Alkyl groups transition state

The influence of alkyl groups has been attributed to the +/ effect operating primarily at the 0- and />-positions (i), and somewhat less strongly at the m-position by relay. Alternatively, the effect is seen as stabilising the transition states for 0- and />-substitution (ii), more than... 

Curiously enough, bulky substituents on nitrogen increase this reactivity towards methyl iodide (119). This has been related to a steric decompression of the thiocarbonyl group in the transition state. Furthermore, knowledge of the ratio of conformers in the starting 4-alkyl-3-i-Pr-A-4-thiazoline-2-thiones and in the resulting 4-alkyl-3-i-Pr-2-methylthiothi-azolium iodides combined with a Winstein-Holness treatment of the kinetic data indicates that in the transition state, the thiocarbonyl bond is approximately 65% along the reaction coordinate from the initial state... 

The regioselectivity of elimination is accommodated m the E2 mechanism by noting that a partial double bond develops at the transition state Because alkyl groups... 

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... 

Potential energy diagram (Section 4 8) Plot of potential en ergy versus some arbitrary measure of the degree to which a reaction has proceeded (the reaction coordinate) The point of maximum potential energy is the transition state Primary alkyl group (Section 2 13) Structural unit of the type RCH2— in which the point of attachment is to a pnmary carbon... 

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

This can be circumvented by choosing alkyl groups with no P H, eg, methyl, neopentyl, trimethylsilylmethyl, phenyl and other aryl groups, and benzyl. The linear transition state for -elimination can also be made stericaHy impossible. The most successful technique for stabilization combines both principles. The pentahaptocyclopentadienyl ring anion (Cp) has six TT-electrons available to share with titanium. Biscyclopentadienyltitanium dichloride... 

Alkyl groups under nonacidic conditions sterically deflect nucleophiles from C, but under acidic conditions this steric effect is to some extent offset by an electronic one the protonated oxirane opens by transition states (Scheme 40) which are even more 5Nl-like than the borderline Sn2 one of the unprotonated oxirane. Thus electronic factors favor cleavage at the more substituted carbon, which can better support a partial positive charge the steric factor is still operative, however, and even under acidic conditions the major product usually results from Cp attack. 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

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. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

Nitroalkanes show a related relationship between kinetic acidity and thermodynamic acidity. Additional alkyl substituents on nitromethane retard the rate of proton removal although the equilibrium is more favorable for the more highly substituted derivatives. The alkyl groups have a strong stabilizing effect on the nitronate ion, but unfavorable steric effects are dominant at the transition state for proton removal. As a result, kinetic and thermodynamic acidity show opposite responses to alkyl substitution. 

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. 

C-alkylation of secondary and tertiary aromatic amines by hexafluoroacetone or methyl trifluoropyruvate is performed under mild conditions [172] (equation 147) The reaction of phenylhydrazme with hexafluoroacetone leads selectively to the product of the C-hydroxyalkylation at the ortho position of the aromatic ring The change from the para orientation characteristic for anilines is apparently a consequence of a cyclic transition state arising from the initial N hydroxy alky lation at the primary amino group [173] (equation 148)... 

Elimination bimolecular (E2) mechanism (Section 5.15) Mechanism for elimination of alkyl halides characterized by a transition state in which the attacking base removes a proton at the same time that the bond to the halide leaving group is broken. 

Extensions of the enamine alkylation to a-tetralones have also been used (245-248), but product yields were lower, presumably due to steric crowding in a transition state where generation of an imonium salt gives rise to a repulsion between a methylene group on nitrogen and a peri aromatic proton. 

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