Solution and Gas-Phase Investigations of Trimethylsilylpropyl-Substituted Pyridinium Ions. Manifestation of the Silicon δ Effect

Computational studies on the <i>N</i>-methyl-2-trimethyl-M-propylpyridinium ions <b>15a</b> (M = Si), <b>15b</b> (M = Ge), <b>15c</b> (M = Sn), and <b>15d</b> (M = Pb) and <i>N</i>-methyl-4-trimethyl-M-propylpyridinium ions <b>16a</b> (M = Si), <b>16b</b> (M = Ge), <b>16c</b> (M = Sn), and <b>16d</b> (M = Pb) provide evidence for a significant through-bond (double hyperconjugative) interaction between the M–CH₂ bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si < Ge < Sn < Pb, in line with the σ-donor abilities of the C–M bond. The through-bond interaction for M = Si has been studied in solution using ¹³C and ²⁹Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation reactions of <b>15a</b> and <b>16a</b> are strongly influenced by the through-bond interaction, with the major fragmentation pathway proceeding via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions <b>1a</b> and <b>2a</b>

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases

Structural Characterization and Unusual Reactivity of Oxosulfido-Mo(V) Compounds: Implications for the Structure and Electronic Description of the Very Rapid Form of Xanthine Oxidase

The first structural characterization of a mononuclear, EPR-active, oxosulfido-Mo­(V) compound related to the very rapid form of xanthine oxidase (VR-XnO) is reported. The compound, [CoCp₂]­[Tp^<i>i</i>PrMo^VOS­(2-OC₆H₄CO₂Et)] [Cp = cyclopentadienyl; Tp^<i>i</i>Pr = hydrotris­(3-isopropylpyrazol-1-yl)­borate], exhibits a distorted octahedral geometry with MoO and Mo⎓S distances of 1.761(5) and 2.215(2) Å, respectively, and an OMo⎓S angle of 107.33(14)°. Significantly, the Mo^V⎓S distance is much shorter than the value of 2.36 Å reported for oxosulfido-Mo­(V) compounds (Singh, R.; et al. Inorg. Chem. 1989, 28, 8) but close to the range established for VR-XnO by protein crystallography. The methyl and phenyl esters were also prepared but the latter is highly reactive and undergoes an intramolecular, radical-based cyclization/elimination reaction to form [CoCp₂]­[Tp^<i>i</i>PrMo^IVO­{2-OC₆H₄C­(O)­S-κ<i>O</i>,κ<i>S</i>}]. This study provides the first definitive measurement of the Mo^V⎓S bond distance in an unambiguously characterized oxosulfido-Mo­(V) compound and supports the presence of a short (ca. 2.22 Å) Mo⎓S bond in VR-XnO. It also demonstrates that the Mo^V⎓S moiety participates in radical-based reactions that are facilitated by the facile redox interplay of Mo and S and by substrates susceptible to radical eliminations

Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond ¹³C–¹³C Coupling Constants of α- and β‑Nopinol

σ_C–C–σ*_C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the ¹³C–¹³C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases