Under review — part of this question (a structure or an option) is not clear in the source scan. 4AB is checking it against the original book; the answer shown may change.
For the following thermal [2+2] cycloaddition reaction, the correct statement about Transition State (TS) and preference for endo product formation is
(a)TS: $\pi 2_s + \pi 2_s$; Me $>$ i-Pr $>$ t-Bu
(b)TS: $\pi 2_s + \pi 2_a$; t-Bu $>$ i-Pr $>$ Me
(c)TS: $\pi 2_s + \pi 2_a$; Me $>$ i-Pr $>$ t-Bu
(d)TS: $\pi 2_s + \pi 2_s$; t-Bu $>$ i-Pr $>$ Me
Answer
Under review — not yet confirmed.
Answer: B
Explanation
A thermal [2+2] cycloaddition of a ketene is allowed as [π2s + π2a], the ketene reacting antarafacially. The bulkier the substituent, the greater the preference for the endo product: t-Bu > i-Pr > Me.