Tetracoordination

The possibihty of six-coordinate sihcon species in aqueous solution has been suggested. Raman studies have indicated, however, that monosilicic acid in solution contains a tetracoordinate sihcon species (37). 

Monophosphams. The similar tetrahedral geometry and bond lengths of tetracoordinate phosphoms(V) compared to those of sulfur(VI) suggested that phosphonic and phosphinic acid groups might act as biososteres for the sulfonic acid moiety in the parent monobactams. The... 

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

Eig. 2. Stmctural representations of chelates where (8) corresponds to M(acac)2 (9) to ML2, L = S-sulfo-S-hydroxyquiaoline (10) to M-Trien (11) to a tetracoordinate metal bound to the hexadentate ligand EDTA and (12) to the Ee(III) chelate of EHPG. Stmcture (13), which emphasizes the spatial... 

Consider the equiUbria in an aqueous system composed of a bidentate ligand HA, eg, the enol form of acetylacetone, and a tetracoordinate metal, stmcture (8). The equations are... 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

Although unsynunetrically substituted amines are chiral, the configuration is not stable because of rapid inversion at nitrogen. The activation energy for pyramidal inversion at phosphorus is much higher than at nitrogen, and many optically active phosphines have been prepared. The barrier to inversion is usually in the range of 30-3S kcal/mol so that enantiomerically pure phosphines are stable at room temperature but racemize by inversion at elevated tempeiatuies. Asymmetrically substituted tetracoordinate phosphorus compounds such as phosphonium salts and phosphine oxides are also chiral. Scheme 2.1 includes some examples of chiral phosphorus compounds. 

In many reactions at carbonyl groups, a key step is addition of a nucleophile, generating a tetracoordinate carbon atom. The overall course of the reaction is then determined ly the fate of this tetrahedral intermediate. 

Section 7.16 Atoms other than carbon can be chirality centers. Examples include those based on tetracoordinate silicon and tricoordinate sulfur as the chirality center. In principle, tricoordinate nitrogen can be a chirality center in compounds of the type N(x, y, z), where x, y, and z are different, but inversion of the nitrogen pyramid is so fast that racernization occurs vit -tually instantly at room temperature. 

Once past the transition state, the leaving group is expelled and carbon becomes tetracoordinate, its hybridization returning to sp. ... 

T. G. Takhirov X-Ray Structural Investigation of Stereochemi-cally Nonrigid Tetracoordinated Metal-Chelates of Zn(II), Cd(II), Hg(II) and Ni(II) on the Base of Derivatives of 4-... 

Planar tetracoordination in metallocycles with group 4 and 5 metals 99ACR494. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Tricoordinate groups, such as sulfinyl (—SO—) and sulfonio (—S + R2), and a tetracoordinate group like sulfonyl (—S02—), possess partial positive charge on the central sulfur atom and hence are electron-withdrawing. The magnitude of the electron-... 

A cobalt(II)-chitosan chelate has been prepared by soaking a chitosan film in C0CI2 aqueous solution. The chitosan chelated Co(II) through both oxygen and nitrogen atoms in the chitosan chain. The tetracoordinated, high-spin Co(II)-chitosan chelate could be used as a catalyst, and the polymerization of vinyl acetate was carried out in the presence of Na2S03 and water at pH 7 and normal temperature. The polyvinyl acetate possessed a random structure [114,115]. 

For a review of mechanisms of nucleophilic substitutions at di-, tri-, and tetracoordinated sulfur atoms, see Ciuffarin, E. Fava, A. Prog. Phys. Org. Chem., 1968, 6, 81. 

Very recently, the first disulfur complexes of a tetracoordinated transition metal PtS2[P(Ar)Me2]2 (Ar=Tbt, Bbt) were synthesized by the reaction of ze-rovalent platinum complexes Pt[P(Ar)Me2]2> generated by treatment of the dichloride complexes PtCl2[P(Ar)Me2]2 with lithium naphthalenide, with elemental sulfur (Scheme 6) [37]. Since the use of excess elemental sulfur also... 

Phosphotriesterase from P. diminuta (PTE) was found to exhibit high hydrolytic activity towards various types of tetracoordinated phosphorus acid esters. Apart from the phosphonothionate 92, phosphoric acid triesters 94 (Equation 45), °" benzenephosphonic acid diester 95 (Equation 46) ° and methyl-phenylphosphinic acid ester 96 (Equation 47) were also stereoselectively hydrolysed under kinetic resolution conditions. Of course, in the case of the latter three kinds of substrates, half of the reacting ester was irreversibly lost due to the formation of achiral phosphorus acids. 

There are also data on pX values of various substituted phenols, as shown in Table 3. The electron-withdrawing effect of the ammonio group is due only to its inductive effect and this can be seen in the higher acidity of the m-substituted compared with the p-substituted phenol the value of is negative, — 0.08. In contrast, all the tricoordinate and tetracoordinate sulfur groups exert a stronger electron-withdrawing effect from the p-position than from the m-position, as is evident from the positive values of This is the same trend as in the acid dissociations of nitrophenols, in which is -I- 0.53. 

The general trend is that boron enolates parallel lithium enolates in their stereoselectivity but show enhanced stereoselectivity. There also are some advantages in terms of access to both stereoisomeric enol derivatives. Another important characteristic of boron enolates is that they are not subject to internal chelation. The tetracoordinate dialkylboron in the cyclic TS is not able to accept additional ligands, so there is no tendency to form a chelated TS when the aldehyde or enolate carries a donor substituent. Table 2.2 gives some typical data for boron enolates and shows the strong correspondence between enolate configuration and product stereochemistry. 

If there is no other interaction, the reaction proceeds through an acyclic TS and steric factors determine the amount of syn versus anti addition. This is the case with BF3, where the tetracoordinate boron-aldehyde adduct does not offer any free coordination sites for formation of a cyclic TS. Stereoselectivity increases with the steric bulk of the silyl enol ether substituent R1.50... 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

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