Ruthenium-catalyzed [2 + 2] cycloadditions between bicyclic alkenes and alkynyl halides.
TL;DR: Ru-catalyzed [2 + 2] cycloadditions between norbornadiene and alkynyl halides were found to occur in moderate to good yields and can be transformed into a variety of products that are difficult or impossible to obtain via directcycloaddition.
About: This article is published in Organic Letters.The article was published on 2004-11-02. It has received 55 citations till now. The article focuses on the topics: Norbornadiene & Moiety.
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TL;DR: This Account highlights some of the recent progress on the development of efficient and practical synthetic methods involving haloalkyne reagents in the laboratory and in others around the world, which showcase the synthetic power of haloalkynes for rapid assembly of complex molecular structures.
Abstract: Inspired by the need for green and sustainable chemistry, modern synthetic chemists have been seeking general and practical ways to construct complex molecules while maximizing atom economy and minimizing synthetic steps. Over the past few decades, considerable progress has been made to fulfill these goals by taking advantage of transition metal catalysis and chemical reagents with diverse and tunable reactivities. In recent years, haloalkynes have emerged as powerful and versatile building blocks in a variety of synthetic transformations, which can be generally conceived as a dual functionalized molecules, and different reaction intermediates, such as σ-acetylene-metal, π-acetylene-metal, and halovinylidene-metal complexes, can be achieved and undergo further transformations. Additionally, the halogen moieties can be retained during the reaction processes, which makes the subsequent structural modifications and tandem carbon-carbon or carbon-heteroatom bond formations possible. As a consequence, impressive effort has been devoted to this attractive area, and some elegant work has been done over the past several years. This Account highlights some of the recent progress on the development of efficient and practical synthetic methods involving haloalkyne reagents in our laboratory and in others around the world, which showcase the synthetic power of haloalkynes for rapid assembly of complex molecular structures. The focus is primarily on reaction development with haloalkynes, such as cross-coupling reactions, nucleophilic additions, and cycloaddition reactions. The designed approaches, as well as serendipitous observations, will be discussed with special emphasis placed on the mechanistic aspects and the synthetic utilities of the obtained products. These transformations can lead directly to heteroatom-containing products and introduce structural complexity rapidly, thus providing new strategies and quick access to a wide range of functionalized products including many synthetically useful conjugated cyclic and acyclic structures that have potential applications in natural product synthesis, materials science, and drug discovery. Importantly, most of these protocols allow multiple bond-forming events to occur in a single operation, thereby offering opportunities to advance chemical synthesis and address the increasing demands for economical and sustainable synthetic methods. We anticipate that a deep understanding of the properties of haloalkyne reagents and the underlying working mechanism can lead to the development of novel catalytic systems to answer the unsolved challenges in haloalkyne chemistry, which, in turn, may be also instructive for other research areas. We hope this Account will help to provide a guideline for researchers who are interested in this fertile area.
217 citations
196 citations
TL;DR: From previous reports, it is realized that after the formation of the nonclassical “norbornonium” cation, the C-7 functionalization can be achieved through a nucleophile rearrangement (Scheme 2), and success in synthesizing C- 7- is achieved.
Abstract: The development of efficient and sustainable procedures for the synthesis of complex molecules is an important task in modern organic chemistry. The direct cleavage of an alkynyl– halogen bond followed by the reconnection of both the alkynyl and halogen ions with the two carbon atoms of an unsaturated carbon–carbon bond provides ready access to highly functionalized products from simple alkynes with excellent atom economy. We previously achieved highly regioand stereoselective bromoalkynylation of internal alkynes for the synthesis of conjugated cis-bromoalkenynes. Subsequent research on this subject revealed a further use of bromoalkynes in complex molecule synthesis. Norbornene derivatives are an appealing group of organic molecules that are convenient starting materials for the synthesis of polymers, solar-energy-storage materials, and bioactive products. In addition, their strained structure and high potential to coordinate to transition metals, as well as their possible industrial applications have attracted considerable research interest. Thus, we decided to react phenylethynyl bromide (2a) with norbornene (1a), expecting to obtain 2-bromo-3-(2-phenylethynyl)bicyclo[2.2.1]heptane (3aa ; Scheme 1). However, when using the previously optimized conditions, we did not detect the formation of 3aa, instead we obtained 2-bromo-7-(2-phenylethynyl)bicyclo[2.2.1]heptane (3a) in excellent yield, as confirmed by H NMR spectroscopy. This unexpected result attracted our interest, since, to the best of our knowledge, no example of a direct 7-alkynyl bromonorbornane formation has been reported. From previous reports, we realized that after the formation of the nonclassical “norbornonium” cation, the C-7 functionalization can be achieved through a nucleophile rearrangement (Scheme 2). Our success in synthesizing C-7-
95 citations
TL;DR: A comprehensive review of recent research trends in the addition polymerization of functionalized norbornenes can be found in this article, where various functional side groups can be incorporated into the monomer units by the Ti-, Ni-, or Pd-catalyzed polymerization, which are available via cycloaddition reactions or norbornadiene-2,5 modifications.
89 citations
TL;DR: This direct coupling provides a rapid and convergent access to oxadiazole core pi-conjugated systems.
Abstract: The direct alkynylation reaction of 1,3,4-oxadiazoles with alkynyl bromides efficiently proceeds in the presence of a copper catalyst at room temperature to create the corresponding heteroaryl−alkynyl linkage in good yields. This direct coupling provides a rapid and convergent access to oxadiazole core π-conjugated systems.
88 citations
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TL;DR: The phytochemical remains of the seven-membered ring formation are still under investigation, but it is clear that the polymethine content of the ring is lower than previously thought, suggesting that it is more likely to be a mixture of 22π and 32σ.
Abstract: 5.7. [32π + 32σ] Cycloadditions 74 5.8. [44π + 22π] Cycloadditions 75 6. Seven-Membered Ring Formation 78 6.1. [44π + 32σ] Cycloadditions 78 6.2. [52π+2σ + 22π] Cycloadditions 79 7. Eight-Membered Ring Formation 79 7.1. [22π + 22π + 22π + 22π] Cycloadditions 80 7.2. [44π + 22π + 22π] Cycloadditions 80 7.3. [44π + 44π] Cycloadditions 81 7.4. [66π + 22π] Cycloadditions 83 8. Ten-Membered Ring Formation 85 9. Conclusion and Remarks 87
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160 citations