Tetracoordinated carbon

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. 

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. 

The most typical and well-represented reactions involve nucleophilic substitution at the sulfonium atom (for example, hydrolysis to give monosulfoxides). This direction of attack is preferred because of the lower energy of S-S bonds and the decreased sterical hindrance at a trisubstituted sulfur atom compared with the hindrance at a tetracoordinated carbon atom. 

Synthesis of Stable Planar Tetracoordinate Carbon Zr/AI Compounds... 

In 1874, varft Hoff [26] and Le Bel [27] independently surmised that tetracoordinate carbon is surrounded by substituents in a tetrahedral geometry. This perception marked the very beginning of modern organic chemistry, which is increasingly being determined by stereochemical argumentation. Some time ago, attempts were made to synthesize stable planar tetracoordinate carbon compounds [28—30]. 

Table 7.1. Compounds 10 containing planar-tetracoordinate carbon.

Figure 7.2. Crystal structure of the bimetallic complex Cp2Zr(ju-r n r 2-Me3SiCCPh)(p-H)AlMe2 10a (X - Cl R = Ph R1 — SiMe3 R2 — Me) the planar-tetracoordinate carbon atom is Cl 5. Adapted by the authors.

R1 = ph R2 = Me) with the double hydrocarbonyl-bridged Cp2Zr(p-C=CPh)(p-CPh—CMeJAlMe 2 complex exhibiting a planar-tetracoordinate carbon atom within the central metallacyclic ring system. Adapted by the authors. 

Bimetallic Boriozirconocene Complexes with Planar Tetracoordinate Carbon... 

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

Scheme 3.2-S. Transformations with conservation of 2e aromaticity. 6a is a derivative of IB (Scheme 3.2-2) and has a planar tetracoordinate carbon atom. The dashed lines of the transition state 7 represent two 3c2e bonds (BBB and CBB).

Transition Metai Chemistry of 1,3-Diynes, Poiy-ynes, and Reiated Compounds 195 8. Planar Tetracoordinate Carbon... 

These results have led us rather far from the apparently straightforward situation indicated by the first set of nuclear quadrupole resonance results, and serve to illustrate how complicated the whole question can become. Whatever the reasons for it however it is clear that the tetrahedral arrangement found in tetracoordinated Carbon compound is much less rigid in the heavier elements of the group. 

Most of the organosilicon compounds contain bonds between the silicon and carbon atom. In the following paragraph the structural chemistry of the Si—C single bond is discussed, mostly in compounds with tetracoordinate silicon and tetracoordinate carbon atoms. The structural chemistry of the Si—C bond in compounds where the carbon coordination state is different, is also discussed. The Si—C bond is markedly polarized and the increase of the bond ionicity by attaching different substituents to either the silicon or the carbon atoms may affect its length. The electronic and steric effects are discussed later. 

This is exotic because one of the carbon atoms is forced to have very unusual pyramidal bonding tetracoordinate carbon normally has its four bonds directed toward the comers of a tetrahedron, but the apical carbon of 1 has all four bonds pointing forward. Without any further investigation of 1 we can thus characterize it as exotic. Of course without further investigation we cannot assert with confidence if it can exist, much less what its properties might be. Semiempirical and low-level ab initio [1, 2] and higher-level ab initio [3] studies on pyramidane have been published, and work on this and related molecules is reviewed [4], SE methods cannot be trusted for molecules like pyramidane because they are parameterized using information, whether experimental or calculated, for normal molecules. 

Unusually short bonds between tetracoordinate carbon atoms... 

The zirconium-benzyne complex 78 reacts with 2 equiv trimethylalumi-num57 or trimethylgallium58 to give 79 and 80, respectively [Eq. (17)]. As shown by X-ray analysis, these complexes contain a planar-tetracoordinate carbon center (C2) at the bridgehead position. Triethylaluminum and diiso-butylaluminum hydride react similarly.57... 

The formation of planar-tetracoordinate carbon is an unusnal sitnation that can be obtained, for instance, by treatment of the in im-generated [Cp2Zr =C H3]+ with one eqnivalent of starting bis(propynyl)zirconocene. In homodimetallic system (57), C36 is a planar-tetracoordinate carbon atom. In fact, it is a part of a C=C donble bond (C37-C36 I.3I7(8)A). C36 is also coimected to the acetylene carbon atom C35 by a Csp Csp a-bond (C36-C35 1.401(8) A) and exhibits an unsymmetrical three-center-two-electron interaction with the adjacent Zr atom (C36 Zrl 2.435(6) A C36-Zr2 2.530(5)A). The central core of three atoms around C36 is coplanar. An activation barrier of AGreatr(190K) = 9.5 0.5kcalmor was determined by studying the dynamic feature of (57). 

Most of the known examples of complexes incorporating a planar-tetracoordinate carbon contain alkyl, aryl, or even bulky substituents on the carbon center. However, recently a... 

The group 4 benzynemetallocenes combined with boron, aluminum, and gallium budding blocks were successftdly employed in the stabihzation of planar-tetracoordinate carbon compounds (equation 26). ... 

In boriozirconocene aUcenes, the chemistry of Csp -Zr and Csp -B bonds differ considerably and this feature allows a sequential route to substituted aUcenes. Furthermore, cleavage of the two types of bonds generally occurs with retention of geometry. On the other hand, heating of aUcynezirconocene complexes with 9-BBN, to 80-90 °C, affords a bimetallic complex (95) showing a planar-tetracoordinate carbon and an wo-BBN structure. 

Gabriel Merino was born in Puebla, Mexico, in 1975. He was educated at Universidad de las Americas, Puebla and Cinvestav (where he studied with Prof. Alberto Vela). After a 2-year postdoctoral stay at TU-Dresden, Germany, where he worked with Prof. Gotthard Seifert and Dr. Thomas Heine in several aspects of magnetic response, he joined the Department of Chemistry at the Universidad de Guanajuato. His research interests include design of molecules containing planar tetracoordinate carbon atoms, study of molecular scalar fields, and electron delocalization. He is a member of the Mexican National System of Researchers. 

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