1-butene, conformation

Figure 4-20 Rotamers (rotational conformers ) of 1-butene (Kevins, Chen, and AlliiiceT, 1996 ).

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

With more substituted terminal alkenes, additional conformations are available as indicated below for 1-butene. 

Further substitution can introduce additional factors, especially nonbonded repulsions, which influence conformational equilibria. For example, methyl substitution at C—2, as in 2-methyl-l-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene the two eclipsed conformations are of approximately equal energy. Increasing the si2e of the group at... 

FIGURE 22.4 New-man projections showing the conformations leading to (a) 1-butene, and (b) frans-2-butene by Hofmann elimination of sec-butyl-trimethylammonium hydroxide. The major product is 1-butene. 

Methyl-3-buten-l-ol, NMR spectrum of, 647 Methylcyclohexane, 1,3-diaxial interactions in, 123 conformations of, 123 mass spectrum of, 4H molecular model of, 123, 293... 

Most preparations of the intermediate racemic 3-allyl(dicarhonyl)cyclopentadieny molybdenum complexes7 2 start from sodium or potassium tricarbonyl(cyclopentadienyl)molybdate8 9 10 (4) for a simple preparation see ref 11, p 493. The allylation of 4 hy homoallylic halides, such as 4-hromo-l-butene, is accompanied by rearrangement and decarhonylationI2. (Z)-if-2-butenyl(dicarbonyl)cyclopentadienylmolybdenum (5), like other comparable complexes, exists as a mixture of endo/exo- conformers, which interconvert rapidly at room temperature12. 

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. 

Schei, S. H. 1984a. 3-Chloro-l-Butene Gas-Phase Molecular Structure and Conformations as Determined by Electron Diffraction and by Molecular Mechanics and Ab Initio Calculations. J. Mol. Struct. 118, 319-332. 

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.19 Side view and projection along chain axis of conformation of chains in crystals of alternating e (hyIenc-c/.v-2-butene copolymer.119...

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

As a final example, we consider the relative stability of the torsional isomers of 2-butene. The six conformations of 2-butene are shown below along with the definitions to be used in the discussion, e.g. the label Css refers to the cis isomer where the in plane hydrogens of the methyl groups are staggered relative to the double bond. 

Fig. 22. Energy difference between the Tee and Cee and Tse and Cse conformations, respectively, of 2-butene. Energies from ab initio calculations.

The experimental probe of the conformational preferences of allyl anions is based primarily on the kinetics of base catalyzed isomerization reactions of olefins and many results have been summarized by Bank180). Kinetic data of base catalyzed isomerization of 1-butene181) show that the less stable cis isomer of 2-butene is formed through a thermodynamically more stable allylic anion ... 

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. 

Arguing as before, we conclude that pi nonbonded attractive interactions are maximized in the SS conformation while sigma nonbonded attractive interactions are maximized in the EE conformation. This conflict between pi and sigma nonbonded attractive interactions is similar to the conflict observed in cis 2-butene in which steric effects resulted in an apparent dominance of sigma over pi aromaticity. 

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