Tobias Fiedler

Dibridgehead Diphosphines that Turn Themselves Inside Out

Molecules and macromolecules that undergo topologically complex dynamic processes—such as knot-forming and multistep folding sequences—have attracted considerable attention from numerous standpoints. However, there is much less awareness that certain types of molecules, including but not limited to macrocyclic bicyclic (macrobicyclic) compounds, are able to turn themselves inside out. This has been termed “homeomorphic isomerization”, even though the process can be degenerate. At the time of a 1996 review, only four (degenerate) cases had been rigorously established by spectroscopic means. Although we are unaware of additional confirmed examples since, such equilibria have been invoked to rationalize the NMR spectroscopic properties of other macrobicyclic compounds. Herein we demonstrate this type of dynamic behavior—in both degenerate and nondegenerate manifestations—with three stereoisomers of a macrobicyclic aliphatic dibridgehead diphosphine (in,in, out,out, and in,out, referring to the orientations of the lone pairs of electrons on the phosphorus atoms). In the nondegenerate case, the lone pairs of electrons are alternately directed in a convergent manner towards an interior domain (in,in) or directed externally (out,out). Thus, such processes can potentially mediate the sequesterization, transport, and delivery of Lewis acid guests. Analogous dynamic behavior has recently been proposed for a hexaaryl dibridgehead diphosphine and other types of aromatic dibridgehead diphosphorus compounds. Our story begins with the platinum dichloride complex trans-1 (Scheme 1), in which three (CH2)14 chains connect the trans-arranged phosphorus atoms. This complex exemplifies a class of compounds termed “gyroscope like”, because of the rapid rotation of the caged MLn moieties in suitably sized systems on the NMR time scale, and their structural similarities to common toy gyroscopes. Treatment of trans-1 with an excess of the nucleophiles NaC CH, LiC CPh, or KC N afforded the macrobicyclic dibridgehead diphosphine 2 as an analytically pure, moderately air-sensitive white powder in 66–91% yield. The dianionic platinum tetrakis(acetylide) complex 3 could also be isolated (35%) when LiC CPh was employed. The three in/out stereoisomers of 2 are depicted in Scheme 1 (middle). In the PtCl2 adduct 1, both lone pairs of electrons of the dibridgehead diphosphine ligand are directed inwards. Treatment of 2 with PtCl2 in C6D6 regenerated 1, which constitutes an overall retention of configuration at the phosphorus atoms. For this reason, it was originally thought that only in,in-2 was produced. For small bicycles, out,out isomers are energetically much more favorable, but computational data for analogous hydrocarbons indicate that in,out isomers become most stable at medium ring sizes, and that in,in isomers become most stable at larger ring sizes. DFT calculations (see the Supporting Information) indicated (as did preliminary molecular mechanics calculations) that in,in-2 was considerably more stable than out,out-2 (6.98 kcalmol ), and somewhat more stable than in,out-2 (1.59 kcalmol ). The longest aliphatic dibridgehead diphosphine reported previously features one (CH2)3 and two (CH2)4 chains, which are much shorter than the (CH2)14 linkers in 2. [11] The isomers of 2 can be regarded as configurational diastereomers that are interrelated by pyramidal inversions at Scheme 1. Synthesis and complexation of the dibridgehead diphosphine 2.