Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Rhodium-Catalyzed C-H Arylations
Kenichiro Itami and co-workers from Nagoya University have developed rhodium complex 1 as a catalyst precursor for the C-H arylation of heteroarenes with aryl iodides (J. Am. Chem. Soc. 2006, 128, 11748. ). Key to the success of the arylation is the use of P[OCH(CF3)2]3 as ligand due to its strongly π-accepting character. Various five-membered heteroarenes were selectively arylated either at the 2- (X = S, O) or 3-position (X = N). To show the generality of this method, anisole and 1,3-dimethoxybenzene were reacted with p-nitrophenyl iodide. For arylated anisole, a mixture of regioisomers was obtained (o:p = 29:71), whereas the latter was arylated regiospecifically at the 4-position in 76% yield.
Hydroformylation of Alkenes
The group of Maurizio Taddei from the UniversitÓ degli Studi di Siena has reported on the rapid synthesis of linear aldehydes via the hydroformylation of alkenes (Org. Lett. 2006, 8, 3725. ). By using a special adapter, the microwave reaction vial containing the reaction mixture was filled with syngas (CO/H2 1:1) up to an internal pressure of 40 psi (ca. 3 bar) and was subsequently subjected to microwave heating. The reaction protocol (Rh-catalyst, Xantphos, [bmim][BF4], toluene, 110 ░C, 4-6 min) was applicable to differently substituted alkenes to obtain the linear aldehydes without notable formation of the branched isomers. Importantly, the ionic liquid was essential for the reaction; without it only 60 ░C could be reached resulting in incomplete conversion of the alkene.
One-Pot Ugi Reaction and Intramolecular O-Arylation
The synthesis of highly functionalized dibenz[b,f][1,4]oxazepin-11(10H)-ones 1 and –carboxamides 2, respectively, has been disclosed by Wei-Min Dai and co-workers from Zhejiang University (Synlett 2006, 2099. ). The oxazepine derivatives 1 and 2 were obtained via a one-pot/two-sequence Ugi reaction and intramolecular O-arylation in good to excellent yields. It turned out that for the Ugi-reaction in the synthesis of derivatives 2 the acidity of the employed acid-component played an important role, resulting in a 49% overall yield for Ar2 = o-Br-Ph with a pKa of 2.85 and decreased yields for less stronger acids. Additionally, a post-modification of derivatives 1 was conducted via a Pd-catalyzed intramolecular N-arylation, which furnished derivatives 3 containing a 2-oxindole unit.
Bruce H. Lipshutz and co-workers from the University of California Santa Barbara have performed several heterogeneous cross-coupling reactions catalyzed by nickel-in-charcoal (Ni/C) (Chem. Asian. J. 2006, 417. ). In combination with microwave heating, the C-C and C-N cross-coupling reactions [Negishi (a), Suzuki (b), aminations (c) and the coupling of vinyl alanes with benzylic chlorides (d)] could be accelerated from several hours to minutes compared to conventional heating giving the products in excellent yields. A reduction of the NiII/C pre-catalyst with n-BuLi at room temperature to the active Ni0/C prior to introduction of the coupling partners is necessary, except for the Negishi-coupling. Here a direct use of NiII/C gives the same results than with Ni0/C.