Strained bonds

Strained bonds will be cleaved particularly readily if there is a reversible reaction by which these bonds might have been formally formed. Such reactions could be the Diels-Alder reaction, aldol additions, Michael additions, or related processes 

Side Reactions in Organic Synthesis. Florencio Zaragoza Dorwald Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31021-5 

Strained cyclohexenes, such as norbornene derivatives, can undergo retro-Diels-Alder reactions even at relatively low temperatures, and this reaction can be used to prepare 1,3-dienes and alkenes (e.g. synthesis of cyclopentadiene by thermolysis of 

Me-eclipsed H-eclipsed Vo O H Okcal/mol -ikeal/mol s-cis s-trans H H 0 H -0.3 kcal/mol 0 kcal/mol 

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In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

Several generalities can be formulated regarding selective reduction of polyolefins. Usually the least hindered double bond is hydrogenated pre ferentially (123), and, with steric hindrance about equal, the most strained bond will be reduced first. Exocyclic olefins are reduced more easily than those in the ring (R)-(+ )-Limonene, 190 g, was shaken with W-4 Raney nickel (12 g) under hydrogen at atmospheric pressure. After 31.9 1 of hydrogen had been absorbed, the solution was filtered. Essentially, pure (R)-( -i- )-carvomenthene was obtained in 96% yield (58). 

It should not be thought that the structure of every intermetallic compound can be treated so simply the discussion of such struetural features as the transfer of electrons between atoms, the occurrence of strained bonds, the significance of relative atomic sizes, and the electron-atom ratio (Hume-Rothery ratio) must, however, be postponed to later papers. 

Epoxides are cylic ethers which are highly reactive because of the strained bond angles of a three-membered ring. Because of the high reactivity of epoxides, they are the starting materials for so-called epoxy resins used for high-strength adhesives. 

Catalysis of strained bond re- 8 arrangements by silver(I) ion (43) The chemistry of organogold com- 13 pounds (94)... 

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

An intriguing reaction has been reported that does not exactly fit into the category of Michaelis-Arbuzov reaction but does involve nucleophilic attack of a neutral trivalent phosphorus for generation of a new C-P bond. Phenyl- and methyldichlorophosphine have been reported to attack the strained cyclopropane ring system of 1,3-dehy-droadamantane, overall adding P-Cl across the most strained bond of the ring system (Equation 3.5).140... 

In the 80/20 specimens, the changes in E (rt) follow a different pattern. There is little change in E (rt) up to a dose of 2000 Mrads. Between 2000 and 4000 Mrads, E (rt) decreases 13%. At 10,000 Mrads, E (rt) is almost 9% greater than a control. Previous DSC studies have shown that during the first 2000 Mrads, there is substantial additional cure (14). Competition between chain scission of strained bonds and further curing reactions could "balance out"... 

Our focus in this chapter shall be primarily upon the surface electrostatic potential, specifically its most positive and most negative values, denoted by Vs, max and Vs,min, respectively. There may be several local and absolute maxima and minima on a given surface. They indicate the most positive and negative sites. The former are often associated with hydrogens, especially acidic ones, and the latter with lone pairs, tt electrons of unsaturated molecules and strained bonds. We have demonstrated that Vs niax and Vs nim correlate well with measures of hydrogen bond donating and accepting tendencies . 

Intramolecular carbene addition reactions have a special importance in the synthesis of strained ring compounds. Because of the high reactivity of carbene or carbenoid species, the formation of highly strained bonds is possible. The strategy for synthesis is to construct a potential carbene precursor, such as diazo compounds or di- or trihalo compounds, which can undergo intramolecular addition to the desired structure. Section E of Scheme 10.5 gives some representative examples. 

Transition Complexes. The numerous collisions between reactant molecules result in a wide distribution of energies among the individual molecules. This can result in strained bonds, unstable forms of molecules, or unstable association of molecules which can then either decompose to give products, or by further collisions return to molecules in the normal state. Such unstable forms are called transition complexes. 

This suggests that excellent atropdiastereoselectivities - the other imaginable diastereomer could not be detected - are due to steric reasons, assuming that one diastereomer, (P,P) or (M,M), is able to arrange itself in a double-helix-like form. The formation of (M.P)- or (P.M)-at-ropdiastereomers is excluded as these are assumed to exhibit extremely strained bond angles 28b. 

The optimized geometry of 4, obtained at the MP2/6-31G level and shown in Figure 22.5, offered more clues about the type of delocalization that is not responsible for the stabilization of 4 than about the type of delocalization that is. If delocalization of the strained bonds between the carbons, a and p to the cationic center, were involved and occurred in the manner depicted in resonance structure C, the bonds between the ipso and a carbons should have considerable double-bond... 

The advantage of this method is that it can be easily used with any number of cation coordination spheres whose bond distances are available. Even one coordination sphere is sufficient to give a trial value though the more that are used the more confidence one can have in the value of Rq. One needs to exercise a little care if only a few coordination spheres are known, since the oxidation state may be unstable except in the presence of strained bonds which could lead to a false value of Rq. There are a number of potential pitfalls in determining bond valence parameters. For example, the inclusion of poorly determined structures in the sample tends to increase the value (and uncertainty) of B with a corresponding decrease in Rq. A critique of these problems has been given by Tytko (1999). 

I8.lt> The P4 molecule in white phosphorus has extremely strained bonds The bond angles in the Tj molecule are only 60°. Therefore the bonds are weak, only 201 kJ mol-1 for each one. The total bond energy of two moles of Pj (962 kJ. Appendix E)is244 kJ less than that of one mole of Pi (1206 kJ. Appendix E). In contrast, the total bond energy of two moles of N. has been calculated57 to be 77 kJ greater than that of one mole of N, ta hypothetical tetrahedral molecule iscslraclund with P4)... 

Dicarhonylation of strained bonds.2 The acyltetracarbonylferracycle (2), obtained on irradiation of quadricyclane (1) in the presence of excess Fe(CO)5, is oxidized to the a-diketone 3 in 81% yield by CuCl2 in aqueous CH3CN. 

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

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