Hyperconjugation positive charge substituents

These data show a decrease in the extent of /J-silyl stabilization with successive methyl substitution. The methyl (and phenyl) substituents stabilize the carbocation by polarization and inductive effects, resulting in a delocalization of positive charge away from the carbocation, and therefore a reduction in hyperconjugative interaction with the fi-substituent bond. 

The /-substituent data in Tables 5 and 6 show that phenyl and alkoxy substituents at silicon are less effective than methyl at stabilizing the partial positive charge build-up on /1-silicon by hyperconjugation. However, the /-trimethylsilyl group does stabilize this build-up of positive charge. 

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

How then is the positive charge in the (OC)9Co3C-substituted carbonium ions delocalized into the cluster substituent In our first preliminary report on this new class of carbonium ions (45), we suggested that their structure presents an especially favorable opportunity for lateral overlap of a filled er-bonding orbital of a metal-carbon bond and a vacant p orbital on an electron-deficient carbonium ion center /3 to the metal (VIII), that is believed responsible for the high... 

In addition to a relatively small inductive effect, two modes of stabilization of positive charge by the P-R3M substituent have been suggested. The first involves the classical cation 7, where the positive charge is stabilized by hyperconjugation (G-p conjugation) between the C-M o bonding orbital and the vacant carbocation... 

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

P-substituent to stabilize the positive charge by P-o-hyperconjugation (6a o 6b) and vice versa, i.e., the more stabilizing the p-substituent the less is the demand for n-delocalization of the positive charge into the aromatic ring. 

A silyl substituent can stabilize the positive charge in a carbocation if the spatial arrangement allows overlap of the C-Si a-bond and the vacant p-orbital at the C -carbon. The hyperconjugative interaction of a p-silyl group, as shown in the MO- and VB-structures in Fig.l, is called the P-silyl... 

In p-silyl substituted benzyl cations with different a-aryl substituents the stabilizing effect of a P-silyl group depends on the electron donating ability of the a-aryl group. The better the a-aryl substituent can stabilize the positive charge, the lower is the need for P-o-hyperconjugative stabilization by the silyl group and the no-bond resonance structure becomes less important (Fig. 6). 

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