Final answer:
In the bromination of (S)-4-(tert-butyl)cyclohex-1-ene, the major diaxial product is formed due to the approach of bromine anti to the leaving group during the transition state, despite the stability preference for the tert-butyl group to be equatorial. The diequatorial product, although more stable, is not the initial major product because of the reaction geometry.
Step-by-step explanation:
The question concerns the bromination of cyclohexenes and why the diaxial product is initially formed as the major product compared to the diequatorial product. In the bromination of (S)-4-(tert-butyl)cyclohex-1-ene, the major product formed is the (1R,2R,4S)-1,2-dibromo-4-(tert-butyl)cyclohexane, where the tert-butyl and bromine substituents are in diaxial positions.
The formed diaxial product signifies that the larger tert-butyl group prefers an equatorial position due to less 1,3-diaxial interaction. However, during the bromination process, the reactive intermediate prefers a conformation that allows the incoming bromine atoms to approach the double bond anti to the leaving group, which in this case leads to both substituents being axial. As the reaction progresses, this diaxial product can undergo ring flipping to become the more stable diequatorial conformer, however, the initial formation is due to the geometry of the bromination transition state. The energy involved in 1,3-diaxial interactions for the tert-butyl group is much higher (11.4 kJ/mol) compared to that of the bromine atoms (1.0 kJ/mol), hence, the diaxial product with tert-butyl in an axial position is not preferred if the reaction did not enforce this arrangement.