Boranes topology

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

More elaborate electron-counting procedures have been established, and these are usually more appropriate for the rationalisation of high-nuclearity clusters. An approach which was originally applied to complex boranes has been extended to carbonyl clusters and a number of topological theories have been described. Although there is some way to... 

The topologies of various cryptands are shown in Fig. 1. Considering the macrobicyclic ligands, each bridgehead may be turned either inward or outward with respect to the molecular cavity (11,12,22). This leads to three topological isomers exo-exo, exo-endo, and endo-endo. Crystal structures of one representative of each have been described. [2.2.2] adopts the endo-endo conformation with both nitrogen lone pairs directed into the cavity, while its bis-borane derivative is exo-exo (23) and the mono-borane derivative of 11.1.11 has the exo-endo form (24) (Fig. 2), with the —BH 1 group outside the cavity. 

The question of the molecular structure of the adducts of tetraborane(8) has now been resolved 144>. The boron-11 nmr spectra of the carbon monoxide and trifluorophosphine adducts have been analyzed 140> in terms of the 2112 topological geometry 141> proposed by Dupont and Schaeffer 142>. The dimethylaminodifluorophosphine adduct of tetra-borane(8) 143> which was prepared by the following reaction is stable at room temperature. 

Finally, several authors including Lipscomb himself, have pointed out the relationship of the styx rules (see above) to Wade s rules55,87,88. The sum of the digits in the topological styx formula for a borane structure gives the number of skeletal electron pairs in the molecule since both approaches factor out the bonds to exopolyhedral substituents. Thus, in the general formula BpHp+q used in the styx rules, q values of 2, 4, 6 and 8 are identified with closo-, nido-, arachno-and hypho-borane structures, respectively. 

A close scrutiny of the seminal report on the boranes reveals that the equivalence traditionally presented between the solid geometrical and the topological pictures of these compounds is incomplete. Because of this mathematical disparity, an alternate geometrical description of the bonding in the boron hydrides is introduced, as well as a new interpretation for the boron bridge. 

Although one could continue to demonstrate the virtues of the proposed nomenclature for the remaining boranes in [2], the models that have been postulated for each such structure seem to be getting further removed from what is truly known about the relevant geometry and topology of the respective compounds. Consequently, although the proposed system of nomenclature addressed the problem, which the presently accepted system did only belatedly and with an extremely convoluted set of terms, locant numbers, etc., it s further development must await until more laboratory results are accumulated. In the present state of knowledge, it is more speculative than is desirable for any canonical system of "scientific" nomenclature. 

Until now we considered the topology of various organic molecules. In inorganic chemistry the topological methods are not so popular. Therefore, in this chapter we shall confine ourselves to only two topics, the topological theory of borane structure and the discussion of the interrelationship between topology, symmetry, and electronic structure of coordination compounds. 

Lipscomb s topological theorem. In 1954 W. Lipscomb with coworkers suggested a so-called topological theory of boranes which he continues to improve even now. On the basis of that theory it became possible to predict,... 

From the Lipscomb theory one can get the topological graphs for any borane of assigned composition and thus predict new structures. Certain limitations should be imposed, though, on the quantities appearing in the balance equations (31)-(34), for example,... 

New refinements. Using the MO method to calculate the optimal geometry of these compounds, it was established that the molecules B2H4, B3H9, and B4H8 have vacant orbitals at a boron atom. To describe the unstable boranes with vacant orbitals, the known topological rules (the balance equations) had to be extended. The new equations for the systems BpHp+g take the form... 

These topological observations relate to the experimentally observed chemistry of the deltahedral borane anions B H 2-. For example the hydrolytic stability of the deltahedral boranes11 decreases in the approximate sequence ... 

The topological approach recently devised for the determination of allowed transition states for nucleophilic and electrophilic reactions of boranes has been revised so as to exclude s/y(—1) valence structures. These are the structures in which one B has no terminal hydrogen, and they should be excluded because of the awkward hybridizations involved. 

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