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Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)

Thursday, April 15, 2004
C. Oliver Kappe
Karl-Franzens-University Graz

Hydrozirconation-Transmetalation-Aldimine Addition Sequence

Hydrozirconation is a mild method for the selective preparation of functionalized organometallics and its compatibility with a range of common protecting groups represents a considerable advantage of these species over traditional organometallic reagents. The group of Peter Wipf at the University of Pittsburgh recently reported (Org. Biomol. Chem. 2004, 2, 443. ) that the hydrozirconation of alkynes (e.g. 1) with zirconocene hydrochloride (2) can be greatly accelerated by microwave irradiation. A synthetically useful one-pot method for the preparation of allylic amides 4 was elaborated where an alkyne was first hydrozirconated by microwave irradiation, followed by rapid addition of imines in the presence of dimethyl zinc.

Ring-Closing Metathesis Reaction

Daniela Balan and Hans Adolfsson of Stockholm University have demonstrated (Tetrahedron Lett. 2004, 45, 3089. ) that functionalized 2,5-dihydropyrroles can be obtained by microwave-mediated ruthenium-catalyzed ring-closing metathesis (RCM). The required bis-olefin precursors 5 were conveniently obtained from aza-Baylis-Hillman adducts. Microwave irradiation for 1-2 min at 100 C of a dilute solution of the diene with 5 mol% Grubbs II catalyst in dichloromethane produces the desired pyrroles 6 in high yield.

Hydration of Alkynes in Superheated Water

Anil Vasudevan and Mary K. Verzal at Abbott Laboratories have found (Synlett 2004, 631. ) that terminal acetylenes can be hydrated under neutral, metal-free conditions using water as solvent. While typically this reaction requires a catalyst such as AuBr3, employing microwave-irradiated superheated water allowed this chemistry to proceed without any catalyst in distilled water. Extension of this methodology led to a one-pot conversion of alkynes to imines (hydroamination, e.g. 7 to 8).

Nucleophilic Aromatic Substitution

A recent report by Amy Takvorian and Andy Combs of Incyte Corporation (J. Comb. Chem. 2004, 6, 171. ) discloses the rapid synthesis of 2-amino-substituted purines by rapid, microwave-assisted nucleophilic aromatic substitution (SN2Ar). Importantly, the authors describe the use of small scale reaction vessels (0.2 mL) for optimization of reaction conditions under optimal reaction concentrations.

C. O. Kappe, Org. Chem. Highlights 2004, April 15.