Exploring the intrinsic polar [4 + 2⁺] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Abstract

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,β-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

Mechanistic aspects of the gas-phase coupling of thioanisole and chlorobenzene to dibenzothiophene catalyzed by atomic Ho+

“…To achieve C-C [9][10][11][12] or C-S [13][14][15][16][17][18][19][20] coupling involved in the ring closure process, activation of C-H, [9][10][11][12][13][14][15][16][17][18][19][20] C-X (X = Br or I) 15,18,20 or C-S 14,17 bonds is a prerequisite. While gas-phase investigations proved helpful to obtain mechanistic aspects for numerous processes, except for a few reports on the formation of sulfur heterocycles in the gas phase, 21 a more detailed understanding of the elementary steps is lacking.…”

“…While gas-phase investigations proved helpful to obtain mechanistic aspects for numerous processes, except for a few reports on the formation of sulfur heterocycles in the gas phase, 21 a more detailed understanding of the elementary steps is lacking. In a previous study on the reactions of atomic Ln + with chlorobenzene, 22 strong indications have been found that Ho + generates efficiently the insertion product Ho(C 6 H 5 )(Cl) + which then promotes HCl elimination in a secondary reaction with chlorobenzene; further, intriguing coupling processes have been observed in preliminary studies on consecutive reactions of Ln + with chlorobenzene and thioanisole in which Ho + exhibited the highest efficiency.…”