Aryl iodides room-temperature reactions

The room-temperature reaction of secondary amines and aryl iodides can be efficiently catalyzed by the ( )-BINAP/Pd system,Eq. (38) [68]. In order to achieve complete conversion, it was necessary to utilize stoichiometric 18-crown-6 as an additive. However, role of the crown ether is not entirely clear. Notably, aryl iodides react exclusively under these conditions while aryl bromides are left unchanged. 

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

The reaction of an aryl diazonium salt with potassium iodide is the standard method for the preparation of aryl iodides The diazonium salt is prepared from a primary aro matic amine m the usual way a solution of potassium iodide is then added and the reac tion mixture is brought to room temperature or heated to accelerate the reaction... 

The palladium-catalyzed cross-coupling of alkenylsilanols has been extensively studied with respect to the structure of both the silicon component and the acceptor halide. The preferred catalyst for coupling of aryl iodides is Pd(dba)2 and for aryl bromides it is [allylPdCl]2. The most effective promoter is tetrabutylammonium fluoride used as a 1.0M solution in THF. In general the coupling reactions occur under mild conditions (room temperature, in 10 min to 12 hr) and some are even exothermic. 

The first examples utilising A-heterocyclic carbenes as ligands in the Buchwald-Hartwig amination involved the in situ formation of the catalyst from the corresponding imidazolium salt and a Pd(0) source. Nolan reported IPr-HCl/PdjCdbalj as a catalytic system for the amination of aryl chlorides in excellent yields, using different types of amines, anilines, and also imines or indoles [142,143] (Scheme 6.46). Hartwig showed later that in some cases the reactions could be performed at room temperature and without anhydrous conditions even for aryl chlorides [ 144]. This was later shown for the less challenging bromides and iodides [145,146]. 

A similarly high performance has been reported for oxime-derived (125) and benzylsulfide-derived (126) palladacycles.438 These precatalysts are effective in the cross-coupling of arylboronic acids,438,439 organotin compounds,440 and terminal acetylenes441 with aryl iodides and bromides, and of activated aryl chlorides. SC-palladacycles can effect the Suzuki-Miyaura reaction even at room temperature. 

A convenient preparation of dihydrobenzofurans has been achieved from the appropriately functionalized ort/ o-halophenol derivatives. Treatment of the aryl iodide (5)-33 with (TMS)3SiH and EtsB in the presence of air at room temperature, gave the aryl radical which cyclized in a 5-exo-trig mode and provided the bicyclic derivatives 34/35 as a 29 1 mixture of diastereoisomers in favour of 34 (Reaction 7.40) [51]. 

In the methylation reaction of 33 with methyl iodide in the presence of potassium carbonate, the A-methyl derivative 35 was formed, which epimerized to 34 on standing in CDCI3 solution at room temperature. This epimerization can be rationalized in terms of the favored N-inside conformation 33 with an equatorial aryl group. On A-methylation, the... 

Good yields are generally observed, especially for methyl ethers. The combination of boron tribromide with dimethyl sulfide has been found to be particularly effective for cleaving aryl methyl ethers.87 Trimethylsilyl iodide cleaves methyl ethers in a period of a few hours at room temperature.85 Benzyl and /-butyl systems are cleaved very rapidly, whereas secondary systems require longer times. The reaction presumably proceeds via an initially formed silyl oxonium ion ... 

In a related three-component reaction procedure, aryl methyl ketones 724 have been combined with aryl aldehydes 725 and urea 726 at room temperature, using trimethylsilyl iodide as catalyst, to give 4,6-diaryl-3,4-dihydro-2(177)-pyrimidinones 727 <2005HCA2996>. A procedure using zinc iodide and microwave irradiation gave similar products <2007T1981>. 

The scope of the use of the inexpensive, commercially available KOSiMc3 as base was examined by Denmark and Sweis. High yields and high stereospecificities were obtained for the coupling of a variety of alkenyldimethyl-silanols and aryl iodides, in DME at room temperature, in very short reaction times. TBS-protected alcohols are not affected by the presence of this base. The authors proposed the formation of a silicon-oxygen-palladium linkage as a pre-association step prior to the transmetallation (Scheme 11). 

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