Electron-richness

Below 90 K, [60]fullerene freezes into an orientational glass in which it adopts a simple cubic stmcture [19]. This low temperature stmcture can be traced to the anisotropic electronic stmcture. Alignment of the electron rich regions of... 

FMO theory requires that a HOMO of one reactant has to be correlated with the LUMO of the other reactant. The decision between the two alternatives - i.e., from which reactant the HOMO should be taken - is made on the basis of which is the smaller energy difference in our case the HOMO of the electron rich diene, 3.1, has to be correlated with the LUMO of the electron-poor dienophile, 3.2. The smaller this HOMO-LUMO gap, the higher the reactivity will be. With the HOMO and LUMO fixed, the orbital coefficients of these two orbitals can explain the regios-electivity of the reaction, which strongly favors the formation of 3.3 over 3.4. 

D-A rxns with electron rich dienes and electron defficient dienophiles work the best. 

D-A rxns with electron deficient dienes and electron rich dienophiles also work well. These are refered to as reverse demand D-A rxns. 

Surprisingly, the highest catalytic activity is observed in TFE. One mi t envisage this to be a result of the poor interaction between TFE and the copper(II) cation, so that the cation will retain most of its Lewis-acidity. In the other solvents the interaction between their electron-rich hetero atoms and the cation is likely to be stronger, thus diminishing the efficiency of the Lewis-acid catalysis. The observation that Cu(N03)2 is only poorly soluble in TFE and much better in the other solvents used, is in line with this reasoning. 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

In the coupling of the allenyl ester 7 with a terminal alkyne, an electron-deficient phosphine (Ph3P) gave the enyne-conjugated ester 8 as the major product, while an electron-rich phosphine (TDMPP or TTMPP) yielded the non-conjugated enyne esters ( )- and (Z)-9[4],... 

In the case of substituted aryl radicals, the results may be slightly different, depending on the polarity of the radicals. With electrophilic radicals the overall reactivity of the thiazole nucleus will decrease and the percentage of 5-substituted isomer (electron-rich position) will increase, in comparison with phenyl radicals. The results are indicated in Table III-28. 

FIGURE 6 4 Electro static potential maps of HCI and ethylene When the two react the interaction is between the electron rich site (red) of ethylene and electron poor region (blue) of HCI The electron rich region of ethylene is associ ated with the tt electrons of the double bond and H IS the electron poor atom of HCI... 

In general alkyl substituents increase the reactivity of a double bond toward elec trophilic addition Alkyl groups are electron releasing and the more electron rich a dou ble bond the better it can share its tt electrons with an electrophile Along with the observed regioselectivity of addition this supports the idea that carbocation formation rather than carbocation capture is rate determining... 

FIGURE 9 3 Electro static potential maps of eth yiene and acetylene The region of highest negative charge (red) is associated with the TT bonds and lies between the two carbons in both This electron rich re gion IS above and below the plane of the molecule in ethylene Because acetylene has two TT bonds a band of high electron density encir cles the molecule... 

The electrophile (E ) m this reaction is mtromum ion (0=N=0) The charge distn bution m mtromum ion is evident both m its Lewis structure and m the electrostatic potential map of Figure 12 2 There we see the complementary relationship between the electron poor region near nitrogen of NO, and the electron rich region associated with the TT electrons of benzene... 

FIGURE 14 1 Electro static potential maps of (a) methyl fluoride and (b) methyllithium The electron distribution is reversed in the two compounds Carbon IS electron poor (blue) in methyl fluoride but electron rich (red) in methyllithium... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

H3P—CH2 where it can be seen that the electron distribution is highly polarized m the direction that makes carbon electron rich The carbon has much of the character of a car banion and can act as a nucleophile toward C=0... 

Both parts of the Lapworth mechanism enol formation and enol halogenation are new to us Let s examine them m reverse order We can understand enol halogenation by analogy to halogen addition to alkenes An enol is a very reactive kind of alkene Its carbon-carbon double bond bears an electron releasing hydroxyl group which makes it electron rich and activates it toward attack by electrophiles... 

Lone pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron rich site to be the oxygen of the carbonyl group and the most electron poor one to be as expected the OH hydrogen... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

Fnedel-Crafts alkylation Alcohols in combination with acids serve as sources of carbocations Attack of a carbocation on the electron rich ring of a phe nol brings about its alkylation... 

The aromatic ring of a phenol like that of an arylamine is seen as an electron rich functional unit and is capable of a variety of reactions In some cases however it IS the hydroxyl oxygen that reacts instead An example of this kind of chemical reac tivity IS described m the following section... 

Phenols are polar compounds but less polar than alcohols They resem ble arylammes m having an electron rich aromatic ring... 

Phenylalanine and tryptophan have side chains that incorporate aromatic rings which are large and hydrophobic The aromatic portion of tryptophan is bicyclic which makes it larger than phenylalanine Tryptophan also has a more electron rich aromatic ring and is more polarizable than phenylalanine Its role is more specialized and it is less abundant m proteins than most of the other ammo acids... 

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

Copolymers of VF and a wide variety of other monomers have been prepared (6,41—48). The high energy of the propagating vinyl fluoride radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (49). The low value of is consistent with titde resonance stability and the e value is suggestive of an electron-rich monomer. 

Preformed Carbocationic Intermediates. Propargyl cations stabilized by hexacarbonyl dicobalt have been used to effect Friedel-Crafts alkylation of electron-rich aromatics, such as anisole, /V, /V- dim ethyl a n il in e and 1,2,4,-trimethoxybenzene (24). Intramolecular reactions have been found to be regio and stereo-selective, and have been used ia the preparatioa of derivatives of 9JT- uoreaes and dibenzofurans (25). 

The other important direct alkylation processes involve reaction of electron-rich olefinic compounds with either tin metal or stannous chloride (tin(II) chloride) in the presence of stoichiometric amounts of hydrogen chloride (22). Butyl acrylate (R = C Hg) was used commercially in this process to prepare the estertin or P-carboalkoxyethyltin chlorides as iHustrated in the foUowing. 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

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