Alkenes thermal reversibility

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

Gallagher, B. M., Pearson, W. H. Thermal cyclization of N-hydroxylamines with alkenes the reverse Cope elimination. Chemtracts Org. Chem. 1996, 9, 126-130. 

We discovered the thermal reversibility of the diene cyclometallation when we attempted cyclization of the diene 9 (Scheme 3) (8,9). The reaction proceeded to completion after two hours at room temperature, but rather than the expected tricyclic alcohol 13, the product was the dimer 11, from the reaction of two monosubstituted alkenes. We repeated the cyclozirconation, but let it proceed for a longer time. After 18 hours at room temperature, a new product had appeared (TLC analysis). After 1.5 hours at 75°C, the reaction was complete. The tricyclic alcohol 13 was isolated in 63% yield from 9. 

This work also shed light on interesting properties of thin films due to the thermally reversible chemistry of the Diels-Alder reaction. Carefully designed, alkene- and diene-functionalized, pulsed plasma polymer thin films will make it possible to control adhesion between any kinds of solid surfaces. 

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. 

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids299 (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism246,300 and thus facilitates the desired stereospecific synthesis301. The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene301. 

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). 

The transfer reaction utilizes a sacrificial alkene to remove the dihydrogen from the pincer or anthraphos complex first, before the oxidative addition of the target alkane. The elementary reaction steps are slightly different from the thermal reaction, which is discussed in the next section, both in their order and their direction. For simplicity, we describe the symmetric reaction where the sacrificial alkene is ethylene and the reactant is ethane (21b). The elementary reaction steps for the mechanism of this transfer reaction involve IVR, IIIR, VIR, VI, III and IV, where the superscript R stands for the reverse of the elementary steps listed in Section III. These reverse steps (IVR, IIIR, and VIR) involve the sacrificial alkene extracting dihydride from the metal to create the Ir(I) species 8, while steps VI, III and IV involve oxidative addition of target alkane, p-H transfer and olefin loss. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

First, it should be repeated that despite early and recent claims, no examples of thermal dissociation of alkenes into carbenes (the reverse reaction of the carbene dimerization) are known.However, in their recent paper, Lemal and co-worker pointed out that electrophiles can catalyze the dissociation of tetraaminoalkenes. 

Alkenes. Addition of diborane and organoboranes to carbon-carbon double bonds is a rapid, quantitative, and reversible transformation. Reversibility, however, is not a severe limitation under the usual reaction conditions, since thermal dissociation of organoboranes becomes significant only above 100°C. The addition is syn and occurs in an anti-Markovnikov manner that is, boron adds preferentially to the less substituted carbon atom. The attack of the reagent takes place on the less hindered side of the reacting alkene molecule. 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

Some big steric or electronic factor is clearly at work. Though the alkene 24 is hindered at one end, the enone barely is and it is electronic factors that dominate. The natural polarity of the alkene is to be nucleophilic at the CH2 group 24a. So in the thermal reaction (that doesn t happen) it could attack the electrophilic end of the enone 25a. One way to predict photochemical 2 + 2 cycloadditions is to suppose that the excited state of the enone reverses the natural polarity from 25b to 25c and the new electrophilic end now combines with the alkene 24. As the alkene is not excited, it behaves in the normal way 24a. This is of course a simplification but it works.9... 

The intramolecular nitrone-alkene cycloaddition reaction of monocyclic 2-azetidinone-tethered alkenyl(alkynyl) aldehydes 211, 214, and 216 with Ar-aIkylhydroxylamincs has been developed as an efficient route to prepare carbacepham derivatives 212, 215, and 217, respectively (Scheme 40). Bridged cycloadducts 212 were further transformed into l-amino-3-hydroxy carbacephams 213 by treatment with Zn in aqueous acetic acid at 75 °C. The aziridine carbaldehyde 217 may arise from thermal sigmatropic rearrangement. However, formation of compound 215 should be explained as the result of a formal reverse-Cope elimination reaction of the intermediate ct-hydroxy-hydroxylamine C1999TL5391, 2000TL1647, 2005EJ01680>. 

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