Protic molecules

Obviously such super bases are suitable only for reactions of aprotic substrates such as hydrocarbons since the reactive alkali metals will be leached from the surface by reaction with protic molecules. 

In Figure 1 are shown three negative ion APIMS spectra of perfluoromethylcy-clohexane (C7F,4) that were obtained under three different ion source conditions. It is well known that perfluorinated molecules of this size attach thermal electrons very rapidly to form molecular anions by the resonance electron capture (EC) mechanism (e + M M"), and this is what is observed in Figure la where pure nitrogen buffer gas has been used. In the use of certain analytical instruments for the detection of such compounds, however, protic molecules such as water or... 

Neutral homoleptic transition amide molecules are for the most part soluble in commonly used aprotic solvents. They are, in effect, hydrocarbon-soluble soiuces of M"ions, and because of the polarity of the M—N bond they can readily be used for the synthesis of a wide variety of reagents. The most common reactions involve the insertion of unsamrated species into the M—N bond or M—N bond cleavage reactions with protic molecules. Their utility can be illustrated by the reactions of some divalent Co) ) amides, shown in Scheme 6.2. 

Clearly those counterions unable to trap water in this way (e.g. SbClg) would not be expected to promote transfer of this type and hence molecular weights of polymers produced would be higher. Independent observation of the propensity of CIO4 to function as an apparent transfer agent in the presence of protic molecules has been reported previously (162). 

Figure 3.48 Hydrogen bond formation between an amine and a protic molecule...

The Br0nsted basicity of a surface is related to its deprotonation ability, which can be probed by investigating the dissociative adsorption of protic molecules (Bailly et al., 2005a Chizallet et al., 2006). The 0Lc2 0Lc H transformation thus induced can be followed by PL, which is one of the few techniques able to simultaneously characterize oxide ions and their protonated forms. The same kind of equilibrium is also involved when a hydroxylated surface is undergoing thermal pretreatment (Section 2.1). PL is thus an interesting tool to evaluate the surface basic properties of alkaline earth oxides. 

The active species too is characterized by rather unusual properties and is different from soluble Ti-peroxides, known for their inertness in the epoxidation of simple olefins. As originally proposed by Clerici et al., the active sites contain Ti-OOH species, stabilized in a cyclic structure by the coadsorption of a protic molecule, generally the alcohol solvent. The oxygen transfer step to the double bond has electrophilic character thus resembling other peroxidic oxidants (Figure 15). 

Isomorphous substitution with a suitable transition metal, notably Ti, produces an oxidation catalyst. The larger ionic radius, coupled with the preference of Ti for octahedral coordination, produces strain in the silica lattice, favouring the splitting of SiO-Ti bonds by a protic molecule, e.g., H2O... 

This proposal was based on adsorption properties of Ti sites, on the reactivity behaviour of organic and inorganic peroxides and on the smooth formation of cyclic structures in hydroperoxides and other compounds. In the alternative species, based on quantum chemical studies, the hydroperoxide group was T] coordinated and did not require stabilization (Figure 18.1b) [72]. In another proposal, also based on DFT calculations, coordination and hydrogen bonding with a protic molecule coexist in the same structure (Figure 18.1c) [73, 74]. 

It is clear from the first part of this chapter that the external medium has a major effect on the catalytic properties of TS-1 and other Ti-zeolites. Early rationahzation suggested that the adsorption of a protic molecule, ROH, had a stabilizing effect for Ti—OOH species. In this view, water and alcohol solvents influenced the catalytic performance through the electronic and steric effects exerted in the oxidation step by the ROH molecule present in the active species (Section 18.4.1.1) [62]. 

The use of aliphatic and aromatic alcohols, amines, and carboxylic acids as proton donor molecules with DTBN reveals that whereas the X-H protons are shifted to low frequency by the radical the C-H protons, other than X-H, are moved to high frequency (223) (Table X). These high frequency shifts are shown to be characteristic of protic molecules and demonstrate conformational or geometrical dependences. Thus, protons lying on a zig-zag path from the -OH or -NH... 

In an Sij2 reaction, such as (1) in a mixed solvent containing a protic component, the anionic nucleophile, the protic component and the dipolar aprotic species all compete for the hydrogen bonds donated by the protic molecules. However, hydrogen-bonding between protic... 

Clerici cites considerable support for the mechanism involving the Ti (q -OOH) species [373]. In TS-1, numerous spectroscopic studies [412] show the adsorption of protic molecules on Ti sites, for example, as observed by XANES by the decrease in intensity of the tetrahedral Ti pre-edge peak on adsorption of water and ammonia [413]. Rate laws suggest that one molecule of alcohol solvent is adsorbed on the active species in the oxidation of alcohols [414] and the... 

G. Belussi, A. Carati, M. G. Clerici, G. Maddinelli, R. MiUini, Reactions of titanium silicalite with protic molecules and hydrogen peroxide, J. Catal. 133 (1992) 220. 

Boron trifluoride is also widely used as a Lewis add in industrial processes. It is less active than aluminium trichloride but has the advantages of being more tolerant of air exposure (it is deactivated but not destroyed by water). It is normally used as a complex, such as the etherate, which gets around the difficulties of working with a gas. Several versions of supported BF3 have been reported. The most recent and possibly most interesting for liquid-phase organic reactions involves the complexation of the Lewis acid with the surface of a silica support in the presence of a protic molecule, typically an alcohol (Figure 4.12).119 120... 

The synthesis of titanium silicalites TS-1 [10] and TS-2 [84-85], with MFI and MEL structure respectively, opened new opportunities in the oxidations with H O. TS-1 and TS-2, the former being the most studied, show similar properties in catalysis. Catalytic sites are isolated titanium atoms, incorporated into the zeolitic framework, in a channel system of about 0.55 nm average diameter [86-88], Different Ti-peroxo species, at lattice position, result from complex equilibria between TS-1, H O, and protic molecules [89-91],... 

The epoxidation of olefins is best carried out in methanol at near room temperature [95-96]. The reaction is fast and selective with unhindered olefins. Yields are almost quantitative. Turnover rates as high as 2 s have been observed for propylene [95]. Proposed active species is a five membered ring peroxo complex, formed by a Ti-OOH species and a protic molecule ROH, at lattice positions [96]. This shows electrophilic properties. However, the epoxidation of allyl chloride and allyl alcohol, is still fast at 40° C. Epichlorohydrin and glycidol are produced with 75% and 85% selectivity, respectively. Styrene yields p-phenyl acetddehyde [97], Recently reported C=C double bond cleavage [98] is likely the result of impure TS-1, as shown by Millini et al. [87]. Allyl methacrylates are epoxidized at allylic position [93]. Dienes and diallylcarbonates are monoepo-xidized [93]. 

As a class, [Fe4S4(SR)4] clusters are unstable to air and are slowly oxidized or decomposed by protic molecules. They are usually found with 5"= 1/2 ground states but examples of pure S = ijl ground states and physical mixtures of the two states are well documented. Both spin states have been found in proteins with EPR spectra that are similar to those of analogue clusters. Electron self-exchange in the systems [Fe4S4(SR)4] is very fast k 10 s at 300 K in... 

The dienophile may also be activated by hydrogen bonding of the carbonyl oxygen with suitable protic molecules and it has been observed that Diels-Alder reactions are accelerated when performed in protic solvents such as 2-butanol. Thus the opportunity exists for the development of an enantioselective alcohol-catalysed asymmetric Diels-Alder reaction. Indeed, Rawal and coworkers have recently discovered that the use of catalytic amounts of the enantiomerically pure diol a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) (8.113) in the Diels-Alder reaction of aminosilyloxydiene (8.114) with 2-substituted acroleins such as (8.24), results in the formation of the cycloadduct (8.115) with high ee. Simiharly, the strong Bronsted acid (8.116) effectively catalyses the Diels-Alder reaction of sUyloxydienes such as (8.117) with a,P-enone (8.105). ... 

The majority of metal-based Lewis acid catalysts used in the HDA reaction are moisture sensitive and are thus usually prepared in situ. The stable and storable zirconium-BINOL Lewis acids developed by Kobayashi and coworkers, effective in the aldol reaction (see Section 7.1) can also be used as asymmetric catalysts in the HDA reaction of aliphatic and aromatic aldehydes with dioxygenated dienes. Metal-free catalytic asymmetric HDA reactions have been developed, utilising enantiomerically pure protic molecules that activate the aldehyde component by hydrogen bonding to the carbonyl group. Rawal and coworkers have achieved up... 

Some binary mixtures of protic molecules have also been reported to form mesophases that are not present in either pure component. For instance, Lawrence observed mesophase formation in 1 2 molar ratios of cholesterol and 1-alkanols containing 12,13,14,16 and 18 carbon atoms [151]. Mixtures of 1-hexadecanol and dodecanoic acid exhibits a lamellar liquid crystalline (smectic) phase [152]. Intermolecular hydrogen bonding must be an important contributing factor to maintain the necessary aggregation for mesomorphism in these cases. For applications of this concept to PLCs, see Chapter 3. 

Figure 10.6. Reaction of an isocyanate with a generic protic molecule.

Subsequently, other [Ln CH(SiMe3)2 3] (Ln = Y, Ce, Lu) have been synthesized by tiie same route, and there appears no reason why the others can be made, except possibly the Lu and Yb compounds. " These pentane-soluble compounds have trigonal pyramidal coordination geometries. One short Ln- CH3 contact is observed in the crystal structure of each molecule, ascribed to an agostic Si interaction (rather than yC- -H- Ln interaction). Reactions of some of these compounds with protic molecules... 

Despite the narrowness of the field, several of the compounds have found real synthetic utility on account of their reactivity, notably but not exclusively with protic molecules. 

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