Chapter 1
Introduction

Wei Zhang and Shu-Li You

1.1 Why Asymmetric Dearomatization Reactions?

Arenes and heteroarenes are widely distributed in nature, and some simple arenes are produced multimillion metric tons annually. They are recognized as fundamental synthetic materials in both academia and industry. The chemistry involving aromatic compounds is thus rich and of prime importance. Tremendous efforts have been devoted to various substitution reactions of aromatic compounds, and many name reactions such as Friedel-Crafts reaction and Sandmeyer reaction have become elementary contents of organic chemistry textbooks. These fully developed processes are undoubtedly essential tools for the total synthesis of natural products. Moreover, they provide chemists with the access to a huge library of aromatic compounds, which are extremely important in the discovery of pharmaceuticals, materials, and other functional molecules. In contrast, dearomatization reactions, another important branch of transformations of aromatic compounds, have been undervalued for a long time despite their potentials to convert simple molecules into complex structures. The specific feature of building quaternary carbon centers and interesting structures makes them straightforward protocols to construct spiro or bridged compounds. In spite of a long history of application in the total synthesis of natural products, only recently the systematic methodology exploration of dearomatization reactions has received huge interest. In this area, asymmetric dearomatization reactions are of particular importance due to the great demand of highly efficient strategies toward the construction of complex chiral molecules.

1.2 Discovery of Aromatic Compounds and Dearomatization Reactions

Although aromatic compounds exist widely in nature, it was only in 1825 that benzene was first isolated by Michael Faraday. Several years later, Eilhard Mitscherlich also obtained this substance and identified its molecular formula as C6H6. He gave it the name "benzin." The highly unsaturated structure of benzene remained controversial for a long time, and various proposed structures were full of imagination. In 1865, Friedrich A. Kekulé postulated that benzene contains a six-membered ring of carbon atoms with alternating single and double bonds. Today, the famous Kekulé formula is still widely used. In 1931, by quantum mechanical calculations, Erich Hückel explained the unique stability of benzene different from other unsaturated hydrocarbons, namely, aromaticity. He differentiated the bonding electrons as p electrons and s electrons, and the Hückel "4n + 2" rule becomes a basis for estimation of aromatic compounds. Around the same time with or after the discovery of benzene, several other aromatic compounds have been discovered, and today, most of them have become extremely important industrial chemical starting materials (Figure 1.1).

Figure 1.1 The representative arenes and heteroarenes and their discovery years.

Great demand on diverse arenes and heteroarenes has stimulated the related synthetic chemistry. Besides some very simple ones coming from petrochemicals (e.g., benzene, toluene, xylene, phenol), the vast majority of aromatic compounds need to be synthesized. Fundamental electrophilic aromatic substitution reactions including nitration, sulfonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation are frequently used synthetic tools, and the nucleophilic aromatic substitution reactions via diazonium ions also provide reliable routes to complicated aryl compounds. Moreover, recent development of transition-metal-catalyzed cross-coupling reactions between aryl halides (or equivalent) and organometallic compounds has enriched the transformations of aromatic compounds, and the direct use of aryl compounds through C-H functionalization is an alternative straightforward method. Meanwhile, the chemistry of heteroarene synthesis has also gained significant progress.

Compared with the chemistry to synthesize aromatic compounds, dearomatization reactions also have a long history. In 1885, the Buchner ring expansion of benzene with ethyl diazoacetate was reported to provide cycloheptatriene under thermal or photochemical conditions, and this methodology was further improved by the introduction of transition metal catalysts (Scheme 1.1) [1, 2]. The Birch reduction was first reported in 1944 to partially hydrogenate benzene to 1,4-cyclohexadiene (Scheme 1.2) [3]. The Reimer-Tiemann reaction discovered in 1876 was originally used for the ortho-formylation of phenols, and an interesting phenomenon was observed later that a dearomative by-product was generated when para-methyl phenol reacted with dichlorocarbene (Scheme 1.3) [4].

Scheme 1.1 Buchner ring expansion reaction (first reported in 1885).

Scheme 1.2 Birch reaction (first reported in 1944).

Scheme 1.3 Reimer-Tiemann reaction.

1.3 Development of Dearomatization Reactions

The history of dearomatization reactions can be dated back to the nineteenth century. Early studies on dearomatization reactions include photochemical processes, transition-metal-mediated processes, hydrogenation processes, enzyme-catalyzed processes, and so on.

In 1957, Blair and Bryce-Smith discovered that fulvene was generated by subjecting pure benzene under the irradiation conditions (Scheme 1.4, eq 1) [5]. Although the conversion of this dearomatization reaction was low, it was believed to be the "first example of the direct isomerization of an aromatic to a nonaromatic hydrocarbon" [5]. Soon after, many related studies emerged, such as the irradiation of substituted benzenes (Scheme 1.4, eq 2) [6]. Meanwhile, the photochemical reaction between benzene and alkene was also investigated (Scheme 1.4, eq 3) [7]. However, this type of dearomatization reaction was not developed into an applicable level due to the multiple reaction pathways to deliver complicated mixtures of dearomatized products.

Scheme 1.4 Dearomatization reactions via photochemical process.

Early studies of transition metal-arene complexes in the 1950s were emphasized on their preparation, as exemplified by the synthesis of C6H5Cr(CO)3 by Fischer in 1957 [8]. Activation of the aromatic ligands by transition metal centers toward nucleophiles was then discovered and explored [9]. Among the versatile transformations of the nucleophilic addition intermediates (Scheme 1.5, A), protonation and other electrophilic trapping generally deliver dearomatized products [10]. However, this type of dearomatization reaction is mediated by stoichiometric amount of transition metal complexes, and recent rapid growth is focused on transition-metal-catalyzed dearomatization processes. Chapters 4, 6, 7, and 9-11 elucidate the detailed development of transition-metal-catalyzed asymmetric dearomatization reactions.

Scheme 1.5 Dearomatization through arene metal complexes.

Dearomatization reactions of aromatic compounds by hydrogenation process (Scheme 1.2) and enzymatic process also have a long history. Chapters 3 and 4 introduce organocatalytic and transition-metal-catalyzed asymmetric dearomatization reactions by hydrogenation process, respectively. Chapter 12 is devoted to the development of enzymatic dearomatization reactions including the details on both history of discovery and current status.

Meanwhile, some interesting approaches taking advantage of the steric effect provide alternative strategies for dearomatization. For instance, Yamamoto and coworkers designed the bulky Lewis acid ATPH that enabled the addition of tBuLi to phenyl methyl ketone occurring at the para position of the phenyl ring, delivering dearomatized product in excellent yield (Scheme 1.6) [11].

Scheme 1.6 Alternative strategy for dearomatization processes.

As attractive strategies, dearomatization reactions are frequently applied to the total synthesis of natural products, providing more efficient synthetic routes, in many cases biomimetic synthesis as a result of inspiration by nature. In 1954, Woodward and coworkers first reported an elegant total synthesis of strychnine, in which the Pictet-Spengler-type dearomatization of indole core was employed as one key step to construct the complex polycyclic framework (Scheme 1.7) [12]. In 1960, Day and coworkers first completed the total synthesis of racemic griseofulvin, in which the dearomatization step was inspired by its biosynthetic pathway (Scheme 1.8) [13]. Numerous fabulous total syntheses were then reported with the application of dearomatization strategy, and Chapter 13 introduces this topic in detail.

Scheme 1.7 Dearomatization step in the total synthesis of strychnine .

Scheme 1.8 Dearomatization step in the total synthesis of griseofulvin.

Despite the wide utilization of dearomatization reactions in total synthesis of natural products, the systematic studies to develop practical dearomatization methodologies are still rare. Many of the known dearomatization methods are limited with scope and selectivity, and...