The Synthetic Methods, Structures, and Properties of the Ca-C σ Bond Organocalcium Containing Compounds

This first chapter is a general introduction to the book in which we give a background on the Ca-Cσ-bond containing organocalcium compounds, which are less studied as compared to other organometallic compounds. We describe the interest in synthesizing the synthesized organometallic intermediates and products as well as their application fields such as chemistry, biology, medicine, pharmacology, and industry. It is indicated that such compounds are not well-known because of the low reactivity of calcium elements, the low solubility of oraganocalcium intermediates and products in common organic solvents, their instability resulting from their high reactivity, cause their degradation and the degradation of their preferred ethereal solvents. Despite the significant difference in electronegativity value between the two elements, the Ca-Cσ-bonds have a covalent polar character. In many cases, the coordination considerably decreases the calcium electropositive character, facilitating the formation of the Ca-Cσ-covalent bond. The difficulties in synthesizing such compounds, their degradation property and catalytic behaviour as well as the spectroscopy technics used to identify their structures are also described.

In this second chapter, we have grouped the physical and chemical properties of elemental calcium and calcium cation species as well as their importance in the human body, especially in the skeleton and cell, as well as their applications in the chemical, electrical, and biological life fields. In a nutshell, we describe the history of the organocalcium Ca-Cσ-bond containing compounds. Then, we expose the encountered challenges and difficulties in the synthesis of organocalcium Ca-C containing compounds: before, during and after the organocalcium preparation and the solutions to overcome these difficulties. We have grouped the calcium activation methods, the nature of the solvent, the substrate structure, temperature and the optimal experimental reaction conditions. 

Compared to other organometallics, the number of organocalcium compounds and their related publications are limited. In this chapter, we grouped different Ca-Cσ-bond containing organocalcium compounds described in the literature, (Alkyl, silyl, cycloalkyl, benzyl, alkenyl, alkynyl, diene, diyne, phenyl, biphenyl, anthracene, halo thiophene and picoline derivatives) with the specificity for each kind of compounds. We discuss the stabilizing factors of structures including: conjugation, hyperconjugation, complexing heteroatom lone pairs and π-bonding electron pairs. The different synthetic processes used to avoid synthetic problems of substrate and calcium source reactivity, intermediate and product stability and solubility, reaction temperature, yields and decomposition reactions of both intermediates and products, as well as the orientation of the reaction sides in some reactions, are also grouped (thiophene and picoline derivatives). The halo aromatic organocalcium compounds are described to be the most stable intermediates, giving the best yields. Some calculations that correlate with experimental results are mentioned. Particular calcium reagents, such as Ca*, CaI2 and particular reactions as well as exchange (transmetalation) reactions are also examined. Some examples of the geometrical structures, which give C-Ca-C bent angles, and bond lengths obtained from X-ray are gathered.

 In addition to the coordination of the calcium to the n electron pairs, some examples of coordination to the π electron pairs are observed resulting from calcium interactions with highly conjugated systems. The complex structures vary from ordinary to close to the unexpected organocalcium inverse sandwich. The organocalcium Ca-C bond containing compounds are highly reactive. They degrade and act as degrading agents on the solvent. The study of the effect of five factors: temperature, concentration, organic group, counter ion and coordinated donor solvent, shows an intramolecular degradation mechanism. Organocalcium complexes are more stable in the THP than in the THF ethereal media. Calcium and organocalcium Ca-Cσ-bond containing derivatives are described as efficient pre-catalysts for catalytic conversions of alkenes by polymerization, hydroamination, hydrosilylation, hydrogenation, and hydrophosphination. The activation process is suggested as an electrostatic interaction between the alkene and Lewis acidic calcium metallic species, leading to both vertical and horizontal polarization of the π bond electron density, resulting in an incentive for nucleophilic attack. Some researchers illustrate that the organocalcium derivatives may be described as “Trojan horses” X-ray constitutes the most efficient technique to characterize the organocalcium structures for crystalized compounds, affording interesting information about angle values and bond lengths. The 43Ca, 13C and 1H NMR spectroscopy constitutes an alternative, secondary and complementary tool for characterizing both soluble, and even insoluble compounds, monitoring the reaction progress and making specific experimental manipulations. The coupling constants 1JCa-C, and 2JCa-C-H are not mentioned and there is no mass spectrometry nor IR spectroscopy studies

Ca-Cσ-bonds containing organocalcium compounds are prepared from different synthetic methods. Several difficulties and challenges are encountered before, during and at the end of the reaction. Organocalcium derivatives are insoluble in common organic solvents and show degrading properties. Ethers are the solvents of choice and THF gave the best results. The products are generally ethereal six-coordinated calcium centers (L4CaRX), with a Ca-C bond length of around 250 pm, depending on many factors. Arylcalcium derivatives are the easiest to prepare and are more stable than the other organocalcium compounds because of the conjugation effect phenomena. Ethereal solvents have a slight influence on the Schlenk-equilibrium type between RCaX or CaR2 and CaX2 . Almost all types of the main classes of σ-bond containing organocalcium derivatives are synthesized. Calcium carbon double bond was never formed. X-ray analysis is the best technique for the characterization of crystallizable organocalcium compounds. The 43Ca NMR does not provide interesting information about the structures. The 13C and 1H NMR data constitute an interesting tool that contributes the elucidation of the structure of the organocalcium compounds, especially for the non-crystalized compounds. Reactivity, chemical behaviour and 13C NMR chemical shift values of organocalcium complexes exhibit more similarities to the organolithium rather than the organomagnesium derivatives and were explained on the basis of their comparable electronegativity values.