Why is the sp3-sp3 reductive elimination between the R1 and norbornene in the Catellani Cycle ''not favorable''?

I have recently started reading a book about C-H activation and I cannot figure out why in the Catellani reaction cycle the R1-norbornene reductive elimination side reaction which can be a major issue isn't favored.The author only briefly mentions sp3-sp3 coupling saying that it isn't favored but it doesn't provide an explanation (Because he probably assumes the reader already knows).


**(Step 6.)**


[![enter image description here](https://i.stack.imgur.com/7Fe9L.png)](https://i.stack.imgur.com/7Fe9L.png)


The book: C-H Activation (Topics in Current Chemistry, 292)




All most all organic reaction mechanisms are given according to the products received after doing variety of kinetic experiments. If you are familiar with reaction progressing energy diagrams, you know all intermediate reactions are reversible to give the thermodynamically most stable product(s). Thus, in the case of choice for $\ce{C\_\mathrm{sp^3}-C\_\mathrm{sp^2}}$ bond formation versus $\ce{C\_\mathrm{sp^3}-C\_\mathrm{sp^3}}$, the final product(s) rececieved go through more stable $\ce{C\_\mathrm{sp^3}-C\_\mathrm{sp^2}}$ bond formation (because reverse reaction require more energy to break).


[![Catellani reaction](https://i.stack.imgur.com/uVfbD.png)](https://i.stack.imgur.com/uVfbD.png)


This is the same situation when palladation of norbornene $(\bf{3})$ ocuurs to give $\bf{4}$ instead of the presence of the equally reactive $\ce{R^2-Y}$ in the reaction mixture, which is a terminal alkene.


Also keep in mind that, there is a possibility to receive tricyclic benzo-norbornane product by $\ce{C\_\mathrm{sp^2}-C\_\mathrm{sp^3}}$ reductiove elimination from $\bf{5}$, based on steric hindrence, solvent used, base used, etc. (Ref.1):


[![Catellani-Suzuki reaction](https://i.stack.imgur.com/vy1zc.png)](https://i.stack.imgur.com/vy1zc.png)




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**References:**


1. Xiaojin Wua and Jianrong (Steve) Zhou, "An efficient method for the Heck–Catellani reaction of aryl halides," *Chem. Commun.* **2013**,*49(94)*, 11035-11037 (DOI: DOI <https://doi.org/10.1039/C3CC46381H>).


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