Bicyclobutonium ions bridged

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

Bisoxo substitution, 44, 48, 58 Bridged cyclobutyl cations, 146, see also Bicyclobutonium ions... 

Bicyclobutonium ions, are bridged carbocations with a pentacoordinated y-carbon which were first discussed as short lived intermediates in the solvolysis reaction of cyclopropylmethyl and cyclobutyl compounds (56, 57, 58, 59, 60, 61). 

The H NMR spectrum shows resonances for (5.95 ppm), Hg/H g/ exo (4.01 ppm), H /Hg/ endo (3.12 ppm) and Hy (0.30 ppm). The NMR data are in accord with a bridged, puckered bicyclobutonium ion structure that is static on the NMR time scale. The 29Si NMR chemical shift of 43.1 ppm for ion 428 indicates that the silicon is involved in stabilization of the positive charge. The stabilization occurs by shifting electron density from the Cy—Si cr-bond across the bridging bond to the formal carbenium carbon Ca. This y-silyl- type of interaction may be termed silicon homohyperconjugation. 

The interpretation of the NMR spectra and structure of the experimentally observed bicyclobutonium ions 40 is confirmed by a reasonably good agreement between the measured C and NMR chemical shifts and the calculated chemical shifts (GIAO-MP2/tzp/dz) of the MP2/6-31G(d) optimized structures of the model cations 44 and 45. An unambiguous proof for the bridging and the stereochemical arrangement of the silyl substituent in the bicyclobutonium ions 40 is the transannular spin-spin coupling between Ha and Hy. The 2D-... 

The experimentally measured coupling constant J(HaHy)exp. = 5.5 Hz is reproduced by calculations for the endo-si y substituted model structure 44 using a finite perturbation level (FPT) 26) (Perdew/IGLO-III) approach J(HaHy)caic. = 5.9 Hz, whereas only 1.2 Hz is calculated for the HaHy-coupling constant in the exo-si y substituted model structure 45. Finally the bonding orbital of the bridging C-C-bond between Cq and Cy in these type of bicyclobutonium ions can be visualized by calculations of the natural bond orbitals (NBO s) (Figure 13). 

At -115 °C the first set of signals disappears within 10 minutes and only the peaks for cation 9 remain (Fig. 8 upper spectrum). Structural assignment for cation 9 was confirmed by HC-COSY-and COSY45-NMR spectra shown in Figs. 9 and 10, respectively. Experiments with (3-CD2-labeled progenitors and quantum chemical model calculations of transition states for 1,3-hydride shifts indicate that the rearrangement of the 1-silyl-substituted bicyclobutonium ion 8 to the 3-silyl-sub-stituted bicyclobutonium ion 9 occurs most probably by a 1,3-hydride shift from Cy to C across the bridging bond. 

In 1967 he again wrote, The second subclass consists of ions such as the bicyclobutonium and the norbornyl cation in its cr-bridging form, which do not possess sufficient electrons to provide a pair for all of the bonds required by the proposed structures. A new bonding concept not yet established in carbon structures is required (emphasis added). 

Hydrolysis (lime-water) of the chloride (207 X=C1) proceeds with retention of configuration to give alcohol (204) this would require that a-bridged ions (IS8) such as bicyclobutonium cation, are unimportant in this reaction (77). 

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