1,3-butadiene, conformations

Figure 25 Geometries of traras, frans-l,4-diphenyl-1,3-butadiene conformers optimized by the AMF calculation.

The cyclooctatetraene-diene complex [( -C4H6)cot]Ti has been reported and has been shown by H NMR spectra to adopt an i -cw- ( -butadiene conformation. ... 

Additional evidence for electron delocalization m 1 3 butadiene can be obtained by considering its conformations Overlap of the two rr electron systems is optimal when the four carbon atoms are coplanar Two conformations allow this coplananty they are called the s cis and s trans conformations... 

The letter sms cis and s trans refers to conformations around the C—C single bond m the diene The s trans conformation of 1 3 butadiene is 12 kJ/mol (2 8 kcal/mol) more stable than the s cis which is destabilized by van der Waals strain between the hydrogens at C 1 and C 4... 

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

There are three derivatives of butadiene having one t-butyl substituent and nine di-t-butyl derivatives. Predict the preferred conformation for each of these 12 compounds. 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

Examine the models 1,3-butadiene in Figure 10.6 on/.earning By Modeling and compare space-filling models of the s-cis and s-trans conformation. 

The photochemical behavior of butadienes has been closely studied. When these compounds are exposed to light, they move from the ground state to an excited state. This excited state eventually returns to one of the ground state conformations via a process that includes a radiationless decay (i.e., without emitting a photon) from the excited state potential energy surface back to the ground state potential energy surface. 

The red line follows the progress of the reaction path. First, a butadiene compound b excited into its first excited state (either the cis or trans form may be used—we will be considering the cis conformation). What we have illustrated as the lower excited state is a singlet state, resulting from a single excitation from the HOMO to the LUMO of the n system. The second excited state is a Ag state, corresponding to a double excitation from HOMO to LUMO. The ordering of these two excited states is not completely known, but internal conversion from the By state to the Ag state i.s known to occur almost immediately (within femtoseconds). 

Solvent polarity is also important in directing the reaction bath and the composition and orientation of the products. For example, the polymerization of butadiene with lithium in tetrahydrofuran (a polar solvent) gives a high 1,2 addition polymer. Polymerization of either butadiene or isoprene using lithium compounds in nonpolar solvent such as n-pentane produces a high cis-1,4 addition product. However, a higher cis-l,4-poly-isoprene isomer was obtained than when butadiene was used. This occurs because butadiene exists mainly in a transoid conformation at room temperature (a higher cisoid conformation is anticipated for isoprene) ... 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

However, exo-selective Diels-Alder reactions are found when a,/J-unsatu-rated exocyclic carbene complexes are used as dienophiles. The fixed s-cis conformation of the vinylcarbene moiety of the complex seems to be responsible for the exo selectivity observed in this reaction. Moreover, the reaction of optically active carbene complexes with 2-morpholino- 1,3-butadienes allows the asymmetric synthesis of spiro compounds [99] (Scheme 53). 

Barluenga et al. have described novel vinylcarbene complexes containing a cyclic BF2 chelated structure which temporarily fixes the s-cis conformation of the exocyclic C=C and Cr=C double bonds. These boroxycarbene complexes behave as dienophiles with 2-amino-l,3-butadienes in a remarkably regio- and exo-selective way. Moreover, high degrees of enantioselectivity are reached by the use of chiral 2-aminodienes derived from (S)-methoxymethylpyrrolidine [101] (Scheme 54). 

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