Butenes chiral conformations

Prepare models of the four isomeric butenes, C4H8. Note that the restricted rotation about the double bond is responsible for the cis-trans stereoisomerism. Verify this by observing that breaking the n bond of cw-2-butene allows rotation and thus conversion to rran5 -2-butene. Is any of the four isomeric butenes chiral (nonsuperposable with its mirror image) Indicate pairs of butene isomers that are structural (constitutional) isomers. Indicate pairs that are diastereomers. How does the distance between the Cl and C4 atoms in mw5 -2-butene compare with that of the anti conformation of butane Compare the Cl to C4 distance in cw-2-butene with that in the conformation of butane in which the methyls are eclipsed. 

In contrast, the diastereoselectivity of the conjugate addition of a chiral alkenylcoppcr-phosphinc complex to 2-mcthyl-2-cyclopentenone was dictated by the chirality of the reagent63. The double Michael addition using the cyclopentenone and 3-(trimethylsilyl)-3-buten-2-one and subsequent aldol condensation gave 4 in 58 % overall yield. The first Michael addition took place from the less hindered face of the m-vinylcopper, in which chelation between copper and the oxygen atom fixed the conformation of the reagent. 

The rearrangement of ammonium ylides 27 derived from chiral amines affords chiral 3-butenals 28 in modest yield120. Critical to the observed diaslereoface selectivity of the rearrangement is the substrate-directed alkylation of amines 26 to afford asymmetric ammonium salts. The observed asymmetric induction is consistent with rearrangement through a transition state conformation in which the more substituted carbon (RL) of the auxiliary occupies an exo orientation. 

Bell, S., Drew, B. R., Guirgis, G. A., Durig, J. R. (2000). The far infrared spectrum, ab initio calculations and conformational energy differences of 1-butene. Journal of Molecular Structure, 555(1-3), 199-219. (b) Broeker, J. L., Hoffmann, R. W., Houk, K. N. (1991). Conformational analysis of chiral aUcenes and oxonium ions ab initio molecular orbital calculations and an improved MM2 force field. Journal of the American Chemical Society, 775(13), 5006-5017. 

Epitaxy of Isotactic Poly (1-butene) Isotactic poly(1-butene) (iPBul) is an archetypical polymorphic polymer with three different structures that differ by the chain conformation, and thus by the unit-cell geometry and symmetry (cf Chapter 2).The three crystal phases could be obtained by epitaxial crystallization on appropriate substrates [48-50]. Most interesting among them are the epitaxy of Form I (trigonal unit cell, threefold helical conformation, racemic phase) and that of Form III (orthorhombic unit cell, fourfold helical geometry, chiral crystal phase). 

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