Metal conditions

Insufficient information about the properties, layout pattern of small defects, potential for their growth in time, usually leads either to an unjustified rejection (repair) or to underestimation of the importance of the defect and, as aconsequence, construction failure. Use of automated computerised means of control allows safe service of the old constructions, periodically repeating the UT and monitoring the development of discontinuities in the metal. The main idea of such policy is periodical UT of development of discontinuities or, in a more general form, monitoring of the metal condition. 

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

The cyclohexylidene ketal, prepared from a catechol and cyclohexanone (AI2O3/ TsOH, CH2CI2, reflux, 36 h), is stable to metalation conditions (RX/BuLi) that cleave aiyl methyl ethers. The ketal is cleaved by acidic hydrolysis (coned. HCl/ EtOH, reflux, 1.5 h, 20°, 12 h) it is stable to milder acidic hydrolysis that cleaves tetrahydropyranyl ethers (1 AHCl/EtOH, reflux, 5 h, 91% yield). ... 

The process here symbolised is probably. . . the chemical reaction between metals and a chemical reagent and the subsequent restoration to a metallic condition... As the artist can see in the landscape graciousness, solemnity, terror, so Zosimos sees the rigour of death and pains of purgation in the turbid seething of the alchemical vessel. ... 

The environment has negative effects on most metals thus, when metallic archaeological objects are eventually found, they are generally in an advanced state of decay. The decay of metals and alloys caused by the chemical action of gases and/or liquids in the environment is known as corrosion. Corrosion processes are natural destructive processes that result in the waste of most metals and alloys. The ultimate result of all corrosion processes is the reversion of most metals from the metallic condition in which they are used, to the chemically combined form in which they naturally occur in the crust of the earth. Rust, the reddish-brown corrosion product that forms on... 

Zhang54 published the first and only account of a non-asymmetric rhodium-catalyzed Alder-ene cycloisomerization of 1,6-enynes.55 The conditions developed by Zhang and co-workers are advantageous in that, similar to the ruthenium conditions developed by Trost, selectivity for 1,4-diene products is exhibited. The rhodium conditions are dissimilar from many other transition metal conditions in that only (Z)-olefins give cycloisomerization products. 

Finally, the primary site of reaction with some heterocyclic systems may not be one that is desired for final elaboration, and in these cases blocking of the initial reaction site is necessary. This can often be achieved by a consecutive metalation and alkylation sequence, but as with N—H protection the overall process, including final removal of the blocking group, must be achievable in reasonable yield. Trialkylsiyl groups have so far received the most attention in this area, because of their ready addition, relative stability under metalation conditions, and facile hydrolysis. 

Entry R Metalation conditions Yield (%) Deprolection Yield (%) Reference... 

If the reaction mixture is allowed to warm before addition of the electrophile, then isomerization to the more thermodynamically stable 4-met-alated derivative can result in the formation of 3,4-disubstituted products. However, if the 4-position is already substituted then metalation and subsequent reaction at C-2 are much easier to achieve, and a wider range of metalation conditions can be used. Direct 2,4-disubstitution can be achieved in those cases where the electrophile is stable in the presence of the base. Normally this involves the use of LDA in the presence of chlorotrimethylsilane. In this case metalation and substitution occur first at C-4 followed by reaction at C-2. 

Substituents Metalation conditions Electrophile Yield (%> Reference... 

Metalation conditions AA Position Electrophile Yield lc4) -AA Reference... 

However, the directed /3-metalation and derivatization of benzol[fc]-l, 4-diazabicyclo[2,2,2]octene has been successfully achieved, and in fact the whole process can be repeated next to the second nitrogen atom, provided that the initially added electrophile is stable to the metalation conditions (Scheme 150) (91KGS798). 

A simple and flexible synthesis of benzoxathiin-2-one from phenols has been reported. The key step in this synthesis is a hitherto unknown anionic rearrangement under direct metallation conditions <2004T5215, 2003SL1474>. 

Starting Material Metalation Conditions Alkylation Agent R3X d.r. (6a-R3/ 6/J-R3) Yield (%) mp (CC) Ref... 

Detonation Velocity by Metallic Transition of Sulfur. Joigneau Thouvenin (Ref 1) reported a large increase in the elec conductivity of cryst sulfur when it was subjected to high transient pressure, but detected no sudden or discontinous transition to metallic condition. It was inferred from their results that a modified system should permit the use of sulfur as an active element of a pressure transducer for measurements in the kilobar range Hauver (Ref 2) prepd a modified system using a thin disc of sulfur, 0.00 50 inch thick and 9/32 inch in diam, insulated in Teflon. A number of tests were performed... 

The majority of metallic calces are only reduced, that is to say, only return to the metallic condition, by immediate contact with a carbonaceous material, or with some substance containing what is called phlogiston. The charcoal that one uses is entirely destroyed during the... 

Only when the d-band is sufficiently weU formed, running over all sites, host and dopant alike, is the fuU benefit of configuration crossover attained. We have seen, with the B phase of (Sm/Gd)S, that when the Sm/ d sites are excluded from the Fermi sea, the ICF conversion percentage becomes relatively low. This is so despite the continuing metallic background of the dopant d gas. With a dopant that is unable to promote metallic conditions the position will be worse. 

On the other hand, LDA metalation of 3-bromopyridine (42) at -70°C yields, after hydrolysis, a mixture of 3- and 4-substituted products (43 and 44) in addition to starting material 42 (Scheme 14) (82T3035). A potential explanation for these results involves the formation of 3,4-pyridyne, which undergoes nonregioselective attack by amine or lithio amide to give 43 and 44. An alternative rationalization is the isomerization of 42 into the 4-isomer 45 under the metalation conditions (see Section II,B,4), followed by the conversion of either isomer into the radical anions 46 which, via the caged radical pairs 47, is converted into 43 and 44 (Radical Anion-Radical Pair = RARP pathway). 

Fluoropyridine also gave 4-substituted products in high yield (75-87%) using LDA [(1 equiv.)/THF/0.5-4hr] metalation conditions (80TL4137 83T2009). Thus, metalation of 3-chloro and 3-fluoropyridine with LDA appears to occur rapidly at the most acidic C4 site. Compared to deprotonation with alkyllithium reagents, which is slow and irreversible, LDA metalation appears to be a fast equilibrium process. 

Application of these metalation conditions to a more diverse series of pyrimidines 384, including a 4-A-pivaloyl derivative, with intervention of a TMSC1 quench leads to 5-TMS products 385 in low yields (Scheme 117). 

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