Single bond formation

To eonsider why the two-orbital two-eleetron single bond formation ease ean be more eomplex than often thought, let us eonsider the H2 system in more detail. In the moleeular orbital deseription of H2, both bonding <5g and antibonding mos appear. 

C. Analysis of Two-Electron, Two-Orbital, Single-Bond Formation... 

Ring-Closure Reactions by C-C Single Bond Formation.140... 

The preparation of polymers from heterocyclic monomers that contain polymerizable functional groups undoubtedly constitutes the most common method of incorporating heterocycles into polymeric materials. Polymer-forming reactions are of two possible types addition reactions and condensation reactions. Addition monomers in general contain a site of unsaturation, i.e. a double or triple bond, through which polymerization occurs by successive single bond formation from one monomer to the next. With condensation monomers a bond is formed between two monomers with concomitant elimination of a... 

We return to single bond formation between As, Sb and Bi atoms later. 

Here, we postulate that gle bonds a formed ing together of atomic nuclei through attractive forces exerted by electrons having paired spins (t4) in overlapping orbitals. This formulation is no particular improvement over what is implied by Lewis structures, except in so far as it provides further appreciation that the electrons involved must have paired spins. Because only two paired electrons can occupy a given orbital, a clear reason i s provided as to why two electrons are involved in single-bond formation rather than 3, 5 or 10. This type of bond is called, in molecular-orbital parlance, a a bon ... 

Chiral oxazolidinones are the most popular auxiharies in the field of asymmetric synthesis. Chen and Sibi [76] employed this auxiliary for the domino asymmetric C-C/C-C single bond formation under convenient radical conditions (Scheme 5.51). During the reaction, the radical intermediates 240 was coordinated onto the Lewis acid Yb(OTf)3 at —78°C to form the desired s-cis conformation. Upon the treatment of this intermediate 240 by 2-methylallyl stannane, the product 241 was generated with a diastereoselectivity of >20 1. Similar radical conditions using a bromooxazolidinone imide source also resulted in allylated products with excellent diastereoselectivity (>50 1) and yield (>90%) even at room temperature. 

FIGURE 8.4 Single bond formation with cubic atoms. Reproduced from A. Stranges, Electrons and Valence,Texas A M University Press, College Station,TX, 1982, p. 212, with permission from the publisher. 

Benzene- and pyridine-derived aryl chlorides underwent C-N single-bond formation in high yields at 80°C employing KOtBu as base. 

Electron-rich and electron-deficient (hetero)arenes underwent C-N single-bond formation at room temperature. It was also demonstrated that K3P04-H20 is a viable base for this transformation, but high reaction temperatures (110°C) were required. 

Despite the exceptional efficiency of the biarylphosphines in the amidation of deactivated aryl chlorides, the C-N single-bond formation of ortho-substituted and... 

The arylation of urea and its derivatives is a challenging task, as the coordination of the substrate or the product can deactivate the catalyst. Nevertheless, a general method for the synthesis of unsymmetrical urea derivatives via Pd-catalyzed C-N single-bond formation was developed by Kotecki et al. [127]. A bipyrazole-derived electron-rich monodentate phosphine 61 was employed in the arylation of N-phenyl- and N-cyclohexyl-substituted urea with aryl bromides and chlorides (Scheme 13.81). 

The preparation of the PPV skeleton can be generally classified into two main strategies (i) olefinations methods to prepare the vinylene double bond and (ii) transition-metal-catalyzed single bond formation between the arene and vinylene components. The former includes Wittig-, Wittig-Horner- and Knoevenagel-type condensations, whereas the latter approach was demonstrated via Heck, Stille, Suzuki Miyaura and McMurry coupling reactions (Scheme 7.5). 

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