Phosphonium- and Benzotriazolyloxy-Mediated Bond-Forming Reactions and Their Synthetic Applications

Abstract

Phosphonium and benzotriazolyloxy (and related) intermediates are easily prepared by the reactions of cyclic amides and ureas with (1H-benzotriazol-1-yloxy)triaminophosphonium hexafluorophosphate related reagents. The former intermediates could also be made available using analogous phosphonium reagents prepared in situ or from commercial sources. These intermediates efficiently lead to carbon-nitrogen, carbon-oxygen, carbon-sulfur, and carbon-carbon bond formations through nucleophilic aromatic substitution reactions with various nucleophiles. A new ­reaction involving the generation of phenols in situ from arylboronic acids and oxygen under palladium(0) catalysis or with boronic acids and hydrogen peroxide is reviewed.

1 Introduction

2 Phosphonium-Mediated Nucleophilic Aromatic Substitution Reactions of Heterocyclic Systems

2.1 Phosphonium-Mediated Carbon-Nitrogen Bond Forming Reactions via Modified Appel Conditions

2.2 Phosphonium-Mediated Carbon-Nitrogen Bond Forming Reactions via Commercially Available Phosphonium Reagents

2.2.1 (1H-Benzotriazol-1-yloxy)tris(dimethylamino)phosphonium Hexafluorophosphate as an Activating Agent

2.2.2 (1H-Benzotriazol-1-yloxy)tripyrrolidinylphosphonium Hexafluorophosphate and Bromotripyrrolidinylphosphonium Hexafluorophosphate as Activating Agents

2.2.3 Solvent and Base Effects

2.3 Reactivity of Various Phosphonium Reagents

2.4 Phosphonium-Mediated Carbon-Oxygen, Carbon-Sulfur, and Carbon-Carbon Bond Forming Reactions

3 Benzotriazolyloxy-Mediated and Related Bond-Forming Reactions of Heterocyclic Systems

4 Phosphonium-Mediated Reaction Mechanisms

4.1 Stepwise Pathways via Phosphonium and 1H-Benzotriazol-1-ol (or Pyridotriazol-1-ol) Adducts

4.2 1H-Benzotriazol-1-ol (or Pyridotriazol-1-ol) Adduct Independent Pathway

5 Palladium-Catalyzed Heteroaryl Ether Formation from Benzotriazolyloxy- or Pyridotriazolyloxy-Substituted Heterocycles with Arylboronic Acids

6 Unusual 1H-Benzotriazol-1-ol Adduct Rearrangement

7 A Tentative Protection and Amination Strategy Involving a 1H-Benzotriazol-1-ol Adduct

8 Conclusion and Outlook

phosphonium - benzotriazoles - nucleophilic aromatic substitutions - aryl ethers - aminations

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While the toxicity profile of hexamethylphosphoramide (HMPA) is well established, that for tris(N,N-tetra-methylene)phosphoric acid triamide (TTPT) has not been reported to the best of our knowledge. Because of the close structural relationship of these compounds, experienced medicinal chemists could easily assume similar toxic effects until TTPT is tested or clear SARs have been established. Therefore, the same precautions should be taken when handling these compounds! One of the shared health concerns is that both liquid chemicals might cause respiratory problems, despite the fact that both liquids have relatively high boiling points (HMPA: 230-232 ˚C at 740 mmHg; TTPT: 140-142 ˚C at 0.1 mmHg) and reasonable flash points (HMPA: 144 ˚C, closed cup; TTPT: 112.8 ˚C, closed cup. See Aldrich material safety data sheets for HMPA (product number 52730) and TTPT (product number 93404).