Electron-rich character

Significant advance in the field of asymmetric catalysis was also achieved with the preparation of l,2-bis(phospholano)benzene (DuPHOS 4) and its confor-mationally flexible derivative (l,2-bis(phospholano)ethane, known as BPE) by Burk et al. [59]. Two main distinctive features embodied by these Hgands, as compared to other known chiral diphosphine ligands, are the electron-rich character of the phosphorus atoms on the one hand and the pseudo-chirality at phosphorus atoms, on the other. These properties are responsible for both the high activity of the corresponding metal complex and an enantioselection indepen-... 

In addition, complexes of P(/-Bu)3 have been shown to catalyze the formation of diaryl heteroarylamines from bromothiophenes.224 Aminations of five-membered heterocyclic halides such as furans and thiophenes are limited because their electron-rich character makes oxidative addition of the heteroaryl halide and reductive elimination of amine slower than it is for simple aryl halides. Reactions of diarylamines with 3-bromothiophenes occurred in higher yields than did reactions of 2-bromothiophene, but reactions of substituted bromothiophenes occurred in more variable yields. The yields for reactions of these substrates in the presence of catalysts bearing P(/-Bu)3 as ligand were much higher than those in the presence of catalysts ligated by arylphosphines. 

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

Trimethylsilyloxy-substituted alkenes are by far the most widely used enol ethers because of their straightforward preparation from the corresponding ketones (equation 20)78-82 -pjjg electron-rich character of silyl enol ethers allows for highly chemoselective cyclopropanations in the presence of additional double bonds (eqnation 21). ... 

In order to increase the electron-rich character of monomer and polymer, leading to reduced oxidation potentials and lower band gaps, researchers at Bayer AG have developed PEDOT [245-248]. By appending the electron donating di-... 

Firstly, copper(I) preferentially formed complexes that incorporated the methylated aldehyde. All three possible products shown in Scheme 1.2 were observed in the reaction of Scheme 1.14. The two aldehydes were incorporated into the product mixture in a molar ratio of 30 70, which indicates a slight thermodynamic preference for the incorporation of the methylated aldehyde, whose more electron-rich character would allow ligands that incorporate this residue to better stabilize the cationic copper(I) center. 

According to Mulliken [9], arenes are classified as electronic donors (D) in measure with their degree of electron-rich character, as evaluated by their ionization potential (IP, gas phase) or oxidation potential (E°ox, solution) [12]. The intermolecular interaction of the arene donor (D) and electronic acceptor (A) spontaneously leads to the electron donor/ acceptor or EDA complex, i.e. 

Similar to the abovementioned silver nhc coordination compounds, carbene chemistry has also been dominant in the field of gold organometallic chemistry. Noteworthy examples include a Au(PPh3)-compound derived from tetraaminoallene, that can be rationalised in terms of a dicarbene with ylide character and which, owing to the electron-rich character of the central carbon atom, offers the potential for dimetallation products.108 Non-activated allenes and alkynes have been found by Lavallo to be readily aminated by cationic carbene gold complexes.109 For this purpose, a 2,6-diisopropylphenyl functionalized cyclic alkylaminocarbene gold(I) complex... 

Silyl enolates are a class of electron-rich, non aromatic compounds which can be described as masked enols or enolates since hydrolysis following their reaction yields ketones they can be purified by distillation or chromatography, and then converted back to the enolate anion. The electron-rich character of these species can be used for oxidation reactions and examples have been described in the preceding sections. In this section, additional examples of chemical, PET and electrochemical redox reactions involving silyl enolates will be discussed, for a better appreciation of these interesting species in organic synthesis. 

The electron-rich character of the aromatic system leads to reactions with electrophiles involving an addition-elimination pathway with the overall effect of substitution... 

Enolate ions are more useful than enols for two reasons. First, pure enols can t normally be isolated. They are usually generated only as short-lived intermediates in low concentration. By contrast, stable solutions of pure enolate ions are easily prepared from most carbonyl compounds by reaction wilh a strong base. Second and more important, enolate ions are much more reactive than enols and undergo many reactions that enols don t. Whereas enols are neutral, enolate ions are negatively cheirged, making them much better nucleophiles. Thus, the a carbon atom of an enolate ion is highly reactive toward electrophiles. An electrostatic potential map of acetone enolate ion, for instance, shows the electron-rich character (red) of the a carbon. 

Despite the rr-electron-rich character of tetrathiafulvalenes, very few reactions with electrophilic reagents have been described, since they usually lead to oxidation rather than to substitution and/or addition. The only known products of attack on sulfur are the 5-oxides [78JOC4394 79JCS(P2)862] and a formally 5-alkylated derivative produced in the reaction of TTF+ with a free radical, rather than by electrophilic attack (Section II.B.4). 

To reduce the strain associated with cyclization, we next stndied acyclic piperidine precnrsors as nucleophiles to ultimately intercept the Mazzocchi, discovery, and cyanohydrin approaches. Diethyl glutaconate 35 conpled readily with flnoro-4-nitroarenes 30a or 30b to give 36 (95%, HPLC Scheme 3.12)." ° Again, however, ring closnre to 37 did not proceed by either S Ar (36a, X = F) or palladinm catalysis (36b, X = Br). The newly formed ally lie anion of 36 presumably delocalized into the nitroaromatic ring to generate electron-rich character inhibiting subsequent nucleophilic Sf Ar substitution or the oxidative addition required to enter palladinm catalysis pathways. 

The electron-rich character of keten acetals (36) means that they undergo cycloaddition with ketens with judicious choice of reactants, this results in the formation of dihydropyran-2-ones, e.g. (37), which are hydrolysed to the dioxo-derivative (38). /3-Keto-esters are cyclized by their reaction with 3-hydroxy-aldehydes and titanium(rv) chloride to form 5,6-dihydropyran-2-ones. Aroylarylacetylenes condense with either ethyl cyanoacetate or aceto-acetate in the presence of a base to form 4,6-diarylpyran-2-ones (39)." ... 

On the basis of the present studies, it appears that the affinity of the 2,5-DMAs for 5-HT2 receptors can be accounted for, primarily, by the llpophiliclty of the 4-position substituent. Other factors, particularly those dealing with length or shape, may also play a role. It is entirely possible that the electronic nature of the 4-position substituents is also involved in affinity (at least for a small subset of 2,5-DMA analogs) but this has been more difficult to demonstrate electronic factors may also be involved in Intrinsic activity, particularly as it relates to altering the electron rich character of the aromatic nucleus of the phenyllsopropylamlnes. The pre-... 

Extension of the [3+4] annulations to more highly oxygenated dienes results in a very effective synthesis of tropones and tro-polones. For example, decomposition of the vinyldiazomethane 4 in the presence of 1-methoxy-l-trimethylsiloxy-1,3-butadiene (46) results in the formation of the cycloheptadiene 47 in 87% yield (Scheme 18). Mild hydrolysis of 47 with citric acid followed by DDQ oxidation results in a very short synthesis of the tropone 48 in 92% yield. The [3+4] annulation with 1-methoxy-l-trimethylsiloxybutadiene is applicable to a range of vinyldiazomethanes, as shown in Table 8. The major limitation is that because of the extremely electron-rich character of the diene, the formation of side products derived from zwit-terionic intermediates dominate over the [3+4] annulation. In general, the side reactions are eliminated by carrying out the reactions in a nonpolar solvent. 

Another reason for generally low performance of PT homopolymers in LED applications is poor electron injection/mobility brought about by electron-rich character of these materials. This can potentially be improved by introducing electron-withdrawing substituents or moieties into the polymer structure. 

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