Quenching directed " /// -substitution

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

Although there is a kinetic barrier to the direct deprotonation of tertiary amines, Ahlbrecht and Dollinger showed in 1984 that the Schlosser superbase, i c-BuLi/f-BuOK, can deprotonate A-methylpiperidine selectively on the methyl group (Scheme 3). This superbase probably yields an a-amino-organopotassium species (and f-BuOLi), but treatment with LiBr effects transmetalation to the more nucleophilic, and less basic, a-amino-organolithium species. Electrophilic quench with several aldehydes and ketones gives substitution products in good yields as typified by the example in Scheme 3. Similarly,... 

Direct lithiation of 2-/r t/-butyl-4(3//)-quinazolinone 76 with jr f-butyllithium and bis(dimethylamino)ethane (TMEDA) followed by quenching with diphenyldisulfide gave the 5-phenylthio derivative 77 in moderate yield, although a mixture of 5- and 8-substituted products was obtained on quenchinq with di-fetZ-butyldisulfide <2004T7983>. 

The general synthesis of the Daniphos ligands starting from enantiomerically pure [(R)-l-(phenylethyl)dimethylamine]chromiumtricarbonyl 1, is depicted in Scheme 1.4.1 [15]. A directed ortho-metallation (DOM) and subsequent quench with a chlorophosphine leads to an enantiomerically pure planar-chiral complex, which after chlorination using ACE chloride (1-chloroethyl chloroformate) is transformed into the desired diphosphine by a nucleophilic substitution without any loss of optical purity (Scheme 1.4.1) [6, 10]. 

QUINAPHOS ligands are usually synthesized in a one-pot-procedure from readily available 8-substituted quinolines [8] via nucleophilic addition of a lithium reagent [9] to the azomethinic double bond and direct quenching of the resulting 1,2-dihydroquinoline amide 1 with a phosphorochloridite derived from enantio-merically pure binaphthol (1) or from 3,3 -di-t-butyl-5,5 -dimethoxybiphenyl-2,2 -diol (m) [10] (Scheme 2.1.5.1, Method A). Alternatively, the anion 1 can be reacted with an excess (in order to avoid multiple substitution) of phosphorous trichloride to obtain the corresponding phosphorous dichloridite 2, which can be isolated (Scheme 2.1.5.1, Method B). In a second step, 2 is converted into 4 by reaction with the desired diol in the presence of triethylamine. 

Suppose that the system is initially uniform with an unstable disordered structure (i.e., r] = 0). For instance, the system may have been quenched from a high-temperature, disordered state, 77 = 1 represents the two equivalent equilibrium ordered variants. If the system is perturbed a small amount by a one-dimensional perturbation in the 2-direction, 77(f) = 6(t) sin(/32). Substituting this ordering perturbation into Eq. 18.26 and keeping the lowest-order terms in the amplification factor, 6(t),... 

The cleavage of the Si—Si bond in the 9,10-disilanthracene 8 dimer by lithium metal in THF allows the synthesis of a dimetalated species 9 (Scheme 4)42. The reaction can be quenched at this point with electrophiles such as methyl iodide. Extended reaction times in the presence of excess metal produced 9,10-dimetalla-9,10-disilaanthracene 10 (M = Li). The corresponding potassium derivative 10 (M = K) was obtained directly from 8 upon treatment with excess metal. These 1,4-dimetalla species 10 have been used to synthesize a variety of civ-substituted disilaanthracene derivatives42. 

---