Palladacycles, preparation

Palladacycles prepared by the addition of furancarbothioamide to a methanol solution of Li2PdCl4 at room temperature are soluble in hexane, chloroform, and moderately soluble in polar solvents DMF and DMSO [187]. These palladacycles are thermally stable, not sensitive to air or moisture, and can be applied effectively in the Heck reaction of aryl halides with terminal olefins and in the Suzuki reaction of aryl halides with arylboronic acids. These reactions were performed under aerobic conditions, leading to turnover numbers... 

It has been shown that palladacycles, prepared from palladium(II) acetate and the tris(o-tolyl)- or trimesitylphosphane, are excellent catalysts for the Heck coupling of triflates and halides including activated aryl chlorides. In this case, oxidation states -i-II and -I-IV of palladium have been invoked in the catalytic cycle. ... 

Chloroarenes and bromoarenes undergo high yield Heck reactions in the presence of a palladacycle prepared from Pd(OAc)2 and tri(t -tolyl)phosphine (equation 20) (66), When this catalyst was us at 140 C none of the palladium deposits which are typical at this temperature with other Pd-catalysts were detected. The catalyst is also more active, allowing a lower catalyst concentration to be used. No added phosphine is necessary, so side reactions involving this ligand do not occur. 4-Chlorobenzaldehyde and /i-butyl acrylate react to give 90 % yield of coupled product (66),... 

Kang J, Yew KH, Kim TH, Choi DH (2002) Preparation of bis [palladacycles] and application to asymmetric aza-Claisen rearrangement of allylic imidates. Tetrahedron Lett 43 9509-9512... 

We have shown that the direct arylation of acrolein toward the synthesis of cinnamaldehyde derivatives was an efficient procedure. Using the palladacycle 1 as catalyst, substituted aldehydes 3 were prepared with up to 87% isolated yield from condensed aiyl bromides (Scheme 21.1, Route 1) that was extended successfully to heteroaiyl bromides, like bromoquinolines (6). Alternatively, the acrolein diethyl acetal was used as olefin and a selective formation of the saturated ester 4 was attained under the same reaction conditions (Scheme 21.1, Route 2). The expected aldehydes 3 were, however, obtained from most of the aiyl halides used under modified conditions. It was shown that the addition of n-Bu4NOAc in the medium... 

This protocol could be extended to a range of different ,/i-unsaturated carbonyl compounds and either activated or deactivated aryl iodides [22], An application of related Heck chemistry to the synthesis of methylated resveratrol (3,4, 5-trihydroxy-( )-stilbene) is shown in Scheme 6.4 [23]. The phytoalexin resveratrol exhibits a variety of interesting biological and therapeutic properties, among them activity against several human cancer cell lines. Botella and Najera have shown that the trimethyl ether of resveratrol (Scheme 6.4) can be rapidly prepared by microwave-assisted Heck reaction of the appropriate aryl iodide and styrene derivatives, using the same oxime-derived palladacycle as indicated in Scheme 6.3. 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

Kuroboshi, M. Waki, Y. Tanaka, H. Palladium-catalyzed tetrakis(dimethylamino)ethy-lene-promoted reductive coupling of aryl halides./. Org. Chem. 2003, 68, 3938-3942. Luo, F.-T. Jeevanandam, A. Basu, M. K. Efficient and high-turnover homocoupling reaction of aryl iodides by the use of palladacycle catalysts. A convenient way to prepare poly-p-phenylene. Tetrahedron Lett. 1998, 39, 7939-7942. 

NHC-palladacycle complexes (125) have been obtained, as depicted in equation (17), from the corresponding dimer (124) and two equivalents of NHC ligand. Conveniently, this route was found compatible with diverse functions and so aminopalladacycles and phosphapalladacycles have been prepared. 

Here again, the reaction involved an intramolecular displacement of the iodide by an ester enolate (Scheme 10). Preparation of stable azapalladacycle ( )-93 commenced with treatment of sulfonamide 90, accessible via A -alkylation of A -trifluoromethanesulfonyl-2-iodoaniline with palladium(O) (Pd2(DBA)3 DBA = dibenzylideneacetone) and tetramethylethylenediamine (TMEDA) to afford palladium(ll) complex 91. An easy ring closure of complex 91 provided palladacycle ( )-92 in 92% yield via addition of /-BuOK (IM in solution in THE, 1.2equiv) at room temperature. Displacement of tetramethylethylenediamine with triphenyl-phosphine delivered palladacycle ( )-93 in quantitative yield. 

The general procedure for the coupling of styrenes with bromo- and chloroarcnes [16] is exemplified by this preparation (Scheme 3-56). In a 100-mL three-necked flask equipped with a reflux condenser, stirrer, and internal thermometer were placed, under a stream of nitrogen, 4-bromoacetophenone (23a-Br) (5.0 g, 25 mmol) [or 4-chloroacetophenone (23a-Cl) (3.3 mL, 3.9 g, 25 mmol) plus TBABr (1.64 g. 5 mmol)], styrene (4.3 mL, 3.9 g, 37 mol), 2,6-di(tert-butyl)phenol (20 mg, as a radical scavenger), NaOAc (2.5 g, 30 mmol), and A(A-dimethylacetamide (50 mL). To the well-stirred suspension was added 12 mg (0.1 mol%) of the palladacycle la, and the mixture was heated at 130 "C for 24 h (54 h with 4-chloroacetophenone). After the reaction mixture had cooled to rt, it was poured into ice-water (200 mL). The precipitate was collected on a filter, carefully washed with water and recrystallized from acetone/water to yield 4.9 g (89%) of 258a (3.8 g, 69% from 4-chloroacetophenone). 

Oxitne-derived palladacycles act as efficient catalysts for the Heck reaction [132], They could be isolated and are found to be thermally stable, not sensitive to air or moisture, and were readily prepared from inexpensive starting material. This was exemplified in the reaction catalyzed by 376 to prepare 377 from 3 and 375. 

Since the first X-ray crystal structure of a palladium(IV) complex was published by Canty [29, 30], a number of groups have undertaken the study of these relatively uncommon intermediates. In an effort to elucidate the role of palladium(IV) and palladium(II) intermediates in the reaction mechanism, Catellani prepared a number of isolable palladacycle complexes in both the (II) and (IV) oxidation states using 1,10-phenanthroline as the ligand (Scheme 8) [31]. Catellani successfully isolated palladium(IV) palladacycle 22, which was subsequently characterized by... 

The newly invented polystyrene-supported palladacycle catalyst was prepared in six steps with high yields. The simple precipitation and filtration process to recycle the catalyst after model reactions for Heck, Suzuki, and Sonogashira reactions is noteworthy.[1]... 

In a preparation of ArCN from ArX and K4Fe(CN)g, the employment of a palladacycle as a catalyst under rather drastic conditions (NMP, 140°) is perhaps of questionable value. 

Formation of dihydroquinol-4-ones from 3-(o-iodoarylamino)propanoate esters involving arylpalladium intermediates which may exist in the palladacycle form and thereby derive special activity for intramolecular attack on the ester group.Under similar conditions, five-membered ring oxime derivatives are prepared. ... 

Nowotny and co-workers prepared polystyrene-immobilized complex 309. The supported complex is labile under typical Heck conditions, the active catalyst being transferred to solution. When the reaction is run at 140 °C, the kinetics are sigmoidal, showing that the active species slowly builds up in solution, and then decomposes. In addition, on the second recycle, all activity is in solution, probably as colloidal Pd(0) and no activity is left within the polymer support. This is consistent with 308 and other supported palladacycles being simply a slow source of active colloidal palladium particles. 

Immobilized dicyclohexylphosphine ligand, 310, has been used as the starting point for the preparation of supported palladacycles 313 and 314. Simply stirring 310 and dimeric palladium complexes 311 and 312 for 1 h in dichloromethane gave 313 and 314, respectively (Scheme 104). They are active Suzuki coupling catalysts on the first use but cannot be recycled. 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

Palladacycles are amply reported in the literature [18]. We shall limit our review to those prepared by norbornene insertion and subsequent cyclization, which are relevant to the synthetic methods treated here. 

A direct way of preparing a palladacycle was found for the special case of m-bromocyanobenzene which was reacted with Pd(PPh3)4 and norbornene in anisole at 105 °C. The palladacycle precipitated in an oligomeric form from the reaction solution in good yields (67% on Pd). The complex contains one molecule of triphenylphosphine per palladium and a coordination site is occupied by the cyano group of another palladacycle unit. (Eq. 11) [21]. 

Alternatively, transition-metal NPs in ILs may be obtained by the simple transfer of the freshly prepared MNP in water [77, 78] or classical organic solvents to the ILs, as for example from the reaction of palladacycles with allenes (Scheme 6.2) [79]. 

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