The increasing order of basicity of the following compounds is
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Basicity of nitrogenous bases is directly proportional to:
1. Availability of the lone pair on the nitrogen atom.
2. Stability of the conjugate acid formed after protonation.
Extended conjugation of the lone pair with double bonds generally decreases direct basicity unless it creates a highly stabilized, symmetric iminium system upon protonation.
Step 1: Understanding the Question:
We need to compare the basicity of the three given nitrogen-containing bicyclic compounds (I, II, and III).
Step 2: Detailed Explanation:
Let us evaluate the availability of the nitrogen lone pair for protonation and the stability of the resulting conjugate acid:
- In compound I: The lone pair of nitrogen is highly conjugated with the adjacent diene system inside the ring. In fact, if we look at the pyrrole-like/cyclopentadiene-like fusion, the nitrogen lone pair is involved in a highly conjugated system, which significantly reduces its availability for protonation, making it the least basic.
- In compound III: There is one double bond conjugated with the amine group (enamine system). The lone pair is partially delocalized, but less so than in the diene conjugated system of I. Therefore, III is more basic than I.
- In compound II: The structure allows for exceptional resonance stabilization of the conjugate acid (protonated form). When protonation occurs on the ring carbon at the terminal end of the conjugated diene system, the positive charge is stabilized through a long-range resonance involving the nitrogen lone pair, which produces a highly stable iminium ion. Alternatively, if we consider direct protonation, the nitrogen's lone pair is coupled with the cyclopentadiene ring in a way that makes the system highly basic (similar to highly basic guanidines or vinylogous systems). This makes II exceptionally basic.
Therefore, the correct increasing order of basicity is:
\[ \text{I} < \text{III} < \text{II} \]
