Nucleophilic capture Subject

However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considCTcd include (1) the spin and charge density distribution in the cyclopropane radical cation (the educt) (2) the spin density distribution in the free-radical product (3) the extent of... 

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

In an effort to provide experimental evidence for the formation of a second oxyallyl cationic intermediate, a gem-dichlorocyclopropane substrate was envisaged with two internal nucleophiles one to participate in the initial interrupted Nazarov reaction, and the other to capture the second cationic species. Surprisingly, when the carefully designed substrate 80 was subjected to the optimized reaction conditions, an alternate mode of trapping occurred to generate the intriguing bridged bicyclic product 81 (Scheme 4.25). 

Steric factors have been mentioned they are not expected to play a major role. In fact, several radical cations have been captured by attack on highly congested centers [154, 156]. The ring-opening substitution of tricyclane radical cation, 81 +, occurred exclusively at the tertiary carbon [215] whereas that of 82 occurred at the tertiary carbon further removed from the dimethyl-substituted bridge, i.e., at the less hindered of two tertiary carbons, not at the quaternary carbon [216]. These results clearly show that the nucleophilic substitution at the cyclopropane one-electron bond is subject to conventional steric hindrance and is not subject to inverse steric effects [154]. 

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