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The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

Nitrogen nucleophiles used to diplace the 3 -acetoxy group include substituted pyridines, quinolines, pyrimidines, triazoles, pyrazoles, azide, and even aniline and methylaniline if the pH is controlled at 7.5. Sulfur nucleophiles include aLkylthiols, thiosulfate, thio and dithio acids, carbamates and carbonates, thioureas, thioamides, and most importandy, from a biological viewpoint, heterocycHc thiols. The yields of the displacement reactions vary widely. Two general approaches for improving 3 -acetoxy displacement have been reported. One approach involves initial, or in situ conversion of the acetoxy moiety to a more facile leaving group. The other approach utilizes Lewis or Brmnsted acid activation (87). 

Conversion to a more facile, sulfur-derived, leaving group can be achieved by treatment with sodium thiosulfate or salts of thio and dithio acids (75,87). Under anhydrous conditions, boron tribromide converts the 3 -acetoxy group to a bromide whereas trimethyl silyl iodide gives good yields of the 3 -iodide (87,171,172). These 3 -halides are much more reactive, even when the carboxyl group is esterified, and can be displaced readily by cyano and by oxygen nucleophiles (127). 

Indeed the cobalt(III) ion is sufficientiy unstable in water to result in release of oxygen and formation of cobalt(II) ion. Under alkaline conditions the oxidation is much more facile and in the presence of complexing agents, eg, ammonia or cyanide, the oxidation may occur with ease or even spontaneously. 

Electrophilic substitution reactions of unsubstituted quinoxaline or phenazine are unusual however, in view of the increased resonance possibilities in the transition states leading to the products one would predict that electrophilic substitution should be more facile than with pyrazine itself (c/. the relationship between pyridine and quinoline). In the case of quinoxaline, electron localization calculations (57JCS2521) indicate the highest electron density at positions 5 and 8 and substitution would be expected to occur at these positions. Nitration is only effected under forcing conditions, e.g. with concentrated nitric acid and oleum at 90 °C for 24 hours a 1.5% yield of 5-nitroquinoxaline (19) is obtained. The major product is 5,6-dinitroquinoxaline (20), formed in 24% yield. 

When activating substituents are present in the benzenoid ring, substitution usually becomes more facile and occurs in accordance with predictions based on simple valence bond theory. When activating substituents are present in the heterocyclic ring the situation varies depending upon reaction conditions thus, nitration of 2(177)-quinoxalinone in acetic acid yields 7-nitro-2(177)-quinoxalinone (21) whereas nitration with mixed acid yields the 6-nitro derivative (22). The difference in products probably reflects a difference in the species being nitrated neutral 2(177)-quinoxalinone in acetic acid and the diprotonated species (23) in mixed acids. 

No practical type B syntheses of quinoxalines are commonly in use, largely because of the fact that type A syntheses are more facile however, some phenazine syntheses of this type are known, particularly those described in the older chemical literature. Hillemann (38CB42) has effected dimerization of 0-bromoaniline by heating its solution in nitrobenzene with K2CO3 and copper powder. The reaction is believed to proceed through the intermediacy of 5,10-dihydrophenazine, but the latter has not been isolated (Scheme 68). 

The isomerization of carbenaoxirane (81) to oxirene seems unlikely, as ab initio calculations predict a more facile conversion to ketene (Figure 3). However, oxirene involvement has been demonstrated for ketene photolysis (Scheme 102) (80PAC1623) and this may proceed through vibrationally excited (81) cold carbenaoxiranes would appear to be even... 

Another way of interpreting absolute risk estimates is through the use of benchmarks or goals. Consider a company that operates 50 chemical process facilities. It is determined (through other, purely qualitative means) that Plant A has exhibited acceptable safety performance over the years. A QRA is performed on Plant A, and the absolute estimates are established as calibration points, or benchmarks, for the rest of the firm s facilities. Over the years, QRAs are performed on other facilities to aid in making decisions about safety maintenance and improvement. As these studies are completed, the results are carefully scrutinized against the benchmark facility. The frequency/consequence estimates are not the only results compared—the lists of major risk contributors, the statistical risk importance of safety systems, and other types of QRA results are also compared. As more and more facility results are accumulated, resources are allocated to any plant areas that are out of line with respect to the benchmark facility. 

The more facile migration of aryl and other unsaturated groups involves bridged intermediates formed by an addition process. In the case of aryl migration, the intermediate is a cyclohexadienyl radical ... 

The use of dimethyl sulfoxide-acetic anhydride as a reagent for the oxidation of unhindered steroidal alcohols does not appear to be as promising due to extensive formation of by-products. However, the reagent is sufficiently reactive to oxidize the hindered 11 j -hydroxyl group to the 11-ketone in moderate yield. The use of sulfur trioxide-pyridine complex in dimethyl sulfoxide has also been reported. The results parallel those using DCC-DMSO but reaction times are much shorter and the work-up is more facile since the separation of dicyclohexylurea is not necessary. Allylic alcohols can be oxidized by this procedure without significant side reactions. 

The azido mesylate may also be reduced with lithium aluminum hydride in the same manner as previously described for iodo azide reductions. The sodium borohydride/cobalt(II)tris(a,a -dipyridyl)bromide reagent may be used, but it does not seem to offer any advantages over the more facile lithium aluminum hydride or hydrazine/Raney nickel procedures. 

Although nitrosation of (l-dicarbonyl compounds becomes increasingly more facile upon successive replacement of the a alkyl groups with perfluoroalkyl groups because of the increased ionization of the perfluorinated enolate (equation 7), the stability of the nitrosodiketone tautomers decreases Thus, 1,1,1-trifluoro pentane-2,4-dione and 1,1,1,5,5,5-hexafluoropentane 2,4 dione mtrosate much faster than penta-2,4 dione but yield ketoximes, which decompose upon workup... 

With a difluoroethylene containing hydrogen and chlonne, where both groups can be replaced by lithium, a mixture of two organolithium compounds is formed in a 2 1 ratio, indicating a more facile replacement of chlorine [63] (equation 30)... 

One way you can help minimize such bias is to involve more facility personnel in a PSM survey than would likely participate in a PSM audit. Broad distribution of a questionnaire among facility staff does not appreciably add to the level of effort required of the PSM team, and may help provide a more balanced perspective. 

Hydrazine, EtOH, 85% yield. Note that the cleavage of an anilide and a benzoylpyrrole is much more facile than that of a typical aliphatic benzamide. 

Examine the transition state for the hydride shift. Calculate the barrier from the more stable initial carbocation. Is the process more facile than typical thermal rearrangements of neutral molecules (.05 to. 08 au or approximately 30-50 kcal/mol) Is the barrier so small (<.02 au or approximately 12 kcal/mol) that it would be impossible to stop the rearrangement even at very low temperature Where is the positive charge in the transition state Examine atomic charges and the electrostatic potential map to tell. Is the name hydride shift appropriate If not, propose a more appropriate name. 

Calculate activation energies for Sn2 reactions of ammonia and trimethylamine with methyl iodide via transition states ammonia+methyl iodide and trimethyl-amine+methyl iodide, respectively. Is attack by ammonia or trimethylamine more facile Rationalize your observation by comparing electrostatic potential maps for the two transition states. Which transition state requires more charge separation Is this also the higher-energy transition state ... 

An unusual sensitivity of this reaction to structure was reported by Ram and Neumeyer (51). When R = H (1), hydrogenolysis could not be effected either directly or by catalytic hydrogen transfer (13), but etherification to give 2 (R = CH3) permitted slow formation of 3, The mild conditions of hydrogenation were required to avoid racemization at the 6a-position. Hydrogenolysis is usually much more facile than is indicated by this example. 

However, reaction (32) is expected to be more facile since it required minimum energy. Gray et al. [115] have found that alkoxy radicals formed by the decomposition... 

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

Broxton and Bunnett (1979) determined the products of the reaction of 4-chloro-3-nitrobenzenediazonium ions with ethoxide ion in ethanol, which is exactly analogous to the reaction in methanol discussed earlier in this section. These authors found 12.8% 4-chloro-3-nitrophenetole, 83% 2-chloronitrobenzene, and 0.8% 2-nitrophenetole. When the reaction was carried out in C2H5OD, the first- and second-mentioned products contained 99% D and 69% D respectively. Dediazoniation in basic ethanol therefore results in a higher yield of hydro-de-diazoniation with this diazonium salt compared with the reaction in methanol. This is probably due to the slightly higher basicity of the ethoxide ion and to the more facile formation of the radical CH3-CHOH (Packer and Richardson, 1975). Broxton and McLeish (1983 c) measured the rates of (Z) — (E) interconversion for some substituted 2-chlorophenylazo ethyl ethers in ethanol. 

It is highly probable that the lesser stability of thiirene dioxides compared with that of the thiirene oxides simply reflects the more facile extrusion of sulfur dioxide relative to that of sulfur monoxide. In fact, the same effect is probably operative in the case of the cis- and trans-diphenylthiirane oxides (16g,h)110 compared with cis- and trans-diphenylthiirane dioxides (17d,e)99 the former were found to be more stable toward thermal decomposition than the latter. 

Addition to a carbon-carbon triple bond is even more facile than addition to a carbon-carbon double bond, and there are now several reports of intermolec-ular [71] and intramolecular reactions [72-74] that produce stable cyclopropene products with moderate to high enantioselectivities. One of the most revealing examples is that shown in Scheme 9 [72] where the allylic cyclopropanation product (30) is formed by the less reactive Rh2(MEPY)4 catalyst, but macrocy-clization is favored by the more reactive Rh2(TBSP)4 and Rh2(IBAZ)4 catalysts and, as expected, the highest enantioselectivities are derived from the use of chiral dirhodium(II) carboxamidate catalysts. 

Compared with the aromatic electrophilic substitution approach, the SNAr approach general requires higher reaction temperatures. The polymers generally have well-defined structures. Therefore, it is more facile to control the structures of die products. In addition, it is more tolerable to some reactive functional groups, which makes it possible to synthesize reactive-group end-capped prepolymers and functional copolymers using functional monomers. 

The observation of Tsuji et al. 148) concerned with copolymerization of 1- or 2-phenyl butadiene with styrene or butadiene illustrates again the importance of the distinction between the classic, direct monomer addition to the carbanion, and the addition involving coordination with Li4. The living polymer of 1- or 2-phenyl butadiene initiated by sec-butyl lithium forms a block polymer on subsequent addition of styrene or butadiene provided that the reaction proceeds in toluene. However, these block polymers are not formed when the reaction takes place in THF. The relatively unreactive anions derived from phenyl butadienes do not add styrene or butadiene, while the addition eventually takes place in hydrocarbons on coordination of the monomers with Li4. The addition through the coordination route is more facile than the classic one. 

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