Electrophilic catalytic system

The discovery of a zirconium-based catalyst able to promote polyolefin depolymerization encourages the search for more electrophilic catalytic systems that could be obtained either by changing the metal center or the inorganic support. 

Of the many reagents, both heterogeneous and homogeneous, that can facilitate chemical reactions, the cycloamyloses stand out. Reactions can be catalyzed with many species such as hydronium ions, hydroxide ions, general acids, general bases, nucleophiles, and electrophiles. More effective catalysis can sometimes be achieved by combinations of catalytic species as in multiple catalysis, intramolecular catalysis, and catalysis by com-plexation. Only the latter catalysis can show the real attributes of an efficient catalytic system, namely speed and selectivity. In analogy to molecular sieves, selectivity can be attained by stereospecific complexation and speed can be likewise attained if the stereochemistry within the complex is correct. The cycloamyloses, of any simple chemical compound, come the closest to these goals. 

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... 

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

A comparison of porphyrin and pincer activity rationalized through reactivity index Porphyrin and pincer complexes are both important categories of compounds in biological and catalytic systems. Structure, spectroscopy, and reactivity properties of porphyrin pincers are systematically studied for selection of divalent metal ions. It is reported that the porphyrin pincers are structurally and spectroscopically different from their precursors and are more reactive in electrophilic and nucleophilic reactions. These results are implicative in chemical modification of hemoproteins and understanding the chemical reactivity in heme-containing and other biologically important complexes and cofactors [45]. 

Indenyl ethers were synthesized via intramolecular carboalkoxylation of alkynes. In this process, a benzylic ether group played a nucleophile role to capture a vinyl gold intermediate obtained by alkyne activation. The first catalytic system tested by Toste and Dube in this study was a mixture of [AuClPPh3] and AgBF4. However, the moderate yield prompted them to research the use of more electrophilic gold(I) complexes such as [AuP(p-CF3-C6H4)3]BF4, which increased the yield of cydized products by 70% [107]. 

The cyclizations of unsaturated precursors 128 can be induced by an electrophile or by a transition metal catalyst as illustrated by the iodonium ion promoted cyclization of allyloxybenzenes 130 (Scheme 76) <2004JA3416> and the preparation of 4,5,7-trimethylchroman 132 from 131 using a RuCl3/AgOTf catalytic system (Scheme 77) <20040L581, CHEC-III(7.08.11.1.2)517>. 

The Cl sequence introduced in Chap. 2.2 represents a mild and catalytic access to chalcones. l,3-Diarylprop-2-en-l-ones are bifunctional electrophilic Michael-systems and per se important three-carbon building blocks in synthetic heterocyclic chemistry [33]. Among many classes of five-, six-, and seven-membered heterocycles the underlying principle is always the Michael-addition-cyclocondensation sequence of chalcones and bifunctional nucleophiles [176-181, 222-229]. Furthermore, chalcones can also participate in cycloadditions, as dienophiles and dipolar-ophiles and furnish in the case of 1,3-dipolar cycloadditions with diazo alkanes pyrazolines [230, 231], with azides triazolines [232], with nitrones isoxazolidines [233] with azomethinylides pyrrolidines [234], or with nitriloxides isoxazolines [235]. Therefore, the mild, catalytic access to chalcones by the CIR excellently sets the stage for the development of consecutive MCRs based upon cyclocondensation strategies. 

If the optically active organoselenium compounds can be used for Tomoda s or Tiecco s catalytic system using diselenide and persulfate (see Sect. 4.1), a catalytic asymmetric oxidation reaction should be possible. The enantioselectivity of the produced allylic compounds may depend on the stereoselectivity of the oxyselenenylation step of chiral selenium electrophiles with prochiral alkenes. Several groups have reported diastereoselective oxyselenenylation using a variety of chiral diselenides in moderate to high diastereoselectivity [5 f, g, i, 25]. The detailed results are reviewed in Chap. 2. 

Homopropargylic alcohols as well as propargylic epoxides and pentynols readily form cyclic ruthenium alkoxycarbenes upon intramolecular nucleophilic addition of the OH group to the electrophilic a-carbon of ruthenium-vinylidene species. Their oxidation in the presence of N-hydroxysuccinimide leads to the formation of penta-lactones. The best catalytic system reported until now for this transformation of but-3-ynols is based on RuCl(C5H5)(cod), tris(2-furyl)phosphine, NaHCOs as a base, in the presence of nBu4NBr or nBu4NPp6, and N-hydroxysuccinimide as the oxidant in DMF-water at 95 °C (Scheme 8.11) [22]. 

Cross-coupling between 1-alkenyl- and arylboron compounds with organic electrophiles have found wide application (see Sect. 1.5.1.1) in organic synthesis [125, 132, 269-271]. The value of this methodology is further realized with the use of alkylboranes, such as B-alkyl-9-borabicyclo[3.3.1]nonanes (B-R-9-BBN), which can be conveniently prepared Ijy hydroboration of olefins [272]. The hydroboration proceeds with high stereoselectivity and chemoselectivity. The choice of phosphines in a catalytic system sometimes affects the chemo- and regioselectivity of the hydroboration. Hydroboration of l-(ethylthio)-l-propyne with catecholborane can be satisfactorily carried out with PdClj (dppf) but the regioselectivity is best for the dppe and dppp complexes of Ni [273]. Notably, PPhj complexes perform poorly. 

The Periana system is currently the most active catalytic system for the C-H activation of methane. The proposed reaction mechanism (Scheme 6) is also based on three steps, C-H activation, oxidation, and functionalization. An important feature of the overall process is that the methyl ester is less reactive with the catalyst than methane. This is attributed to greater inhibition of the presumed electrophilic reaction of the C-H bonds of methylbisulfate in comparison with methane as a result of the electron-withdrawing ability of the bisulfate group... 

---