Transition states alkenes

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

Syntheses of alkenes with three or four bulky substituents cannot be achieved with an ylide or by a direct coupling reaction. Sterical hindrance of substituents presumably does not allow the direct contact of polar or radical carbon synthons in the transition state. A generally applicable principle formulated by A. Eschenmoser indicates a possible solution to this problem //an intermolecular reaction is complex or slow, it is advisable to change the educt in such a way. that the critical bond formation can occur intramolecularly (A. Eschenmoser, 1970). 

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

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... 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

Fig. 6.2. Enthalpy differences of starting alkenes and transition states in bromination.

Comparison of the data for methoxide with those for t-butoxide in Table 6.4 illustrates a second general trend Stronger bases favor formation of the less substituted alkene. " A stronger base leads to an increase in the carbanion character at the transition state and thus shifts the transition state in the Elcb direction. A linear correlation between the strength of the base and the difference in AG for the formation of 1-butene versus 2-butene has been established. Some of the data are given in Table 6.5. 

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more El cb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethylammonium salts, may favor a more El cb-like transition state because their inductive and field effects increase the acidity of the p protons. 

Another stereochemical feature of the Diels-Alder reaction is addressed by the Alder rule. The empirical observation is that if two isomeric adducts are possible, the one that has an unsaturated substituent(s) on the alkene oriented toward the newly formed cyclohexene double bond is the preferred product. The two alternative transition states are referred to as the endo and exo transition states ... 

The observed stereoehemistry of [2jc -I- 2jc] eyeloadditions is in accord with the stereochemieal predietions that ean be made on the basis of this model. For example, E-and Z-2-butene give stereoisomerie products with ethoxyketene. For monosubstituted alkenes, the substituent is vieinal and cis to the ethoxy group in the eyelobutanone product. This is exactly the stereoehemistry predicted by the model in Fig. 11.16, sinee it maximizes the separation of the alkyl and ethoxy substituents in the transition state. 

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