Double bond strain

Flo. 4. Activation enthalpies for alkene oxidation and extrusion as a function of strain in the double bond. Legend El, directly measured activation enthalpies , T calculated from the relationship AH (extrusion) — double bond strain — AH% (oxidation) = 5.2 kcal/mol. 

Cyclodimerization of fluorinated alkenes was observed as early as 1947 during the pyrolysis study of polytetrafluoroethylene (PTFE) [54]. The ability of fluorinated alkenes to dimerize with themselves has been attributed to the energy relief of fluorinated double bond strain [4]. In the same fashion, the aryl trifluorovinyl ether groups (Ar—O—CF=Cp2) underwent thermally activated [2-1-2] cycloaddition to form bisaryloxy-substituted PFCB rings. Scheme 14.9 depicts the model reaction of cycloaddition of aryl trifluorovinyl ethers. The thermal cyclodimerization of methyl... 

Owing to the convex shape of the wall of Cgo, all the 30 double bonds, located exocyclic to the pentagons, are strained and thus more reactive than formal double bonds in aromatic compounds. Because the electron density is much higher on the [6,6] ring junctions than on the [5,6] jnnctions, most of the chemical reactions of fullerenes tend to occur across these sites. Cycloaddition reactions (widely applied for functionalization of fullerenes), as well as additions of nucleophiles and free radicals, provoke a hybridization change in the carbon atoms involved from a trigonal sp to a tetrahedral sp. This release of the double bond strain is the driving force of such reactions. 

For a long period of time after the discovery of catalytic cycloalumination of imsaturated compounds, 1,1-disubstituted olefins were considered to be nonreactive in these reactions. It was suggested [175] that unlike acyclic olefins with low-reactivity 1,1-disubstituted double bonds, strained cyclic unsaturated compounds with an activated methylidene bond, for example, methylidene cyclobutanes or methylidene cyclopropanes, would react with EtsAl in the presence of transition... 

The driving force for the addition of -butyIlithium onto ethylene is the relief of double bond strain that amounts to 22-23 kcal/mol. When allylic or benzylic organoalkalis combine with ethylene, their resonance energy of 10-20 kcal/mol is sacrified and this diminishes accordingly the reaction enthalpy. All the more astonishes the ease with which 3-methyl-2-butenylpotassium ("prenylpotassium"), 3-ethyl-2-pentenyl-potassium, l-phenylcyclopropylpotassium, " or 1,2,3,4-tetrahydrocarbazol-ylpotassium add onto ethylene. 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Sulfonium ylides may be added to C N double bonds to yield aziridines in a formal [1 -t-2]-cycloaddition. Alkyl azides are decomposed upon heating or irradiating to yield ni-trenes, which may also undergo [ 1 + 2 -cycloaddition reactions to yield highly strained hetero-cycles (A.G. Hortmann, 1972). 

The coupling of 1,8-diiodonaphthalene (25) with acenaphthylene (26) affords acenaphth[l,2-a]acenaphthylene (27). It should be noted that the reaction involves unusual trans elimination of H—Pd—1[32], This tetrasubstituted double bond in 11 reacts further with iodobenzene to give the [4, 3, 3]propellane 28 in 72%. This unusual reaction may be accelerated by strain activation, although it took 14 days[33]. 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

The greater stability of more highly substituted double bonds is an exam pie of an electronic effect The decreased stability that results from van der Waals strain between cis substituents is an example of a steric effect... 

Bulky tert butyl groups are cis to one another on each side of the double bond and van der Waals strain destabilizes the alkene... 

Torsional strain (Section 3 1) Decreased stability of a mole cule associated with eclipsed bonds trans (Section 3 11) Stereochemical prefix indicating that two substituents are on opposite sides of a ring or a double bond (Contrast with the prefix cis )... 

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymeri2ation (see Table 1) (24). When corrected for the heat of fusion, it is significantly less than the theoretical value of —83.7 kJ/mol (—20 kcal/mol) for the process of converting a double bond to two single bonds. The steric strain apparentiy is not important in the addition step, because VDC polymeri2es easily. Nor is it sufficient to favor depolymeri2ation the estimated ceiling temperature for poly (vinyhdene chloride) (PVDC) is about 400°C. 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

The principle of microscopic reversibility requires that the reverse process, ring closure of a butadiene to a cyclobutene, must also be a coiuotatory process. Usually, this is thermodynamically unfavorable, but a case in which the ring closure is energetically favorable is conversion of tra s,cis-2,4-cyclooctadiene (1) to bicyclo[4.2.0]oct-7-ene (2). The ring closure is favorable in this case because of the strain associated with the trans double bond. The ring closure occurs by a coiuotatory process. 

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