Chiral phosphinines

Relatively little has appeared in the past year on the synthesis and reactivity of the six membered, potentially aromatic, phosphinine ring system. A route to the first C2-asymmetric phosphinine (147), derived from (+ )-camphor, has been developed from the reaction of a pyrylium salt precursor with P(TMS)3. The chiral phosphinine is a crystalline, air-stable solid and forms complexes with metal ions, therefore having some potential as a ligand in homogeneous eatalysis." A pyrylium salt-P(TMS)3 final step was also used in the synthesis of a series of 2-(2 -halo)triarylphosphinines (148), subsequently shown to form a series of tungsten(0) and rhodium(I) complexes involving coordination of the phosphorus sp lone pair to the metal." Interest has continued in studies of the coordination chemistry of phosphabarrelenes, derived from the established reactions of phosphinines... 

Interest in the synthesis and reactivity of the six-membered, potentially aromatic, phosphinine ring system has also continued, but at a much lower level than in recent years. New synthetic work includes the application of the pyrylium salt route to phosphinine synthesis, this time starting from pyrylium salts bearing chiral substituents to give the related chiral phosphinines," and the development of new routes to the 2-phosphanaphthalene (137)" ° and the phosphinine-2-aldehyde (138). Also reported is an approach to the synthesis of l,2-dihydro-l,4,2-benzo-diazaphosphinines," cationic gold(I) complexes of 2,4,6-tri-t-butyl-l,3,5-triphosphabenzene," and the synthesis of some X -phosphinines from phosphonium-iodonium ylides." " ... 

The first C2-symmetrical chiral phosphinine 12 was recently reported by Gamer and co-workers and prepared from the corresponding biscamphorpyrylium salt and P(SiMe3)3 (Scheme 6.6) [35]. 

The first successful asymmetric reaction using a chiral phosphinine-based ligand was reported by our group [32]. The chiral BINOL-substituted phosphinine-phosphite 11 (Fig. 6.5) was obtained quantitatively from the hydroxy-substituted phosphinine 36 by reaction with the corresponding phosphorchloridite in the presence of an amine (Scheme 6.15). 

Chiral Phosphinine-Based Ligands for Asymmetric Catalysis... 

Fig. 6.21 Gold-catalyzed 1,6-enyne cyclization and Au(I)Cl complex containing the axially chiral phosphinine ligand 10...

Interest in the chemistry of the 6ji-phosphinine (phosphabenzene) system has continued, although considerably fewer papers have appeared compared to recent years. The synthesis, coordination chemistry and catalytic applications of phosphi-nines have been reviewed." New chiral bidentate phosphinine ligands (220) have been prepared and their coordination chemistry and applications in rhodium-catalysed asymmetric hydrogenations assessed." The diphosphinine (221) continues to find new applications as a ligand in homogeneous catalysis" and a new mode of coordination to a metal has been identified for the phosphinine (222), two such ligands simultaneously bridging a Mn-Mn bond." ... 

The synthesis and reactivity of the six membered ring, potentially aromatic, phosphinine system has received further attention. The first examples of a new class of axially chiral phosphorus ligands, the atropisomeric phosphinines (204), have been prepared and isolated in an enantiopure state." Also reported are syntheses of the wide-bite-angle diphosphinine (205)" and the diphosphinine analogue of terpyridine (206), a new class of neutral, 7i-acceptor PNP-pincer ligand." Studies of the reactivity of phosphinines include the first observations of protonation, alkylation and silylation in systems usually viewed as extremely weak bases," " the proto-desilylation of 2,6-disubstituted silylphosphinines," a theoretical study of... 

Until recently, the implementation of chirality to phosphinines was in fact limited to the introduction of chiral auxiliaries within the phosphinine-framework, such as (+) camphor (7), oxazoline- (8), dicyclohexyl-1,2-ethanediol (9) or binaphthyl-groups (11) (Fig. 6.5) [29-32]. 

Our group started to investigate the synthetic access to polymethyl-substituted phosphinines in order to generate axial chirality. The introduction of an 6>-xylyl group in 6-position and an additional -CH3 group in 5-position of the heterocycle via the pyrylium salt route gave a racemic mixture of the (5 a) and (R ) enantiomer of phosphinine 10 (Fig. 6.5, only the enantiomer is shown) [33, 34]. Chiral resolution could be achieved by chiral HPLC. The rotational barrier of AG = 109.5 kJ/mol was determined experimentally by means of racemization experiments. 

A very recent example on the application of phosphinines in homogeneous catalysis was reported by Neumann [69]. The chiral oxazoline-functionalized phosphinine 8 (Fig. 6.5), first reported by Breit et al., was applied in the Ir-catalyzed hydrogenation of several highly substituted unfunctionalized and functionalized alkenes and imines (Scheme 6.17). 

Gamer and co-workers applied the first chiral C2-symmetric phosphinine 12 (Scheme 6.6) in the Pd-catalyzed asymmetric hydrosilylation of styrene (Scheme 6.19) [35]. The trichlorosilane product was converted to 1-phenylethanol by Tamao-Fleming oxidation and showed an enantiomeric excess of 27% by chiral GC analysis. The relatively low level of asymmetric induction is attributed to the size and orientation of the bridgehead methyl groups. 

Muller C, Pidko EA, Staring AJPM, Lutz M, Spek AL, van Santen RA, Vogt D (2008) Developing a new class of axial chiral phosphorus ligands preparation and characterization of enantiopure atropisomeric phosphinines. Chem Eur J 14 4899-4905... 

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