PubMed2003
Norbornyl cations of the group 14 elements Si --> Pb have been synthesized from substituted 3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the C=C double bond of the 3-cyclopentenemethyl substituent (pi-route to norbornyl cations). The norbornyl cations 4a (E = Si, R = Me), 4e (E = Si, R = Et), 4f (E = Si, R = Bu), 4g (E = Ge, R = Bu), 4h (E = Sn, R = Bu), and 4i (E = Pb, R = Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, delta((29)Si) = 80-87, delta((13)C)(CH=) = 149.6-150.6; 4g, delta((13)C)(CH=) = 144.8; 4h, delta((119)Sn) = 334, delta((13)C)(CH=) = 141.5; 4i, delta((207)Pb) = 1049, delta((13)C)(CH=) = 138). The significant deshielding of the vinylic carbon atoms (Deltadelta((13)C)) relative to those of the precursor (Deltadelta((13)C) = 19.3-20.3 (4a,e,f), Deltadelta((13)C) = 14.6 (4g), Deltadelta((13)C) = 11.1 (4h), Deltadelta((13)C) approximately 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in the case of 4h and 4i (J(CSn) = 26 Hz, J(CPb) = 16 Hz) give experimental evidence for the intramolecular interaction and the charge transfer between the positively charged element and the remote C=C double bond. The experimental results are supported by quantum mechanical calculations of structures, energies, and magnetic properties for the norbornyl cations 4a,b (E = Ge, R = Me), 4c (E = Sn, R = Me), 4d (E = Pb, R = Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb) level of theory. The calculated (29)Si NMR chemical shifts for the silanorbornyl cations 4a,e,f (delta((29)Si) = 77-93) agree well with experiment, and the calculated structures of the cations 4a-f reveal their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e-i a formally 3 + 1 coordination for the element atom with the extra coordination provided by the C=C double bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group 14 element norbornyl cations 4a,e-i exhibit only negligible interactions with the aromatic solvent, and they are, depending on the nature of the element group, stable at room temperature in aromatic solvents for periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the norbornyl cations 4a,e-h breaks down and nitrilium ions with the element in a tetrahedral environment are formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal environment for the lead atom with the C=C double bond of the cyclopentenemethyl ligand and the nitrogen atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + DeltaZPVE level indicate that the thermodynamic stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol(-)(1). In contrast, the intramolecular stabilization energy E(A) of the norbornyl cations 4a-d decreases, suggesting reduced interaction between the C=C double bond and the electron-deficient element center in the plumbacation compared to the silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.