Stereoselective P₅-Deltacyclene Alkylation, an Efficient Route to New Asymmetric P-C-Cage Compounds

Abstract

Tetra-tert-butyl-P 5 -deltacyclene 5 represents one of only two asymmetric P-C cage compounds, which are available in highly enantiomerically enriched versions. This paper reports about stereoselective substitution reactions of 5 to develop the chemistry of optically active P-C cages further. Electrophilic substitution of the only secondary phosphorus atom P1 of the cage with methyl and benzyl groups was achieved with 92 % and Ͼ99 % de, but the yields of the reactions are limited due to competing processes. The uncatalyzed hydrophosphination reaction of a monosubstituted allene and two α,β-unsaturated 922 carbonyl compounds with 5 proved to be the method of choice. cis-Butanone-P 5 -deltacyclene 12 is formed in 92 % yield and with Ͼ99 % de and cis-pentanone-P 5 -deltacyclene 13a is accessible with Ͼ99 % de for P1 and 92 % de for the attached carbon atom at the same time. Besides stereoselectivity, the hydrophosphination reaction of 5 performs with a good regioselectivity. The chiral cage 5 controls the stereoselectivity of its reactions for the cage elements as well as for the α position of a substituent.A novel aspect of this field of chemistry are optically active derivatives of chiral P-C cage compounds. [6][7][8] As a specific property of the derivatives of 3, a once established enantiomeric excess remains untouched by all reactions tested to date which do not disintegrate the molecules such as destannylation, [6] protonation, [7] deprotonation, [9] complexation, [7,9] oxidation, [10] and cage rearrangement including cage inversion. [9] In spite of its five P-lone pairs and a dynamic equilibrium between two epimers, P-H P 5 -deltacyclene 5 (Scheme 1) proved to be a highly selective ligand with metals of interest for catalytic applications. It forms only one single complex species in high yield for each of its enantiomers with the (benzene)Ru II Cl 2 fragment, for example. [7] Scheme 1. Preparation of P-H P 5 -deltacyclene 5 and iso-P 5 -deltacyclene 8. Journal of Inorganic and General ChemistryZeitschrift für anorganische und allgemeine Chemie ARTICLE ated [5 -H] -. [7,9] P-alkylation of a P-C cage compound has been reported for the first time for tetraphosphacubane (1), [11] and an asymmetric P-C cage has been used as well. [12] Cationic mono organylation and alkynylation products [1R] + are accessible with suitable electrophiles. 5 should form the cation [5R] + if treated in the same way as 1. A successive deprotonation of [5R] + with bases seemed to be possible and would lead to alkylated neutral cages, whose ligand properties with respect to metal cations would not be hampered by a positive charge. In spite of the bulkiness of the tetra-tert-butyl-P 5 -deltacyclene cage, the P-H function of 5 might be active as a hydrophosphination agent towards activated unsaturated organic compounds and form neutral alkyl-tetra-tert-butyl-P 5 -deltacyclene derivatives directly. In the case of prochiral organic substrates, the stereochemical aspects of the reaction are of interest for the potential use of the cages as ch...