A Plethora of Carbene Interconversions on the C₅H₄S Energy Surface:  A Computational Study

Abstract

The potential energy surface for the reaction of atomic carbon with thiophene has been studied computationally. Intermediates which are energetically viable include the 2- and 3-thienylcarbenes 8 and 11, thiacyclohexa-3,5-dien-2-ylidene, 10, and thiacyclohexa-2,3,5-triene, 6. In accord with experimental data, 6 and 8 are in equilibrium. The lowest-energy pathway for rearrangement of 6 to 8, which is endothermic by 14.5 kcal/mol, involves ring opening to Z-2-penten-4-ynthial which then recloses to carbene 8. A 1,4 addition of C across the diene system in thiophene generates an ylid which rearranges with little or no barrier to cyclopentadienethione, the global minimum on this potential energy surface.

Theoretical study of reactions of atomic carbon with some π-systems

“…Attempts to locate electronic minima for related products with pyrrole, furan and thiophene were unsuccessful. Pyrrole, furan and thiophene formed carbenes analogous those reported by Shevlin et al [1] (see ESI). Details of the properties of these systems are beyond the scope of this study and will be reported on separately.…”

“…These structures are likely to be kinetically reactive as shown by Shevlin, et al [1]. They may provide a missing link in the proposed mechanisms of reactions of atomic carbon with thiophene reported by Shevlin and co-workers [1] and may lay on the energy surface of C 6 H 5 intermediates leading to benzynes [2,3]. Future studies will focus on detailed studies of possible rearrangements and related compounds formed between heteroatoms and a variety of p systems.…”

“…Many of these unique structures show significant bonding between the carbon and ring carbons. These structures are likely to be kinetically reactive as shown by Shevlin, et al [1]. They may provide a missing link in the proposed mechanisms of reactions of atomic carbon with thiophene reported by Shevlin and co-workers [1] and may lay on the energy surface of C 6 H 5 intermediates leading to benzynes [2,3].…”

“…Pioneering experimental and computational studies by Shevlin and co-workers involving reactions of free carbon atoms with a variety of heterocyclic 6-p systems revealed the fascinating scope of these reactions [1]. Detailed computational studies on the carbon:thiophene system revealed ''A Plethora of Carbene Interconversions'' leading to a series of isomeric products and intermediates.…”

Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

“…Such an explanation would also seem to be at odds with the relatively high thermal barrier (>30 kcal/mol) separating singlet 3-thienylcarbene ( 13 ) and α -thial-methylenecyclopropene ( 9 ). 24 The more likely explanation, therefore, involves the excited state of the diazo compound. As observed in several other systems, 45,46 the excited state of diazo compound 1 may partition between two pathways: N 2 loss, to give 3-thienylcarbene ( 13 ), and N 2 loss with concomitant rearrangement, to give α -thial-methylenecyclopropene ( 9 ) without the intervention of the carbene.…”