2- Methyl-2-butene stereochemistry

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

The stereochemistry of HC1 or DC1 addition to a wide variety of alkenes has been examined. Addition of HC1 to m-2,3-dideutero-2-butene affords a mixture of erythro and threo 2-chlorobutanes.40 1-Methyl-cyclopentene37 and 1,2-dimethylcyclopentene41 give almost exclusively tertiary chlorides formed by anti... 

The structures of the products formed in the photoadditions to 3-ethoxyisoindole-none and precedent from other types of photoaddition reactions suggest a potential 1,4-diradical intermediate. To test for such an intermediate, the stereochemistry of addition to cis- and 2-butene has been examined50. Irradiation of 50 in the presence of cis or trans-2-butene in methylene chloride solvent gave a mixture of the same four products, although in different proportions. Two cycloadducts 59 and 60 and two ene products 61 and 62 are formed. The stereochemistry of the methyl substituents of 59 and 60 has been assigned from 13C NMR spectral data. 

However, the utility of this Zweifel synthesis was limited in the past by the limited availability of dialkylboranes, because direct hydroboration leads cleanly to the formation of dialkylboranes only in the case of relatively hindered alkenes such as 2-methyl-2-butene and cyclohexene. More generally, the hydroboration fails to stop at the R2BH stages. Recent developments have provided a general preparation of a variety of dialkylboranes via the hydridation of dialkylhalo-boranes. Thus, dialkylvinylboranes prepared via the hydridation of dialkylhalo-boranes in the presence of an alkyne, react with iodine under basic conditions to produce disubstituted alkenes (Eq. 58) and trisubstituted alkenes (Eq. 59) of established stereochemistry. These results indicate a mechanism analogous to that... 

Reaction of (55) with trani-2-butene also proceeds with excellent asymmetric induction to give a 15 1 mixture of (58) and the diastereomer at C-3 in 85% yield. The stereochemistry at C-2 is the same in both isomers. Mixtures of isomers are produced since endo and exo transition states give different products with an internal alkene. The major isomer (58) is formed via the less-hindered exo transition state (57). These reactions are stepwise, rather than concerted, since cii-2-butene is isomerized to ri-ans-2-butene under the reaction conditions. SnCU-catalyzed addition of (55) to cis-4-methyl-2-pentene results in isomerization of the alkene to the trans isomer, which reacts exclusively at C-3 giving (59) in 86% yield (Scheme 11). 

Both cis- and trans-2-butene give cyclopropane with retention of the alkene geometry. Propene, styrene, 2-methyl-2-butene lead to the predominant formation of the thermodynamically less stable cis-cyclopropane (see Table 4). However, this cannot be explained, as suggested earlier, by a preferred stereochemistry of a hypothetical metalla-cyclobutane intermediate. 

This reaction was first reported by Nenitzescu in 1931. It is the formation of an a,p-unsaturated ketone directly by aluminum chloride-promoted acylation of alkenes with acyl halides. Therefore, it is known as the Darzens-Nenitzescu reaction (or Nenitzescu reductive acylation), or Nenitzescu acylation. Under such reaction conditions, Nenitzescu prepared 2-butenyl methyl ketone from acetyl chloride and 1-butene and dimethylacetylcyclohex-ene from acetyl chloride and cyclooctene. However, in the presence of benzene or hexane, the saturated ketones are often resolved, as supported by the preparation of 4-phenyl cyclohexyl methyl ketone from the reaction of cyclohexene and acetyl chloride in benzene, and the synthesis of 3- or 4-methylcyclohexyl methyl ketone by refluxing the mixture of cycloheptene and acetyl chloride in cyclohexane or isopentane. This is probably caused by the intermolecular hydrogen transfer from the solvent. In addition, owing to its intrinsic strain, cyclopropyl group reacts in a manner similar to an oleflnic functionality so that it can be readily acylated. It should be pointed out that under various reaction conditions, the Darzens-Nenitzescu reaction is often complicated by the formation of -halo ketones, 3,)/-enones, or /3-acyloxy ketones. This complication can be overcome by an aluminum chloride-promoted acylation with vinyl mercuric chloride, resulting in a high purity of stereochemistry. ... 

Scheme 6.47. The reaction of singlet methylene (hCH2) and triplet methylene f CH2) with an alkene. In (a), the singlet methylene (hCH2) adds across the double bond with retention of alkene stereochemistry, that is, the methyl groups in (Z)-2-butene are on the same side of the double bond while in the product, (Z)-l,2-dimethylcyclopropane, they are on the same side of the three-membered ring. Alternatively, in (b), triplet methylene ( CH2) reacts with the loss of alkene stereochemistry. The orbital representations were created using the MOViewer in WebMO version 6.0.002p.

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