Imines molecular 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,... 

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... 

Figure 2-27. The rc-molecular orbitals of an imine. The HOMO is occupied by a pair of electrons and the LUMO is vacant.

Let us consider a real case - the hydrolysis of an imine, R2C=NR The Tt-molecular orbitals of an imine are shown in Fig. 2-27. 

If the metal possesses two electrons in the appropriate d orbital, the molecular orbitals XF1 and 4 are filled. The important orbital is which is derived (in part) from the old 7t -level of the ligand. Placing electron density within this orbital results in a build-up of electron density in the tt-symmetry orbitals on the carbon atom of the imine. This will result in a repulsion being experienced by any incoming nucleophile, and a deactivation of the imine towards nucleophilic attack. 

Figure 2-29. Molecular orbital diagram showing the interaction of a metal d orbital with the 7t-mole-cular orbitals of an imine. If the metal possesses electrons in the relevant d orbital, there will be electron density in Tj.

Nucleophilic Reactions of Aromatic Heterocyclic Bases Heterocyclic aromatic compounds containing a formal imine group (pyridine, quinoline, isoquinoline, and acridine) also react readily with nucleophilic reagents. A dihydro-derivative results, which is readily dehydrogenated to a new heteroaromatic system. Since the nucleophile always attacks the a-carbon atom, the reaction formally constitutes an addition to the C=N double bond. An actual localization of the C=N double bond in aromatic heterocyclic compounds is incompatible with molecular orbital theory. The attack of the nucleophilic reagent occurs at a site of low 77-electron density, which is not... 

Recently, it has been shown1 by molecular orbital calculations that the ionized vinylamine 34 (Scheme 26) is more stable than the ionized ethylideneimine isomer 35 by 122 kJ mol"The reverse is true for neutral compounds, the imine being more stable than enamine by 22 kJ mol-1. Moreover, the conversion of 34 to 35 by 1,3 H-transfer requires 265 kJ mol-1 of critical energy. 

The stereochemistry of the 1,3-dipolar cycloaddition of the heteroaromatic iV-imines has been investigated in some detail by using the reaction of phenanthridine N-benzoylimine with a series of activated olefins such as JV-methylmaleimide, maleic anhydride, diethyl maleate, methyl acrylate, methyl methacrylate, and methyl trans-crotonate (e.g., Eq. 30).202 The adducts from the former three have the all-cis stereochemistry. These results are rationalized in terms of secondary molecular orbital interactions. With acrylates such stereospecificity is lost, suggesting that this effect is of lesser importance in these cases (see Table II). 

Mechanistic aspects of both the Ni and Pd di-imine catalysts systems have been addressed using density functional, molecular orbital, and molecular mechanics calculations in an extensive series of pa-pers. ° - 2 2 It is beyond the scope of this review to present a comprehensive account of this complex body of work, but a few features should be noted. In general, calculations have been consistent with the experimental observations available. Calculated barriers to insertion in the [(diimine)Pd(olefin)R]+ complexes are in reasonable agreement with experiment. For example, the calculated barrier to insertion in [(2,6-i-PrPh)2DABMe2]Pd(C2H4)CH3)]+ is 14.1 kcaF mol, which compares favorably with the experimentally determined barrier of 17.3 kcaFmol. The barrier to insertion in the Ni analogue was calculated to be 13.2 kcal/mol, which is very close to the measured barrier of 13.5 kcaFmol. ° Other calculations significantly underestimate this barrier. ... 

The imines derived from furan-2-carbaldehyde and 2-acetylfuran show the problem of (Ey, (Z)-isomers relative to the 0=N bond. A H and C NMR study on some A-alkylimines (52) has been performed <87SA(A)1055>. NMR spectra show that these imines exist only as a single (E)- form. The same conclusion has arisen from analysis of electronic absorption spectra of related compounds (R = H, R = Me) with the aid of molecular orbital calculations <84B0844>. 

Correspondingly, diazomethane (42) which is isosteric with 43 may be protonated at the terminal C and N atoms, respectively. Molecular orbital theoretical studies of the protonation of ketene (102) and diazomethane (101) have revealed that the products of the thermodynamically controlled protonation reactions in the gas phase are the C-protonated species. And it appears that in solution C protonation is still favored thermodynamically. C-protonated derivatives of ketenes (103) and diazocompounds (104) have been obtained in super-acid media at low temperatures. O-protonated ketene 43a in its most stable conformation of C symmetry (102) is Isosteric with ketene imine (49) and the most stable allenyl anion 11a (1 ). 

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