Electrophilic approach on norbornene systems. Preferential endo attack of halogens on hindered 2-phenylnorbornenes

Abstract

The results from the electrophilic addition of bromine to croíí-7-bromo-o-phenyIbenzonorbornadiene (3) and syn-7-bromo-2-phenylnorbornene (2) suggest that initial approach of the bulky electrophile to these hindered systems is endo. This is in contrast to typical stepwise electrophilic additions to hindered norbornene systems without the phenyl group where exo attack is usually observed. The observations are interpreted to indicate that some stereoelectronic factor must be operating, at least in part, in controlling the preference for exo approach of an electrophile to a norbornene system.

Recent Advances in Transition Metal-Catalyzed Reactions of Oxabenzonorbornadiene

“…Gandeepan et al used Co-catalyzed C-H bond activation to synthesize dihydroepoxybenzofluorenone derivatives 7 from OBD 1a and aromatic/vinylic amides 6 (Scheme 2 ). Normally, a [4+ 2 ] or [4+ 1 ] cycloaddition product is observed with transitional metal-catalyzed C-H activation reactions between secondary amides and alkenes. This was however, an unusual annulation reaction which produced a [3+ 2 ] cycloaddition product.…”

“…Normally, a [4+ 2 ] or [4+ 1 ] cycloaddition product is observed with transitional metal-catalyzed C-H activation reactions between secondary amides and alkenes. This was however, an unusual annulation reaction which produced a [3+ 2 ] cycloaddition product. Co(OAc) 2 was found to be the optimized catalyst, with Ag + from Ag 2 CO 3 being used to oxidize Co(II) to the catalytically active Co(III).…”

“…Steric hindrance from OBD and the quinolyl moiety prevent reductive elimination to a six-membered lactam. Therefore, intermediate 10 is formed through an intramolecular nucleophilic addition of the C-Co bond to the amide carbonyl; the [3+ 2 ] cycloaddition step. Protodemetalation and elimination of the quinolyl moiety produce the final product 7 and regenerate the Co (III) species, which is catalytically active (Scheme 3 ) [ 40 ].…”

“…Zhou et al developed a method for ARO of OBD 1a with amine nucleophiles 40 , producing highly enantioselective ring-opened products 41 (Scheme 19 ). Their optimized catalyst and chiral ligand system, [Ir (COD)Cl] 2 and (R)-xylyl-phanephos, was discovered by chance when studying [2+ 2 ] cycloaddition reactions with OBD. A wide scope of primary and secondary amine nucleophiles was studied, giving moderate to good yields of the products accompanied by high enantioselectivity.…”

“…The exo face is composed of a five-membered ring, which includes the bridging oxygen, and the more sterically hindered endo face is composed of a six-membered ring, which excludes the bridging oxygen [ 1 ]. There are many structural features of OBD which contribute to its reactivity: i) a free alkene, ii) a bridging oxygen atom which allows for the coordination of electrophiles, Lewis acids and metals, and iii) ring strain imparted by the bicyclic structure [ 2 ]. OBD has ring strain energy of 36.1 kcal/mol and the alleviation of this strain is a driving force behind ring-opening reactions of OBD [ 3 ].…”