Alcohols transition metal complexes

A great variety of suitable polymers is accessible by polymerization of vinylic monomers, or by reaction of alcohols or amines with functionalized polymers such as chloromethylat polystyrene or methacryloylchloride. The functionality in the polymer may also a ligand which can bind transition metal complexes. Examples are poly-4-vinylpyridine and triphenylphosphine modified polymers. In all cases of reactively functionalized polymers, the loading with redox active species may also occur after film formation on the electrode surface but it was recognized that such a procedure may lead to inhomogeneous distribution of redox centers in the film... 

Figure 4.5 Simplified mechanism of the racemization of sec-alcohols catalyzed by transition metal complexes.

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Lewis acids such as SnCl4 also catalyze the reaction, in which case the species that adds to the alkenes is H2C —O— SnC. Montmorillonite KIO clay containing zinc(IV) has been used to promote the reaction. The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form allylic alcohols. A typical example is. ... 

Oxidative addition of the O-H bond to transition metal complexes gives hydrido(hy-droxo), hydrido(alkoxo) or hydrido(carboxylato) complexes (Eq. 6.1), but web-characterized complexes obtained as primary products from the reaction of the compound, XO-H (XO-H = water, alcohol, and carboxylic acid) with late transition metals are quite rare [1]. Furthermore, the crystal stractures of very few complexes of this type have been reported. In this section we will survey late transition metal complexes resulting from activation of water, alcohol, and carboxylic acid. 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

It should also be noted that this polymerization system is not disturbed in the presence of alcohol and water. Similar polymerizations with nickel [278,279] and iron [280] complexes have also been reported. The structures of the transition metal complexes are shown ... 

Although this classic picture evolved from "soft, mononuclear transition metal complexes suffices to explain a great deal of carbon monoxide chemistry, it is not clear that it is complete or accurate for understanding processes whereby CO is reduced, deoxygenated, and/or polymerized to form methane, long-chain hydrocarbons, alcohols, and other oxocarbons, especially in cases where heterogeneous catalysts or "hard" metals are involved (6, 7, ,9,J 0). This deficiency of information has led to the search for new modes of carbon monoxide reactivity and to attempts to understand carbon monoxide chemistry in nontraditional environments ... 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

The isomerization of allylic alcohols provides an enol (or enolate) intermediate, which tautomerizes to afford the saturated carbonyl compound (Equation (8)). The isomerization of allylic alcohols to saturated carbonyl compounds is a useful synthetic process with high atom economy, which eliminates conventional two-step sequential oxidation and reduction.25,26 A catalytic one-step transformation, which is equivalent to an internal reduction/oxidation process, is a conceptually attractive strategy due to easy access to allylic alcohols.27-29 A variety of transition metal complexes have been employed for the isomerization of allylic alcohols, as shown below. 

The direct conversion of alcohols and amines into carbamate esters by oxidative carbonylation is also an attractive process from an industrial point of view, since carbamates are useful intermediates for the production of polyurethanes. Many efforts have, therefore, been devoted to the development of efficient catalysts able to operate under relatively mild conditions. The reaction, when applied to amino alcohols, allows a convenient synthesis of cyclic urethanes. Several transition metal complexes, based on Pd [218— 239], Cu [240-242], Au [243,244], Os [245], Rh [237,238,246,247], Co [248], Mn [249], Ru [224,250-252], Pt [238] are able to promote the process. The formation of ureas, oxamates, or oxamides as byproducts can in some cases lower the selectivity towards carbamates. 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

Many transition-metal complexes have been reported as catalysts of this reaction, including [lr(g-Cl)(coe)2]2 [74] and [lrH2(solv.)(PPh3)][SbF6] [75]. The latter catalyst appeared to be a very active and highly selective. The hydroxyl group can be selectively silylated, even in the presence of other potentially reactive C=C and C=0 groups. The order of relative reactivities of alcohol isomers is secondary alcohol > primary alcohol > tertiary alcohol. 

Enantioselective transition metal-catalyzed allyhc alkylation has stimulated immense interest due to its potential synthetic utihty [1 b]. Although excellent enantioselectivities have been obtained for a wide variety of cychc and acyclic aUyhc alcohol derivatives, using a wide range of chiral transition metal complexes, the ability to also control regioselectivity has proven challenging. In hght of the excellent selectivities observed for rhodium-catalyzed allyhc substitution, it would seem reasonable to assume that the enantioselective rhodium-catalyzed version may provide the definitive solution to this problem. 

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

In sharp contrast to the transition metal-catalyzed allylic alkylation of allylic alcohols and their derivatives (see Chapter 11.03) where 77 -allyl-transition metal complexes are key intermediates, the benzyiic alkylation of benzyiic alcohols and their derivatives catalyzed by transition metal complexes has been quite unexplored, although 7] -benzyl-transition metal complexes have often been considered to explain the regioselectivity of transition metal-catalyzed addition to vinylarenes (Scheme 21). A3,63a... 

Carbonylations of olefins, acetylenes, halides, alcohols, amines, nitro compounds, etc., promoted by transition metal complexes are very important in both industrial and laboratory organic syntheses. The mechanisms of those reactions have been studied extensively, especially for those associated with commercial processes. " The research... 

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