Lithium Carbenoids

Abstract

Introduction General Reactivity and Structure Investigations Lithium Carbenoids as Reactive Intermediates in Synthesis Conclusions Acknowledgments

A practical guide for using lithium halocarbenoids in homologation reactions

“…Carbenoidic reagents play a prominent role within the plethora of homologating agents [ 3 – 6 ]. The term carbenoid was introduced by the pioneers in the field Closs and Moss who defined their chemical reactivity “qualitatively analogous to those of carbenes without necessarily being free divalent carbon species” [ 7 ].…”

“…The concomitant presence of an electron-donating and electron-withdrawing substituent at the carbon center determines the so-called ambiphilicity of these reagents [ 5 , 10 ]. Thus, carbenoids display a dual reactivity ranging from nucleophilic to electrophilic [ 6 , 11 , 12 ]. Depending on the experimental conditions, they may selectively exhibit only one of these two properties [ 13 – 17 ]: it is normally accepted that the nucleophilic behavior is shown at low temperatures, while their electrophilicity comes into play at higher temperatures (Scheme 1 ) [ 6 , 18 , 19 ].…”

“…Thus, carbenoids display a dual reactivity ranging from nucleophilic to electrophilic [ 6 , 11 , 12 ]. Depending on the experimental conditions, they may selectively exhibit only one of these two properties [ 13 – 17 ]: it is normally accepted that the nucleophilic behavior is shown at low temperatures, while their electrophilicity comes into play at higher temperatures (Scheme 1 ) [ 6 , 18 , 19 ]. This key characteristic of carbenoid reagents can be explained taking into consideration structures, which in principle can provide two different ionization forms.…”

“…Given these premises, one may individuate two different reactions categories in which carbenoids are involved: (1) nucleophilic additions (eventually followed by elimination); (2) cyclopropanation-type processes (Simmons–Smith like chemistry) [ 20 , 21 ]. It is important to stress that carbenoids of lithium and magnesium, because of their excellent nucleophilicity, do react predominantly as carbanions [ 6 , 13 – 16 ]. On the other hand, less nucleophilic carbenoids such as zinc or rhodium linked ones exhibit preferentially an electrophilic behavior [ 4 , 22 ].…”