state ion 8a and ca. 12 kJ/mol above that of the ion-molecule complex 18. The proton transfer step for the corresponding trimethylsilyl-substituted ion 2a was found to be 225 kJ/mol above the ground state ion but only ca. 4.8 kJ/mol above the ion-molecule complex, 14, at the same level of theory.19 The slightly higher barrier for proton transfer from the trimethylgermyl cation to the pyridine enamine within the ion-molecule complex 18 compared with the analogous reaction for the silicon derivative, ion-molecule complex 14, is consistent with the expected lower acidity of the R-protons of the trimethylgermyl cation compared to that of the R-protons of the trimethylsilyl cation.20 An interesting point to note is that while hydride abstraction was observed within the ion-molecule complex derived from the silicon-containing ions 1a and 2a,19 this pathway was not observed to occur for 7a and 8a.24 This may reflect differing hydride affinities of trimethylgermyl cation and its silicon counterpart. To further explore the gas -phase chemistry of germaniumsubstituted pyridinium ions 7 and 8 under collision-induced dissociation conditions, experiments were conducted on the ((triethylgermyl)methyl)- and ((tert-butyldimethylgermyl)methyl)pyridinium ions 7b,c and 8b,c and their corresponding N-CD3 analogues 7e,f and 8e,f. CID of the N-methyl 2- and 4-((triethylgermyl)methyl)pyridinium ions 7b and 8b, m/z 268 and 271 for the N-methyld3 2- and 4-derivatives 7e and 8e, resulted in the formation of Et3Geþ, m/z 161, followed by sequential loss of ethylene from this ion to give ions m/z 133 and m/z 105, assigned to þ GeEt2H, and þGeH2Et, respectively (Figure 5). There was no evidence for the formation of ions at m/z 108 (for N-CH3 derivatives) or m/z 111 (for N-CD3 derivatives), indicating that, if an IMC is formed similar to that shown in Scheme 5, then proton transfer from the
Figure 5. CID of ions (A) 7b, (B) 8b, (C) 7e, and (D) 8e. Asterisks denote the peaks subjected to CID.
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Figure 6. CID of ions (A) 7c, (B) 8c, (C) 7f, and (D) 8f. The asterisks denote the peaks subjected to CID. Scheme 6. Gas-Phase Behavior of (Triethylgermyl)methyl-Substituted Pyridinium Ions 7b and 8b
Scheme 7. Gas-Phase Behavior of (Trimethylgermyl)methyl-Substituted Pyridinium Ions 7c and 8ca
a
Independent MS3 experiments (data not shown) confirm the consecutive nature of these fragment ion reactions.
triethylgermyl cation to the enamine intermediate does not compete with simple dissociation of this complex (Scheme 6).
ESI/MS analysis of N-methyl 2- and 4-((tert-butyldimethylgermyl)methyl)pyridinium ions 7c and 8c and the corresponding N-CD3 derivatives 7f and 8f gave the
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expected ions at m/z 268 and m/z 271, respectively (Figure 6). CID of each of these pyridinium ions produced an ion at m/z 161 corresponding to Me2tBuGeþ, in addition to an ion at m/z 119, corresponding to Me3Geþ. There is no evidence for the formation of ions at m/z 108 for 7c or 8c or m/z 111 for 7f or 8f. It appears that, upon formation of the presumed ion-molecule complex (Scheme 7), dissociation occurs before proton transfer from the Me2tBuGeþ cation to the enamine can occur. Once the Me2tBuGeþ ion dissociates from the presumed IMC, it then rearranges with loss of propene to give Me3Geþ cation in competition with fragmentation to give GeHMe2þ (m/z 105), which can further fragment to give ion m/z 89, tentatively assigned as CH2dGeHþ. Phillipou21 has shown that Me2tBuSiþ generated from a number of sources using electron ionization rearranges readily to give Me3Siþ; however, rearrangement to give the corresponding silicon-containing ions SiHMe2þ and CH2dSiHþ was not reported.
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Crystallography. Intensity data were collected with an Oxford Diffraction Sapphire CCD diffractometer using Cu KR radiation (graphite crystal monochromator, λ = 1.541 84 A˚); the temperature during data collection was maintained at 130.0(1) K using an Oxford Cryosystems cooling device.
The structures were solved by direct methods and difference Fourier synthesis. Thermal ellipsoid plots were generated using the program ORTEP-322 integrated within the WINGX23 suite of programs. Crystal Data for 7c. C14H24F3GeNO3S, Mr = 415.99, T = 130.0(2) K, λ=1.541 84 A˚, orthorhombic, space group Pbca, a= 10.7763(3) A˚, b=11.2920(3) A˚, c=31.2541(8) A˚, V=3803.2(2) A˚3, Z=8, Dc=1.453 mg M-3, μ(Mo KR)=3.579 mm-1, F(000)= 1712, crystal size 0.6 0.3 0.1 mm, 19 014 reflections measured, 3775 independent reflections (Rint = 0.041), final R=0.0442 (I > 2σ(I)) and Rw(F2)=0.1305 (all data). Crystal Data for 8c. C14H24F3GeNO3S, Mr = 415.99, T = 130.0(2) K, λ = 1.541 84 A˚, monoclinic, space group C2/c, a = 30.8277(6) A˚, b = 10.5744(2) A˚, c =11.2514(2) A˚, β = 94.261(2)°, V=3657.6(1) A˚3, Z=8, Dc =1.511 mg M-3, μ(Mo KR) = 3.721 mm-1, F(000) = 1712, crystal size 0.24 0.19 0.02 mm, 7561 reflections measured, 3521 independent reflections (Rint=0.04), final R=0.0396 (I > 2σ(I)), Rw(F2)=0.0989 (all data). Synthesis. All reactions were conducted in oven-dried glassware under a nitrogen atmosphere. Solvents were anhydrous, and all fine chemicals were used as received from Sigma-Aldrich Chemical Co. Petroleum spirit refers to the fraction that boils at 40-60 °C. Analytical thin-layer chromatography (TLC) was carried out using aluminum-backed 2 mm thick Merck silica gel 60 GF256. Compounds were visualized under UV 365 nm light or by iodine. 1H and 13C NMR spectra were recorded as solutions in solvents indicated on an Inova 400 or Inova 500 spectrometer. 1H and 13C spectra were reported as chemical shift (ppm) followed by (in parentheses, where applicable) integration, multiplicity, coupling constant, and peak assignment. High-resolution mass spectra were generated as described below. General Procedure for the Preparation of the ((Trialkylgermyl)methyl)pyridines. To a solution of the required methylpyridine in ether was added 1 mol equiv of MeLi (1.6 M in ether), and the solution was refluxed for 30 min. This solution was cooled to room temperature and stirred for 1 h before being further cooled to -78 °C (dry ice/acetone) prior to the addition of the corresponding trialkylgermanium chloride (1.2 mol equiv). The mixture was stirred at room temperature for a further 1 h. The reaction mixture was quenched with water (20 mL) and extracted into ether (mL). The solution was washed with NaOH (10 mL) and the ether layer washed with water (2 mL) and dried (MgSO4). The solvent was removed under reduced pressure and the compound purified via dry-flash column chromatography (petroleum spirits/ether) to yield the ((trialkylgermyl)methyl)pyridine. Data for 10a. Yield: 2.2 g, 13.3 mmol, 59%. 1H NMR (500 MHz, CDCl3): δ 8.41 (1H, d, J=7.42 Hz, H4), 7.48 (1H, ddd, J=7.7, 7.6, 2.0 Hz), 6.93 (2H, m), 2.44 (2H, s), 0.13 (9H, s), 13 C NMR (125 MHz, CDCl3): δ 161.5, 148.4, 135.2, 121.1, 118.4, 28.9, -2.70. ESI/MS: [M þ CH3]þ 226.064 65 (calculated for C10H18GeNþ 226.064 55). Data for 9a. Yield: 0.96 g, 4.60 mmol, 50%. 1H NMR (500 MHz, CD3CN): δ 8.34 (2H, m), 6.94 (2H, m), 2.26 (2H, s), 2.34 (9H, s). 13C NMR (125 MHz, CDCl3): δ 152.1, 150.2, 124.1, 26.5, -2.518. ESI/MS: [M þ CH3]þ 226.064 60 (calculated for C10H18GeNþ 226.064 55). Data for 10b. Yield: 0.18 g, 0.71 mmol, 31%. 1H NMR (500 MHz, CD3CN): δ 8.36 (1H, m), 7.53 (1H, ddd, J = 7.7, 7.6, 2.0 Hz), 7.05 (1H, m, H1), 6.99 (1H, m, H3), 2.16 (2H, s), 1.00 (9H, t, J = 7.9 Hz), 0.75 (6H, q, J = 7.7 Hz). 13C NMR (100 MHz, CD3CN): δ 159.4, 150.0, 137.2, 124.1, 121.7, 24.6,
(20) The measured proton affinities of Me2MdCH2 (M=Si, Ge) are 945.5 and 851.1 kJ/mol, respectively: Hunter, E. P.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27 (3), 413–656. (21) Phillipou, G. Org. Mass Spectrom. 1977, 12 (4), 261.
(22) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (23) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (24) Despite numerous attempts, we have been unable to find a TS for hydride transfer. However, the calculated energy of the products of this reaction, which is 218.4 kJ/mol, lies 14.5 kJ above the proton-transfer transition state.
Conclusion Computational studies on ((trimethylgermyl)methyl)pyridinium ions 7a and 8a have been carried out and establish that σC-Ge-π hyperconjugation is a major factor in the stabilization of these ions by the germanium substituent. Comparison with the corresponding silicon-substituted ions 1a and 2a reveals that σC-Ge-π hyperconjugation is slightly stronger than σC-Si-- hyperconjugation. In both cases hyperconjugation is stronger in the 2-substituted pyridinium ions (2a and 8a) than in the 4-substituted ions (1a and 7a). The effects of σC-Ge-π hyperconjugation have been demonstrated in the crystal structures of the ((tert-butyldimethylgermyl)methyl)pyridinium ions 7c and 8c; thus, the Ge-CH2 bond is lengthened compared to the standard bond distance and the CH2-Ar bond is shortened. Comparison of the structures of 7c and 8c with the corresponding siliconsubstituted pyridinium ions 1c and 2c shows that structural effects of σC-Ge-π and σC-Si-π hypeconjugation are similar in magnitude. Analysis of these ions using ESI/MS reveals that 7a and 8a give strong signals of the expected pyridinium ions, which upon CID result in heterolytic cleavage of the weak Me3GeCH2 bond, to give an ion-molecule complex, which then follows two competing pathways: dissociation to give a free trialkylgermyl cation in competition with proton transfer to the enamine intermediate. The latter pathway is competitive only for the (trimethylmethyl)germyl derivatives 7a and 8a.
Experimental Section
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9.6, 8.5. ESI/MS: [M þ CH3]þ 268.111 38 (calculated for C13H24GeNþ 268.111 50). Data for 9b. Yield: 0.10 g, 0.40 mmol, 9%. 1H NMR (500 MHz, CD3CN): δ 8.32 (2H, m), 7.01 (2H, m), 2.27 (2H, s), 1.02 (9H, q, J=7.8 Hz), 0.75 (6H, t, J=7.8 Hz). 13C NMR (125 MHz, CD3CN): δ 152.6, 150.2, 124.3, 21.7, 9.1, 4.5. ESI/MS: [M þ CH3]þ 268.111 36 (calculated for C13H24GeNþ 268.111 50). Data for 10c. Yield: 0.96 g, 3.8 mmol, 76%. 1H NMR (500 MHz, CD3CN): δ 8.37 (1H, d, J=4.9 Hz), 7.54 (1H, ddd, J=7.7, 7.6, 2.0 Hz), 7.05 (1H, d, J=7.8 Hz), 7.00 (1H, m), 2.45 (2H, s), 0.99 (6H, m), 0.02 (9H, m). 13C NMR (125 MHz, CD3CN): δ 163.5, 149.7, 136.7, 123.0, 120.0, 26.6, 27.6, 22.0, -6.6. ESI/MS: [M þ CH3]þ 268.111 26 (calculated for C13H24GeNþ 268.111 50). Data for 9c. Yield: 0.87 g, 3.4 mmol, 76%. 1H NMR (500 MHz, CD3CN): δ 8.33 (2H, m), 6.87 (2H, m), 2.19 (2H, s), 1.00 (6H, m), 0.00 (9H, m). 13C NMR (125 MHz, CD3CN): δ 151.5, 149.2, 123.3, 22.4, 27.3, 21.6, -7.3. ESI/MS: [M þ CH3]þ 268.111 57 (calculated for C13H24GeNþ 268.111 50). General Procedure for the Preparation of N-Methyl 2- and 4-((Trialkylgermyl)methyl)pyridines 7a-c and 8a-c. A solution of the required pyridine derivative (100 mg) in deuterioacetonitrile (0.5 mL) was treated with neat methyl triflate (1.05 equiv). Data for 8a. 1H NMR (400 MHz, CD3CN): δ 8.65 (1H, d, J= 6.4 Hz), 8.40 (1H, ddd, J= 7.8, 7.8, 0.9 Hz), 7.94 (1H, d, J = 7.8 Hz), 7.85 (1H, t, J=6.9 Hz), 2.93 (2H, s), 4.13 (3H, s), 0.31 (9H, s). 13C NMR (100 MHz, CD3CN): δ 162.5, 146.4, 144.8, 128.8, 123.6, 47.1, 26.4, -1.0. Data for 7a. 1H NMR (500 MHz, CD3CN): δ 8.35 (2H, d, J= 6.8 Hz), 7.54 (2H, d, J=6.6 Hz), 4.17 (3H, s), 2.67 (2H, s), 4.2 (3H, s) 0.21 (9H, s). 13C NMR (125 MHz, CD3CN): δ 165.4, 144.5, 126.5, 47.8, 29.7, -2.5. Data for 8b. 1H NMR (500 MHz, CD3CN): δ 8.47 (1H, d, J= 6.4 Hz), 8.60 (1H, m), 8.21 (1H, m), 7.68 (1H, d, J=8.3 Hz), 4.09 (3H, s), 2.84 (2H, s), 1.02 (9H, m), 0.92 (6H, m). 13C NMR (100 MHz, CD3CN): δ 163.0, 146.2, 144.8, 128.9, 123.6, 46.6, 22.6, 8.9, 5.7. Data for 7b. 1H NMR (500 MHz, CD3CN): δ 8.34 (2H, d, J= 6.8 Hz), 7.55 (2H, d, J=6.8 Hz), 4.16 (3H, s), 2.67 (2H, s), 1.03 (9H, m), 0.83 (6H, m). 13C NMR (125 MHz, CD3CN): δ 166.1, 144.5, 126.7, 47.7, 25.4, 8.86, 4.66. Data for 8c. 1H NMR (500 MHz, CD3CN): δ 8.41 (1H, d, J= 6.4 Hz), 8.21 (1H, ddd, J= 7.1, 7.3, 0.8 Hz), 7.64 (1H, d, J = 8.3 Hz), 7.59 (1H, m), 4.06 (3H, s), 2.83 (2H, s), 0.15 (6H, m), 1.10 (9H, m). 13C NMR (125 MHz, CD3CN): δ 162.8, 146.4, 144.9, 128.9, 123.7, 46.7, 25.8, -5.98, 27.2, 23.6.
Karnezis et al. Data for 7c. 1H NMR (500 MHz, CD3CN): δ 8.37 (2H, d, J= 6.8 Hz), 7.6 (2H, d, J=6.8 Hz), 4.18 (3H, s), 2.66 (2H, s), 0.07 (6H, s), 1.04 (9H, s). 13C NMR (125 MHz, CD3CN): δ 165.8, 144.5, 126.7, 47.6, 26.0, 27.5, 23.2, -7.10. Mass Spectrometry. All mass spectrometry experiments were carried out using a commercially available hybrid linear ion trap and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer (Finnigan LTQ-FT, Bremen, Germany), which is equipped with an electrospray ionization source. Stock solutions were prepared using 1 mmol of the pyridine derivative in 1 mL of acetonitrile and methylated, to form pyridinium ions, using 2-3 drops of iodomethane or deuterated iodomethane. The solutions where introduced into the electrospray ionization source via a syringe pump operating at a rate of 5 μL/min. Typical ESI conditions were employed and involve a needle potential of 4.0-5.0 kV, a heated capillary temperature of 200 °C, sheath air, ca. 3-25. The pyridinum precursor ion was mass selected with a window of m/z 1 and then subjected to CID using a corresponding normalized collision energy of 25-45% and an activation Q of 0.25 for a period of 30 ms. All highresolution mass spectrometry experiments were carried out on the same instrument. The [M þ CH3]þ pyridinium ions were mass-selected in the LTQ using standard procedures and were then analyzed in the FT-ICR MS to generate the highresolution tandem mass spectrum. Calibration was done via the automatic calibration function using the suggested LTQ calibration solution, consisting of caffeine, MRFA, and Ultramark solution.
Acknowledgment. We thank the Australian Research Council for financial support for financial support (DP0770565) and an award of an APA to A.K. We also thank the Victorian Partnership for Advanced Computing and the Victorian Institute for Chemical Sciences High Performance Computing Facility for the computational time and Professor Carl Schiesser for all his helpful advice with regard to the computational aspect of this paper. Supporting Information Available: Tables giving atomic coordinates for the calculated structures and CIF files giving crystal data for 7c and 8c. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files for 7c and 8c have also been deposited at the Cambridge Crystallographic Data Centre and assigned CCDC codes 749246 and 749247, respectively.
