Bicyclobutonium ions, stability

The cartoon-like drawing of the structure of the parent bicylobutonium ion C4H7+ 36 is adopted from an ingenious forward-looking paper of Olah and coworkers in 1972, 61) long before routine 13C-FT-NMR spectroscopy and routine ab initio quantum chemical calculations were available, which envisaged correctly the stabilization mode of the parent bicyclobutonium ion to arise from the interaction of the backside lobe of the Cy-Hendo sp3 orbital with the empty carbenium carbon p-orbital at Ca. 

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 same technique was used950 to generate the l -(ferf-butyldimethylsilyl)bicy-clobutonium ion 529, which undergoes fast 1,3-hydride shift upon increasing temperature to yield 3-e ferf-butyldimethylsilyl)bicyclobutonium ion 530 [Eq. (3.135)]. Ion 530 has a static structure, which is due to the efficient stabilization by the y-ewiio-trialkylsilyl substituent. 

The ab /w/nWIGLO/NMR method has been used to determine the relative distribution and stability difference of the cyclopropylcarbinyl cation and cyclobutyl cation in solu-tion. Agreement between IGLO chemical shifts and experimental shifts could only be obtained when assuming a rapid equilibrium between the two cations. Over the range of temperatures considered (-61 to -132° C), a cyclobutyl cation structure with an axial H atom and short 1,3-distances of 1.65 A (bicyclobutonium ion structure) was found to be more stable by 0.5 kcalntoT For the gas phase, however, the cyclopropylcarbinyl cation was calculated to be 0.26 kcalmoT more stable [MP4/6-31G(d)//MP2/6-31G(d) calculations including vibrational corrections]. ... 

Although Roberts originally ascribed the stability of cyclopropylcarbinyl cations to hyperconjugation, he later proposed that it arose from a bonding interaction between one of the rear carbon atoms of the cyclopropane ring and the CHj group to form a bicyclobutonium ion, (48) and (49). 

Another way to envision bicyclobutonium is to start from the homoallyl cation and write an interaction between the cationic carbon and both carbons of the double bond. We already analyzed the stabilization of cations by a homoallyl interaction in reference to Figure 11.7, and so the bicyclobutonium ion should not be too surprising. 

---