Dedicated to Professor Dieter Seebach on the occasion of his 60th birthday Cyclooligomerization reactions of phosphaalkynes in the coordination sphere of transition metals in which the metal complex fragment is incorporated in the product have played an important role in the development of phosphaalkyne chemistry.[1] Significant examples are the cyclodimerization of tert-butylphosphaacetylene (5a) to give the tricyclic zirconium complex 1, [2] cyclotrimerizations to give the vanadium and hafnium complexes 2[3] and 3, [4] and the formation of the zirconium and hafnium complexes 4[5] by cyclotetramerization. Treatment of 1, 3, and 4 with hexachloroethane leads to detachment of the metal complex fragments and formation of tBu-substituted 1,3,5,7-tetraphosphacubane (cyclodimerization of the residual molecule), [6] 1,3,5-triphosphabenzene (14a), [4] and 1,3,5,7-tetraphosphabarrelene. [5] Reactions of 2 with the same reagent have not yet been investigated. We now report on the chemoselective cyclooligomerization [7] of phosphaalkynes 5 a ± e with the vanadium(v) compounds tBuNVCl 3´D ME (6) [8] (DME 1,2-dimethoxyethane) and tBuNVCl 3 (13) [9] to produce the title compounds in good yields.Reaction of 5 a ± e with 6 in a molar ratio of 4:1 in toluene followed by column chromatography on alumina affords the previously unknown azatetraphosphaquadricyclanes 7 a ± e as yellow solids in yields of 47 ± 76 % (not optimized; Table 1).The elemental analyses and mass spectra (Table 2) indicate that the heteropolycyclic compounds are formed from four molecules of the phosphaalkyne 5 with loss of one molecule of an alkyne RCCR (see Scheme 1) and incorporation of the imide ligand from the vanadium compound 6.As an example the NMR data of 7 a are discussed here (for further spectroscopic data of the compounds 7, see Table 2).[ Table 2. Selected spectroscopic data for the azatetraphosphaquadricyclanes 7.[a]

published as an Advance Article on the web 2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene (1) reacts selectively with a mixture of the primary lithium alkoxides 2a,b and the corresponding alcohols 3a,b to afford the novel 2,4,6-tri-tert-butyl-1,3,5-alkoxy-1,3,5-triphosphacyclohexane derivatives 4a,b; treatment of 4a with sulfur results in oxidation of all phosphorus atoms to furnish the triphosphane sulfide 5.

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Eine kleine Variation des Vanadiumreagens hat ein vollkommen anderes Produkt zur Folge: Durch Cyclooligomerisierung reagieren Phosphaalkine 2 mit tBuN=VCl3⋅DME unter Einbau des Imidofragments zu den Azatetraphosphaquadricyclanen 1 (DME=1,2‐Dimethoxyethan), mit dem entsprechenden Lewis‐Basen‐freien Vanadium‐Reagens werden dagegen ohne Einbau dieses Fragments die 1,3,5‐Triphosphabenzole 3 erhalten.

Cycloaddition reactions of the kinetically stabilized phosphaalkynes 1 with the imidovanadium(v) trihalides 9 furnish the 1,2,4-azaphosphavanada(v)cyclobutenes 10. The stability of these novel metallacyclic compounds depends solely on the substitutents of the imido unit. Thus, the imidovanadium(v) species 9 with tertiary alkyl groups on the N atom form stable addition products with 1 while in the cases of compounds 9 with a lower degree of substitution at N (primary and secondary alkyl groups) the primarily formed adducts 10 undergo irreversible decomposition to afford the 1H-1,2,4-azadiphospholes 13. Reactions of an excess of the phosphaaalkyne 1 with the vanadium complexes 9 furnish the corresponding triphosphabenzenes 8 in good yields (36-68%). A catalytic reaction course has been demonstrated for the all-tert-butyl system 1a/9a in which the metallacyclic species 10a serves as the catalytically active species. Poisoning of the catalyst leads to a second reaction pathway, which results in formation of the azatetraphosphaquadricyclanes 16. By means of the stepwise use of different phosphaalkynes 1a,b this methodology provides the first access to the differently substituted triphosphabenzenes through cyclotrimerization.