Stability structural features

The initial transient formed, rearranges in a reaction that involves the ring contraction step in reaction (74). The lifetime of this intermediate is considerably longer than that reported for any other intermediate with a copper(II)-carbon bond in aqueous solution (85-87,101,136), suggesting the stabilized structure featuring the metallocycle. This intermediate decomposes via heterolysis of one of the copper(II)-carbon -bonds followed by homolysis of the second to form the cyclopentyl-methanol radical in reactions (75) and (76), which reacts with Cu + to form the final product cyclopentanecarbaldehyde (89). 

Bethanechol incorporates the stabilizing structural features of both methacholine and carbachol. Thus it is like carbachol in that no appreciable cholinesterase hydrolysis takes place, and like methacholine it does not stimulate ganglia or skeletal muscle. This dmg appears to show some specificity of action in the gastrointestinal and urinary tracts. Thus, it has clinical applications in postoperative distention of the abdomen and bladder in the latter case it relieves the urinary retention that results following surgery. The drug is even orally effective. 

The two most important structural features that stabilize the enol of a (3 dicarbonyl compound are... 

What structural features are responsible for the reactivity order of carboxylic acid derivatives Like the other carbonyl containing compounds that we ve studied they all have a planar arrangement of bonds to the carbonyl group Thus all are about the same in offering relatively unhindered access to the approach of a nucleophile They differ m the degree to which the atom attached to the carbonyl group can stabilize the carbonyl group by electron donation... 

Calculations at several levels of theory (AMI, 6-31G, and MP2/6-31G ) find lower activation energies for the transition state leading to the observed product. The transition-state calculations presumably reflect the same structural features as the frontier orbital approach. The greatest transition-state stabilization should arise from the most favorable orbital interactions. As discussed earlier for Diels-Alder reactions, the-HSAB theory can also be applied to interpretation of the regiochemistry of 1,3-dipolar cycloaddi-... 

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

The transition state is closer in energy to the car bocation and more closely resembles it than the alkyloxonium ion. Thus, structural features that stabilize car bocations stabilize transition states leading to them. It follows, therefore, that alkyloxonium ions derived from tertiary alcohols have a lower energy of activation for dissociation and are converted to their- corresponding carbocations faster than those derived from secondary and primar y alcohols. Simply put more stable carbocations are formed faster than less stable ones. Figure 4.17 expresses this principle via a potential energy diagran. 

Certain structural features can make the keto-enol equilibrium more favorable by stabilizing the enol fonn. Enolization of 2,4-cyclohexadienone is one such exanple ... 

E. C. Taylor and his co-workers have demonstrated an important principle in the ring-opening of pyridopyrimidines and other fused pyrimidine systems to o-aminonitriles. They have demonstrated that based-catalyzed cleavage of a 4-substituted pyrimidine will occur provided that (a) the anion formed by the attack by the base at the 2-position can be stabilized by appropriate structural features in the remainder of the molecule and (b) that the substituent attached to the 4-position is capable of departure with its bonding pair of electrons in... 

The following discussion concerns the thermal liquidus ranges available in different ionic liquids, as functions of cation and anion structure and composition. In particular, those structural features of cation and anion that promote these properties (while providing other desirable, and sometimes conflicting characteristics of the liquid, such as low viscosity, chemical stability, etc.) and variations in liquidus ranges and stabilities are the focus of this chapter. 

I-Oialkoxy carbonyl compounds are a special class of chiral alkoxy carbonyl compounds because they combine the structural features, and, therefore, also the stereochemical behavior, of 7-alkoxy and /i-alkoxy carbonyl compounds. Prediction of the stereochemical outcome of nucleophilic additions to these substrates is very difficult and often impossible. As exemplified with isopropylidene glyceraldehyde (Table 15), one of the most widely investigated a,/J-di-alkoxy carbonyl compoundsI0S, the predominant formation of the syn-diastereomer 2 may be attributed to the formation of the a-chelate 1 A. The opposite stereochemistry can be rationalized by assuming the Felkin-Anh-type transition state IB. Formation of the /(-chelate 1C, which stabilizes the Felkin-Anh transition state, also leads to the predominant formation of the atm -diastereomeric reaction product. 

Another structural feature that increases carbocation stability is the presence, adjacent to the cationic center, of a heteroatom bearing an unshared pair," for example, oxygen," nitrogen," or halogen. Such ions are stabilized by resonance ... 

Many carbanions are far more stable than the simple kind mentioned above. The increased stability is due to certain structural features ... 

The definitions of the cluster ligands of the [3Fe-4S] cluster and of the related structural features are quite useful to predict cluster types in other Fds of known sequence, as well as to determine the nature of the cluster coordinating atoms (and variability) and their control on the type and performances of the metal sites, in particular in terms of cluster stability, cluster interconversion capability, and acceptance of other metals at the cluster. 

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Bulk structures of oxides are best described by assuming that they are made up of positive metal ions (cations) and negative O ions (anions). Locally the major structural feature is that cations are surrounded by O ions and oxygen by cations, leading to a bulk structure that is largely determined by the stoichiometry. The ions are, in almost all oxides, larger than the metal cation. It does not exist in isolated form but is stabilized by the surrounding positive metal ions. 

The preparation and investigation of the thietane oxide system (5a) is largely associated with stereochemical and conformational studies . The investigation of the thietane dioxides (5b) is substantially related to the chemistry of sulfenes , the [2 -I- 2] cycloaddition of which with enamines is probably the method of choice for the synthesis of 5b . The study of the thiete dioxide system (6) evolved, at least in part, from the recognition that the unstable thiete system 183 can be uniquely stabilized when the sulfur in the system is transformed into the corresponding sulfone , and that the thiete dioxide system is very useful in cycloadditions and thermolytic reactions. The main interest in the dithietane oxides and dioxides (7) appears to lie in the synthetic challenge associated with their preparation, as well as in their unique structural features and chemical behavior under thermolytic conditions . ... 

Acyclic Tricoordinate Radicals As one can easily note, the above examples of the cyclic radicals of Si and Ge atoms have rather particular structures featuring the cyclic delocalization of the odd electrons. The simple tricoordinate acyclic radicals of the type RsE (E = Si, Ge, Sn, Pb), lacking the stabilization effects of the cyclic Jt-delocalization, constitute another, more general and even more challenging, class of stable organometallic radicals. " Consequently, the search for such highly symmetrical species appeared to be of primary importance for synthetic chemists. 

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