Trapping of a tert-adamantyl peroxyl radical in the gas phase

Abstract

A bridgehead adamantyl peroxyl radical has been prepared and isolated in the gas phase by the reaction of a distonic radical anion with dioxygen in a quadrupole ion-trap mass spectrometer.

Investigation of the gas phase reactivity of the 1-adamantyl radical using a distonic radical anion approach

“…can be generated by oxidative decarboxylation of the 1,3-adamantane dicarboxylate dianion 4 (Scheme 2), 4 in a similar manner to the production of distonic 4-carboxylato-1-phenyl radical anions from benzene dicarboxylate dianions. 18 However, low yields from this process have been encountered in the presence of some gas-phase reagents due to the preponderance of ionic reactions of the dianion.…”

“…Electronic structure calculations at the B3LYP/6-31+G(d)//HF/6-31+G(d) level of theory suggest that an activation energy of 81 kJ mol -1 must be overcome for conversion of 3 into 8. 4 It was previously argued that this barrier was sufficiently large to prevent isomerisation given; (i) the gentle method of radical formation, (ii) the fact that CID does not produce energetic secondary collisions in ion traps and (iii) the rapid cooling (5-10 ms) 27,28 of ions by collisions with the helium buffer gas. Further experimental evidence for the bridgehead structure of 3 is presented below.…”

“…We have previously addressed the possibility that the radical in question may instead be the ring-opened isomer 8 (Scheme 3). 4 Tertiary radical 8 is stabilised by resonance interactions with the adjacent carboxylate group and by relief of ring strain relative to 3, thus making it the most likely structural alternative to 3. Electronic structure calculations at the B3LYP/6-31+G(d)//HF/6-31+G(d) level of theory suggest that an activation energy of 81 kJ mol -1 must be overcome for conversion of 3 into 8.…”

“…8 Such calculations are supported by experimental measurements of spin density at radical centres that reveal an increase from 83 to 88% upon transition from 1-adamantyl to, the even further distorted, 1-bicyclo[1.1.1]pentyl radical. 2 In a previous computational study from our own laboratory 4 we have shown that the 1-adamantyl radical 2 is pyramidal about C 1 but has shorter Cα-Cβ bonds, longer Cβ-Cγ bonds, larger Cβ-Cα-Cβ angles and smaller Cα-Cβ-Cγ angles than adamantane 1 (Scheme 1). These observations are supported by previous calculations by Schaefer and coworkers 3 and are consistent with the radical centre is slightly flattened by H-Cβ-Cα hyperconjugation.…”

“…2 This experimentally derived value is congruent with gas phase structure calculations that predict a bond angle of 113º for the 1-admantyl radical using density functional methods. 3,4 Such deviation from the 118º for the near planar tert-butyl radical, [5][6][7] demonstrates the degree of pyramidalisation around the bridgehead radical. Further computational studies, predict that as the geometry of a carbon-centred radical deviates increasingly from its most stable planar conformation, the energy increases since spin delocalisation mechanisms such as resonance and hyperconjugation are compromised.…”