Unusual reactivity

Very unusual reactivity has been observed (82-84) with the tricyclic orthoamides 122 and 123. This can be readily explained on the basis of the stereoelectronic effects due to the three nitrogen electron pairs. 

Tile chemical shift of the methine proton in orthoamide is 2.3 ppm. It is therefore at a much higher field than that of orthoamide 122. This remarkable difference of 2.7 ppm can be ascribed to a dramatic stereoelectronic effect. The origin of the unusual spectroscopic properties of orthoamide 123 presumably is the antiperiplanar relationship of the central C-H bond to the three lone pairs. This arrangement permits mixing of the lone pair orbital with the antibonding orbital of the central C-H bond (o ). As a result, the electron density at the methine hydrogen increases and the central C-H bond is weakened. Indeed, this hydrogen has a notably small chemical shift. 

Tlie orthoamides 1 and 123 are therefore completely different one adopts a conformation in which the central C-H bond is synperiplanar to the adjacent lone pairs (122A) while the other takes a conformation in which the central C—H bond is antiperiplanar (123A). These two compounds are there- 

Treatment of orthoamide 123 with equimolar amounts of aqueous hydrochloric acid yielded the salt 1 from which 223 can be regenerated by neutralization. NMR indicates that salt 124 and bicyclic formamidinium ion 126 interconvert rapidly at 70°C. This is explained by a conformational change from 124 into 125 which permits elimination of the amine group with stereoelec-tronic control (125 126). Finally, addition of excess hydrochloric acid to orthoaraide 123 precipitated the bicyclic dichloride 127. 

Orthoamide cleanly reduced mercuric acetate in ethanol at 25 C to mercury or mercurous acetate. The organic product formed is guanidinium salt 129 (X= OAc). Similarly, iodine in methanolic potassium carbonate at 25°C oxidized orthoamide 123 to guanidinium iodide 129 (X= l). On the other hand, orthoamide does not react with mercuric acetate even in boiling ethanol. Syn-elimination of mercury and acetic acid from complex 130 must be slow but anti-elimination from complex 211 2 or HgX2) must occur readily. 

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The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Mesityl Oxide. Mesityl oxide (MSO) (4-metliyl-3-penten-2-one) is an oily colorless liquid with an unpleasant odor. It exhibits the versatiUty and unusual reactivity associated with conjugated a,P unsaturated carbonyl compounds (172). On standing ia air, mesityl oxide slowly forms bis(3,5,5-trimethyl-l,2-dioxolanyl)-3-peroxide (173). 

Because of the unusual reactivity of the DCPD molecule, there are a number of wide and varying end use areas. The primary uses in the U.S. are DCPD-based unsaturated polyester resins (36%) hydrocarbon type resins, based on DCPD alone or with other reactive olefins (39%) EPDM elastomers via a third monomer ethylidenenorhornene or DCPD (16%) and miscellaneous uses (9%), including polychlorinated pesticides, polyhalogenated flame retardants, and polydicyclopentadiene for reaction injection mol ding (39). 

The 5,6-amide function has been found to be unusually reactive in the 2,4-disubstituted pteridin-6-one series, forming the corresponding 6-alkoxy derivatives, e.g. (141), with alcoholic HCl directly (62CB755, 70CB735). 

Materials that demonstrate unusual reactivity with water are identified as and materials that possess oxidizing properties shall be identified by the letters Z. Other special hazard symbols may be used to identify radioactive hazards, corrosive hazards, substances that are toxic to fish, and so on. 

The bottom space is primarily used to identify unusual reactivity with water. A W with a line through it alerts personnel to the possible hazard in use of water. This space may also be used to identify radiation hazetrd by displaying the propeller symbol or oxidizing material by displaying OXY. 

Compare CCl distances in the reactants. If shorter bonds are stronger, which reactants will show unusual reactivity, and in what direction Compare the cation structures with each other and with the reactants. If the structural changes in reaction (1) are normal , then what unusual changes, if any, occur in reactions (2) and (3) ... 

Unusual reactivities of mechano-radicals have been reported in a few instances. To explain the pseudo first-order kinetics and the high yield of linear block copolymers formed during the mechanochemical degradation of a mixture of... 

One of the most dramatic recent developments in metal carbene chemistry catalyzed by dirhodium(II) has been demonstration of the feasibility and usefulness of intermolecular carbon-hydrogen insertion reactions [38, 91]. These were made possible by recognition of the unusual reactivity and selectivity of aryl- and vinyldiazoacetates [12] and the high level of electronic control that is possible in their reactions. Some of the products that have been formed in these reactions, and their selectivities with catalysis by Rh2(S-DOSP)4, are reported in Scheme 10. 

The Cp Ti(amidinate) fragment provides a particularly useful platform for the synthesis of novel titanium imido complexes and the study of their unusual reactivity. Numerous single-, double-, and cross-coupling and imido-transfer reactions were investigated using these compounds. The synthetic routes leading to CpTi and Cp Ti amidinate derivatives are outlined in Scheme... 

Astudyofthereactionsofthecomplexes [CH3COOCHRCH2Co(DMG)2-py], where R is H or CH3, shows that the (8-carbon atom is unusually reactive. Hydrolysis gives alcohols (C0CH2CHROH), while methanolysis... 

Due to an interest in studying their unusual reactivity (vide infra), several attempts were made to maximize yields of the strained dimers 74. Lengthening reaction times and decreasing substrate concentrations in the cyclooligomerization experiments proved fruitless. In response to this situation, a stepwise synthesis of the tetrahexyl-substituted dimer was developed as shown in Scheme 18. Surprisingly, Hay coupling of 77 resulted in an improved yield of the tetramer (45 vs 8 %) and a substantial decrease in the yield of dimer (13 vs 30%). This product distribution was unexpected, since intramolecular reactions are typically much faster than intermolecular reactions. 

There are some special cases where tetrahedral intermediates are unusually stable there are three phenomena which lead to this stability enhancement. The first is an unusually reactive carbonyl (or imine) compound which is very prone to addition. An example of such a compound is trichoroacetaldehyde or chloral, for which the covalent hydrate can be isolated. A simple way to recognize such compounds is to think of the carbonyl group as a (very) stabilized carbocation, bearing an substituent. 

FIGURE 4.44 Magnesium intermediates may account some unusual reactivity. 

For preparative purposes, the most important AA/V -carbonylbisazole is A -carbo-nyldiimidazole (abbreviated in the subsequent text as CDI), which as the first member of this family was synthesized in 1956 by the senior author.[29],[30] The unusual reactivity of this compound is demonstrated by its hydrolysis, which occurs instantaneously at room temperature with drops of water, causing effervescence of carbon dioxide. 

Silenes and saturated silicon compounds that are coordinated by donors and that thus have higher coordination numbers at the silicon centres exhibit unusual reactivities [1]. An interpretation of these experimental findings is possible when the structural features of the stable model compounds [C6H4CH2N(CH3)2]2SiCl2 (1) [2], [C6H4CH2N(CH3)2]3SiH (4), and [C6H4CH2N(CH3)2]4Si (5) are considered [3]. 

The unusual reactivity pattern of benzylamine and carbon tetrachloride under the influence of metal carbonyl catalysts is paralleled by the behavior... 

Many 1,4-diazabutadiene adducts of dialkylzinc compounds show unusual reactivities (see also Section 2.06.10.7), such as the intramolecular electron transfer from zinc to the chelating ligand and the subsequent dimerization of these radicals.127 In solution, the carbon-carbon coupled dimer [MeZn(ButN=CHCH=NBut)2ZnMe] 71 is in equilibrium with its radical monomers (Scheme 56). Addition of potassium to a THF solution of the dimer produced cleanly 72, the first heteroleptic alkyldiamidozincate. 

A bis(secondary/tertiary) ligand reacted with diphenylzinc to afford the bis(phenylzinc) complex 88 (Scheme 67).148 The unusual reactivity of this dinuclear species was demonstrated by its interactions with trimethylphosphine oxide and iV,A-dimethylacetamide. In both cases, the products contained eight-membered rings incorporating both zinc atoms the structure of the bis(trimethylphosphine oxide) product is shown in Figure 46. 

Many zwitterionic organometallic compounds (betaines) are known in which positively and negatively charged centers of different nature are separated by various spacer groups. Substances of this type always exhibit the pronounced specific physical properties and high and unusual reactivity. Several types of organometallic betaines have been reviewed recently.1 4... 

A free radical is best defined as an atom or molecule containing one or more unpaired electrons, or in the illuminating expression of our French colleagues, Electrons cdlibataires.1 The unpaired electron exists alone in an orbital and therefore has a spin and, resulting magnetic moment uncompensated by the oppositely directed spin and magnetic moment of another electron. This has certain physical and chemical consequences which serve to show the presence of a free radical. But it must be emphasized that unusual reactivity alone (the chemical consequences) is never conclusive proof that a substance is a free radical. 

The initial product of the inhibition step is not known in this case and may be a molecular complex.8 The direct reaction of the ethane with the peroxy radical is an example of a covalent compound giving a reaction resembling that of a related free radical. The molecular weight determination by Gomberg was therefore a necessary part of the proof that he was dealing with radicals and not merely an unusually reactive hydrocarbon. The presence of free radicals has since been confirmed by measurements of the paramagnetic susceptibility and the paramagnetic resonance absorption.9-10 The latter evidence also rules out an alter-... 

Halogen substitution is expected to increase the electrophilicity of the carbenes, and in particular lh with four fluorine substituents is expected to be highly electrophilic and of unusual reactivity. All the carbenes le-g could be matrix-isolated by irradiation of their corresponding quinone diazides 2 in argon at 8-10 K.24 68,62 Again, the thermal reaction in (Vdoped matrices results in the formation of quinone oxides 7, which show the expected photochemical rearrangement to the spiro dioxiranes 8 and finally lactones 9. 

The perhalogenated carbenes lg and lh are of unusual reactivity towards molecular hydrogen and hydrocarbons.62 Annealing of H2- or CH4-doped argon matrices containing the carbenes lg or lh at 30-45 K rapidly results in the formation of insertion products (Scheme 10). With H2 2,5-cyclohexadienone (14) is formed and with CH4 the 4-methyl-2,5-cyclohexadienone (16). The... 

Relatively polar diphosphine 8 has an elongated P-P bond, and thus exhibits unusual reactivity. The reaction of 8 with acrylonitrile or methyl acrylate proceeds at 50 °C in a chemo- and regioselective manner to afford the 1,2-addition product with the PPh2 group attached at the terminal position (Equation (64)).165 Tetrachlorodiphosphine reacts with cyclohexene to give trans-adduct presumably via an ionic pathway.166... 

Para-enriched hydrogen offers considerable advantages for the NMR identification of transient intermediates [12d, 34]. PHIP experiments carried out in situ under PASADENA conditions are especially powerful in this regard. The PHA-NEPHOS [MM]-derived Rh catalyst is unusually reactive, with turnover possible even at —40°C. This high reactivity, coupled with good enantioselectivity, provides an ideal case for characterizing the elusive Rh dihydrides. 

Three salicylate (2-hydroxybenzoate) anions, which have unusual reactivity towards bromine that has been attributed to intramolecular proton transfer assisting electrophilic attack (Tee and Iyengar, 1985, 1990), exhibit modest catalysis (k /k2u = 3 to 10) and have KTS values similar to phenols. Pyridones and their /V-methyl derivatives, three heteroaromatic acid anions, and four phenoxy derivatives show comparable catalysis (k //c2u = 1.7 to 9.5) and Krs values (Table A4.4). 

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