Alkene bond rotation

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination ... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

Since the additions are normally stereospecific with respect to the alkene, if an open-chain intermediate is involved it must collapse to product more rapidly than single-bond rotations that would destroy the stereoselectivity. 

The cis alkenes are more reactive and more selective than their trans counterparts. As with the Evans system, this reaction is not stereospecific. Acyclic cis alkenes provide mixtures of cis and trans aziridines. cis-p-Methylstyrene affords a 3 1 ratio of aziridines favoring the cis isomer, Eq. 67, although selectivity is higher in the trans isomer. A fascinating discussion of this phenomenon, observed in this system as well as the Mn-catalyzed asymmetric oxo-transfer reaction, has been advanced by Jacobsen and co-workers (83). Styrene provides the aziridine in moderate selectivity, Eq. 68, not altogether surprising since bond rotation in this case would lead to enantiomeric products. 

If the alkene can exist as cis and trans isomers then we need to be aware of the stereospecificity of the reaction. If the reaction involves an excited triplet state then the biradical formed will be able to undergo bond rotation in the lifetime of the excited state. The reaction is, therefore, nonstereospecific, forming a mixture of oxetane isomers from either alkene isomer ... 

There continues to be an increasing level of activity centered about the use of porphyrin catalysts for the epoxidation of alkenes of various configurations. For example, the sterically encumbered fra/w-dioxoruthenium(VI) porphyrin (26) was found to catalyze the epoxidation of a variety of alkenes in yields from fair to excellent e.g., 27 -> 28). Kinetic studies on a series of para-substituted styrenes point to a mechanism which proceeds via a rate-limiting benzylic radical formation. The high degree of stereoretention in cir-alkenes was attributed to steric crowding which prevents C-C bond rotation of the intermediate radical. This same steric bulk prevents the familiar side-on approach of the alkene substrate, so that a head-on approach is postulated <99JOC7365>. 

Another characteristic of photocycloaddition to electron-rich alkenes is the loss of any stereochemistry of the starting alkene in the oxetane structure. An example is the formation of practically the same mixture of geometric isomers of 2,2-diphenyl-3,4-dimethyloxetane from benzophenone and either cis- or trans- 2-butene (equation 103). This is understandable on the basis of the diradical intermediate having a sufficiently long lifetime for bond rotations to occur. 

Stereochemical equilibration can arise from bond rotation in the 1,4-diradical and/or reversibility of diradical formation resulting in stereochemical equilibration of the starting alkene. 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

Alkenes which have no symmetry planes perpendicular to the plane of the double bond such as Pmr-butene-2 or propene can coordinate to platinum in two enantiomorphous ways (77) and (78). If an optically active ligand is also bound to platinum(H), then two diastereoisomers are found which can be separated by fractional crystallization657,658 or by HPLC.659 Both cis and trans isomers of complexes PtCl(N—0)(alkene) have beenprepared, where N—O is an anion derived from an amino add (equations 235a and 235b).660-664 Epimerization cannot occur by simple rotation of the alkene about its bond axis, but only by a mechanism involving cleavage of the platinum(II)-alkene bond. 

Because of the stepwise formation of the two new carbon-carbon bonds of the cyclopropane product, and because of the intermediacy of the carbanionic adduct (8), the double-bond configuration of the alkene substrate may be lost, depending upon the lifetime of the intermediate relative to carbon-carbon single-bond rotations. Consequently, the stereospecificity of these cyclopropanations varies from one case to another.132... 

Cis-trans isomerism in alkenes arises because the electronic structure of the carbon-carbon double bond makes bond rotation energetically unfavorable at normal temperatures. Were it to occur, rotation would break the pi part of the double bond by disrupting the sideways overlap of two parallel p orbitals (Figure 23.2). In fact, an energy input of 240 kj/mol is needed to cause bond rotation. 

Ring opening of cz s-2,3-dimethyloxirane by triphenylphosphine has been modeled using the B3LYP functional with the 6—3lG(d) basis set.43 The calculations suggest that the first step of the reaction is an SN2 process with simultaneous C-C bond rotation giving an oxaphosphetane intermediate that decomposes to the frans-alkene no betaine intermediate is formed. 

The sulfoxonium ylid 78 is more stable and is therefore liable to do conjugate rather than direct addition (chapter 21). The intermediate eliminates dimethyl sulfoxide 79 to give the cyclopropane 76. The intermediate is long lived and the single bond that was the alkene can rotate so the geometry of the alkene is lost. In this case we expect the more stable trans cyclopropane to be formed by choice. 

As shown by the two simple compounds in Figure 1.11, the two carbon atoms connected by a double bond in alkenes cannot rotate relative to each other. For this reason, another kind of isomerism, called cis-trans, is possible for alkenes. Cis-trans isomers have different parts of the molecule oriented differently in space, although these parts occur in the same order. Both alkenes illustrated in Figure 1.11 have a molecular formula of C4H8. In the case of m-2-butcnc, the two CH3 (methyl) groups attached to the C=C carbon atoms are on the same side of the molecule, whereas in trans-2-butene they are on opposite sides. 

Irradiation of phenyldisulfide cleaves the weak S—S bond to give a pair of thiophenyl radicals. One of these then adds to the less-substituted olefinic carbon of 25 to generate the tertiary alkyl radical 26 (Scheme 14.5). Bond rotation then ensues to give the more stable rotamer 27 in which there is minimal steric repulsion between the C(7)-methyl and the tetrahydrofuran framework. Elimination of the thiophenyl radical from 27 finalises the isomerisation to alkene 19. 

Bond rotation is not possible for a C = C double bond since this would require the o bond to be broken. Therefore, isomers of alkenes are possible depending on the relative position of the substituents. These can be defined as the cis or fans, but are more properly defined as (Z) or (E). 

In the transition state of the epoxidation of alkenes with a percarboxylic acid the C=C axis of the alkene is rotated out of the plane of the percarboxylic acid group by 90° ( spiro transition state ). In this process, four electron pairs are shifted simultaneously shifted. This very special transition state geometry make peracid oxidations of C=C double bonds largely insensitive to steric hindrance. The epoxidation given in Figure 3.20 provides an impressive example. 

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