When I was learning about the reactions of epoxides from my textbook, I came across the following mechanism for arene oxide-phenol rearrangement in the presence of acid:
NIH Shift - National Institute of Health Shift (c.f. Wikipedia)
In the above example, there is no substituents on the ring and hence the final product remains irrespective of which $\ce{C-O}$ bond in the arene oxide cleaves to open the ring. Later, the author presented the following reaction where the ring has an electron donating group - methyl group:
Here the final product depends on which of the two $\ce{C-O}$ bonds in the arene oxide is broken. Based on the general reaction mechanism provided by the textbook, I derived the following mechanism to form the product prescribed by the author:
Before writing the above mechanism, I first arrived at a different product by following the mechanism given below:
The reason I initially chose to cleave the left $\ce{C-O}$ bond in "Reaction Mechanism - 2" was, it gives rise to the carbocation 2(a) where the methyl group is at the para position. I've learnt that methyl group stabilizes a positive charge by hyperconjugation. Since, resonance effect is felt only at the ortho and para positions and not at the meta position, the carbocation 2(a) is more stable than 1(a) from "Reaction Mechanism - 1". Moreover, in the first mechanism, NIH shift involves a 1,2 Deuterium shift whereas in the second it's a 1,2 Hydrogen shift. I find a deuterium shift to be slightly energy expensive process due to its higher mass and stronger bond with carbon compared to a hydrogen shift.
The only reason why I think the first mechanism is better than the second is the product 1(b) is more stable than 2(b). But I don't understand why is the second reaction not preferred over the first even though it has two factors supporting (stability of 2(a) over 1(a) and hydrogen shift over deuterium shift) it.
I faced similar contradictory results while writing the mechanisms for the following reactions (from the same textbook):
My questions are:
Are the reactions provided by my textbook correct?
If yes, why is the cleavage of one $\ce{C-O}$ bond of the epoxide preferred over the other even though it produces a comparatively less stable carbocation? Are there any other factors which determine which bond cleaves?
Is arene-oxide phenol rearrangement "specific" or highly "selective"? Or in simpler terms, does the rearrangement exclusively form only one product or does it form some small amount of the other product too?
Image source: My own work :) | Made using Marvin Sketch.
Textbook: Organic Chemistry by Solomons, Fryhle, and Snyder | Chapter 10: Alcohols and Ethers | Section 10.14: Reactions of Epoxides