Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Rh-Catalyzed Transannulation of 1,2,3-Triazoles
The groups of Vladimir Gevorgyan and Valery Fokin from the University of Illinios and The Scripps Research Institute have reported on the synthesis of imidazoles via the transannulation of 1-sulfonyl triazoles with nitriles using rhodium(II) octanoate as catalyst (J. Am. Chem. Soc. 2008, 130, 14972. ). When applying microwave heating no inert atmosphere is necessary compared to performing the reactions under conventional heating at 80 °C where - in addition - longer reaction times of 12-24 h are needed. Furthermore, the authors were successful in combining the synthesis of 1-sulfonyl triazoles - via the Cu(I)-catalyzed cycloaddition of sulfonyl azides with alkynes - and the transannulation in a one-pot two-step sequence.
Synthesis of 4-Keto-4,5,6,7-tetrahydrobenzofurans
Alain Wagner, Rachid Baati and coworkers from Université Louis Pasteur, France, have disclosed the synthesis of 4-keto-4,5,6,7-tetrahydrobenzofurans via an intramolecular cyclization of triketones (Tetrahedron Lett. 2008, 49, ASAP. ). Key to the success was the employment of 4 equiv. TMSCl in MeOH as a source of anhydrous HCl. In general excellent product yields are obtained, only for 1,3-dicarbonyls with an amide functionality a lower yield (56%) of the corresponding oxazole is achieved. Whereas acyclic and 7-membered triketones furnished the products in high yields, 5-membered triketones were completely unreactive and only starting material was recovered.
One-Pot Sonogashira Coupling-NaOH-Mediated Cyclization
The synthesis of 2-substituted indoles by a one-pot two-step sequence starting from o-iodoanilines and terminal alkynes was described by the group of Roberto Sanz from Universidad de Burgos, Spain (Synlett 2008, 3006. ). The first step includes a Sonogashira coupling leading to o-alkynylanilines which were subsequently cyclized under mediation of NaOH to the 2-substituted indoles. Both steps were performed under microwave heating which had the advantage of employing less NaOH (4 vs. 10 equiv.) and reduced reaction times compared to conventional heating (1-2 h at rt for step 1 and up to 6 h at 140 °C for step 2). In addition, an arylthio group can be selectively introduced at the C-3 position in a one-pot three-step procedure.
o-Alkylation of Aryl Iodides with Secondary Alkyl Halides
Mark Lautens and coworkers from the University of Toronto have developed Pd-catalyzed norbornene-mediated intramolecular o-alkylations of aryl iodides with secondary alkyl iodides and bromides for the synthesis of polycylic heterocycles (J. Org. Chem. 2008, 73, ASAP. ). These alkylations proceed in a domino process with subsequent termination steps such as Heck reactions, direct arylations or Buchwald-Hartwig aminations in one-pot forming new C-C or C-N bonds. In the domino intramolecular o-alkylation/Heck reaction tricyclic heterocycles are formed with separable exo- and endocyclic double bond isomers for products with an O-containing 5-membered ring. Pyrrole, indole and thiophene containing substrates undergo an intramolecular o-alkylation/direct arylation sequence delivering tetra- and pentacyclic heterocycles. In addition, the reaction sequence occurs in a stereospecific manner which was validated by the use of enantioenriched substrates where a minimal loss of ee was obtained for phenol tethered alkyl iodides. However, secondary alkyl bromides gave the product in more dimished ee.