Posts Tagged ‘SN2’
Organometallic chemistry has vastly expanded the practicing organic chemist’s notion of what makes a good nucleophile or electrophile. Pre-cross-coupling, for example, using unactivated aryl halides as electrophiles was largely a pipe dream (or possible only under certain specific circumstances). Enter the oxidative addition of polarized bonds: all of a sudden, compounds like bromobenzene started looking a lot more attractive as starting materials. Cross-coupling reactions involving sp2– and sp-hybridized C–X bonds beautifully complement the “classical” substitution reactions at sp3 electrophilic carbons. Oxidative addition of the C–X bond is the step that kicks off the magic of these methods. In this post, we’ll explore the mechanisms and favorability trends of oxidative additions of polar reagents. The landscape of mechanistic possibilities for polarized bonds is much more rich than in the non-polar case—concerted, ionic, and radical mechanisms have all been observed.
Oxidative additions of aryl and alkenyl Csp2–X bonds, where X is a halogen or sulfonate, proceed through concerted mechanisms analogous to oxidative additions of dihydrogen. Reactions of N–H and O–H bonds in amines, alcohols, and water also appear to be concerted. A π complex involving η2-coordination is an intermediate in the mechanism of insertion into aryl halides at least, and probably vinyl halides too. As two open coordination sites are necessary for concerted oxidative addition, loss of a ligand from a saturated metal complex commonly precedes the actual oxidative addition event.
A critical difference between the transition metals and the organic elements is the ability of the former to exist in multiple oxidation states. In fact, the redox flexibility of the transition metals and the redox obstinacy of the organic elements work wonderfully together. Why? Imagine the transition metal as a kind of matchmaker for the organic elements. Transition metals can take on additional covalent bonds (oxidation), switch out ligands (substitution), then release new covalent bonds (reduction). The resulting organic products remain unfazed by the metal’s redox insanity. Talk about a match made in heaven!
The following series of posts will deal with the first step of this process, oxidation. More specifically, we’ll discuss the oxidation of transition metals via formal insertion into covalent bonds, also known as oxidative addition (OA). Although we often think of oxidative addition as an elementary reaction of organometallic chemistry, it is not an elementary mechanistic step. In fact, oxidative addition can proceed through a variety of mechanisms. Furthermore, any old change in oxidation state does not an oxidative addition make (that almost rhymes…). Formally, the attachment of an electrophile to a metal center (e.g., protonation) represents oxidation, but we shouldn’t call this oxidative addition, since two ligands aren’t entering the fray. Instead, we call this oxidative ligation (OL).