Optically active P₅-deltacyclenes: selective oxidation, ligand properties, and a diastereoselective rearrangement reaction

Abstract

a Cage-chiral tetra-tert-butyl-P 5 -deltacyclene 5 is accessible as a pair of highly enriched enantiomers 5'and 5''. The only secondary phosphorus atom P1 of the cage can be selectively oxidized by reaction with t-BuOOH. The P1-oxo species 9a' and 9a'', allow the direct determination of their ee values. Oxidation occurs with the complete retention of the optical activity of the compounds. The chiroptical properties of 9a' and 9a'' are strongly dominated by their cage chirality, the oxygen atom does not contribute significantly. Elemental sulfur and selenium oxidize P5 with high preference to yield P5-thio-and P5-seleno-P 5 -deltacyclenes 10 and 11 of the intact cages again. Longer reaction time and more than stoichiometric amounts of selenium, leads to tri-seleno-P 5 -tetracycloundecane 12, a partially opened oxidized rearrangement product. The ligand properties of racemic 9a were determined. Diphosphetane phosphorus atom P2 of 9a is the active donor center to bind a Cr(CO) 5 fragment, but a tautomerization of 9a takes place if [(benzene)RuCl 2 ] 2 is added. A hydrogen atom migrates from P1 to the oxygen atom to form a phosphinous acid ligand. The lone pair of P1 is regenerated and acts as the active ligand function of the cage in this case. As for 5, the base n-BuLi induces an efficient cage rearrangement reaction of 9a, where P1 and the neighboring carbon atom C4 containing its t-Bu substituent change places. C4 moves to its new position without breaking the bond with P5, this way forming the novel P1-oxo-P 5 -norsnoutene cage in a highly diastereoselective process.

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

“…An exclusive cis orientation of triorganyl stannyl substituents of P1 has been observed earlier,, transition metals bind with high preference to the lone pair of P1 in the cis position,, and the P‐H epimers of 5 displace an equilibrium of 87:13 with a cis orientation of H1 for the favored epimer 5a . A balance of steric interactions between the tert ‐butyl groups of C2 and C4 on the side of the cage and the incoming substrates control the stereochemistry of P1.…”

“…The first aspect was anticipated because P 5 ‐deltacyclenes proved their rigidity already in cases of even stronger modifications of the cage periphery. This includes introduction of bulky and chiral triorganyl stannyl groups,, oxidation with oxygen, sulfur, or selenium, and complexation with some transition metals , …”

“…A hint on a closely related reactivity of P1 and P5 of 5 was identified by oxidation of the cage. Mainly the oxidant decides, which one of the two nuclei reacts …”

“…A novel aspect of this field of chemistry are optically active derivatives of chiral P‐C cage compounds . 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, protonation, deprotonation, complexation,, oxidation, and cage rearrangement including cage inversion . In spite of its five P‐lone pairs and a dynamic equilibrium between two epimers, P‐H P 5 ‐deltacyclene 5 (Scheme ) proved to be a highly selective ligand with metals of interest for catalytic applications.…”