A palladium(II) catalyst,
in the presence of Selectfluor, enables
the efficient and chemoselective transformation of primary amides
into nitriles. The amides can be attached to aromatic rings, heteroaromatic
rings, or aliphatic side chains, and the reactions tolerate steric
bulk and electronic modification. Dehydration of a peptaibol containing
three glutamine groups afforded structure–activity relationships
for each glutamine residue. Thus, this dehydration can act similarly
to an alanine scan for glutamines via synthetic mutation.
The conversion of alkynes to their
corresponding vinyl triflates
in the presence of stoichiometric TMS-triflate was greatly facilitated
by the triflate salt of several transition metal catalysts most especially
Zn(OTf)2. Products are formed in high regioselectivity
under mild conditions. Internal alkynes bearing an aryl substituent
afford vinyl triflates with a modest preference for the Z-isomer especially with larger substituents. A mechanism is put forward
to explain the unique role of silicon in this system.
The dehydration of primary amides to generate their respective nitriles is an isohypsic, or redox neutral, method to install a valuable functional group into a molecule. This process has been known for over 175 years, typically using highly reactive dehydrating agents and high temperatures. Recently, transition metal-catalysis has enabled reactivity under milder reaction conditions, while also tolerating a wide range of functional groups. The removal of water from primary amides is still energetically uphill so a stoichiometric dehydrating agent is required. The dehydrating agents used in recent years can be rather benign, such as acetonitrile to generate acetamide. This Minireview focuses on metalcatalyzed reactions to convert primary amides into nitriles and is organized based on the dehydrating agent.[a] Dr. Figure 1. Bioactive compounds containing a nitrile. Scheme 1. Methods for dehydration of amides. 2 3 4 5 6 7 8
Conditions for the first palladium-catalyzed chemoselective protodecarboxylation of polyenoic acids to give the desired polyenes in good yields are presented. The reactions proceed under mild conditions using either a Pd(0) or Pd(II) catalyst and tolerate a variety of aryl and aliphatic substitutions. Unique aspects of the reaction include the requirement of phosphines, water, and a polyene adjacent to the carboxylic acid.
Phenylcyanocarbene was generated by the reaction of azide with a hypervalent iodonium alkynyl triflate and reacted in situ with 21 different carbocyclic and heterocyclic aromatic compounds. These reactions led to more complex products that frequently underwent subsequent rearrangements. The reactivity was further explored in a mechanistic study to ascertain the chemoselectivity and stereospecificity.
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