Functionalization
Reactions Characteristic of a Robust Bicyclic Diphosphane Framework
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Abstract
The 3,4,8,9-tetramethyl-1,6-diphospha-bicyclo-[4.4.0]deca-3,8-diene
(P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub>) framework
containing a P–P bond has allowed for an unprecedented selectivity
toward functionalization of a single phosphorus lone pair with reference
to acyclic diphosphane molecules. Functionalization at the second
phosphorus atom was found to proceed at a significantly slower rate,
thus opening the pathway for obtaining mixed functional groups for
a pair of P–P bonded λ<sup>5</sup>-phosphorus atoms.
Reactivity with the chalcogen-atom donors MesCNO (Mes = 2,4,6-C<sub>6</sub>H<sub>2</sub>Me<sub>3</sub>) and SSbPh<sub>3</sub> has allowed
for the selective synthesis of the diphosphane chalcogenides OP<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (87%), O<sub>2</sub>P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (94%), SP<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (56%), and S<sub>2</sub>P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (87%).
Computational studies indicate that the oxygen-atom transfer reactions
involve penta-coordinated phosphorus intermediates that have four-membered
{PONC} cycles. The P–E bond dissociation enthalpies in EP<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> were measured via
calorimetric studies to be 134.7 ± 2.1 kcal/mol for P–O,
and 93 ± 3 kcal/mol for P–S, respectively, in good agreement
with the computed values. Additional reactivity with breaking of the
P–P bond and formation of diphosphinate O<sub>3</sub>P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> was only observed
to occur upon heating of dimethylsulfoxide solutions of the precursor.
Reactivity of diphosphane P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> with azides allowed the isolation of monoiminophosphoranes
(RN)P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub>(R = Mes,
CPh<sub>3</sub>, SiMe<sub>3</sub>), and treatment with additional
MesN<sub>3</sub> yielded symmetric and unsymmetric diiminodiphosphoranes
(RN)(MesN)P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (91%
for R = Mes). Metalation reactions with the bulky diiminodiphosphorane
ligand (MesN)<sub>2</sub>P<sub>2</sub>(C<sub>6</sub>H<sub>10</sub>)<sub>2</sub> (nppn) allowed for the isolation and characterization
of (nppn)Mo(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Cl(CO)<sub>2</sub> (91%), (nppn)NiCl<sub>2</sub> (76%), and [(nppn)Ni(η<sup>3</sup>-2-C<sub>3</sub>H<sub>4</sub>Me)][OTf] showing that these
ligands provide an attractive preorganized binding pocket for both
late and early transition metals
