Donor component

In a cross-coupling benzoin condensation of two different aldehydes, usually a mixture of products is obtained, with the ratio being determined by the relative stabilities of the four possible coupling products under thermodynamic control. If, however, an acyl silane, e.g. 5, is used as the donor component, the a-silyloxy-ketone 6 is obtained as a single product " ... 

Similar to DHAP aldolases, the 3-hexulose 6-phosphate aldolase found in Methylomonas Ml 5 is highly specific for the aldol donor component D-ribulose 5-phosphate, but accepts a wide variety of aldehydes as replacement for formaldehyde as the acceptor. With propanal,... 

A number of lyases are known which, unlike the aldolases, require thiamine pyrophosphate as a cofactor in the transfer of acyl anion equivalents, but mechanistically act via enolate-type additions. The commercially available transketolase (EC 2.2.1.1) stems from the pentose phosphate pathway where it catalyzes the transfer of a hydroxyacetyl fragment from a ketose phosphate to an aldehyde phosphate. For synthetic purposes, the donor component can be replaced by hydroxypyruvate, which forms the reactive intermediate by an irreversible, spontaneous decarboxylation. 

X-ray diffraction analysis of crystalline poly(schiff base)s and their low molecular models shows that the formation of molecular complexes is accompanied by an increase in interplanar distances and, in a number of cases, by complete amor-phization. Molecular complexes of poly(schiff base)s with Br2 decompose with time, because of the bromination of the donor components, forming C—Br bonds. Substitution of hydrogen by bromine in phenyl groups occurs only in cases in which these groups are not included into the main polymeric chain. 

Zollinger and coworkers (Nakazumi et al., 1983) therefore supposed that the diazonium ion and the crown ether are in a rapid equilibrium with two complexes as in Scheme 11-2. One of these is the charge-transfer complex (CT), whose stability is based on the interaction between the acceptor (ArNj) and donor components (Crown). The acceptor center of the diazonium ion is either the (3-nitrogen atom or the combined 7r-electron system of the aryl part and the diazonio group, while the donor centers are one or more of the ether oxygen atoms. The other partner in the equilibrium is the insertion complex (IC), as shown in structure 11.5. Scheme 11-2 is intended to leave the question open as to whether the CT and IC complexes are formed competitively or consecutively from the components. ... 

Typically, lyases are quite specific for the nucleophilic donor component owing to mechanistic requirements. Usually, approach of the aldol acceptor to the enzyme-bound nucleophile occurs stereospedfically following an overall retention mechanism, while the facial differentiation of the aldehyde carbonyl is responsible for the relative stereoselectivity. In this manner, the stereochemistry of the C—C bond formation is completely controlled by the enzymes, in general irrespective of the constitution or configuration of the substrate, which renders the enzymes highly predictable. On the other hand, most of the lyases allow a reasonably broad variation of the electrophilic acceptor component that is usually an aldehyde. This feature... 

Apparently, all DHAP aldolases are highly specific for (25) as the donor component for mechanistic reason [30-33], a fact which requires an economical access to this compound for synthetic applications. Owing to the limited stability of (25) in solution, particularly at alkaline pH, it is preferentially generated in situ to avoid high stationary concentrations. 

Ion-Radical Organic Chemistry in Its Practical Applicability Donor components tor organic metals... 

Even acetaldehyde can be used as the donor component with A -Boc imines... 

The concept of hydrogen bonding is constantly evolving from classical hydrogen bonds to nonclassical (or nonconventional) hydrogen-bonded complexes. Here, on the basis of new experimental and theoretical data and new approaches to this problem, the nature of a proton-donor component and a proton-acceptor site is reformulated completely. In addition, experimental criteria that have been used successfully earlier for the detection of hydrogen bonds are also changed. 

According to these criteria, a dihydrogen bond is formed when in the presence of a proton acceptor, the v(OH) band intensity of a free proton-donor component decreases and a new low-frequency broad v(OH) band (part of the dihydrogen-bond complex) appears. As in the case of classical hydrogen bonds. 

Except for the direct van t Hoff method, the enthalpies of dihydrogen bonds can be estimated from the frequency shifts, Av (HX), or the integral intensity, A (XH), determined in the IR spectra for proton-donor components, HX ... 

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

A similar nonlinear dependence (Figure 8.4) has been reported by Alkorta and co-workers [13]. The authors calculated a set of dihydrogen-bonded complexes where LiH plays the role of a strong proton acceptor while the C-H bond represents a proton-donor component whose strength changes within wide limits. The analytic expression of this dependence is... 

Figure 8.9 Electron density p at bond critical points found in the H- H directions versus the H- H distance in an H2 molecule and dihydrogen-bonded complexes of Li-H, BeH2, BH3, BH4, LiCCH, and CH4 with various proton-donor components HE, HNF3+, HNH3+, HNC, HCN, HCCSiHs, HCCH, HCCF, HCCCH3, HCCLi, and CH4. The taboo domain between 0.9 and 1.15 A, shown as the rectangle, corresponds to the case when the dihydrogen-bonded complexes are not stable enough to yield a free H2 molecule, or are nonexistent. (Reproduced with permission from ref. 18.)...

The ready availability of the transketolase (TK E.C. 2.2.1.1) from E. coli within the research collaboration in G. A. Sprenger s group suggested the joint development of an improved synthesis of D-xylulose 5-phosphate 19, which was expensive but required routinely for activity measurements [27]. In vivo, transketolase catalyzes the stereospecific transfer of a hydroxyacetyl nucleophile between various sugar phosphates in the presence of a thiamine diphosphate cofactor and divalent cations, and the C2 donor component 19 offers superior kinetic constants. For synthetic purposes, the enzyme is generally attractive for its high asymmetric induction at the newly formed chiral center and high kinetic enantioselectivity for 2-hydroxyaldehydes, as well as its broad substrate tolerance for aldehyde acceptors [28]. 

Scheme 1. Classification of lyases according to their donor components...

Typically, because of mechanistic requirements the lyases are highly specific for the nucleophilic donor component. This includes the necessity for a reasonably high substrate affinity as well as the general difficulty of binding and anchoring a rather small molecule in a fashion that restricts solvent access to the carb-anionic site after deprotonation and shields one enantiotopical face of the nucleophile in order to secure correct diastereofacial discrimination (Figs. 2 and 3) [51]. Usually, approach of the aldol acceptor to the enzyme-bound nucleophile occurs stereospecifically following an overall retention mechanism,... 

Representatives of all kinds have been explored for synthetic applications while mechanistic investigations were mainly focussed on the distinct FruA enzymes isolated from rabbit muscle [196] and yeast [197,198]. For mechanistic reasons, all DHAP aldolases appear to be highly specific for the donor component DHAP [199], and only a few isosteric replacements of the ester oxygen for sulfur (46), nitrogen (47), or methylene carbon (48) were found to be tolerable in preparative experiments (Fig. 7) [200,201], Earlier assay results [202] that had indicated activity also for a racemic methyl-branched DHAP analog 53 are now considered to be artefactual [203]. Dihydroxyacetone sulfate 50 has been shown to be covalently bound via Schiff base formation, but apparently no a-deprotonation occurred as neither H/D-exchange nor C-C... 

Functionally and mechanistically reminiscent of the pyruvate lyases, the 2-deoxy-D-ribose 5-phosphate (121) aldolase (RibA EC 4.1.2.4) [363] is involved in the deoxynucleotide metabolism where it catalyzes the addition of acetaldehyde (122) to D-glyceraldehyde 3-phosphate (12) via the transient formation of a lysine Schiff base intermediate (class I). Hence, it is a unique aldolase in that it uses two aldehydic substrates both as the aldol donor and acceptor components. RibA enzymes from several microbial and animal sources have been purified [363-365], and those from Lactobacillus plantarum and E. coli could be induced to crystallization [365-367]. In addition, the E. coli RibA has been cloned [368] and overexpressed. It has a usefully high specific activity [369] of 58 Umg-1 and high affinity for acetaldehyde as the natural aldol donor component (Km = 1.7 mM) [370]. The equilibrium constant for the formation of 121 of 2 x 10M does not strongly favor synthesis. Interestingly, the enzyme s relaxed acceptor specificity allows for substitution of both cosubstrates propional-dehyde 111, acetone 123, or fluoroacetone 124 can replace 122 as the donor [370,371], and a number of aldehydes up to a chain length of 4 non-hydrogen atoms are tolerated as the acceptor moiety (Table 6). 

Like the DHAP aldolases, the class II 3-hexulose 6-phosphate aldolase from a unique formaldehyde-fixing system of the methylotrophic bacterium Methyl-omonas Ml5 utilizes a ketose phosphate, i.e. D-ribulose 5-phosphate (128), as the aldol donor component for which it has a stringent requirement [374], On the... 

Because of its ready availability and stability, the transketolase from yeast has been studied more frequently than enzymes from other sources for mechanistic as well as synthetic purposes. A valuable feature is that during the first round of a ping-pong catalytic cycle the natural ketose donor component(s) can... 

With benzaldehyde 144 or halogenated derivatives (Cl, F) as acceptors the yeast-PDC-catalyzed addition proceeds with almost complete stereoselectivity to furnish the corresponding (R)-configurated 1-hydroxy-1-phenylpropanones 145 [447]. For practical reasons, whole yeast cells are most often used as the catalyst, with only small loss of enantioselectivity [423,424]. The conversion of benzaldehyde in particular has gained industrial importance because the acyloin is an important precursor for the synthesis of L-(-)-ephedrine [448]. Otherwise, the substrate tolerance is remarkably broad for aromatic aldehydes on the laboratory scale, however, yields of acyloins are usually low because of the prior or consequent reductive metabolism of aldehyde substrate and product, giving rise to considerable quantities of alcohol 146 and vicinol diols 147, respectively [423,424,449], The range of structural variability covers both higher a-oxo-acids (e.g. -butyrate, -valerate) as the donor component, as well as a,/J-un-saturated aldehydes (e.g. cinnamaldehyde 148) as the acceptor [450]. 

Two isomeric naphtho[l,8-fec 4,5-fe c ]dithiophene (syn-NDT) and naph tho[l,8-5c 5,4-5 c Jdi thiophene (anfz -NDT) and their dithienyl derivatives which served as good donor components for molecular-based conductors and conducting polymers were developed. Absorption spectra of 453-457 also provide information on the extent of re-conjugation between... 

Thus far only eight different types of five-membered heterocycles with two heteroatoms, one of these being tellurium, are known. Of special interest are those containing S,Se, or Te as the second heteroatom in the ring. Such compounds afford electron-donor components of charge-transfer complexes or radical cation salts, exhibiting properties of the so-called organic metals. 

The problem of the very low solubility of the majority of tellurium-containing donor component of charge-transfer complexes as well as of the radical cation salts derived from them puts certain limitations on their use and study. But the synthesis of a number of structurally modified derivatives of TTeF was accomplished. Thus, soluble derivatives of tetra-tellurafulvalenes 67 were synthesized in 20% (R = H, R = Me) to 35% (R = R = Me) yields and their structures were proved by cyclic voltame-try studies (87MI6 88MI5). 

Photoexcitation of the donor component (Fig. 2, F) is followed by electron transfer to the C6o molecule (Fig. 2,12). However, photoexcitation of amines is possible only in the UV-range. 

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