Introduction to Crossover Reactions

What is/are Crossover Reactions?

Crossover Reactions - Radical–polar crossover reactions, that is, single-electron redox events allowing for the interconversion between radical and ionic intermediates, make it possible to connect radical and polar processes in the same synthetic transformation. ^[1] To access uncharted chemical space, the adoption of photoredox catalysis in DELs, including Ni-catalyzed manifolds and radical/polar crossover reactions, has enabled the construction of novel structural scaffolds through regulated odd-electron intermediates. ^[2] Taking advantage of the stabilization effect of a silyl group, with a bulky but positive nature, a range of less investigated organosilanes could be generally accessed via photoredox-catalysed radical-polar crossover reactions. ^[3] An increase in the number of methyl substituents at the 2-position of the cyclic acetal improved the stability of the generated carbocation, which led to copolymerization with both frequent crossover reactions and at high polymerization rates. ^[4] This proposal has been proven by germylene-exchange crossover reactions in which free 1tBu and [M(CO)5 {Ge(tBu2 bzamC(OEt)Me)CH2 SiMe3 }] (5'M ; M=Cr, W) were formed when complexes 3M were treated at room temperature with the germylene Ge(tBu2 bzam)CH2 SiMe3 (1tmsm ). ^[5] Lactic acid-derived 1,3-dioxolan-4-ones (DOLOs), which do not undergo cationic homopolymerization, were demonstrated to yield copolymers with oxiranes through a cationic copolymerization via frequent crossover reactions. ^[6] In the 2nd part, "radical-metal crossover reactions", metal centered radical species and more generally longer-lived transition metal complexes able to react with radicals are discussed - a field that has flourished recently. ^[7]

Polar Crossover Reactions

Radical–polar crossover reactions, that is, single-electron redox events allowing for the interconversion between radical and ionic intermediates, make it possible to connect radical and polar processes in the same synthetic transformation. ^[1] To access uncharted chemical space, the adoption of photoredox catalysis in DELs, including Ni-catalyzed manifolds and radical/polar crossover reactions, has enabled the construction of novel structural scaffolds through regulated odd-electron intermediates. ^[2] Taking advantage of the stabilization effect of a silyl group, with a bulky but positive nature, a range of less investigated organosilanes could be generally accessed via photoredox-catalysed radical-polar crossover reactions. ^[3]

Frequent Crossover Reactions

An increase in the number of methyl substituents at the 2-position of the cyclic acetal improved the stability of the generated carbocation, which led to copolymerization with both frequent crossover reactions and at high polymerization rates. ^[1] Lactic acid-derived 1,3-dioxolan-4-ones (DOLOs), which do not undergo cationic homopolymerization, were demonstrated to yield copolymers with oxiranes through a cationic copolymerization via frequent crossover reactions. ^[2]