Imines orbitals

Stereospecific addition of CN at a chelated imine - orbital steering... 

The other reactant in a dipolar cycloaddition, usually an alkene or alkyne, is referred to as the dipolarophile. Other multiply bonded functional groups such as imine, azo, and nitroso can also act as dipolarophiles. The 1,3-dipolar cycloadditions involve four it electrons from the 1,3-dipole and two from the dipolarophile. As in the D-A reaction, the reactants approach one another in parallel planes to permit interaction between the tt and tt orbitals. 

Acyclic phosphazenes (phosphazo derivatives, phosphine imines, phosphoranimines) continue to attract interest. A review of the three coordinate materials, RN=PR =X has appeared. " Several molecular orbital calculations have been reported. An ab initio treatment of the PN energy surface suggests that this species is best regarded as having a dative phosphorus-nitrogen double bond rather than a triple bond and the phosphonitrene, once formed,... 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

Cycloadditions give rise to four-membered rings. Thermal concerted [2+2] cycloadditions have to be antarafacial on one component and the geometrical and orbital constraints thus imposed ensure that this process is encountered only in special circumstances. Most thermal [2+2] cycloadditions of alkenes take place by a stepwise pathway involving diradical or zwitterionic intermediates [la]. Considerably fewer studies have been performed regarding the application of microwave irradiation in [2+2] cydoadditions than for other kinds of cydoaddition (vide supra). Such reactions have been commonly used to obtain /1-lactam derivatives by cycloaddition of ketenes with imines [18-20,117,118],... 

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

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... 

The reaction mechanism proposed for the LiBr/NEta induced azomethine ylide cycloadditions to a,p-unsaturated carbonyl acceptors is illustrated in Scheme 11.10. The ( , )-ylides, reversibly generated from the imine esters, interact with acceptors under frontier orbital control, and the lithium atom of ylides coordinates with the carbonyl oxygen of the acceptors. Either through a direct cycloaddition (path a) or a sequence of Michael addition-intramolecular cyclization (path b), the cycloadducts are produced with endo- and regioselectivity. Path b is more likely, since in some cases Michael adducts are isolated. 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

Surprisingly, imino carbon shifts of strained 2 7/-azirines [431] do not differ substantially from those of open-chain imines (Section 4.9.3). On the other hand, the alkene carbon shifts of l-azabicyclo[2.2.2]oct-2-ene do not reflect the polarization characteristic of other enamines (Section 4.9.2) (+ )-M electron donation of the bridgehead enamino nitrogen cannot take place since the lone-pair orbital and the p orbitals of the it bond are... 

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

The diazomethane-imine addition can be interpreted in terms of the FMO theory.51 Calculations on diazoalkanes and benzylideneanilines lead to two orbital interactions, a HOMOdiazoalkane-LUMOimine and a LUMOdiazoalkane-HOMOiminc, but regardless of which one is the favored interaction, the coefficients of the atomic orbitals of the bonding atoms favor the formation of a 1,2,3- and not a 1,2,4-triazoline.338 And indeed, the 1,2,4-triazolines proposed by Mustafa327 have never been observed. 

The addition of diazoalkanes to alkenes and alkynes has been shown to be a HOMOdiazoalkane-LUMOdipo,arophi,e controlled reaction,5153 and because the orbital energies of imines are comparable to those of electron-poor olefins, the reactivity of diazoalkanes toward imines may also be considered a HOMOdiazoalkane-LUMOimine favored interaction.338... 

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