Catalytic reactions aryl chloride case

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]. 

Simple Pd salts and complexes which contain neither phosphines nor any other deliberately added ligands are well known to provide catalytic activity in cross-coupling reactions. Such catalytic systems (often referred to as ligand-free catalysts ) often require the use of water as a component of the reaction medium.17 In the majority of cases such systems are applicable to electrophiles easily undergoing the oxidative addition (aryl iodides and activated bromides), although there are examples of effective reactions with unactivated substrates (electron-rich aiyl bromides, and some aryl chlorides).18,470... 

Apart from intuitions based on experimental observations and support from computational work, the arguments in favor of Pd(II)/Pd(IV) mechanisms in the Heck reactions catalyzed by Pd pincer complexes are scarce. On the contrary, there is conclusive evidence indicating that in many cases the actual catalytic species results from the decomposition of pincer complexes [62, 76, 77, 97,100, 101, 103]. This conclusion can probably be extended to all systems that achieve exceptionally high TON numbers, such as 2 and 3, since the rate of the processes based on Pd(II)/Pd(IV) cycles would be always Hmited by the low reactivity of Pd(II) toward aryl halides. The observed influence of pincer ligands on the catalytic activity or the ability to catalyze difficult couplings (e.g., with aryl chlorides) can be rationalized on the basis of their ability to regulate the production of the actual catalytic species [11, 12, 96]. This, however, does not prevent the possibility that, in some specific cases, pincer complexes could act as true molecular catalysts for the Heck reaction or other closely related processes. In recent years, a couple of examples have been provided that demonstrate this possibility, as discussed below. 

Since then, statements that palladium nanoparticles or sols are true catalysts in a given catalytic system have become not infrequent. Indeed, in many cases it is clearly seen that, after the addition of precatalyst, a brownish-grey colour of palladium sol appears immediately and hansmission electron microscope measurements reveal the nanoparticles of metal. From the discussion of various preformed sols in Mizoroki-Heck reactions (Scenarios 1-3), it becomes evident that palladium nanoparticles display rather hmited activity with respect to scope, TONs and TOF values. On the other hand, in the discussion of various SRPCs, we see many examples of high activity and quite extended scope of substrates reaching to aryl chlorides. 

It was shown that palladacydes 1 [3c, 24] prepared from palladium] I) acetate and tris(o-tolyl)- or trimesitylphosphine are excellent catalysts for the Heck coupHng of triflates and halides including certain aryl chlorides. In some of these cases, a possible involvement of oxidation states +II and +IV in the catalytic cycle has been considered [25]. Similarly, other palladacydes such as 3 [26e,h] or 6 [27] have been used in the Heck reactions (Figure 8.1) [24, 26, 28]. It has been proposed that, at least for NC palladacydes, the reaction proceeds through the classical phosphine-free Pd(0)/Pd(II) catalytic cycle and that the active catalysts are actually slowly formed palladium clusters [29]. Besides classical palladacydes, complexes with pincer-type ligands such as 2 [30] have become very popular in palladium catalysis [31]. 

It has been observed that reductive elimination can also occur for aryl-Pd-carbene complexes. Such complexes were investigated in mechanistic studies on Heck coupling and catalyst decomposition routes. Reductive elimination products with direct imidazolium-aryl coupling were observed and in one case fully characterized. Such products provide direct evidence of the Heck coupling mechanism and of intermediates in the catalytic cycle. Important mechanistic studies on the oxidative addition of aryl chlorides to a 14-electron Pd(0)(carbene)2 complex have demonstrated that oxidative addition occurs via a dissociative process and this step is probably the rate-determining step in the amination of aryl chlorides. Aryl-carbene reductive coupling was observed in this study of the amination reaction, and directly coupled aryl-imidazolium compounds were isolated. A further study on an (aryl)Pd(carbene) complex has also demonstrated that such complexes undergo facile reductive elimination to form aryl-imidazolium salt. ... 

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