Synthesis
of the tricyclic 1,3-dithiole-2-thione-derived 1,4-dihydro-1,4-diphosphinine
is presented using a base-induced ring formation protocol and chloro(diethylamino)(1,3-dithiole-2-thion-4-yl)phosphane
as the starting point. P-oxidation reactions of dihydrodiphosphinine
by chalcogens led to bis(P-oxide), bis(P-sulfide), or bis(P-selenide), respectively; all
tricyclic compounds were obtained as cis/trans mixtures.
1,4-Dihydro-1,4-diphosphinine was converted into 1,4-dichloro-1,4-dihydro-1,4-diphosphinine.
This compound is almost insoluble in organic solvents, furnished selectively
the trans-bis(amino) derivative upon a 2-fold P-substitution
reaction with the weak nucleophile potassium bis(trimethylsilyl)amide,
and reacted also with alcohols ROH (R = nBu, iPr, tBu) to give cis/trans mixtures of
the corresponding bis(alkoxy) derivatives. Furthermore, the dichloro
derivative could be reduced to a 1,4-diphosphinine using PnBu3, but, unfortunately, the stubbornly insoluble product
could be neither purified nor crystallized. Despite this, we achieved
a thermal [4 + 2] cycloaddition reaction of this first CPS-ternary
compound with diethylacetylene dicarboxylate to obtain the corresponding
diphosphabarrelene, thus providing indirect evidence for the aromatic
tricyclic diphosphinine. Detailed density functional theory studies
on the formation of 1,4-diphosphinine provided insights into formation
pathways as well as NMR, IR, and UV/vis data.
