Involved in the reductive branch of L-leucine fermentation. Catalyzes the irreversible beta/alpha-elimination of water from (R)-2-hydroxyisocaproyl-CoA to yield isocaprenoyl-CoA. This beta/alpha-dehydration depends on the reductive formation of ketyl radicals on the substrate generated by injection of a single electron from the ATP-dependent activator protein HadI. The enzyme is specific for the R-isomer. (R)-2-hydroxy-4-methylpentanoyl-CoA = 4-methylpent-2-enoyl-CoA + H2O Binds 1 [4Fe-4S] cluster. Activated by HadI. Part of the heterodimeric complex HadBC composed of (R)-2-hydroxyisocaproyl-CoA dehydratase alpha (HadB) and beta (HadC) subunit. Belongs to the FldB/FldC dehydratase alpha/beta subunit family.