Covalent derivative

A variety of complexes of the thionyl imide anion [NSO] with both early and late transition-metal complexes have been prepared and structurally characterized. Since both ionic and covalent derivatives of this anion are readily prepared, e.g., K[NSO], McsMNSO (M = Si, Sn) or Hg(NSO)2, metathetical reactions of these reagents with transition-metal halide complexes represent the most general synthetic method for the preparation of these complexes (Eq. 7.10 and 7.11). ... 

Numerous metal azides have been characterized (p. 417) and covalent derivatives of non-metals are also readily preparable by simple metathesis using either NaN3 or aqueous solutions of... 

Finally, it is of interest to compare the estimates of covalency contributions for Ir(IV) hexahalides deduced by Allen et al. (11) from spectroscopic data, with those obtained by Owen and Thornley (85, 86) from ESR results. These latter authors attributed the reduction of below the free-ion value, entirely to symmetry restricted covalency, deriving the expression 0bsd = N (Cd +s , >), where the normalising constant, N , is equal to (1 —4a S + and [Pg.153]

Dilithium perfluoropinacolate, Li2[(CF3)2CO]2, obtained from metallic lithium and hexafluoroacetone, reacts with TiC to form the covalent derivative (19). ... 

As a third group of fullerene-like systems whose synthetic development is identified as a major future challenge are structurally defined single-walled carbon nanotubes (SWNTs) with uniform helicity and length, including their covalent and non-covalent derivatives. SWNTs can be considered as elongated giant fullerenes. 

A second method requires the formation of diastereomeric salts or covalent derivatives, which are in a mobile equilibrium in solution ( First-Order Asymmetric Transformation"). Again, one of the diastereomers crystallizes ( Second-Order Asymmetric Transformation ). 

Compounds of unknown chirality to which a chiral reference is added in order to form a covalent derivative, a salt, or even a solvate. In the latter case, a compound of known chirality is incorporated by way of cocrystallization in the crystal structure of a compound of unknown absolute configuration, or vice versa. 

Iron, as found in the porphyrin derivative hemoglobin, complexes CO to form a stable metal carbonyl. Iron also forms a variety of metal carbon monoxide derivatives such as the homoleptic Fe(CO)5, Fe2(CO)9 and Fe3(CO)i2, the anionic [Fe(CO)4] and its covalent derivative Fe(CO)4Br2, [CpFe(CO)2] and its alkylated covalent derivatives CpFe(CO)2-R with its readily distinguished n (and and a (and / ) iron carbon bonds. By contrast. Mg in its chlorin derivative chlorophyll, which very much resembles porphyrin, forms no such bonds with CO nor is there a rich magnesium carbonyl chemistry (if indeed, there is any at all). 

Bis(tributyltin) oxide, BtijSnOSnBuj, is a high-boiling oil, bp 180°C/2 mm, with a penetrating, unpleasant, sweetish smell. It is readily available. Mixing with an alcohol R-OH gives a tributylstannyl ether according to Eq. (5), a reversible reaction that may be drawn to completion by azeotropic removal of water. These ethers are moisture-sensitive. They are undoubtedly covalent derivatives, yet their reactions with electrophiles are, broadly speak-... 

A topological study of the l,2,4-thiatriazole-5-thiolate anion and some covalent derivatives has been undertaken <2006MI1, 2006JMS(770)13>. In a recent review article, the CHIH-DFT determination of the molecular structure and infrared and ultraviolet spectra of azathiophenes, including 1,2,3,4-thiatriazoles, is discussed <2007MI1>. 

Covalent enzyme catalysis involves the formation of a transient covalent bond between an enzyme and its substrate. Below are the general structures of commonly encountered so-called acyl-enzyme intermediates and other covalent derivatives. 

Hydrogen pentafluorooxoselenate (VI) is the main source of the extremely electronegative ligand [SeOFs]. This ligand and [TeOFs] are, for example, the only groups that form stable covalent derivatives of the types Xe(SeOFs)2, Xe(TeOFs)4, U(TeOFs)6, and others. In contrast, the chemistry of the SOFs... 

Tributylstannyl ethers are covalent derivatives with the nucleophilic properties of soft metal alkoxides they react with benzyl bromide on heating in non-polar solvents in the presence of tetraalkylammonium bromides or iodides to give the O-benzyl ether [Scheme 4.166]. 309 310 The moisture sensitive tributylstannyl ethers are formed by reaction of an alcohol with bis(tributyltin) oxide (bp... 

The cyclic sulfur imides are weak Bronsted acids that are readily deprotonated by strong bases. In the case of SyNH, deprotonation produces the thermally unstable, yellow SyN ion, which decomposes to the deep blue S4N ion (equation 28). Some covalent derivatives of SyN have been isolated. The most important are the mercniy (11) compounds, PhHgSyN and Hg(SyN)2. 

Fig. 7.4. (Left) Schematic representation of the proposed TCDD recognition site showing the covalently derived amine groups at the edge of the cavity and the non-covalently positioned 7i-complexing comonomer residue co-planar with the ligand. (Right) Structures of aromatic comonomers used in this study pentafluorostyrene (8), l,4-bis-(3/4-vinylbenzoxy)-benzene (9) and l-methoxy-3,5-bis-(4-vinylbenzyloxy)-benzene (10). Adapted from [23].

A less frequent but nonetheless interesting problem arises in the chemical modification of liquid, or low-melting, active principles in solid prodrugs, suitable for tablet or capsule preparation. Indeed solid dosage forms are still the most widely used for the administration of medicines, as well for patient acceptability and convenience for product stability and ease of manufacture. Their preparation implies that the active principle can itself be handled as a stable solid, an objective that is usually attained by one of the following strategies formation of a salt or a molecular complex, formation of a crystalline covalent derivative, introduction of symmetry. 

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