Chapter 1
Fundamentals of the Stereochemistry of Organophosphorus Compounds

1.1 Historical Background

Natural processes are subordinate to geometrodynamics - the theory describing physical objects, geometrical spacetime, and associated phenomena completely in terms of geometry, and her elder sister - symmetry. Symmetry/asymmetry is one of the basic concepts in modern natural science [1]. Research into this field began in the Middle Ages, when the birefringent properties of calcite were discovered. In 1669, Bartholinus observed the double refractive properties of the calcite Iceland spar. Later, in 1801, the mineralogist Haui found that quartz crystals are enantiomorphic, representing mirror images of one another. In 1815, another French naturalist J.-B. Biot discovered that certain chemical compounds rotate the plane of a beam of polarized light [2]. Biot constructed the first polarimeter and he also discovered that many natural compounds exhibit optical activity, that is, they rotate the plane of circularly polarized light. Studying crystals under a microscope, Biot discovered two types of crystals. The sample consisting of crystals of one type turned polarized light clockwise and that from another type in the opposite direction. A mixture of the two types of crystals had a neutral effect on polarized light. The nature of this property remained a mystery until 1848, when Louis Pasteur proposed that it had a molecular basis originating from some form of dissymmetry [3]. Pasteur separated the left and right hemihedral crystals of the sodium-ammonium salt of D,L-tartaric acid under a microscope, and connected the opposite optical activity to the mirror image of these crystals. Pasteur termed the mixture creating polarization as dissymetric and the phenomenon as dissymmetry (asymmetry). The term chirality was proposed by Lord Kelvin in 1894 and introduced into chemistry by Mislow in 1962. Dissimmetry, as discovered by Pasteur, is found in nature, whereas compounds obtained from living organisms are chiral or nonracemic. In 1852, Pasteur discovered that resolution could also be achieved by using a chiral base (quinine and brucine) and by using microorganisms. He discovered that paratartaric acid could be separated under the influence of optically active natural bases such as quinine or brucine. Pasteur developed a method for the separation of paratartaric acid with the help of Penicillium glaucum, leading to the formation of levorotatory tartaric acid, thus creating the basis for microbiological separation of racemates. J. Wislicenus came to the conclusion that the right- and non-superimposable levorotatory lactic acids have an identical structure, and he noticed that the only difference between the isomers is the order in which the radicals are distributed in space [4]. The origin of chirality itself was finally discovered in 1874, when van't Hoff and Le Bel independently proposed that this phenomenon of optical activity can be explained by the assumption that the four saturated chemical bonds between carbon atoms and their neighbors are directed toward the corners of a regular tetrahedron [5]. This concept led to the explanation for the observed optical activity by recognizing that a carbon atom with four different substituents exists in two mirror images: that is, it is chiral. The study of enantioselective reactions began with Emil Fisher [6], who studied the addition of hydrogen cyanide to sugars. In 1912, Bredig and Fiske [7] described the first catalytic enantioselective reaction. They studied the addition of hydrogen cyanide to benzaldehyde catalyzed by cinchona alkaloids. Although the mandelic acid that they obtained after hydrolysis of the initially formed benzcyanohydrin was of low optical purity (3-8%), Bredig and Fiske showed that it was possible to synthesize optically active compounds out of achiral precursors by using a chiral catalyst. Unlike Fischer, Marckwald performed an enantioselective reaction upon an achiral, unnatural starting material, although with a chiral organocatalyst [8]. In a paper titled "Ueber asymmetrische Syntheses," Marckwald gave the following definition of asymmetric synthesis: "Asymmetric syntheses are those reactions which produce optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes." Fifty years later, Horst Pracejus reported the asymmetric organocatalytic reaction of methyl(phenyl)ketenes with alcohols catalyzed by alkaloids, leading to the formation of enantiomerically enriched esters of a-phenyl-propionic acid [9].

Louis Pasteur (1822-1895)

Hermann Emil Fischer (1852-1919)

The first work devoted to the asymmetric synthesis of aminophosphonates by catalytic hydrogenation of unsaturated phosphonates was published approximately 30 years ago. The development of enantioselective synthesis was initially slow, largely owing to the limited range of techniques available for their separation and analysis. It was not until the 1950s that real progress began with the development of new techniques. The first of these was X-ray crystallography, which was used to determine the absolute configuration (AC) of an organic compound by Bijvoet et al. [10]. During the same period, methods were developed to allow the analysis of chiral compounds by NMR, either using chiral derivatizing agents (CDAs), such as Mosher's acid [11], or europium-based shift reagents, of which Eu(DPM)3 was the earliest [12]. Chiral auxiliaries were introduced by Corey and Ensley in 1975 [13] and featured prominently in the work of D. Enders. Around the same time, enantioselective organocatalysis was developed and enzyme-catalyzed enantioselective reactions became more and more common during the 1980s, particularly in industry, with their applications including asymmetric ester hydrolysis with pig-liver esterase. The emerging technology of genetic engineering has allowed the tailoring of enzymes to specific processes, permitting an increased range of selective transformations.

Today, the asymmetric synthesis of organophosphorus compounds is an extremely dynamic research domain in modern chemistry. Contributions to the development of asymmetric synthesis was made by many outstanding chemists. Thus, L. Horner studied the electrochemical cleavage of quaternary phosphonium salts leading to the discovery that tertiary phosphines with three different substituents are chiral [14, 15]. This knowledge formed the basis of the pioneering work of Horner on enantioselective catalysis, especially enantioselective homogeneous hydrogenation [15], which was published independently in the same year as the work of W. S. Knowles [16] - work that was honored by the Nobel Prize and which was based on the chiral phosphines discovered by Horner [15]. Knowles developed one of the first asymmetric hydrogenation catalysts by replacing the achiral triphenylphosphine ligands in Wilkinson's catalyst with chiral phosphine ligands. He developed an enantioselective hydrogenation step for the production of L-DOPA (3-(3,4-dihydroxyphenyl)-L-alanine), utilizing the DIPAMP ligand. L-DOPA later became a mainstay for treating Parkinson's disease. Noyori Ryoji won the Nobel Prize in Chemistry together with W. S. Knowles for the development of the atropoisomeric ligand BINAP (2,2´-bis(diphenylphosphino)-1,1´-binaphthyl) and study of chirally catalyzed hydrogenation [17]. In 1985, Schöllkopf et al. [18] reported asymmetric hydrogenation of N-[1-(dimethoxyphosphoryl)-ethenyl] formamide, using a rhodium catalyst with (+)-DIOP chiral ligand to afford the L-(1-aminoethyl) phosphonate in good yields and 76% ee enantioselectivity. The initially formed formamide was hydrolyzed with concentrated hydrochloric acid to give the aminophosphonic acid. Crystallization from water/methanol increased the enantiomeric purity of the product up to 93% ee.

Significant contribution to the development of asymmetric synthesis of organophosphorus compounds was made by Henry Kagan, a member of the French Academy of Sciences. He developed C2-symmetric phosphinic ligands, including DIOP, for asymmetric catalysis. These ligands have wide practical applications in the chemical industry [19].

The Japanese chemist Imamoto developed many types of phosphine ligands, which found practical applications [20]. The French chemist Juge created the accessible "ephedrine" method for the preparation of chiral phosphines named "the Juge-Stephan method." Together with Imamoto, he developed phosphine-boranes [21]. The American chemist William McEwen developed the fundamentals of the stereochemistry of organophosphorus compounds [22]. The Polish chemists Kafarsky [23] and Mikolajchyk [24] conducted important research studies in the application of phospha and sulfur reactants for the preparation of bioactive and natural compounds. Pietrusiewicz et al. [25], Kielbasisky and Drabowich [24, 26] are now continuing these studies. Methods for asymmetric synthesis and the synthesis of chiral organophosphorus compounds are of great interest to a number of powerful industrial firms and scientific research institutes, notable among them being the Leibniz Institute for Catalysis at the University of Rostock (LIKAT), the largest publicly funded research institute in Europe. Professor A. Börner of the Institute has been working on the development of new phosphinic chiral ligands and their practical applications [27]. In addition to those mentioned above, hundreds of highly professional chemists in many scientific centers are working in the domain of...