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Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism

Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism
Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism
Mesoionic heterocycles derived from 1,3-thiazolium-4-olates (thioisomunchnones) undergo thionation with aryl isothiocyanates to afford the corresponding 4-thiolate derivatives. Here, we document this transformation in detail, giving a crystallographic characterization of the solid-state structures. From the mechanistic viewpoint, the formal thionation process could be consistent with a [2 + 2] reaction of the exocyclic C-O bond of the thioisomunchnone with the C=S double bond of the isothiocyanate moiety, which would be competing with a (3 + 2) process as usual in mesoionic rings. Theoretical computations at the [B3LYP/6-31G(d):PM3] level, in which only bond-forming and bond-breaking reactions and neighboring atoms are treated at the DFT level, do reproduce the experimental results and rule out the expected pathway. Calculations instead suggest the existence of a four-step domino pathway through several polar intermediates that agrees with the electronic nature of the substituents involved. The mechanistic hypothesis has further been corroborated by an experiment with isotopically C-13-labeled PhNCS that unambiguously shows the way in which the exchange reaction occurs.
0022-3263
3698-3705
Cantillo, David
87273376-7777-4726-bbf9-505104580fad
Avalos, Martin
75b746b8-64d3-4451-be7a-c7c536ed6602
Babiano, Reyes
b7ffd91c-bacd-4372-ab99-fc30ac2450ae
Cintas, Pedro
29979233-8382-47a8-bde7-1faf4869308c
Jimenez, Jose L.
d071067d-ffdd-4655-bb18-256ca94c9f06
Light, Mark Edward
cf57314e-6856-491b-a8d2-2dffc452e161
Palacios, Juan C.
2c06e379-89b3-46b1-95c8-7f5057732f9f
Cantillo, David
87273376-7777-4726-bbf9-505104580fad
Avalos, Martin
75b746b8-64d3-4451-be7a-c7c536ed6602
Babiano, Reyes
b7ffd91c-bacd-4372-ab99-fc30ac2450ae
Cintas, Pedro
29979233-8382-47a8-bde7-1faf4869308c
Jimenez, Jose L.
d071067d-ffdd-4655-bb18-256ca94c9f06
Light, Mark Edward
cf57314e-6856-491b-a8d2-2dffc452e161
Palacios, Juan C.
2c06e379-89b3-46b1-95c8-7f5057732f9f

Cantillo, David, Avalos, Martin, Babiano, Reyes, Cintas, Pedro, Jimenez, Jose L., Light, Mark Edward and Palacios, Juan C. (2009) Thionation of mesoionics with isothiocyanates: evidence supporting a four-step domino process and ruling out a [2+2] mechanism. Journal of Organic Chemistry, 74 (10), 3698-3705. (doi:10.1021/jo900028c).

Record type: Article

Abstract

Mesoionic heterocycles derived from 1,3-thiazolium-4-olates (thioisomunchnones) undergo thionation with aryl isothiocyanates to afford the corresponding 4-thiolate derivatives. Here, we document this transformation in detail, giving a crystallographic characterization of the solid-state structures. From the mechanistic viewpoint, the formal thionation process could be consistent with a [2 + 2] reaction of the exocyclic C-O bond of the thioisomunchnone with the C=S double bond of the isothiocyanate moiety, which would be competing with a (3 + 2) process as usual in mesoionic rings. Theoretical computations at the [B3LYP/6-31G(d):PM3] level, in which only bond-forming and bond-breaking reactions and neighboring atoms are treated at the DFT level, do reproduce the experimental results and rule out the expected pathway. Calculations instead suggest the existence of a four-step domino pathway through several polar intermediates that agrees with the electronic nature of the substituents involved. The mechanistic hypothesis has further been corroborated by an experiment with isotopically C-13-labeled PhNCS that unambiguously shows the way in which the exchange reaction occurs.

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Published date: 15 May 2009

Identifiers

Local EPrints ID: 67652
URI: https://eprints.soton.ac.uk/id/eprint/67652
ISSN: 0022-3263
PURE UUID: 6ef98435-77d2-4119-95cb-03121aa1ce81
ORCID for Mark Edward Light: ORCID iD orcid.org/0000-0002-0585-0843

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Date deposited: 02 Sep 2009
Last modified: 05 Nov 2019 01:56

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