Osmathiazole Ring: Extrapolation of an Aromatic Purely Organic System to Organometallic Chemistry

TL;DR

Scientists created a new organometallic ring structure and compared its aromatic properties to organic thiazole rings.

Contribution

The paper introduces an osmathiazole ring and explores its aromaticity and reactivity in organometallic chemistry.

Findings

The osmathiazole ring is more aromatic than osmaoxazoles but less than organic thiazole.

Compound 5 undergoes vicarious nucleophilic substitution of hydride with phenylacetylene.

The osmathiazole ring shows distinct reactivity compared to osmathiazolium cycles.

Abstract

An osmathiazole skeleton has been generated starting
from the cation
of the salt [OsH(OH)(≡CPh)(IPr)(PiPr3)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene;
OTf = CF3SO3) and thioacetamide; its aromaticity
degree was compared with that of thiazole, and its aromatic reactivity
was confirmed through a reaction with phenylacetylene. Salt 1 reacts with the thioamide to initially afford the synthetic
intermediate [OsH{κ2-N,S-[NHC(CH3)S]}(≡CPh)(IPr)(PiPr3)]OTf (2). Thioamidate and alkylidyne ligands of 2 couple
in acetonitrile at 70 °C, forming a 1:1 mixture of the salts
[OsH{κ2-C,S-[C(Ph)NHC(CH3)S]}(CH3CN)(IPr)(PiPr3)]OTf (3) and [Os{κ2-C,S-[CH(Ph)NHC(CH3)S]}(CH3CN)3(IPr)]OTf (4). Treatment of 3 with potassium tert-butoxide produces the NH-deprotonation of its five-membered ring
and gives OsH{κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (5). The
osmathiazole ring of 5 is slightly less aromatic than
the osmathiazolium cycle of 3 and the purely organic
thiazole. However, it is more aromatic than related osmaoxazoles and
osmaoxazoliums. There are significant differences in behavior between 3 and 5 toward phenylacetylene. In acetonitrile,
the cation of 3 loses the phosphine and adds the alkyne
to afford [Os{η3-C3,κ1-S-[CH2C(Ph)C(Ph)NHC(CH3)S]}(CH3CN)2(IPr)]OTf (6), bearing a functionalized allyl ligand. In contrast, the osmathiazole
ring of 5 undergoes a vicarious nucleophilic substitution
of hydride, by acetylide, via the dihydride OsH2(C≡CPh){κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (7), which releases H2 to yield Os(C≡CPh){κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (8).

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