N-Jt bonding

M = Na, K, Rb, Cs, or Ag) D4h symmetry. Spectra indicate that metal—N Jt-bonding is more pronounced for Pt than ... 

So how does the Mitsunobu reaction work The first step involves neither the alcohol nor the nucleophile. The phosphine adds to the weak N=N Jt bond to give an anion stabilized by one of the ester groups. 

An electrocyclic reaction is one in which a jt-system consisting of n Jt bonds interconverts with a cyclic system with (n-1) jt-bonds and one extra Jt-bond. 

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. 

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... 

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. 

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... 

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.)... 

Figure 2.12 Interaction of a C=C bond and a carbonyl group in propenal. The Jt — it band, shown as transitions (a) and (b), occurs at lower energy (longer wavelength) as a result of conjugation. The n —> Jt, band shown as transitions (c) and (d), likewise occurs at longer wavelength as a result of conjugation...

In order to understand the origin of the breakdown of the SR methods away from equilibrium, consider the torsional potential in ethylene (Figure 2). While at its equilibrium geometry ethylene is a well-behaved closed-shell molecule whose ground and n-valence excited states can be described accurately by SR models (except for the doubly excited Z-state), it becomes a diradical at the barrier, when the Jt-bond is completely broken (13). Thus, at the twisted geometry all of ethylene s Jt-valence states (N, T, V, and Z) are two-configurational, except for the high-spin components of the triplet. 

It is hardly surprising that the high-spin T state is not affected by the 7t-bond breaking indeed, since both n electrons in the T state are of the same spin, there is no Jt bond in this state to begin with ... 

This makes the acetylene molecule linear, i.e. bond angles of 180°, and there are two n bonds with electron density either side of this axis. The properties of an alkyne, like acetylene, are also special in that the Jt bonds are again much more reactive than the a bond. 

When we consider double bonds to oxygen, as in carbonyl groups (C=0) or to nitrogen, as in imine functions (C=N), we find that experimental data are best accommodated by the premise that these atoms are sp hybridized (Figure 2.22). This effectively follows the pattern for carbon-carbon double bonds (see Section 2.6.2). The double bond is again a combination of a ct bond plus a jt bond resulting from overlap of p atomic orbitals. The carbonyl... 

To achieve this stabiiization, the p orbitai on nitrogen needs to be iined np with the carbonyi jt bond. The immediate conseqnences of this are that five bonds in the peptide iinkage must be copianar. There is no free rotation about the N-C bond, because it is invoived with a partiai doubie-bond system. Of... 

Our analysis of the changes in the SC wavefunction along the reaction path strongly suggests that the two tt bonds on the butadiene fragment and the ethene Jt bond break simultaneously, and that the formation of the two new o bonds that close the cyclohexene ring and of the cyclohexene n bond also takes place almost in parallel. If we wish to express all of this using full- or half-arrows, it would be most appropriate to use half-arrows, as in scheme B from the Introduction ... 

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