Liminal

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

The ynaminoketone vinylogs react with 1,3-dipoles (C,N-disubstituted nitii-limines, benzonitrile A-oxide) in a regio- and stereospecific fashion at the triple... 

To minimize and essentially eliminate contamination, and to liminate mix-ups, equipment, utensils, and containers are to be thoroughly cleaned and adequately stored... 

Loupy and Soufiaoui described a comparative study of the reactivity of diphenylnitri-limine 200 with several dipolarophiles under microwave irradiation in the absence of solvent using a solid mineral support or phase-transfer catalysis (PTC) conditions (Scheme 9.62) [30b]. The results showed that the best yields of adducts were achieved upon impregnating KF-alumina with a mixture of the hydrazynoyl chloride 199 and the dipolarophile followed by irradiation of the mixture in a focused oven. Reaction of this mixture under solid-liquid PTC conditions with KF-Aliquat under microwaves afforded lower yields of cycloadducts, perhaps owing to the partial decomposition of Aliquat at the reaction temperature (140 °C). In all cases, worse yields were obtained by classical heating under comparable reaction conditions (time and temperature). 

Not only is Tsukuba Science City in a liminal institutional position in Japan, but it also brings together Japanese and foreign scientists and even Japanese scientists who have found themselves shunned by broader Japanese society for having been abroad working in foreign universities for many years. 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

Diastereoselectivities of up to 90% were observed for the cycloadditions of jV-galaclosy-limines 108 (Piv = pivalyl R = 2-furyl, 2-thienyl-, 4-FC6H4, 4-ClC6f or 3-pyridyl) to isoprene in the presence of zinc chloride to form the tetrahydropyridines 109 and 110 (equation 62)55. 

This view has been challenged with more recent evidence indicating that AT-[(acyloxy)methyl] derivatives of both primary and secondary amides (8.170, Fig. 8.21) undergo decomposition by the same mechanisms, namely a) an acid-catalyzed process involving protonation followed by formation of an /V-acyliminium species (Fig. 8.21, Reaction a) b) a pH-independent heterolytic cleavage forming the same /V-acyliminium species (Fig. 8.21, Reaction b) and c) a base-catalyzed pathway, which for /V-[(acyloxy)methyl] derivatives of AT-methylamides is the normal mechanism (Fig. 8.21, Reaction c), but for AT-[(acyloxy)methyl] derivatives of primary amides involves substrate deprotonation followed by /V-acy limine formation (Fig. 8.21, Reaction d) [218],... 

Fig. 4 Diphosphine ligand 3. Summary of the best results obtained in the Ir-catalyzed hydrogenation of Alary limines...

Gann (KoUoidchem. Beihefte, vni. 127,1916) has likewise measured the adsorption of various precipitating anions by colloidal aluminium hydroxide and found that although the liminal concentrations necessary for precipitation are very different yet precipitation is caused on adsorption of equivalent concentrations as indicated by the following figures. 

Anion Liminal concentration millimols. per litre Adsorbed milli-eqnimlents... 

The effect of valency on precipitatory power is most marked, the polyvalent ions being much more effective than the monovalent ions. As a rough approximation the liminal concentrations, i.e. the concentrations in milligram ions, per litre necessary to cause coagulation vary with the valency in the proportion valency 1 2 3,... 

The alteration in sensitiveness of a suspension to electrolytes by the addition of a non-electrolyte was first noted by Blake (Amer. Jour, of Soi. xvi. 439, 1903) who observed that the quantity of potassium alum necessary to convert a red gold into a violet form had to be increased on the addition of ether to the suspension the liminal concentration without ether was found to be 00035 whilst in an aqueous solution saturated with ether 00151 equivalent per litre were required. 

Kruyt and van Duin (Kolloidchemie Beihefte, v. 269,1914) have examined the alteration in sensitiveness to electrolytes of a suspension of arsenious sulphide to which various non-electrolytes had been added. They found that the influence of various nonelectrolytes on the sensitiveness of the suspension ran parallel to the adsorption of the non-electrolytes from aqueous solution by powdered charcoal and that the most capillary active non-electrolyte exerted the greatest effect on the liminal concentration required for precipitation. Further it was observed that the addition of non-electrolytes lowered the liminal concentrations, i.e. increased the sensitiveness of the suspension to uni- and trivalent ions but increased the liminal concentrations, i.e. decreased the sensitiveness for divalent and tetravalent cations as will be noted from the following tables. 

Isoamyl alcohol cone, millimols. per litre Liminal concentrations of ECl Ratio... 

This hypothesis (Rideal, ProG. Oamb. Phil. Sog. xxiv. 101, 1924) has recently been tested experimentally by Weiser Jour. Phys. Chem. xxxviii. 1255, 1924) in which both the liminal concentrations required for precipitation and the actual adsorptions of the precipitating ions in the presence of various sensitising agents were determined. 

Two main liminations have, however, become evident the hrst is that molecules whose gas-phase ionization energy exceeds > 9.5 eV cannot be oxidized by ionized solid Ar. The reason for this limitation is unclear, because the process is exothermic (the ionization energy of solid Ar is 13.9 eV, that of organic molecules in Ar is typically lowered by 1 eV in solid Ar relative to the gas phase ). Perhaps the localization of the spin and charge onto the substrate entails a Frank-Condon barrier that cannot easily be surmounted at 12 K. 

When they subjected the allenylzinc reagent to the Hoffmann test for configurational stability,29 Poisson, Chemla and Normant found that at — 50 °C, racemization does not occur at a significant rate (equation 36)30,31. Accordingly, when the racemic allenylzinc reagent was added slowly to the /V-benzy limine of (R)-mandehc aldehyde at — 50 °C, a 1 1 mixture of the anti,syn and anti,anti adducts was isolated in 65% yield. However, when the addition process was reversed, a 3 1 mixture favoring the matched anti,anti adduct was formed in 53% yield, suggestive of a partial kinetic resolution. 

Benzoqvinone-l-imino-4-me thy limine, CH3.N CB-H4 NH mw 120.15, N 23 32% col prisms, mp 64 -7°, very unstable.compd, dec on standing in less than 1 hr, becoming a brn tarry product. Other props prepn are given in Refs 1 2 Refs l)Beil 7,620 2)R.Willstatter C.W.Moore, Ber 40,2671(1907)... 

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