Palladium catalysis cyclization

Transition-Metal Catalyzed Cyclizations. o-Halogenated anilines and anilides can serve as indole precursors in a group of reactions which are typically cataly2ed by transition metals. Several catalysts have been developed which convert o-haloanilines or anilides to indoles by reaction with acetylenes. An early procedure involved coupling to a copper acetyUde with o-iodoaniline. A more versatile procedure involves palladium catalysis of the reaction of an o-bromo- or o-trifluoromethylsulfonyloxyanihde with a triaLkylstaimylalkyne. The reaction is conducted in two stages, first with a Pd(0) and then a Pd(II) catalyst (29). 

A few synthetic applications of palladium catalysis appeared this year. The palladium-catalyzed cyclization of amino allenes 58 occurs with coupling of aryl iodides or vinyl triflates at the 3-position <990L717, 99SL324>. The cyclization can also proceed by the exo-trig pathway, but under suitable reaction conditions the piperidine 59 is prepared selectively. The intramolecular cyclization of amines onto N-allylbenzotriazoles similarly affords piperidines <99JOC6066>. 

The cyclization of alleneamines under palladium catalysis proceeds with coupling at the 3-position with aryl iodides and vinyl triflates <19990L717, 1999SL324>. The cyclization can also proceed by the 4-a o-/ttg-pathway, and, under suitable conditions, piperidines are produced exclusively (Equation 28). 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

Oxygen heterocycles can be prepared in several different ways via palladium catalysis, even though oxygen is a poorer nucleophile than nitrogen. For example, p, y-alkynic ketones are cyclized to fhrans in the presence of Pd(dba)2 and PPhs (equation 96). 

Palladium catalysis has also been used, at least indirectly, in polycyclizations of acyclic dienynes or enediynes. While the mechanisms of these cyclizations have not been rigorously proven, the results are consistent with a stereoselective 1,6-ECRC as one of the key carbon carbon bond forming events. For example, it has been reported111 that the palladium(0)-mediated cycliza-tion of 79 affords the chiral tricyclic compound 81 in 67 % yield and > 95 % diastereomeric... 

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