Phosphoranes chiral

The synthesis of an optically active chiral phosphorane in which the optical activity is solely connected with the P atom implies, for instance, the preparation of a trigonal bipyramid or a tetragonal pyramid with five different achiral ligands which would, at least theoretically, entail the possible formation in the first case of ten or in the second case of fifteen diastereoisomers requiring separation, a formidable task even in the absence of stereomutation. The problem has nevertheless been solved using three different approaches exploiting the particular properties of spirophosphoranes, as hereafter detailed. 

Synthesis of Chiral Phosphorane. (5)-(/ )-PPFA has been converted to an enantiomerically pure ferrocenylphosphonium salt... 

An example of chain elongation developed in palytoxin synthesis is given below. As shown in Scheme 11.53, compound 239 was reacted with the chiral phosphorane 240 to furnish the Z isomer... 

Wittig reactions of cyclohexanone derivatives in their inclusion compounds with the chiral host 5 gave optically active reaction products. For example, when a mixture of finely powdered 1 1 inclusion compound of 5b with 4-methyl-(15a) or 4-ethylcyclo-hexanone (15b) and phosphorane (16) was kept at 70 °C, the Wittig reaction in the solid state was completed within 4 h. To the reaction mixture was added diethyl ether-petroleum ether (1 1), and then the precipitated triphenylphos-phine oxide was removed by filtration. The crude product left after evaporation of solvent from the filtrate was distilled in vacuo to give (-)-4-methyl- (17a) of 42% ee (73 % yield) or (-)-4-ethyl-l-(carboethoxymethylene)cyclohexane (17b) of 45 % ee (57 % yield), respectively [9], Similar reaction of the 1 1 inclusion compound of 5c and c /. v - 3,5 - dime th y I c y c I o h e x a n o n e (18) with 16 gave 19 of 57 % ee in 58 % yield [9],... 

As can be seen from the following scheme, cleavage of the optically active ate-complex 57 with protons should lead to a chiral, optically active phosphorane, B, provided this has a rigid trigonal-bipyramidal configuration. 

The phosphorane (70) shown in Scheme 8 is analogous to (65) discussed in Scheme 7, except that the dimethyl-compound (70) lacks the chiral center in one of the ring carbons,77 which is present in (65). The n.m.r. spectrum of (70) shows77 two methoxy signals at — 8°, but only one such signal at ca. +62. The interpretation of the data is quite simple below certain temperatures, (70) exists as two frozen isomers, (70-(l 4) (2 5))... 

The bis(methylene)phosphoranes hold two carbon atoms doubly bonded to the same phosphorus atom. The molecule is chiral in the solid state due to the propeller-like orientation of the silyl groups as demonstrated in Fig. 2. 

Relatively strong bases are used for the deprotonation of phosphonate reagents, and the phosphonate-stabilized carbanions formed are more basic than the corresponding phosphorane reagents. Such conditions may be incompatible with base-sensitive aldehydes and ketones, causing epimerization of chiral compounds or... 

As illustrated in Eq. (10), a chiral phosphonium ion can undergo attack by a nucleophile at any one of four different faces or six different edges, thus placing the entering ligand in the a and e positions, respectively. In the general case, when all five ligands are different, and in the absence of special constraints (see Sect. 3.2) 20 isomeric phosphoranes, which are interconnected by 30 pseudorotation steps, are thus produced from both enantiomers of the phosphonium ion. Because of the possibility for reaction via this complex intermediate manifold, interpretation of the stereochemical consequences of... 

Fig. 8 thus provides a useful topological representation of the stereochemical relationships of the displacement reactions in Eq. (11). In applications to chemical systems, the phosphonium ion may be chiral or prochiral i) (as in 18, Rl = R2 see Sect. 3.2) however, the vertex-isomer relationship among the derived phosphoranes is dependent on which type of center is involved. 

If chiral, all vertices represent chiral moleculesjjnantiomers are related through the center of symmetry of Fig. 8 (e.g., 15 and 15 are enantiomers), and the phosphoranes in each subset are enantiomers of those in the partner subset... 

Tetramethyldiphosphine and hexafluoroacetone gave the phosphoranes (29), (30), and (31) in which the chiral centres at phosphorus led to doubling of resonances in their n.m.r. spectra. ... 

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