Addition Reactions of exo-Trimethylene-2-norbornene

Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (^Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (^Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.

The Reaction of 5-Substituted-2-Norbornenes with Phenylselenyl Chloride

In dichloromethane, 5-substituted-2-norbornenes add phenylselenyl chloride across the double bond to give adducts in which the phenylselanyl substituent is exo and the chloro endo. The relative yields of the two regioisomeric adducts are reported for several of these. For most, the formation of the 2-chloro-3-phenyselanyl adduct is favoured. The main exceptions are when the 5-substituent is exo and a good resonance donor, where the 3-chloro-2-phenylselanyl adduct is the major product. Possible factors influencing the product distribution are discussed.