Alkene bond reversibility

However, (TMS)3Si radicals are found to add to a variety of double bonds reversibly and therefore to isomerize alkenes [19]. An example is shown for the interconversion of ( )- to (Z)-3-hexen-l-ol and vice versa by (TMS)3Si radicals (Reaction 5.1). Figure 5.1 shows the time profile of this reaction under standard experimental conditions (AIBN, 80 °C). The equilibration of the two geometrical isomers is reached in ca 10 h, and the percentage of Z/E = 18/82 after completion corresponds to an equilibrium constant of = 4.5. The difference in the stability of the two isomers in 2-butenes, i.e., AG°( -isomer) - AG° (Z-isomer) = — 3.1kJ/mol, corresponds to K = 3.5, since... 

ZJE isomerization of the alkene bond in A -enecarboxyIic acids occurs during Kolbe electrolysis because the intermediate radical reacts reversibly with this function to form a cyclopropane [92], This process leads to a partial loss of stereochemistry in the synthesis of long chain alkenes [93]. However, it does not present stereochemical problems during the synthesis of cycloalkenes such as chaul-moogric acid [94]. 

Thermochemical data correspond with a decrease in the platinum(0)-alkene bond strength ir the sequence C2H4 > PhCH=CH2 > cis-PhCH=CHPh > frans-PhCH==CHPh. Displacemem reactions show an expanded stability order for platinum(O) complexes to be TCNE> PhCfeCH > alkenes.801 The relative weakness of alkene complexes relative to alkyne com plexes of platinum(O) is the reverse of that found with platinum(II).802... 

Furthermore, cis isomers are more easily hydrated than their trans counterparts, although exceptions are known. Thus, the hydration of cis-1,2-dicyclopropylethylene is 2.5 times faster than its trans isomer.282 Strain introduced into a ring by the incorporation of a trans double bond reverses the trend, thereby making the hydration of franr-cyclooctene more rapid than the cis compound.283 Smaller ring alkenes are also rather sluggish towards hydration.284... 

Actinide hydrides are rather hydridic and nndergo rapid reactions typical of snch systems for example, reactions with ketones and halocarbons provide actinide alkoxides and halides, respectively. Rapid qnantitative addition of terminal alkenes (the reverse of /S-hydride elimination) is consistent with the known bond disruption enthalpies (eqnation 66). ... 

The regiochemistry of the elimination depends on the type of elimination process that occurs. The El process favors the formation of the more substituted alkene because reversible protonation of the double bond occurs and creates an equilibrium mixture that favors the more stable product. The E2 regiochemistry is controlled by the need to minimize steric interactions in the transition state the size of the base is important because one proton may be more accessible than another, as in Figure 4.21. The ElcB regiochemistry is determined by the loss of the most acidic proton. Elimination reactions can produce different stereoisomers, for example, cis and trans alkenes. Since the trans isomer is usually of lower energy because of steric reasons, it usually predominates over the cis isomer in the product mixture. 

Alkene metathesis is a transition-metal-catalyzed reaction in which alkene bonds are cleaved and redistributed to form new alkenes [1-3]. The reaction proceeds through the formal [2 + 2] cycloaddition of an alkene and a metal alkylidene to yield a metallocyclobutane intermediate (Scheme 1). The productive retrocydoad-dition of this intermediate generates a new metal alkylidene and a new alkene product. These processes are generally reversible, and the reaction is under thermodynamic control. 

Figure 32 The 124 , 124Z, 124 , and 124Z equilibrium system in which intermolecular host-guest complexation of 1-adamantol competes with cinnamate self-complexation in 124Z and E to Z isomerization about the amide bond occurs while the E stereochemistry about the cinnamate alkene bond is retained. Photoisomerization by irradiating at 300 nm switches the stereochemistry about the cinnamate alkene bond to Z to produce the 125 , 125Z, 125 , and 125Z system in which no competitive cinnamate self-complexation occurs. The cinnamate alkene photoisomerization is reversed by irradiation at 254 nm to complete an on-off photoswitching process.

Like the stannyl radicals [18], (TMS)3Si radicals are own to add to a double bond reversibly and therefore, to isomerize alkenes [19,20]. That is, these radicals add to (Z)-or ( )-alkene to form radical 2 or 3, respectively. Interconversion between the two radical adducts by rotation around the carbon-carbon bond, followed by p-scission can then lead to the formation of either (Z)- or (i )-alkene, depending on the radcal-alkene combination. 

Both parts of the Lapworth mechanism enol formation and enol halogenation are new to us Let s examine them m reverse order We can understand enol halogenation by analogy to halogen addition to alkenes An enol is a very reactive kind of alkene Its carbon-carbon double bond bears an electron releasing hydroxyl group which makes it electron rich and activates it toward attack by electrophiles... 

The bond highlighted m yellow is the peptide bond ) Pencyclic reaction (Section 10 12) A reaction that proceeds through a cyclic transition state Period (Section 1 1) A honzontal row of the penodic table Peroxide (Section 6 8) A compound of the type ROOR Peroxide effect (Section 6 8) Reversal of regioselectivity oh served m the addition of hydrogen bromide to alkenes brought about by the presence of peroxides m the reaction mixture... 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

The reaction with bromine is very rapid and is easily carried out at room temperature, although the reaction is reversible under some conditions. In the case of bromine, an alkene-Br2 complex has been detected in at least one case. Bromine is often used as a test, qualitative or quantitative, for unsaturation. The vast majority of double bonds can be successfully brominated. Even when aldehyde, ketone, amine, so on functions are present in the molecule, they do not interfere, since the reaction with double bonds is faster. 

Thermal cleavage of cyclobutanesto give two alkene molecules (cyclorever-sion, the reverse of 2 -I- 2 cycloaddition) operates by the diradical mechanism, and the [ 2s -I- o2a] pathway has not been found " (the subscripts a indicate that cr bonds are involved in this reaction). 

---