Jt-bonding

Once a particular 6-membered ring is selected as a site for applying the Diels-Alder transform, six possible [4 + 2] disconnections can be examined, i.e. there are six possible locations of the Jt-bond of the basic Diels-Alder retron. With ring numbering as shown in 36, and... 

The alkyl derivatives of Au were discovered by W. J. Pope and C. S. Gibson in 1907 they include some of the most familiar and stable organo compounds of the group, and are notable for not requiring the stabilizing presence of jt-bonding ligands. They are of three types ... 

Scheme 1). Introduction of a jt bond into the molecular structure of 1 furnishes homoallylic amine 2 and satisfies the structural prerequisite for an aza-Prins transform.4 Thus, disconnection of the bond between C-2 and C-3 affords intermediate 3 as a viable precursor. In the forward sense, a cation ji-type cyclization, or aza-Prins reaction, could achieve the formation of the C2-C3 bond and complete the assembly of the complex pentacyclic skeleton of the target molecule (1). Reduction of the residual n bond in 2, hydro-genolysis of the benzyl ether, and adjustment of the oxidation state at the side-chain terminus would then complete the synthesis of 1. 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

Cyclization of an organolithium tethered to a suitably positioned carbon-carbon jt-bond is a thermodynamically favorable process that proceeds in a totally regioselective exo-fashion with a high degree of stereocontrol via a transition state in which the lithium atom is intramolecularly coordinated with the remote rc-bond.9 The stereochemical outcome of the cyclization of a substituted 5-hexenyllithium follows from the preference of the substituent to occupy a pseudoequatorial position in the chair-like transition state depicted below.7... 

In this case the excited molecules produced on interaction with radiation undergo spin reversal to yield a triplet state with a much longer lifetime than that of the singlet excited state. One or more jt-bonds are broken in the triplet state since one of the n-electrons affected is in an antibonding n molecular orbital. This means that the o-bond is free to rotate and cis and trans isomers can be formed next to each other on recombination of the double bond. 

Scheme 18 a,b Attraction between and the substituents of alkenes a a lone pair and b a phenyl Jt bond... 

Class 111-type behavior is representative of reactions in which jt bonds have to be broken. It is the typical behavior of reactions in which CO or N2 bond activation is rate limiting. 

Ethylene, C2H4, can adsorb in two modes the weaker Jt-bonded ethylene, in which the C=C double bond is above a single metal atom, or the stronger di-cr bonded ethylene in which the two C-atoms of the ethylene molecule bind to two metal atoms (Fig. 6.37). We consider the (111) metal surface. Hydrogen adsorbs dis-sociatively and is believed to reside in the threefold hollow sites of the metal. 

Neurock and coworkers [M. Neurock, V. Pallassana and R.A. van Santen, J. Am. Chem. Soc. 122 (2000) 1150] performed density functional calculations for this reaction scheme up to the formation of the ethyl fragment, for a palladium(lll) surface. Figure 6.38(a) shows the potential energy diagram, starting from point at which H atoms are already at the surface. As the diagram shows, ethylene adsorbs in the Jt-bonded mode with a heat adsorption of 30 kj mol and conversion of the latter into the di-a bonded mode stabilizes the molecule by a further 32 kJ mol . ... 

Spectroscopic evidence that ethylene preferably hydrogenates directly via the jt-bonded adsorbate and not via the more stable di-<7 bonded state has been provided by Somorjai and coworkers by in situ sum-frequency generation measurements [P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai,y. Am. Chem. Soc. 118 (1996) 2942]. 

The "Crystal Orbital Overlap Population" (COOP) [20] shows (Fig. 4) that all levels arising below the Fermi level are a and Jt bonding and the highest energy levels are ct and n antibonding however the specific COOP curves for each Mo-0 distance (Fig. 5) show a... 

Iron(II) complexes are often included in studies when complexes are prepared from a large number of different metal ions. 2-formylpyridine thiosemicarbazone, 5, forms brown [Fe(5)2A2] (A = Cl, Br) when prepared in ethanol and [Fe(5-H)2] from aqueous alcohol solution [156], All of these complexes are diamagnetic. The resonance Raman and infrared spectra of [Fe(5-H)2] were examined in detail [130] and coordination occurs via the pyridyl nitrogen, azomethine nitrogen and thiol sulfur. There is appreciable d-d sulfur-to-iron(II) Jt-bonding. Solution studies of iron(II) complexes of some 5-substituted-2-formylpyridine thiosemicarbazones have been reported [157], but no solids... 

The major breakthrough in the development of such RsE species was achieved several years ago by Sekiguchi et al., who reported the isolation and full identification of a homologous series of (t-Bu2MeSi)3E (E = Si, Ge, Sn) radicals without Jt-bond conjugation. " All of these radicals, kinetically and thermodynamically stabilized by the bulky electropositive silyl substituents, were prepared by the same very simple and straightforward method, which clearly danonstrated the generality of this synthetic approach. 

Silica has often been modified with silver for argentation chromatography because of the additional selectivity conferred by the interactions between silver and Jt-bonds of unsaturated hydrocarbons. In a recent example, methyl linoleate was separated from methyl linolenate on silver-modified silica in a dioxane-hexane mixture.23 Bonded phases using amino or cyano groups have proved to be of great utility. In a recent application on a 250 x 1-mm Deltabond (Keystone Scientific Belief onte, PA) Cyano cyanopropyl column, carbon dioxide was dissolved under pressure into the hexane mobile phase, serving to reduce the viscosity from 6.2 to 1 MPa and improve efficiency and peak symmetry.24 It was proposed that the carbon dioxide served to suppress the effect of residual surface silanols on retention. 

Ethyne has two jt bonding orbitals at right angles to each other and a resultant jt electron density that is cylindrically symmetric with respect to the internuclear axis. Complexes of ethyne with HF [133], HC1 [134], HBr [135], C1F [66], CI2 [47], BrCl [50], Br2 [92] and IC1 [95] have been characterised by... 

X-ray structural analyses reveal that the jt-bonding of dihalogens, halocar-bons and halides to arene donors and acceptors are characterized mostly by over-the-rim coordination in which the dihalogen acceptor generally follows the position of highest electron density on the aromatic donor, and the arrangement of halide donor mostly follows the LUMO shape of the aromatic acceptor. 

Allenynes 160 were also cyclized chemo- and regioselectively to methylen-eyclopentane derivatives 161 and 162 using Rh(acac)(CO)2 as the catalyst and silanes or alkoxysilanes as the reductant (Eq. 32) [96]. The major product resulted from initial insertion of the internal Jt-bond of the allene into the Rh-Si bond. Only 1,1-disubstituted allenes were used for this reaction others may show less selectivity for the internal Jt-bond of the allene. 

FIGURE 1.10 Various possible surface species on a Pt or Pd (111) surface. A and B represent possible locations of 1,2-di-a-Cj 2-cyclohexane, and C, D, and E represent possible locations of Jt-complexed Jt-C -cyclohexene. Full complements of hydrogens are assumed at each angle and terminal that is not either a- or Jt-bonded to a surface site as indicated by a small circle. Half-hydrogenated states, which are mono-a-C -adsorbed species (where n is the number of the carbon attached to the surface), would be represented by one small circle at the carbon bonded to a surface site. F, G, and I represent possible locations of Jt-C -cyclohexene. F shows the three carbons of the Jt-allyl moiety adsorbed in three adjacent three-point hollow sites and G shows it over one three-point hollow site, whereas I shows adsorption over the approximate tops of three adjacent atoms. (Note Label H is not used to avoid confusion with hydrogen, which is not shown.)... 

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