On the basis of isolated diastereomeric triorganylstannyl-P5 -deltacyclenes 7' and 7'', almost pure enantiomers of their destannylation products 8' and 8'' are now available. These stereochemically inert cage chiral species contain a configurationally labile P1H1 group that defines two epimers 8 a and 8 b of each of the enantiomers, which are connected by a rapid equilibrium. Mirror-symmetric circular dichroism (CD) spectra of the enantiomeric cages are compatible with the identification of epimers. A simulation of the CD spectrum of the major epimer 8'a relates the cage chirality of the system to the observed chiroptical effects. Both cage epimers and two of the phosphorus cage atoms are active as ligands with respect to [M(CO)5 ] fragments of Cr, Mo, and W. Four almost isoenergetic regio- and stereoisomers of the resulting mononuclear complexes are formed for these metals, but only one of the isomers per metal crystallized in the case of the racemic series of the complexes. The enantiopure versions of cages and cage complexes, however, did not crystallize at all, a well-known phenomenon for chiral compounds. CD spectra of the optically active complex isomer mixtures are close to identical with the CD spectra of the related free cages and point again to the chiral cages as the dominant source of the CD effects of the complexes. [(Benzene)RuCl2 ] complexes of the cage ligand 8 behave totally differently. Only a single species 12=[(benzene)RuCl2 ⋅8 b] is formed in almost quantitative yield and the minor epimer 8 b plays the role of the ligand exclusively. The reaction works as well for the separated enantiomeric cage versions to yield the highly enriched enantiomers 12' and 12'' separately. An efficient kinetic resolution process was identified as the main reason for this finding. It is based on a high stereo- and regiochemical flexibility of the PC cage ligand that is capable of adjusting to the specific requirements of a suitable transition-metal complex fragment. Such ligand flexibility is regularly observed in metalloenzymes, but is a very rare case in classical and organometallic complex chemistry.

Cage-chiral P 5 -deltacyclene 1 is available as a pair of highly enriched P-C cage enantiomers 1′ and 1′′, which exist as pairs of epimers a and b. Deprotonation of cage atom P1 initiates a rearrangement reaction, in which P1 and the neighboring carbon atom C4, together with its substituent, change places to form optically active iso-P 5 -deltacyclene enantiomers 6′′ and 6′, again as pairs of epimers. The CD spectra of related pairs of optically active cages 1 and 6 consist of almost mirrorsymmetric curves, an indicator of mirror-symmetric cage structures. This surprising result was verified by an absolute structure determination of the W(CO) 5 complex 9a′′. With the exception of the two cage nuclei that had changed places in relation to starting material 1′, all other cage nuclei of rearrangement [a] 691 Scheme 2. Base-induced cage-rearrangement reactions of 1′ and 1′′ to form 6′′ and 6′, respectively. Expected and experimentally observed reaction pathways.

Diastereomeric Pd and Pt complexes with a highly rigid chelate cage ligand core have been designed on the basis of alkylated C 2 -symmetric tetra-tert-butylhexaphosphapentaprismanes R 2 P 6 C 4 tBu 4 . The ligands are accessible by substitution of the diiodo derivative I 2 P 6 C 4 tBu 4 with Grignard and organyllithium reagents. Depending on the optical properties of the organyl group, racemic or diastereomeric dialkylhexaphosphapentaprismanes R 2 P 6 C 4 tBu 4 can be treated with suitable Pd II or Pt II precursor complexes to form neutral square-planar cis-[(R 2 P 6 C 4 tBu 4 )PdCl 2 ] and cis-[(R 2 P 6 C 4 tBu 4 )-PtCl 2 ] complexes, respectively. Monoalkylation products RIP 6 C 4 tBu 4 were also observed in the reaction mixtures, but in most cases were inactive as chelate ligands towards Pd II [a]

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.

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