Isoelectronic structures

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

Table 4. Distance ratio Xlgas/Dgalt for isoelectronic structures...

A DFT study of the geometrical, electronic, and energetic parameters of fluoro derivatives of diphenylborates 9 and 10 and isoelectronic structures where the central atom has been substituted by carbon and nitrogen has also been carried out <2005TA755>. The relative energy of the homochiral versus heterochiral complex has been linearly correlated with the central atom and fluorine substitution on the different positions of the aromatic ring. 

Our group has devoted two articles for the study of systems where the chiral subunits are bonded to a boron or to a metallic atom (Scheme 3.28). In both cases, the weakness of the bonds formed can be easily broken. In the first case, a series of bisdiphenylborates and the effect of the fluoro substitution on the relative stability of the homo vs. heterochiral complexes have been studied by means of DFT calculations (B3LYP/6-31G ) [29]. In addition, the corresponding isoelectronic structures, in which the boron has been substituted by a carbon or a positively charged nitrogen, have been considered. In all the cases, the homo and heterochiral complexes present D2 and S4 symmetries, respectively. 

I. Alkorta et al., Effect of fluoro substituents and central atom nature on chiral derivatives of bisdiphenylborates and isoelectronic structures. Tetrahedron. Asymm. 16, 755-760 (2005)... 

Most probably, phosphorylated forms of these inhibitors are transition state analogues, in which methyl group represents -NH3+ substituent. Crystal structures of both MetSox-GS91 and PPT-GS94 complexes were measured and solved, however, due to the isoelectronic structure of oxygen atom and methyl group at... 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

Benzothiopyrano[2,3-c/]pyrimidines(5-deaza-10-thiaflavines) are of special biological interest because of their isosteric and isoelectronic structure to 5-desazaflavine. The synthesis starts from 6-chloro-5-formyl-3-meth-yluracil and thiophenol and subsequent cyclization of the phenylthio intermediate with PPA. Base-catalyzed reduction with secondary alcohols leads to a 5//-benzothiopyrano[2,3-fi(]pyrimidine (78TL2803 81JHC1329). 8-Substituted 5-deazaflavins have been made by a simple approach (cf. (85JHC841) (Scheme 101). 

Problem 2.69. Define the term isoelectronic, and give two examples of isoelectronic structures. 

Singlet carbenes will rearrange to isoelectronic structures in which all atoms have an octet of electrons if hydrogen atoms are located on adjacent carbon atoms. 

Carbon monoxide and isonitxile, which have an isoelectronic structure and serve as TT-acid ligand on transition metals, form various stable transition metal complexes. One of the most fundamental and important reactions with carbon monoxide and isonitrile in organometallic chemistry is their insertion into the carbon-metal bond of organometallic compounds (Scheme 1). 

Solution 18 The structures that are isoelectronic have the same geometry. Isoelectronic structures are... 

---