Final answer:
To direct addition to the less substituted sp² carbon, use sterically hindered reagents like bulky boranes in hydroboration-oxidation reactions or bulky hydride donors like selectride reagents to overcome steric hindrance.
Step-by-step explanation:
To have a stronger preference for the less substituted sp² carbon, you would typically want to use compounds or reactants that favor Markovnikov's rule, which states that in the addition of a protic acid (HX) to an alkene, the acid hydrogen (H) gets added to the more substituted carbon of the double bond, leaving the less substituted carbon to bond with the halide (X). However, for reactions favoring addition to the less substituted carbon, one can use sterically hindered bases or bulky reactants like bulky boranes (for hydroboration-oxidation reactions) or sterically hindered hydride donors such as selectride reagents.
These reagents approach the alkene in a way that avoids the more substituted carbon due to steric hindrance, adding to the less substituted side. Hydroboration-oxidation, in particular, results in anti-Markovnikov product distribution having the hydrating reagent add to the less hindered carbon. The sp² hybridization of carbons and bond angles (120°) also play a role in the accessibility of the carbon atoms to reactants.