Experimental and DFT Studies on the Identity Exchange Reactions between Phenyl Chalcogen Iranium Ions and Alkenes

Electronic and Steric Effects on the Reactivity of Seleniranium Ions with Alkenes in the Gas Phase

Gas phase ion-molecule reactions between seleniranium ions, R-c-SeCH2CH2+, and cis-cyclooctene were used to probe electronic and steric effects of substituents on kinetics and branching ratios. The second-order rate coefficients increased in the order p-OMeC6H4 < C6H5 < p-BrC6H4 < p-CF3C6H4 < m-NO2C6H4, giving a Hammett plot with R2 = 0.98 and ρ = +1.66. The two main pathways include direct transfer of the selenium moiety to the incoming alkene (π-ligand exchange) and the less favored ring-opening by attack at an iranium carbon to give a cis-bicyclic selenonium ion as supported by density functional theory (DFT) calculations. Branching ratios of each pathway indicated that electron-withdrawing groups directed more attack at carbon than selenium in agreement with previous solution-phase results. Increased steric bulk on selenium was investigated by changing the R group from a methyl to t-butyl, which not only shut down π-ligand exchange but also significantly reduced the overall reactivity. Finally, the reactivity of the iranium ion derived from Se-methylselenocysteine was investigated and shown to react faster and favor π-ligand exchange as the leaving group was changed from ethene to acrylic acid.

Experimental and theoretical investigations into the mechanisms of haliranium ion π-ligand exchange reactions with cyclic alkenes in the gas phase

Haliranium ions are intermediates often involved in complex cyclisations, where their structure allows for control over stereospecific outcomes. Extending previous studies into their structure and reactivity in the gas phase, this work focuses on the bimolecular reactivity of ethyl bromiranium and iodiranium ions with cyclic alkenes. The products observed via mass spectrometry were broadly attributed to either addition by cyclohexene at the iranium carbon or attack at the heteroatom to undergo associative π-ligand exchange. The model proposed was supported by both kinetic experiments and DFT calculations, where the rate of parent ion consumption proceeded at the collision rate (Br: k₂ = 1.25 × 10^-9 and I: k₂ = 1.28 × 10^-9 cm³ molecule^-1 s^-1) with the subsequent partitioning dependent on the relative stability of the initial intermediates and the relatively large barriers present in the addition pathway. Exploration of the effect of cycloalkene ring strain on the iodiranium ion reactivity was conducted with a series of crossover experiments with 50 : 50 mixtures of either cyclohexene or cis-cyclooctene and styrene, where the outcomes were dependent on the competing ring strain relief gained by reaction with each neutral. The nature of the exchange transition state was determined to be pseudocoarctate following both natural bond orbital (NBO) and anisotropy of the induced current density (ACID) analysis.

Computational Study of the Donor-Acceptor Interactions Underlying the Variable Oxygen Probe

The variable oxygen probe (VOP) is a crystallographic technique that has been used to explore the relative donor abilities of various filled orbitals ranging from vicinal lone pairs to polarized heteroatom-carbon bonds, remote π functionalities, and strained carbon-carbon (CC) bonds. In this study, the donor-acceptor interactions which underlie the VOP have been explored in the gas phase using density functional theory on the model systems 1-13 with natural bond orbital analysis of the various donor-acceptor interactions involving both neutral and charged σ* antibonding orbitals as the acceptor probes. Updated values for the VOP slopes of 1-13 were shown to relate qualitatively with the sum of all significant donor-acceptor interactions present in these derivatives. Application of the VOP to calculated structures of 1-13 with various -OR substituents revealed a similar relationship between the C-OR bond distance to pK_a (ROH). However, the VOP slopes in the gas phase were significantly smaller in magnitude than those obtained from crystal structural data, likely due to the valence form (C⁺-OR) being disfavored in the former, highlighting the advantage of the VOP as an experimental technique to discriminate donor ability more effectively than calculated structures.

Attempts to Synthesize a Thiirane, Selenirane, and Thiirene by Dealkylation of Chalcogeniranium and Thiirenium Salts

Thiiranium salts [Ad2 SR]+ X- (5, 8, 9, 11, 12; X- =Tf2 N- (Tf=CF3 SO2 ), SbCl6 - ) and seleniranium salts [Ad2 SeR]+ X- (14, 16, 17, 23-25; X- =Tf2 N- , BF4 - , CHB11 Cl11 - , SbCl6 - ) are synthesized from strained alkene bis(adamantylidene) (1). The disulfides and the diselenides (Me3 SiCH2 CH2 E)2 (4, 13), (tBuMe2 SiCH2 CH2 E)2 (7, 22), and (NCCH2 CH2 E)2 (10, 15; E=S, Se) have been used. The thiirenium salts [tBu2 C2 SR]+ X- (34) and [Ad2 C2 SR]+ X- (35, 36) are prepared from the bis-tert-butylacetylene (2) and bis-adamantyl-acetylene (3) with R=Me3 SiCH2 CH2 and tBuMe2 SiCH2 CH2 . Attempts to cleave off the groups Me3 SiCH2 CH2 , tBuMe2 SiCH2 CH2 , and NCCH2 CH2 resulted in thiiranes 27, 30. No selenirane Ad2 Se (33) is formed from seleniranium salts, instead cleavage to the alkene (1) and diselenide (13, 15) occurs. The thiirenium salt [Ad2 C2 SCH2 CH2 SiMe3 ]+ Tf2 N- (35) does not yield the thiirene Ad2 C2 S (37), the three-membered ring is cleaved, forming the alkyne (3) and disulfide (4). All compounds are characterized by ESI mass spectra, NMR spectra, and by quantum chemical calculations. Crystal structures of the salts 8, 12, 25, 17, 26, 36 and the thiiranes 27, 30 are presented.