Electron deactivation

Carbene Reactions. The best procedure for preparing dihalocarbene adducts of olefins consists in stirring a haloform—methylene chloride solution with an excess of concentrated aqueous caustic soda in the presence of hen 2y1triethy1 amm onium chloride. Even stericahy hindered and electronically deactivated compounds give excellent yields (32). Mixed dihalocarbenes, CXY (X,Y = E, Cl, Br, I), except for CE2, can be prepared. 

The HO formation reaction R2 is in competition with electronic deactivation back to the ground state of the oxygen atom... 

Due to electronic deactivation of the substrate, hydrodechlorination of the dichlorophenol is discriminated in comparison to the monosubstituted substrate. 

Dimethoxybenzoylimidazolium chloride also reacts rather slowly because of steric hindrance and electronic deactivation of the carbonyl group by the two methoxy groups. The synthesis of 2,6-dimethoxybenzoyl chloride was carried out in chloroform at 90 °C in a bomb tube to give a yield of 60% after 3 h.[41... 

Dissociation of axial ligands has been followed by picosecond spectroscopy for a number of metalloporphyrins. For the well-known photodissociation of O2 and CO from hemoglobin and myoglobin the photoproducts appear very early < 10 psec. Dissociation of basic axial ligands such as pyridine and piperadine occurs within the lifetime of the excited state for Ni(II), Co(III) as well as for Fe(II) porphyrins. Whether the ejected species is "hot" with energy from the electronic deactivation of the porphyrin is not known, but the dissociation process does not appear to be dependent upon the wavelength of the excitation pulse (30,32). 

Perphthalic acid <550SC(3)619) is an even milder reagent that works even at —20 to 20 °C. Performic, permaleic and pertrifluoroacetic acids are strong oxidizing agents and they are recommended for AT-oxidation of the least reactive substrates. The last of the three is the most commonly used, especially for the oxidation of highly deactivated substrates (Scheme 16) such as perchloropyridine (electronic deactivation) (74MI20501) and 2,6-dibromopyridine (electronic and steric deactivation) (equation 35). 

A more clear-cut demonstration of electronic deactivation was provided55 by the cyclization of ethyl 5-(2-diazo-l-oxoethyl)octanoate. In this case, there should not be any steric discrimination, vet very substantial selectivity was observed. In subsequent work, ethers and tosylates were shown to induce similar remote deactivation50. 

In complete equilibrium, the ratio of the population of an atomic or molecular species in an excited electronic state to the population in the groun d state is given by Boltzmann factor e — and the statistical weight term. Under these equilibrium conditions the process of electronic excitation by absorption of radiation will be in balance with electronic deactivation by emission of radiation, and collision activation will be balanced by collision deactivation excitation by chemical reaction will be balanced by the reverse reaction in which the electronically excited species supplies the excitation energy. However, this perfect equilibrium is attained only in a constant-temperature inclosure such as the ideal black-body furnace, and the radiation must then give -a continuous spectrum with unit emissivity. In practice we are more familiar with hot gases emitting dis-... 

Type III (no homodimerization) Acrylonitrile," protected 3° allylamines" Vinyl trialkoxysilanes, vinyl siloxanes 1,1-Disubstituted olefins, " non-bulky trisubstituted olefms, vinyl phosphonates, " vinyl phosphine oxides,phenyl vinyl sulfone, acrylonitrile, 4° allylic carbons (all alkyl substituents), protected 3° allylic alcohols, 7,Aolefm of 2-subst. 1.3- butadienes, 7,Aolefin of electronically deactivated 1.3- butadienes ... 

Type IV (spectators to CM) 1,1-Disubstituted olefms " 1,1-Disubstituted olefms, disub. o ,/ -unsaturated carbonyls, 4° allylic carbon-containing olefins, perfluorinated alkane olefins, 3° allylamines (protected)" Vinyl nitro olefins, protected trisubstituted allyl alcohols, a,/ -olefin of 2-subst. 1.3- butadienes, a,/ -olefm of electronically deactivated 1.3- butadienes ... 

Finally, we discuss briefly the results of Donovan and Husain126 on the relaxation of I(52Pi->i), for which AE = 7603 cm-1. The problem is even more complex than the foregoing, because the splitting is large and the extent of vibrational excitation accompanying electronic deactivation is not known. Atomic iodine in the 5 2Pi state can be produced by flash photolysis of various iodides,... 

Scheme 7.9. Acylation of sterically or electronically deactivated anilines [33-36],...

Another class of problematic amines are a-amino nitriles, which are readily accessible from ketones, amines, and cyanide. Like a-amino acids, these amines are electronically deactivated and less basic and nucleophilic than purely aliphatic amines, and are therefore difficult to acylate. Some a-amino nitriles or the corresponding acylated derivatives can, furthermore, decompose into imines and cyanide if reaction temperatures are too high or if the bases used are too strong (Scheme 7.11). 

Unsymmetric diamines can sometimes be acylated selectively at one amino group only, if one of the amino groups is sterically or electronically deactivated. If the difference in reactivity is only small, acylating agents of low reactivity will usually be required. Some polyamino heteroarenes can be monoacylated or monoalkylated with astonishingly high regioselectivity (Scheme 7.13), which is, however, difficult to predict. 

Because 2-haloanilines are strongly electronically deactivated with regard to oxidative addition, it may be expected that there is sufficient disparity in reactivity between N-benzyl-2-chloroaniline and 4-chloroanisole for them to undergo the sequential coupling reaction. Very little carbazole product is formed, however, the major product being the intermediate 5a. In this case it seems that the rate of the... 

It appears that such electronic deactivation by a collision process is very improbable. The dissipation of energy from the lowest vibrational level of an excited electronic state usually occurs as a result of emission of energy as radiation. 

Chloroaryls, which are electronically deactivated and thus resistant to enter to the oxidative addition, were coupled with phenyltributyltin (PhSnBus) by using tricyclohexylphosphine (Pcys) adducts of palladium in K3PO4 and 1,4-dioxane to yield the corresponding biaryls in good yield. ... 

For example, when 5,8-dimethyl-[l,2,4]triazolo[4,3- ]pyrazine was refluxed with 10% sodium hydroxide solution for 60 hours, a 20% yield of the isomeric 5,8-dimethyl-[l,2,4]triazolo[l,5-a]pyrazine was obtained. No rearrangement occurred in acid or under the influence of heat (68JHC485). The poor yield obtained in this isomerization may be due to two factors (i) electronic deactivation and steric crowding caused by the 5-methyl group and/or (ii) the great tt-electron density associated with C-5 (as determined by HMO calculations) for [l,2,4]triazolo[4,3-a]pyrazines in general (68JHC485). 

Catalyst Ru-4 exhibits overall superior activity and improved substrate scope relative to catalyst Ru-2. For example, Ru-4 completes simple metathesis reactions, such as the RCM of diethyl diallylmalonate or the ROMP of cyclooctadiene, at rates several orders of magnitude greater than with Ru-2. In addition, whereas catalyst Ru-2 is unreactive toward sterically congested or electronically deactivated substrates, Ru-4 successfully mediates the formation of tetra-substituted olefins in five- and six-mem-bered rings systems [9], as well as CM to form tri-substituted olefins and products containing electron-withdrawing substituents [10]. 

The cyclopropanation of alkenes with dihalocarbenes, CX2 or CX X, except for CF2, can be efficiently executed by the PTC procedures (CHX3/strong aqueous base/catalyst), which are well-documented in books and reviews . The dichloro- and dibromo-carbene additions by the PTC procedure have successfully been applied for many alkenes, conjugated polyolefins and allenes. Satisfactory results are also reported for the reactions of sterically hindered olefins as well as electronically deactivated olefins, which frequently... 

Reactivity in electrophilic aromatic substitution depends, then, upon the tendency of a substituent group to release or withdraw electrons. A group that releases electrons activates the ring a group that withdraws electrons deactivates the ring. 

G releases electrons activates G withdraws electrons deactivates... 

In this reaction, partial racemization of some homochiral amino acids was observed. In addition, the combination of a hindered amino acid and an electronically deactivated aldehyde may lead to low yields. Bis-alkylation side reactions of aliphatic aldehydes may generate 1-10% of tertiary amines. 

Electronically deactivated amines react only very slowly. Here, cycHzation of the acyclic peptoid dimer occurs mainly during the TEA post-treatment. 

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