Concerted reactions stereospecificity

Section 10 12 Conjugate addition of an alkene (the dienophile) to a conjugated diene gives a cyclohexene derivative in a process called the Diels-Alder reaction It is concerted and stereospecific substituents that are cis to each other on the dienophile remain cis m the product... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Offer an explanation for the facility of the reaction, as compared to the vinylcyclopropane rearrangement of hydrocarbons, which requires a temperature above 200°C. Consider concerted reaction pathways which would account for the observed stereospecificity of the reaction. 

The chemical reactions through cyclic transition states are controlled by the symmetry of the frontier orbitals [11]. At the symmetrical (Cs) six-membered ring transition state of Diels-Alder reaction between butadiene and ethylene, the HOMO of butadiene and the LUMO of ethylene (Scheme 18) are antisymmetric with respect to the reflection in the mirror plane (Scheme 24). The symmetry allows the frontier orbitals to have the same signs of the overlap integrals between the p-or-bital components at both reaction sites. The simultaneous interactions at the both sites promotes the frontier orbital interaction more than the interaction at one site of an acyclic transition state. This is also the case with interaction between the HOMO of ethylene and the LUMO of butadiene. The Diels-Alder reactions occur through the cyclic transition states in a concerted and stereospecific manner with retention of configuration of the reactants. 

Apart from their intrinsic interest, these electrocyclic reactions have considerable synthetic carbon-carbon bond-forming importance because of their rigid stereospecificity, which is much greater than in the vast majority of other, non-concerted reactions involving biradical or bipolar intermediates. 

This means that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific but at most stereoselective. The concerted reactions, including SN2 displacements, E2 elimination of alkyl halides, anti and Syn addition to alkenes are all stereoselective. In the case of chiral or geometric substrates the nature of the product depends on the unique stereoelectronic requirement of the reaction. These are examples of stereospecific reactions. 

A concerted reaction is a chemical reaction in which all bond breaking and bond formation occurs in a single step in which reactive intermediates are not involved. Concerted photoreactions tend to be stereospecific, occurring from the vertical excited state. 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer ... 

Singlet carbenes add stereospecifically to olefins in a concerted reaction mechanism. 

Typically, Sn2 reaction requires a backside attack. The C—X bond weakens as nucleophile approaches. All these occur in one step. This is a concerted reaction, as it takes place in a single step with the new bond forming as the old bond is breaking. The Sn2 reaction is stereospecific, always proceeding with inversion of stereochemistry. The inversion of... 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

Trialkylsilyl vinyl ketenes (72) have been shown to react stereoselectively with a-benzotriazolyl organolithium species to give highly substituted cyclopentenones. The selectivity was found to be kinetic, not thermodynamic, in origin. Several possible mechanisms have been proposed (Scheme 10). It has been suggested that the observed stereoselectivity may result from torquoselectivity in a concerted reaction, or from stereospecific conrotatory cyclization of cation (73), formed stereoselectively because of the interaction shown between the electron-withdrawing group Z and the metal ion.75... 

Reaction of dichloroketene with cis or trans cyclo-octene suggests that it is a concerted reaction each gives stereospecifically a different stereoisomer of the adduct cis-15 gives cis- 16 while trans-17 gives trans-18. The marked hydrogen atoms should make this clear. The very reactive trans -cyclo-octene 17 gives a 100% yield so there is no room for any 16 in the product.3... 

The reaction of 1,2-dithietes with unsaturated compounds has also been investigated. 1,2-Dithietes were found to react with alkenes and alkynes to give the cycloadducts stereospecifically, which indicates the concerted reaction between ethane-1,2-dithione, the valence isomer of 1,2-dithiete, and dienophiles <2000JOM(611)106, 1999JOC8489>. 

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