Enantiomeric Discrimination in a Reiterative Domino Coupling Process: CuI-mediated Syn Cyclotrimerization of Racemic Polycyclic Trimethylstannyl Bromonorbornadienes This work was funded by MURST (Rome) within the national project “Stereoselezione in Sintesi Organica. Metodologie e Applicazioni”.

Abstract

The challenge to generate all‐syn architecture has prompted the synthesis of syn cyclotrimers 2 from racemic bromotrimethylstannyl polycyclic alkenes 1. The syn‐selective CuI‐catalyzed cyclotrimerization process is both regio‐ and chemoselective.

Role of Copper in the Stereoselective Metal-Promoted Cyclotrimerisation of Polycyclic Alkenes

“…As far as the Cu I ‐catalysed cross‐coupling reaction is concerned, from our experimental data5b and the known behaviour of organocuprate species,10 the following mechanism can be proposed (Scheme ).…”

“…This seems not to be the case, however, since it conflicts strongly with the observed formation of dibrominated dimers 11 and 12 (Figure 4). 5b Clearly, a homocoupling process (formally between the carbon atoms originally attached to the tin residues) takes place and this fact could be considered as an experimental evidence in support of the formation of a Cu III intermediate 10f…”

“…In this manner, the lithium salt restores the capability to generate organic nucleophiles in the alkenylcuprous species. Indeed, if the reaction is carried out in the absence of lithium salt, the coupling of (trialkylstannyl)alkenes occurs under radical conditions due to the preferred C−Cu homolytic cleavage 5b,12…”

“…In this context, we have recently reported on the first highly selective routes to either syn ‐or anti ‐cyclotrimers. In fact, while Cu(NO 3 ) 2 ‐promoted cyclotrimerisation8 proved to be nonstereoselective, (Scheme 1, route A), subjection of bromobenzonorbornadiene to Pd 0 ‐catalysed reaction conditions (Heck reaction)5a selectively afforded anti ‐cyclotrimers (Scheme , route B), whereas Cu I ‐mediated cross‐coupling of bromo(trimethylstannyl)benzonorbornadiene provided a direct route to syn ‐cyclotrimers (Scheme 1, route C) 5b…”

7 Aromatic chemistry

“…The anti/syn selectivity was carefully investigated on the basis of stereochemical discrimination. 207,208 The Heck reaction of (1R)-2-iodobornene gave trimeric benzene products in low yield. 209 The nonplanar fused aromatic system 180 was prepared by the palladium-catalysed cyclotrimerization of 9,10-didehydrophenanthrene.…”