Stability tetrasubstituted

In this work the possibility of the existence of 1,2-dihydro isomer with the core structure 42 was not considered. Recently, however, it was shown that 1,2-dihydropyridazines could be prepared by careful electroreduction of the corresponding pyridazines, and that their stability depends significantly on the ring substitutions. Thus, dimethyl l,2-dihydropyridazine-3,6-dicarboxylate 43a (R = H) is reasonably stable and rearranges into the 1,4-dihydro tautomer 43b only at a more negative potential, while the tautomerization in its tetrasubstituted analog 43a (R = COOMe) occurs more readily (Scheme 14) [00TL647]. 

Most monomers have an asymmetric substitution pattern and the two ends of the double bond are distinct. For mono- and 1,1-disubstituted monomers (Section 4,3.1) it is usual to call the less substituted end "the tail" and the more substituted end "the head". Thus the terminology evolved for two modes of addition head and tail and for the three types of linkages hcad-to-tail, hcad-to-hcad and tail-to-ta.il. For 1,2-di-, tri- and tetrasubstituted monomers definitions of head and tail are necessarily more arbitrary. The term "head" has been used for that end with the most substituents, the largest substituents or the best radical stabilizing substituent (Scheme 4.4). 

The stability of phosphinous amides depends, to a large extent, on the substituents at phosphorus and nitrogen. Normally, tetrasubstituted and N,P,P-trisubstituted phosphinous amides are stable and well-known compounds. The parent compound H2PNH2 is a volatile compound that is formed on hydrolysis of a solid state solution mixture of magnesium phosphide and magnesium... 

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

Five-membered carbocycles are the most easily formed [45, 107, 196, 200, 202,203]. Five-membered carbocyclic rings can be formed (with 2% MoF6 as the catalyst) even when the double bond is tetrasubstituted (Eq. 24) [200]. The stability of the catalyst toward the free OH group in this case is noteworthy. Evidently this particular t-butoxide-like alcohol does not react with this particular catalyst for steric reasons. Six-membered carbocyclic rings are also formed readily (Eq. 25) [200], as are seven-membered rings, especially if one takes advantage of a Thorpe-Ingold effect (e.g., Eq. 26) [20] or a similar conformational predisposition for the double bonds to remain near one another. 

In this section we will consider the effect of nonbonded interactions upon the relative stability of the cis and tram isomers of symmetrically tetrasubstituted ethylenes. 

Butynone 93 and tetrasubstituted allene 94 combine to form acetyl cyclopenta-diene 95 (Eq. 13.30) [32]. The annulation does not proceed in satisfactory yield in the case of unsubstituted trimethylsilyl allene 96 (Eq. 13.31) [32]. This is presumably due to diminished stabilization of the cationic intermediates in the case of 96, which allows competing reaction pathways to erode the yield. 

Hopf 52> has recently synthesized a number of tetrasubstituted [2.2]paracyclophanes of type 23,23 a remains unchanged on heating for several hours at 250 °C. This thermal stability is compatible with the compound having an anti conformation. 

The calculated energy change for this reaction gives the difference in olefinic strain between the alkene and cyclohexene, and the olefinic strain of the latter is negligible. With tri- and tetrasubstituted alkenes, it is more appropriate to use methylcyclohexane or 1,2-dimethylcyclohexane as the unstrained reactant since it is known that alkyl substitution stabilizes double bonds. The values of olefinic strain for a number of alkenes are summarized in Figure 15.2. Compounds with high OS are expected to be relatively reactive. 

Ca-Tetrasubstitution (Section 10.3) is recommended when the stabilization of specific backbone conformations ((3-turns, a/310-helices, or the fully extended structure) is the target of modifications.1612 The central Ca-trisubstituted atom of an a-amino acid may also be replaced by an -NH- or an -NR- group, thereby producing a-azapeptides (Section 10.4). 7,13 ... 

Selected nickel(II) complexes with TV-substituted diamines are also shown in Table 41. It is usually found that as the number and size of the substituents increase, the number of the coordinated diamines decreases, as does the stability of the complexes which are, in general, sensitive to moisture. The complexes are soluble in dry solvents without dissociation or decomposition. Pseudotetrahedral coordination is stabilized by increasing the steric hindrance on the donor atoms. In the series of tetrasubstituted diamines the pseudotetrahedral species are stabilized in the order Me4en < Me4pn < Me4tmd and NCS < Cl < Br < I (Table 41).646... 

Figure 10 Comparison of stability constants for complexes of [18]crown-6, tetrasubstituted analogs and a spherical...

The comparison of lower rim tetrasubstituted calix[4]arene 13a with the corresponding pentasubstituted calix[5]arene 13b (both compounds immobilised in the cone conformation) revealed that the higher number of ferrocene units in 13b does not positively influence the complexation ability of the receptor. The stability constants of 13a and 13b with chloride anion were found to be 55 and 15 M"1, respectively. As indicated by cyclic voltammetry, both receptors are selective towards the dihydrogen phosphate anion (cathodic shift -160 mV). 

As discussed, the major product of the elimination in Figure 4.36 is produced under thermodynamic control. As such, it is surprising that it contains a trisubstituted C=C double bond, while the minor product has a tetrasubstituted one. The stability of a C=C double bond normally increases with each additional alkyl substituent. However, here an opposing effect dom-... 

The deprotonation of an iminium ion (formula A in Figure 7.27) to give an enam-ine is reversible under the usual reaction conditions. Therefore, the most stable enam-ine possible is produced preferentially. Figure 7.28 emphasizes this using the example of an enamine formation from a-methylcyclohexanone (i.e., from an asymmetrical ketone). The enamine with the trisubstituted double bond is produced regioselectively and not the enamine with the tetrasubstituted double bond. Since the stability of olefins usually increases with an increasing degree of alkylation, this result is at first... 

With its tetrasubstituted carbon, the zirconaaziridine 2i remains ligand-free in THF, but in the presence of PMe3 it forms a PMe3 adduct. Complex 2j [33], prepared in a similar manner with the Rosenthal reagent 8 [34], is the only zirconaaziridine with a tetrasubstituted carbon that has had its structure determined by X-ray studies. The stability of its pyridine adduct reflects the fact that pyridine is a stronger donor ligand than THF. 

In most reactions of 2-pyrones, alkyl or aryl Grignard halides have been used. The best results have been achieved for 4,6-disubstituted 2-pyrones because of the general stability of 2,2,4,6-tetrasubstituted 2//-pyrans. Thus,... 

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