Tetra‐tert‐butyltetraphosphacubane: The First Thermal Cyclooligomerization of a Phosphaalkyne

Abstract

Dedicated to Professor Gottfried Mark1 on the occasion of his 60th birthdayThe extremely rapid development of phosphaalkyne chemistry has focused above all on cycloaddition reactions and ligand properties.''I Of particular interest have been the cyclodimerization and cyclotrimerization reactions in the coordination sphere of metals. Thus, tert-butylphosphaacetylene 5 can be dimerized, for example, to give cobalt complexes containing diphosphacyclobutadiene (1 [' , 31) and diphosphabicyclo[l .I .O] butane ligands (Z14'). Complexes with cyclotrimer ligands derived from 1,3,5-triphosphabenzene 3IS1 and its Dewar derivatives 4161 are also known. + 1 2 3 4The use of metal complexes in cyclooligomerization reactions is not always necessary, as shown by the thermal cyclotetramerization of 5. Although most phosphaalkynes are not particularly thermally stable,['] the behavior of these species upon heating has not been studied.When neat phosphaalkyne 5 is heated for a long period of time at 130 "C and the product distilled, pale yellow crystals are obtained which exhibit a high melting point and are stable toward air. Elemental analysis and mass spectrometry revealed that the product is a tetramer of 5, compound 7. The stepwise decomposition of the tetramer to the monomer occurs upon electron-collision-induced fragmentation. ['] The N M R spectra of 7 support a tetraphosphacubane structure with alternating C and P atoms. The 13C{1H) N M R spectrum (100.62 MHz, C6D6) shows three multiplets at 6 = -29.07,21.57, and 30.64 (Fig. 1). These represent the [*I Prof. Dr. M. Regitz, Dip1.-Chem. T. Wettling, Dip1.-Chem. J. Schneider, Dip].-Chem. 0. Wagner, Prof.

Spontaneous polymerization of phenylphosphaethyne

“…The contribution from the minor peak lying upfield (␦ P ϭ Ϫ130) may be due to ( 3 , 3 ) phosphorus atoms in three-membered rings. 9 Although the 31 P-NMR spectrum of the thermolysis products from t-butylphosphaethyne is dominated by the strong singlet at 258 ppm from the cubic tetramer, 3 there are two broad groups of well-resolved peaks centered at ␦ P ϭ 50 and Ϫ100 analogous to those observed in Figure 1(b). Although these shared spectral features may indicate similarities in the oligomerization pathways of the two monomers, the absence of a signal a Gel permeation chromatography analyses were run in THF at 30°C through two linear Millipore m-Ultrastyragel columns at 1 mL/min under RI detection using a polystyrene calibration.…”

“…1(b)]. Although much of the line broadening of these signals can be attributed to the existence of different regio-and stereoisomers in the oligomers, it is likely that the peak includes resonances from oligomers bearing not only saturated ( 3 , 3 ), but unsaturated ( 3 , 2 ) phosphorus functionalities as well. The bulk of the major 31 P resonance (␦ P ϭ 60) lies in the chemical shift range consistent with 3 , 3 phosphaalkanes (␦ P ϭ 20 -100).…”

“…2(c)]. The spectrum of the labeled oligomer was dominated by broad peak at ␦ C ϭ 51 assigned to sp 3 carbons of the cyclooligomers.…”

“…2 Longer COP bonds and the lack of pendant organic groups attached to phosphorus make phosphaalkynes potential precursors to conjugated polymers. However, few details concerning their polymerization chemistry are known, save that t-butylphosphaethyne and a few other sterically stabilized phosphaalkynes undergo thermally induced (130 -180°C) 3 or Lewis acid-mediated cycloaddition reactions 4 -9 to give polyhedral oligomers (primarily the tetramer, but the hexamer has also been isolated) 7 rather than linear oligomers or polymers. Spontaneous "polymerization" reactions of phosphaalkynes with less sterically demanding substituents (RAH, CH 3 , Me 3 Si, phenyl) have also been reported, 10 -13 but none of the products have been characterized.…”

“…However, few details concerning their polymerization chemistry are known, save that t-butylphosphaethyne and a few other sterically stabilized phosphaalkynes undergo thermally induced (130 -180°C) 3 or Lewis acid-mediated cycloaddition reactions 4 -9 to give polyhedral oligomers (primarily the tetramer, but the hexamer has also been isolated) 7 rather than linear oligomers or polymers. Spontaneous "polymerization" reactions of phosphaalkynes with less sterically demanding substituents (RAH, CH 3 , Me 3 Si, phenyl) have also been reported, 10 -13 but none of the products have been characterized. In this report, we have investigated phenylphosphaethyne (PhC'P, 1) to determine if its spontaneous oligomerization leads to discrete polyhedral oligomers analogous to those formed from tbutylphosphaethyne, or if higher molecular weight or even crosslinked polymers form.…”