Stille cross-coupling conditions

Sometimes organoboranes or stannanes are either unavailable or too unstable under Suzuki or Stille cross-coupling conditions. The Sonogashira reaction, however, does not involve the synthesis or handling of stoichiometric amounts of M -R... 

Despite being activated by the nitrogen atom, 2-chloroquinoline (25) is still a poor substrate for the Stille cross-coupling reactions, though yields are usually improved under Negishi conditions. For instance, the coupling of... 

Stille cross coupling reactions usually proceed under mild neutral conditions. 2-Substituted thiazoUnes can be obtained by the cross coupling reaction of 2-bromothiazolines with various tributylstannyl compounds. Previous attempts at a palladium mediated coupling of 2-trimethylstannylthiazoUne led to only decomposition of the substrate <96TL4857>. 

A concise synthesis of photoactivatable 4-benzoyl-L-phenylalanines and related peptides was described by G. Ortar et al. using a carbonylative Stille cross-coupling as the key step. Surprisingly, when the coupling was attempted with tyrosine triflate derivatives, it proved to be unsuccessful. However, 4-iodo-phenylalanine derivatives reacted smoothly under standard conditions to give the corresponding 4-benzoyl derivatives. 

In another study, an allyltin anchored on a macroporous polystyrene added to the aldehyde of p-bromobenzaldehyde under Lewis acid conditions, but underwent a Stille cross-coupling using Pd(0) catalysis, and in both cases the reactions were completely chemoselective. ... 

Numerous works on the optimization of [ F]-fluoridations and the preparation of specific [ F]-labeled radiotracers using diaryliodonium salts have been published. Wiist and coworkers have developed a convenient access to 4-[ F]fiuoroiodobenzene (28) employing 4,4 -diiododiaryliodonium salt 27 as a precursor (Scheme 7.8) [71-73]. 4-[ F]Fluoroiodobenzene (28) has been further utilized in Sonogashira or Stille cross-coupling reactions for the preparation of numerous radiotracers. For example, the Stille reaction with 4-[ F]fiuoroiodobenzene has been used for the synthesis of radiotracers for monitoring COX-2 expression by means of PET. By using optimized reaction conditions F-labeled COX-2 inhibitors 29 and 30 could be obtained in radiochemical yields of up to 94% and 68%, respectively, based upon 4-[ F]fiuoroiodobenzene (28) [72]. 

Ionic Hquids are excellent solvents for Stille cross-couplings under mild reaction conditions allowing catalyst recovery [21]. For example, when l-butyl-3-methylimidazoHum tetrafluoroborate (BMIM BF4) is used [21a], recycling of the solvent and catalyst system has been shown to give little loss of activity even afler five cycles (Scheme 6.8). 

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