Heck reaction of aryl iodides

In experiments with a supported palladium catalyst, Pd/C, satisfactory yields were obtained without the use of phosphine ligands for the Heck reactions of aryl iodide with acrylonitrile, styrene, and methyl methacrylate in the ionic liquid [BMIM]PF6 (259). The addition of triethylamine improved the yields. The Pd/C remained in the ionic liquid only. The ionic liquid containing Pd/C can be reused as... 

The use of ultrasound as a way of supplying energy to a reaction system has been widely studied recently.32 Ultrasonically assisted (cleaning bath of frequency 50 KHz) Heck reactions of aryl iodides in [C4C4im]Br and [C4C4im][BF4] have been reported to exhibit considerably enhanced reaction... 

Figure 3.51 A multi-functionalised ionic liquid composition (MFILC) containing an amino functionalised imidiizolium salt for the Heck reaction of aryl iodides and bromides.

Verkade s P,N-ligand L13 also catalyzes the Heck reaction of aryl iodides. It was thus shown that the Buchwald-Hartwig amination could be appended to a Heck reaction using Pd/L13.47 The operation is performed by first adding an aryl iodide to the catalyst solution and an aminostyrene 10 at 60 °C. This gives an intermediate N-aryl aminostyrene 11 to which 2 equiv of a second aryl iodide is added at 110 °C. This gives a second aryiation of the aminostyrene as well as the Heck arylation of the olefin (12). 

Zhang and Wang reported the use of Pd nanopartides (5nm) stabUised by poly(ethylene glycol) (PEG) as an efficient and recyclable catalyst for the Heck reaction of aryl iodide and bromides with ethyl acrylate or styrene [31]. Leaching of Pd species in solution was evidenced by ICP-AES analyses. The authors concluded from a filtration experiment made on a cold reaction mixture that the activity was due to heterogeneous systems. However, the method used allows the dissolved Pd species to redeposit on the support. That is, more probably, the activity observed with these catalysts was due to (temporarily) leached Pd spedes. 

Quite a few groups have performed additional research trying to answer the question as to whether the Heck reaction takes place on the surface of the clusters or if it occurs entirely or partially in solution. De Vries and coworkers applied electrospray MS (negative mode) on the Heck reaction of aryl iodides with butyl acrylate. They uncovered a number of monomeric anionic species which led them to propose the catalytic cycle depicted in Scheme 10.6 [57, 58]. 

As stated earlier, it is very important to keep the size of the clusters down to prevent formation of palladium black. Apart from the kinetic approach described above a mechanical approach is also possible [76]. Two groups have described an ultrasound promoted Ugand-free Heck reaction of aryl iodides [77, 78]. In one case, the yield of product was found to be 8-fold higher than under comparable thermal conditions. 

Brase and coworkers [29] also reported highly enantioselective desymmetrization of rran5-bicyclo[4.4.0]decadienes 81-84, which were prepared through Birch reduction of naphthalene, epoxidation and epoxide-opening reaction (Figure 13.11). The position of C=C double bonds also differed from the previous report [25], Although a Mizoroki-Heck reaction of aryl iodide 81 with the usual chiral ligands such as BINAP, DIOP i(2R,3R)- or (25,35)-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane)... 

The first time that a Mizoroki-Heck reaction was conducted in a flnorons system was in 1999 when Sinou and coworkers [71] applied Pd2(dba)3 (dba = dibenzylideneacetone) or Pd(OAc)2 and a perflnorinaled phosphine (e.g. 28-30) in a perflnorinated-nonflnorons solvent mixture in the Mizoroki-Heck reaction of aryl iodides (4h, 80 °C). In this system, the Mizoroki-Heck prodnct was soluble in acetonitrile and the catalyst was dissolved in the fluorous phase. However, some ligand was lost due to its partial solubility in acetonitrile and some of the palladinm was reduced to palladium black. Thus, after each run with the recovered catalyst, lower conversions were obtained. 

Scheme 4 Palladium-catalyzed Heck reaction of aryl iodides." Reagents and conditions (i) PdAS-V, EtsN (1.5 equivalents), toluene, 100 °C.

Shao and coworkers synthesized prolme-derived complex 30 (Figure 4.9), which was active for Heck reactions of aryl iodides and bromides in water [41]. Better results were obtained when electron-rich aryl iodides were used as substrates. 

Miura and coworkers reported the first palladium catalyzed intermolecular carbonylative Heck reaction of aryl iodides and five membered olefins in the presence of carbon monoxide (Scheme 1.28) [99]. The expected 2,5-dihydrofuran product was not observed in the carbonylative coupling of 4-iodoanisole and 2,3-dihydrofuran as depicted in Scheme 1.28. Instead, isomerization of the double bond via reinsertion of the palladium(II) hydride followed by another ]S-hydride elimination formed the vinyl ether exclusively. 

Table 7.2 Carbonylative heck reaction of aryl iodides in the two-chamber system... 

Scheme 6.36 Heck reaction of aryl iodides with olefins using 52 in water.

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