Stereochemistry of nitrogen

Conjugated nitrogen, in compounds where it is bound to three other atoms, presents an interesting stereochemical problem which raises a number of points which were not covered in Chapter 1. 

In order to form bonds effectively in these directions, the nitrogen atom needs to use tetrahedral hybrid AOs (cf. methane. Section 1.10). We therefore need to promote the nitrogen atom into a valence state where its five valence electrons occupy tetrahedral hybrid AHs, each one-fourth s and three-fourths p (Fig. 3.27b). The effective occupation of the 2s and 2p AOs in such a state is then five-fourths 2s and fifteen-fourths 2p. To get there from the ground state involves in effect the promotion of three-fourths of an electron from the 2s AO to the 2p. The energy required to do this is supplied by the 

FIGURE 3.26. Distribution of formal charge (a) in the a-naphthylmethyl anion (b) in a-naphthylamine. 

The repulsion between the hydrogen atoms could be still further reduced if the molecule became planar. The nitrogen atom would then have sp hybridization (Fig. 3.27c) with three electrons in orbitals which are (js + Ip) and two in the remaining 2p AO. The effective valence state would then be sp. To get here from the tetrahedral configuration would require further promotion of one-fourth of an electron from the 2s to the 2p AO apparently the hydrogen atoms are sufficiently content with their situation when nitrogen is tetrahedral for this further promotion not to be worthwhile. There is therefore a barrier to inversion in ammonia and in its alkyl derivatives of about 6 kcal/mole. This is the energy required to convert the pyramidal molecule into the planar intermediate. 

Since only the 2py AO can Tc-bond to the adjacent carbon atom, the overlap of T with the carbon 2p AO is thus 0.82 times the overlap of 2p AOs in Fig. 3.28a. Since the CN resonance integral should be more or less proportional to the overlap of the AOs in question, the n interaction energy for tetrahedral 

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So far in our study of organic chemistry, we have devoted considerable time to the spatial arrangement of atoms and groups attached to carbon atoms, that is, to the stereochemistry of carbon. Now let us look briefly at the stereochemistry of nitrogen. 

The last three, the NH2, NH , and N ions, are found in the salt-like amides, imides, and nitrides of the most electropositive metals, but with the exception of the amide ion the stereochemistry of nitrogen is based on N with no lone pairs and N with one lone pair. These two states of the nitrogen atom correspond to the classical trivalent and pentavalent states, now preferably regarded as oxidation rather than valence states. 

The stereochemistry of nitrogen is summarized in Table 18.1, in which the primary subdivision is made according to the number of orbitals used by bonding and lone pairs of electrons. The first main subdivision includes the simpler hydrogen and halogen compounds, and we shall deal with these first in the order of Table 18.1, except that amides and imides will be included with ammonia. The second and third sections include most of the oxy-compounds, and since it is... 

Benzil acts like a ketone in that it forms oximes with hydroxylamine-The oximes are of exceptional interest, since our knowledge of the stereochemistry of nitrogen proceeds from them. Benzil forms two monoximes and three dioximes. The constitution of these compounds will be discussed later, under the preparation of benzophenone-oxime ... 

Although nitrogen is in a similar position to carbon in being more a ligand atom than a central atom, there are a number of structures which have particular relevance to the stereochemistry of nitrogen and are worthy of discussion in this section. 

The chemistry and stereochemistry of aminoboranes containing the siLicon—nitrogen—boron linkage have been the subject of numerous studies. Many of these compounds are useful precursors to other B—N systems including diboryl-amines (45) and B—H substituted aminoboranes (46). A series of... 

Several factors influence the diastereoselectivity of the Pictet-Spengler condensation to form 1,3-disubstituted and 1,2,3-trisubstituted tetrahydro-P-carbolines (39 and 40, respectively). The presence or absence of an alkyl substituent on the nitrogen of tryptophan has a large influence on the relative stereochemistry of the tetrahydro-P-carboline products formed from a condensation reaction with an aldehyde under various reaction conditions. 

The regioselective ring-opening of vinyloxiranes by nitrogen nucleophiles offers an attractive route to vie-amino alcohols, compounds of much recent interest. As with oxygen nucleophiles, the stereochemistry of the reaction can be controlled by choice of reaction conditions aminolysis of 25, for example, affords anti-amino alcohol 26 in excellent yield and diastereoselectivity (Scheme 9.21) [48, 96, 97], and... 

Tables XXVIII and XXIX, respectively. Excellent correlations were obtained for all three sets. The stereochemistry of the sy -methyl ketoximes is discussed by Charton and Charton (73). The values of pj obtained for the trans-heterovinylene sets are not in good agreement with each other. Two sets gave values of 54 and 55, respectively, and the third set gave a value of 35. The difference in pj values cannot be accounted for. A value of 54 to 55 for pj suggests the possibility of some exaltation between substituent and reaction site such as that which occurs in para-substituted phenols and anilines. To demonstrate this with certainty requires that the value of pj be determined for a set of imines bearing a reaction site on the nitrogen which will not interact strongly with substituents. No such set of data is extant in the literature at the present time.

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