Isoelectronic replacement

Dihydro-lH-l,2-azaboroles derive from cyclopentadiene by the isoelectronic replacement of a C=C group by a BN moiety ". The neutral rings react in xs boiling Fe(CO)j without any other solvent to form red-brown iron-dihydro-1,2-azaborolyldi-carbonyl dimers as their cis and transisomers in 56% yield, or with CojfCOg in petroleum ether at 60-80°C to the half-sandwich complex (dihydro-1,2-azaborolyl)Co(CO)2 (40%). The CO group can easily be substituted by olefins ... 

In the second type of mixed hydride, hypercarbon atoms feature alongside boron atoms in the polyhedral molecular skeleton. Most of these mixed hydrides contain more boron atoms than carbon atoms, and their formulae and structures can be understood simply by isoelectronic replacement of B by C, B by C+, or BH units by C atoms in the parent borane. They are therefore known as carbaboranes, though the shorter name carboranes, coined soon after their discovery, "- is that most often used for this important category of compound and will be used here. Their now well-documented chemistry, " particularly structural and bonding aspects, is the concern of the present chapter. 

Entries in the second column of Figure 3.1 and in Figures 3.3 and 3.4 show the deltahedral fragment skeletal shapes most commonly encountered among boranes and carboranes of formulae B H +4, CB iH +3, C2B 2H +2, and so on, all of which can be regarded as derived from hypothetical anions [B H ]4 by protonation and/or isoelectronic replacement of B by C. Their molecular formulae are sometimes written as CxB H +4, where x + y = n, the total number of skeletal atoms. The skeletal structures of these nido species. 

Isoelectronic replacement of a carbanionic carbon by a heteroatom gives much more stable compounds, and such 5,5-bicyclic aromatic systems have received considerable attention. In these compounds, sulfur, nitrogen and oxygen can also be incorporated into fully conjugated systems, unlike the 5,6-compounds, where only nitrogen can be used. Because of the variety of such systems, it is difficult to generalise about reactivity, but electrophilic substitution, which can take place in either ring, has been most widely reported with occasional examples of nucleophilic displacements and lithiations. Some representative reactions and self-explanatory syntheses are shown below." ... 

Finally, the conceptual isoelectronic replacement of C-C by B-N can be applied to carba-boranes, thus leading (by appropriate synthetic routes) to azaboranes in which one or more of the cluster vertices of the borane is occupied by an N atom. So far, the following species have been characterized, the relevant cluster geometries and numbering schemes being given by the indicated structures on pp. 153-85 arachno-... 

M. Frederickson, J. R. Coggin, and C. Abell, Vinyl fluoride as an isoelectronic replacement for an enolate anion Inhibition of type II dehydroquinases, Chem. Commun., (2002) 1886-1887. 

By isoelectronic replacement of the nitrosyl ligand with a carbyne unit, we derived a new type of system, which was realized in the form of the complex W(sCMes)(dmpe)2H [41]. A strong 7t-accepting ability is also attributed to the carbyne ligand, so that a electronically related environment of the hydride could be expected, compared to related nitrosyl substituted species. It turns out that W(NO)(dmpe)2H reacts with excess benzophenone about five times faster than the corresponding carbyne complexes. This factor of enhancement, however, is in kinetic terms a minor one, which means that these complexes have overall comparable reactivity. For the insertion of benzophenone into the W-H bond of W(CMes)(dmpe)2H, an inverse and very pronounced deuterium isotope effect is observed (kj /k = 1/3). This points to existence of a late transition state in the rate determining step, which must have to do with the actual H-transfer. 

Mixed metal clusters have also become very numerous and well known. These range from simple cases, where obvious isoelectronic replacements are made, as exemplified by Co4(CO)i2 and RhCo3(CO)i2, to very large and complex species such as [Ni9Pt3(CO)2iH]. There is no end in sight to the possibilities for more mixed metal clusters. 

Therefore, starting in the 1980s, medicinal chemists discovered that the selective introduction of fluorine into biologically active molecules exercised an influence on activity there are numerous reports of compounds incorporating fluorine as either a bioisosteric replacement for hydrogen or an isoelectronic replacement for the hydroxyl group [ 1,2],... 

Isoelectronic replacement of a carbanionic carbon by a hetero atom gives much more stable compounds such 5,5-bicyclic aromatic systems have received considerable attention pyrrolo[2,l-b]thiazole is one such example. Although we have not had sufficient space to describe these in this short chapter, their properties and synthesis follow the general principles discussed for example, imidazothiazoles are prepared from substituted thiazoles in a way analogous to the preparation of imidazopyridines, from substituted pyridines. 

We looked at some metalacycles with saturated rings in Eqs. 3.25-3.26, and we have seen several metalacyclic descriptions of complexes in this chapter (e.g., 5.6,5.9). Isoelectronic replacement is a general strategy for finding... 

Carboranes were first reported in 1962. Much of the early work was done in the USA as part of a classified project on borane chemistry, seeking novel rocket fuels, an object which proved abortive. Since then thousands of compounds of this type have been reported. Carboranes have structures which are related to those of the boranes by isoelectronic replacement of groups (e.g. B" or BH by C BH or BH by CH). They may belong to closo, nido or arachno series, although the last are relatively uncommon. The usual entry to carborane chemistry is to heat a borane with an alkyne or to subject the mixture to a silent electric discharge. Thus nido-BjH, and ethyne at 200°C give mainly nido-I S-C BJl, while at 500°C a... 

Closo. Isoelectronic replacement e.g. BH by CH or additional electron required. ... 

Table 2.5 lists reagent electron counts and also tells us about possible isoelectronic replacements of one ligand by another. For example, an X group can replace an L ligand without any change in the electron... 

Isoelectronical replacement of the a-carhon of common amino acids with a trivalent nitrogen atom results in loss of chirality and in... 

Thiadiazolo [3,4 - c ] [1,2,5 Jthiadiazoles.—1,2,5 -Thiadiazolo[3,4- c][l, 2,5]thia-diazole (192) is a novel, exceptionally interesting, compound, the ring system of which is derived by the isoelectronic replacement of all peripheral atoms of naphthalene by hetero-atoms. The resulting symmetrical structure is aromatic, as manifested by its planarity, shortened bond lengths, and stability, which is considerably greater than that of the thieno[3,4-c]thiophens. ... 

Newton and Fhrenson (34) obtained a mininum enerpy confipuration for the Hydronium ion which was planar, contrary to expectations. A more expensive computation by Kollman and Pender (351 was repui ed to obtain pyramidal distortion(36). Die substitution of Lithium cation for proton in the hydronium ion is not an isoelectronic replacement, and the arpument by analopy need not hold. Tlie suppestion is nevertheless painfully clear that the ion should not be planar. Since hiph accuracy Quantum Mechanical computations are quite expensive, one can perhaps understand v hy "frozen peometry water molecules have been used in these Lithium-water complex ion computations, and therefore why planar, triponal structures have always been found to be most favored. 

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