Final answer:
The correct answer is option 2. The starting alkene configuration is preserved in the Simmons-Smith reaction due to its concerted mechanism, where bond-making and breaking occur simultaneously with no intermediates that could cause isomerization or rearrangement.
Step-by-step explanation:
The configuration of the starting alkene is preserved in the Simmons-Smith reaction because the reaction is believed to occur in a concerted manner. This means that all the bond-making and breaking processes occur simultaneously in a single step.
In such concerted reactions, there is typically no intermediate formed that would allow for isomerization or rearrangement of the starting material. Thus, the alkene configuration you start with is the configuration you end up with in the product. Competing mechanisms such as SN1 or E1, which proceed through a carbocation intermediate, could lead to a mixture of isomers due to rearrangement.
However, the Simmons-Smith reaction avoids this by proceeding in a concerted fashion, which is typically faster than the stepwise alternatives. Additionally, the reagents involved in the Simmons-Smith reaction, typically diiodomethane (CH2I2) and a zinc-copper couple, form a highly reactive carbenoid species that inserts into the double bond of the alkene with retention of configuration.