Direct transformation of silyl enol ethers into functionalized allenes

Adventures in 1,1-Diphenylacetone Dianion Chemistry

The present personalized account highlights reactions of the dianion of 1,1-diphenylacetone and its applications. From a study directed towards the regioselectivity of such reactions a new allene synthesis via allene dianions was found by serendipity. In addition, reactions of allene dianions with various electrophiles have been studied.

1 Introduction

2 Cyclizations of the Dianion of 1,1-Diphenylacetone

2.1 Reaction with Bis(cyclopentadienyl)zirconium Dichloride

2.1 Reactions with Dichlorosilanes

2.1 Reactions with Difluorosilanes

2.1 Reactions with Ketones and Dielectrophiles

3 Regioselectivity of Dianion Reactions

4 Allene Dianions from Silyl Enol Ethers

5 Scope of the Reactions of Allene Dianions

6 Reactions of Allene Dianions with Ketones

7 Reactions of Allene Dianions with Nitriles

8 Conclusions

One-Pot Assembly and Synthetic Applications of Geminal Acyl/Alkoxy Tetrasubstituted Allenes

Polysubstituted allenes are useful synthetic intermediates in many applications, offering structural complexity, modularity, and their axial chirality in further transformations. While acyl and alkoxy-substituted allenes are known, there are currently few examples of allenes containing both functionalities and no reports of geminally substituted acyl/alkoxy allenes being isolated and characterized. Herein, we report the synthesis of tetrasubstituted allenes featuring a novel geminal acyl/alkoxy substitution. These unique "push-pull" allenes are bench-stable and exhibit interesting reactivity in several applications.

Hydroalumination of Ketenimines and Subsequent Reactions with Heterocumulenes: Synthesis of Unsaturated Amide Derivatives and 1,3-Diimines

The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom. Furthermore, domino reactions with more than 1 equiv of the electrophile or by subsequent addition of two different electrophiles are possible and lead to polyfunctional amide derivatives of the biuret type which are otherwise not easily accessible.

Acid-catalyzed domino reactions of tetraarylbut-2-yne-1,4-diols. Synthesis of conjugated indenes and inden-2-ones

The reaction of tetraarylbut-2-yne-1,4-diols with electron-rich aromatic compounds at room temperature, under p-TsOH catalysis, affords substituted polycyclic aromatic indene derivatives through a domino reaction involving the formation of a cationic allenylium intermediate. This species can undergo a series of competitive intramolecular cascade reactions, leading to a conjugated inden-2-one. This simple method allows the efficient synthesis of substituted indenes and inden-2-ones, in two steps, from aromatic ketones.

Beyond the Divinyl Ketone: Innovations in the Generation and Nazarov Cyclization of Pentadienyl Cation Intermediates

The requirement for new strategies for synthesizing five-membered carbocycles has driven an expansion in the study of the Nazarov cyclization. This renewed interest in the reaction has led to the discovery of several interesting new methods for generating the pentadienyl cation intermediate central to the cyclization. Methods reviewed include carbon-heteroatom ionization, functionalization of a double bond, nucleophilic addition, or electrocyclic ring opening. Additional variations employ unconventional substrates to produce novel pentacycles, such as the iso- and imino-Nazarov. Herein, we provide an overview of these unconventional, yet highly useful versions of the Nazarov cyclization.