Pyridine compounds, nitrogen orbitals

CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

Fig. 1. Occupancy of 2sp2 (a, b, c) and 2p, (d) nitrogen orbitals by electrons ( ) in pyridine-like compounds. The orbital designated by c is non-bonding (the shape of the orbitals is somewhat distorted for clarity).

New examples of the preparation of stannylamines, largely by established methods, are shown in Equations (164) and (165). Sodium bis(trimethylstannyl)amine can be prepared from sodium amide and a stannylamine,499 and can act as the precursor to other bis- and tris(stannyl)amino compounds.499 500 The tristannylamines are planar about the nitrogen, with the unshared pair in a 2p orbital. Tris(chlorodimethylstannyl)amine, with five-coordinate tin, is close to centrosymmetrical, and reacts with pyridine to give a tetrastannabicyclo[3.1.1]heptane.501... 

If the nitrogen atom s orbitals in a pyridine-like compound are assumed to be sp2 hybridized, like carbon orbitals, the live 2s and 2p electrons obviously must be allotted to the individual orbitals as shown in Fig. 1. Overlap of the 2sp2 hybrid orbitals of the heteroatom with similar orbitals of the neighboring atoms results in the formation of (T-bonds overlap of its 2pz orbital with the 2pz orbitals of other... 

Two kinds of donors can be considered (1) a-donors that possess nonbonding electrons, such as various nitrogen bases (e.g. amines, pyridines, and nitriles), oxygen bases (e.g. alcohols, ethers, and carbonyl compounds), and organic sulfides and selenides, forming a-a complexes (2) tt-donors in which the donor function is performed by bonding n-orbitals such as in aromatic compounds (e.g. benzene and polycyclic aromatics), forming a-n complexes. 

The triplet state di-TC-methane reactivity of the methanoquinoline systems (61, 62) have been studied. The results of the irradiations are shown in Schemes 3 and 4 where it can be seen that each compound usually affords two products. The results indicate that the pyridine nitrogen does not manifestly alter the photochemical behaviour of the compounds. A detailed analysis of the reasons for the observed regioselectivity is made on the basis of molecular orbital theory. The di-ic-methane reactivity of the benzonorbornadienes (63) still excites considerable interest. Paquette and Burke have studied the triplet state reactivity of the derivatives (63) in an attempt to establish the influence of bridgehead substitution. Thus the irradiation of (63a) results in the formation of both isomers (64a) and <65a) in 42 and 58 It respectively. Similar yields are shown for the irradiation of (63b) when the two products are obtained in similar yields to the above. However, irradiation of the derivative (63c) results in the formation of only the cyclized product (64c) while the derivative with the bridgehead cyano (63d) yields both (64d) and (65d) but in 10 and 90 X yields respectively. The authors present arguments to explain the observed specificities leading to products of either bridgehead control or vinylic control of the biradical... 

Transition metal atoms coordinated by dithiocarbamate ligands through four sulfur atoms have both electronic (free d-orbitals) and steric (free octahedral sites) propensity to form adducts with nitrogen bases, since nitrogen atoms have one sp -hybrid orbital, which is occupied by the lone pair of stereochem-ically active electrons. To characterize newly prepared adducts of dimethyl-, diethyl- and morpholinedithiocarbamate zinc(II) complexes with cyclic N-donor bases, such as pyridine, piperidine, hexamethyleneimine and morpholine, compounds 28-35 have been prepared and studied by and CP/MAS... 

Note that the procedure described here can be extended without difficulty to atoms P which contribute severed orbitals and several p electrons of different symmetries carbon atoms in the sp hybridization state of acetylene and allene-type compounds 28>, heteroatoms with one n electron and a lone pair, like oxygen in the carbonyl group or nitrogen in pyridine 20,21). 

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