On baker nathan hyperconjugation

Recently I have been reading about the [history of the Baker Nathan reaction](http://www.scs.illinois.edu/~mainzv/HIST/bulletin_open_access/v37-2/v37-2%20p82-90.pdf).



> 
> In addition to the large
>  rate increase for Me vs H, all the other alkyl groups
>  showed decreases with respect to Me in a regular manner
>  as would be predicted by reduced hyperconjugative
>  ability. 
> 
> 
> 


By drawing resonance structures, it is easy to see the $\mathrm{Me < Et < {}^iPr <} {}^t\mathrm{Bu}$ predicted by the inductive effect:


[![enter image description here](https://i.stack.imgur.com/NV6cd.png)](https://i.stack.imgur.com/NV6cd.png)


Therefore the more electron donating (i.e. the stronger the I+ effect) the *para* group is , the more stabilized the transition structure is (black $\delta^+$ on the left diagram become smaller hence less electrostatic repulsion in the $\pi$ system, hence blue $\delta^+$ and the $\delta^-$ can be larger thus facilitating the reaction). Hence $\mathrm{Me < Et <} {}^i\mathrm{Pr <} {}^t\mathrm{Bu}$.


[![enter image description here](https://i.stack.imgur.com/bmnKQ.png)](https://i.stack.imgur.com/bmnKQ.png)


However, for hyperconjugation, the $\sigma$ bonds overlap with the $\pi$ system resulting in electrons to become more delocalised at the C-C bond between the alkyl group and the $\pi$ system. Therefore the more hyperconjugations present, once again the more stablised the transition structure hence a faster reaction.


Since both $\sigma\_\mathrm{CC}$ and $\sigma\_\mathrm{CH}$ bonds adjacent to the para carbon can potentially engage in hyperconjugation (and naively the $\sigma\_\mathrm{CC}$, being larger in lobe size, should expect to overlap better with the $\pi$ system), does that mean the experimentally observed trend that $$\mathrm{Me > Et > {}^iPr >} {}^t\mathrm{Bu}$$


suggest hypderconjugation of $\sigma\_\mathrm{CC}$ is much weaker than that of $\sigma\_\mathrm{CH}$ hence justifying the quoted message?



> 
> If hyperconjugation of $\sigma\_\mathrm{CC} < \sigma\_\mathrm{CH}$ what is the underlying molecular orbital explanation?
> 
> 
> 


