Fe-Catalyzed Cross-Dehydrogenative Coupling Reactions

Topics in Current Chemistry, Jul 2016

Cross-dehydrogenative coupling (CDC), which enables the formation of carbon–carbon (C–C) and C–heteroatom bonds from the direct coupling of two C–H bonds or C–H/X–H bonds, represents a new state of the art in the field of organic chemistry. Iron, a prominent metal, has already shown its versatile application in chemical synthesis. This review attempts to provide a comprehensive understanding of the evolution of cross-dehydrogenative coupling via iron catalysis, as well as its application in synthetic chemistry.

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Fe-Catalyzed Cross-Dehydrogenative Coupling Reactions

Leiyang Lv 0 1 Zhiping Li 0 1 0 Department of Chemistry, Renmin University of China , Beijing 100872 , China 1 & Zhiping Li Cross-dehydrogenative coupling (CDC), which enables the formation of carbon-carbon (C-C) and C-heteroatom bonds from the direct coupling of two C-H bonds or C-H/X-H bonds, represents a new state of the art in the field of organic chemistry. Iron, a prominent metal, has already shown its versatile application in chemical synthesis. This review attempts to provide a comprehensive understanding of the evolution of cross-dehydrogenative coupling via iron catalysis, as well as its application in synthetic chemistry. This article is part of the Topical Collection ''Ni- and Fe-Based Cross-Coupling Reactions'', edited by Arkaitz Correa. Cross-dehydrogenative coupling (CDC); Iron catalysis; C-C bond; C-X bond 1 Introduction The development of selective, efficient, sustainable, and environmentally benign synthetic methodologies for the formation of carbon–carbon (C–C) and C– heteroatom bonds is an area of great interest to chemists, and one that is being actively pursued. Carbon–hydrogen (C–H) bonds exist broadly in a variety of organic molecules. Catalytic functionalization of C–H bonds has evolved as a powerful tool for organic synthesis, which not only provides an atom-economic alternative method, but also opens new routes to the target molecules. This trend is evidenced by reports of the ever-increasing utilization of C–H bonds as substrates for cross-coupling reactions [1–6]. Among reported methods, crossdehydrogenative coupling (CDC), which enables C–C and C–heteroatom bond formation from the direct coupling of two C–H bonds or C–H/X–H bonds, has emerged as the most attractive—and also the most challenging [7–10]. It is worth noting that in most cases, hydrogen gas (H2) is not produced in CDC transformation, and an appropriate sacrificial oxidant is generally needed. The obvious benefit of this strategy is that there is no need for preparation and isolation of activated reagents, or for pre-functionalization of easily available chemicals, thus improving atom and step economy. However, the conundrum that chemists must confront is how to overcome the low reactivity of C–H bonds and achieve site-selective functionalization of one C–H bond in the presence of all others. Li et al. have pioneered work addressing this challenge, and have made significant contributions in developing a series of synthetic methodologies in this field [11–16]. Iron, as one of the most abundant metals, is particularly attractive given its low cost, non-toxicity, and environmentally benign character. Various iron complexes have been incorporated into biological systems, with resulting low toxicity that is critical in the pharmaceutical and food industries. In addition, versatile ironcatalyzed organic transformations have been achieved over the past few decades. Several instructive and significant reviews have been published on this fascinating chemistry from various perspectives [ 17–22 ]. The current review focuses mainly on the evolution of iron-catalyzed CDC through C–H bond oxidation. Advances in other metal-mediated and metal-free CDC reactions have already been well documented and are beyond the scope of this work. In general, iron-catalyzed CDC results mainly in the formation of C–C, C–N, and C–O bonds. This review is structured around the hybridization of both of the C– H coupling partners. We hope that this paper provides a comprehensive overview of this topic, sheds light on new perspectives, and inspires chemists to work towards further improving and expanding the application of CDC. 2 Coupling of C(sp3)–H with X(sp3)–H Iron-catalyzed direct C–H oxidation for the construction of C–C and C–X bonds (X=O, S, N, P, etc.), with its remarkable potential for step efficiency, atom economy, and environmental sustainability, has emerged as one of the most significant tools in synthetic organic chemistry. Oxidative C(sp)–H and C(sp2)–H cross-coupling for the formation of C–C bonds has garnered much attention and has seen great progress over the past decade. However, oxidative couplings involving C(sp3)–H bonds remain challenging, given their low reactivity and lack of suitable coordination site for the iron catalyst. The following section will focus on advances in iron-catalyzed CDC reactions involving C(sp3)–H bonds. These transformations are classified by the type of C–H bonds, including benzylic C–H bonds and C–H bonds adjacent to heteroatoms (Fig. 1). The general reaction pathway of iron-catalyzed CDC is depicted in Fig. 2. The reaction with benzylic substrates proceeds as follows (Eq. 1): the initial hydrogen abstraction of the substrate by the oxidant generates the carbon radical I. Then I is further oxidized by the Fe catalyst through single-electron transfer (SET) to give the radical cation II, which is trapped by a nucleophile to give the final product. The process involving C(sp3)–H bonds adjacent (...truncated)


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Leiyang Lv, Zhiping Li. Fe-Catalyzed Cross-Dehydrogenative Coupling Reactions, Topics in Current Chemistry, 2016, pp. 38, Volume 374, Issue 4, DOI: 10.1007/s41061-016-0038-y