Water tolerance, metal-catalyzed

Because of the slightly acidic nature of the sp C-H bonds, the reaction of metal acetylides with various electrophiles is one of the most general strategies in organic transformations.1 Traditionally, such reactions are carried out by using alkali metal acetylides which are air and water sensitive. On the other hand, there is much interest in developing transition-metal catalyzed terminal alkyne reactions involving soft and more stable C-M bonds as reaction intermediates, because many such reactions can tolerate water. 

The enantioselective alkylation of indoles catalyzed by C2-symmetric chiral bisoxazoline-metal complexes 90 encouraged many groups to develop superior asymmetric catalysts which are cheap, accessible, air-stable and water-tolerant. Other analogs of the bisoxazoline-metal complex 90 as chiral catalysts and new Michael acceptors have also been studied. The enantioselective alkylations of indole derivatives with of-hydroxy enones using Cu(II)-bis(oxazoline) catalysts 93 and 94 provided the adducts in good yields... 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

One of the most distinguishable characteristics of the metal-catalyzed living radical polymerization is that it affords polymers with controlled molecular weights and narrow MWDs from a wide variety of monomers under mild conditions even in the presence of a protic compound such as water. This permits the synthesis of a vast number of polymers with controlled structures such as end-functionalized polymers, block copolymers, star polymers, etc., where they are widely varied in comparison with those obtained by other living polymerizations. This is primarily due to the tolerance to various functional groups and the polymerizability/controllability of various vinyl monomers as mentioned above. 

A very successful way to improve regiospecificity and to minimize branching was the synthesis through transition-metal-catalyzed reactions of monomeric 2,7-dihalogenated fiuorenes. The palladium-catalyzed synthesis of mixed biphenyles from phenylboronic add and aryl bromide discovered by Suzuki et al. [64] tolerates a large variety of functional groups and the presence of water. This method can also be used to prepare perfectly alternating copolymers. 

Thus, the reaction in aqueous Cu(N03)2 solution proceeded about 800 times faster than in water alone and 250000 times faster than in acetonitrile. This Lewis-acid-catalyzed aqueous Diels-Alder reaction presumably occurs via a transition state such as 2.50 with bidentate complexation to the metal as shown. In a related study, Kobayashi reported that scandium triflate can be used as a water-tolerant Lewis-acid catalyst for a Diels-Alder reaction in (9 1) tetrahydrofuran-water (THF-H2O) [30], though this mixed solvent system cannot take advantage of the hydrophobic effect observed in pure water or highly aqueous mixtures. 

Transition metal-catalyzed routes are also known for the conversion of arylboronic acids into phenols. Of the copper-catalyzed approaches, the methods that are tolerant to air and water are particularly practical due to the operational simplicity. One example of this chemistry entailed the use of copper sulfate as the catalyst (Scheme 2.34 and Example 2.5) [44]. Using 1,10-phenanthroline as a supporting ligand for the copper center, a wide array of electron-deficient and electron-rich arylboronic acids were converted into phenols in excellent yields. Even bulky arylboronic acids such as 2,6-dimethylphenylboronic acid were successfully converted (87%). In related work, copper oxide has also been successfully used to prepare phenols in water and under air from arylboronic acids bearing an assortment of electron-donating and electron-withdrawing groups (Schane 2.35) [45]. 

In the transition-metal-catalyzed hydrothiolation reactions, dithioketals 4 as double thiolation products are sometimes formed as byproducts. Since dithioketals 4 are synthetically useful as carbonyl equivalents, selective synthesis of 4 fi om alkynes and thiols is attractive. Indium bromide (InBr3) as a water-tolerant green Lewis acid catalyst is found to catalyze efficiently the hydrothiolation of alkynes with arene-and alkanethiols to produce the corresponding dithioketals in excellent yields (Scheme 20) [64]. 

This class of PC catalysts has also been extensively studied as a potential substitute for Pt as they are low cost, methanol tolerant and have reasonably good activity and remarkable selectivity toward ORR [194]. They normally catalyze a direct 4e reduction of O2 to )deld water. The major drawback of this kind of catalyst is of low stability in acidic media [195]. However, when the catalyst is heat treated, the activity and stability of transition metal macrocycle complex (TMMC) are improved significantly [194]. The molecules of TMMC have a square planar structure with the metal ion symmetrically surrounded by four nitrogen atoms these nitrogen atoms are from each member of the ring systems which, in turn, are connected by carbon atoms (porphyrins) or nitrogen atoms (phthalocyanines). 

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