An Overview on Ynamines
Alkynes represent one of the most important and versatile building blocks in organic synthesis. Heteroatom-substituted alkynes, which can be considered as subgroups of alkynes, have also been vastly utilized in developing synthetic methods. In particular, ynamines [1-amino-alkynes or N-alkynyl amines] became the most valuable subgroup of alkynes after the establishment of their practical synthesis in the 1960's. The first attempt at preparation of an ynamine was reported by Bode1,2 in 1892. While well-characterized ynamines were reported in 19583 and 1960,4 a practical synthesis was not achieved until the effort led by Viehe5 in 1963 in addition to other subsequent works. In the ensuing twenty years, the synthetic significance of ynamines in organic and organometallic chemistry was firmly established by the work of many creative synthetic chemists. These elegant pioneer works have been informatively and carefully reviewed by Viehe in 19676 and 1969;7 Ficini in 1976;8 Pitacco and Valentin9 in 1979; Collard-Motte and Janousek10 in 1986; Himbert11 in 1993; and most recently by us12,13 and Katritzky14.





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The synthetic eminence of ynamines is well merited because of the predicable regioselectivity in their transformations as shown by the generalization in Scheme i, and more importantly, because they are inherently highly reactive. However, this latter attribute is also the source of the limitation that has seriously hampered the development of ynamine chemistry, thereby shortening the period of its prominence in synthesis. Ynamines are very sensitive toward hydrolysis, as protonation of the electron-rich alkynyl motif affords reactive keteniminium intermediates, which upon trapping with water leads to simple amides in a rather expensive manner (Scheme i). This hydrolytic instability has caused much difficulty in the experimental preparation and general handling of ynamines, and more detrimentally, rendered ynamine chemistry inaccessible.




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Scheme i



Consequently, the synthetic utility of ynamines has suffered a dramatic decline during the last thirty years.15 The most glaring limitations have been in the development of intramolecular and stereoselective reactions.7–14 The only reported intramolecular reaction of ynamines was Genet and Kahn's acid catalyzed addition of a hydroxyl group to an ynamine [i→ii in Scheme ii] in 1980,16 and although clever, it constitutes a hydrolytic process.






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Scheme ii



Besides Reinhoudt's17 sole account in 1987 reporting hetero-[4 + 2] cycloadditions of chiral ynamine iii with nitroalkenes that led to cycloadducts iv in modest de, the only other notable studies were reported ten years later by Fischer18 showcasing [2 + 2] cycloadditions of chiral ynamides v and vi with vinylidene chromium carbene complexes, and another three years later by Pericas19 in their Pauson-Khand cycloadditions using chiral ynamines vii.

Ynamides: A Modern Functional Group For The New Millennium

Ynamides display an exceptionally fine balance between stability and reactivity. They also offer unique and multiple opportunities for the inclusion of nitrogen-based functionalities into organic molecules, and are emerging as especially useful and versatile building blocks for organic synthesis. Recent breakthroughs in the preparation of these substrates have revitalized interest in nitrogen-substituted alkynes, and the beginning of the 21st century has witnessed an ever-increasing number of publications reporting the development of new reactions or synthetic sequences starting from ynamides. This Review highlights major developments in this area.

Ynamides: versatile tools in organic synthesis.

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.

http://pubs.acs.org/doi/pdf/10.1021/ar5001499

Haloalkynes: a powerful and versatile building block in organic synthesis.

We report here infrared spectra of protonated and lithiated valine with varying degrees of hydration in the gas phase and interpret them with the help of DFT calculations at the B3LYP/6-31++G** level. In both the protonated and lithiated species our results clearly indicate that the solvation process is driven first by solvation of the charge site and subsequently by formation of a second solvation shell. The infrared spectra of Val·Li+(H2O)4 and Val·H+(H2O)4 are strikingly similar in the region of the spectrum corresponding to hydrogen-bonded stretches of donor water molecules, suggesting that in both cases similar extended water structures are formed once the charge site is solvated. In the case of the lithiated species, our spectra are consistent with a conformation change of the amino acid backbone from syn to anti accompanied by a change in the lithium binding from a NO coordination to OO coordination configuration upon addition of the third water molecule. This change in the mode of metal ion bindin...

Infrared spectroscopy of hydrated amino acids in the gas phase: protonated and lithiated valine.

Silver-Catalyzed Csp−H and Csp−Si Bond Transformations and Related Processes

Ruthenium-catalyzed [2 + 2] cycloadditions of ynamides.

Asymmetric induction in ruthenium-catalyzed [2+2] cycloadditions between bicyclic alkenes and a chiral acetylenic acyl sultam.

1,2-Naphthalene oxides and imines can be rapidly accessed through a ruthenium-catalyzed isomerization of readily available 7-oxa/azabenzonorbornadienes. These mild reaction conditions were found to be tolerant to various functional groups and the isomerization is highly regioselective.

Karine Villeneuve

Papers

Ruthenium-catalyzed [2 + 2] cycloadditions of ynamides.

Asymmetric induction in ruthenium-catalyzed [2+2] cycloadditions between bicyclic alkenes and a chiral acetylenic acyl sultam.

Ruthenium-catalyzed isomerization of oxa/azabicyclic alkenes: an expedient route for the synthesis of 1,2-naphthalene oxides and imines.

Ruthenium-catalyzed [2 + 2] cycloadditions between bicyclic alkenes and alkynyl halides.

Rhodium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides.