Alkyl esters, covalent

An inorganic ester may be thought of as a compound that contains one or more alkyl groups covalently bonded to the anion of a ternary inorganic acid, such as HN03. 

These observations do not, however, mean that TBT carboxylates and TBTCl are ionic in nature. After detailed analysis of the physical evidence such as the low specific conductance and dipole moment of trialkyltin halides, Neumann has concluded that they have no "salt-like constitution" (6). Bonding in the trialkyltin carboxylates also is essentially similar to that in covalent alkyl esters, as evidenced by the low dipole moment of 2.2D for tributyltin acetate in benzene, as compared to 1.9D for alkyl acetates (7). 

Proteinaceous Surfactants Prepared by Covalent Attachment of L-Leucine / -Alkyl Esters to Food Proteins by Modification with Papain... 

Covalent Attachment of l -Leucine n-Alkyl Esters to Food Proteins for Improving Their Functionality (10,11)... 

Table III. Conditions Used to Improve the Functionality of Gelatin (Substrate) Through Covalent Attachment of L-Leucine n-Alkyl Ester (Nucleophile) with the Use of Papain (Enzyme)...

Covalent species such as alkyl esters and halides may be in equilibrium with either onium or carbenium ions [Eq. (45)]. However, only heterocyclic monomers are... 

Arai et al. [141] described a particular enzymatic reaction for producing a surface-active protein. A highly hydrophobic amino acid was covalently bound to a hydrophilic protein in an enzyme-catalyzed process for this purpose. The covalent attachment of L-Leu n-alkyl ester to gelatin in the presence of papain as catalyst resulted in a proteinaceous surfactant [141,142] with very good emulsifying properties. 

Addition of an alkyl halide or an alkyl ester to a tertiary nitrogen compound. Here the binding of the alkyl is to the nitrogen, and the trivalent nitrogen is often assumed to be converted to a pentavalent linkage. In reality, the nitrogen possesses four ordinary covalencies and one electrostatic bond, e.g.,... 

Figure 15 Covalent incorporation of amino acid esters into proteins by a transpeptidation reaction catalyzed by proteases. The reaction is based on the stereospecific nucleophilic attack of the —NHj group of L-amino acid alkyl ester on the carboamide bond of the polypeptide activated by the enzyme.

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

The diazeniumdiolate anions react with electrophiles to produce stable covalent compounds (Fig. 3.9) [213, 216]. These compounds have the ability to act as prodrugs, releasing nitric oxide only when metabolically or enzymatically converted to the diazeniumdiolate anion [217-219]. Several compounds ofthis class have been synthesized by reaction of alkyl or aryl halides, sulfate esters, epoxides, etc. with the ionic diazeniumdiolates [220, 221]. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

The first suggestion of a practical form of antidotal therapy came in 1949 from Hestrin, who found that acetylcholinesterase (AChE) catalyzed the formation of acetohydroxamlc acid when incubated with sodium acetate and hydroxylamine. Critical in vitro studies in the next decade led to the development of a practical approach to therapy. The crucial concept in these studies was the recognition that the compound formed when AChE reacted with a phosphorus ester was a covalent phosphoryl-enzyme Intermediate similar to that formed in the hydrolysis of acetylcholine. 3 Wilson and colleagues, beginning in 1951, demonstrated that AChE inhibited by alkyl phosphate esters (tetraethyl pyrophosphate, TEPP) could be reactivated by water, but that free enzyme formed much more rapidly in the presence of hydroxylamine. 0 21 Similar results... 

The first, and fundamental, piece of evidence necessary for a discussion of the detailed mechanism of any chemical change is the identification of the covalent bonds formed and broken this may or may not be the same thing as the identification of the products of the reaction. In the case of ester hydrolysis or formation the alternatives involve the cleavage or formation of bonds from oxygen to the carbon atom of either an alkyl or an acyl group, and it is in principle, and generally also in practice, a simple matter to distinguish between these alternatives. 

Alkyl halides react with nucleophiles, reagents that can supply an electron pair to form a covalent bond, to give a product in which the nucleophile takes the place of the halogen. Table 6.1 gives fifteen examples of such nucleophilic substitution reactions, which can be used to convert alkyl halides to alcohols, ethers, esters, amines, thiols, alkyl cyanides, or acetylenes. 

Fig. 9. HREELS spectra of functionalized silicon surfaces prepared via photochemical reactions with H/Si(lll). In each case R represents a saturated alkyl chain (9 or 10 carbon atoms long) covalently attached to the Si surface. The methyl and acid terminated surfaces were prepared via reactions with decene and undecylenic acid respectively while the thienyl terminated surface was prepared by reaction of thienyl Li with an ester terminated surface. The dashed line at 1500 cm-1 represents the typical low frequency cut-off for ATR-FTIR measurements on silicon.

The covalent C-B bonds of organoboronic acids and esters are very inert to ionic and radical reactions, thus allowing functionalization of remote sites other than the B-G bond (Equations (94)-(97)). Bulky diols such as pinacol have been used as the protecting group of B(OH)2 because of their high stability to nucleophiles and water and silica gel in amination of 316,474 hydroboration-amination of 317,475 Wittig reaction of 3 18,476 and oxidation-alkylation of 319.477... 

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