The reactions of 1,1-diamino-2,2-diphenyl-substituted diphosphines featuring various degrees of P-P bond polarization with different alkynes were investigated. All diphosphines reacted with alkynes carrying one or two electron withdrawing carboxylic ester moieties under cleavage of the P-P bond and stereospecific phosphinyl-phosphination at the triple bond to give unsymmetrical ethane-1,2- bisphosphines. Several of the products were further converted into chelate complexes upon reaction with group-10 metal dihalides. All isolated compounds were characterized by analytical and spectroscopic data, and several of the new ligands and complexes by single-crystal X-ray diffraction studies.
Studies of the reactions between an unsymmetrically substituted 1,1-diaminodiphosphine and electron poor alkenes revealed that, in contrast to the regioselective 1,2-addition of the P−P bond to α,β-unsaturated esters and nitriles with terminal double bonds, ethyl(vinyl)ketone reacted via 1,4-addition and α,β-unsaturated esters or ketones with internal double bonds failed to react at all, presumably owing to the deactivating influence of the alkyl groups. Reaction of 1 with maleic N-phenylimide proceeded stereoselectively under cis-addition but diesters of maleic and fumaric acid gave mixtures of diastereomeric 1,2-bisphosphines. The addition products were characterized by 31P NMR before being converted into palladium complexes that were isolated and comprehensively characterized by spectroscopic data and in most cases by X-ray diffraction studies. Monitoring the reactions of 1 with maleic and fumaric diesters by NMR revealed that both E/Z-isomerization of alkene starting materials and epimerization of stereogenic centers in 1,2-bisphosphines take place and allow isolation from the mixtures of diastereomeric ligands of complexes featuring a uniform stereochemistry of the C2 backbone.
2‐Diphenylphosphanyl‐1,3,2‐diazaphospholidines were prepared via metathesis from 2‐chloro‐1,3,2‐diazaphospholidines and LiPPh2. For some of the products, symmetrisation to tetraphenyldiphosphane and 2,2′‐bis‐1,3,2‐diazaphospholidinyls was observed. Most of the derivatives were characterised by single‐crystal X‐ray diffraction, which showed that all compounds studied feature elongated exocyclic P–Cl or P–P‐bonds, respectively. The extent of this bond lengthening is in the P‐phosphanyl‐substituted species similar and in the P‐chloro‐derivatives less pronounced than in corresponding CC‐unsaturated 1,3,2‐diazaphospholenes. Structure correlation involving comparison of exocyclic P–X and endocyclic P–N distances suggests that n(N)/σ*(P–X) hyperconjugation contributes strongly to the bond lengthening and induces a perceptible weakening of the P–P bonds in 2‐diphenylphosphanyl‐1,3,2‐diazaphospholidines, which should render these compounds interesting substrates for P–P bond activation reactions.
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