Cis-dienes

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

Diels-Alder reaction, 492 characteristics of, 492-497 dienes in, 496-497 dienophiles in. 493-494 electrostatic potential map of. 493 endo stereochemistry of, 495 HOMO in. 1188-1189 LUMO in. 1188-1189 mechanism of. 493 s-cis diene conformation in, 496-497... 

The other possibility disrotatory motion) would have one moving clockwise while the other moves counterclockwise the cis isomer would have given the cis-cis diene (shown) or the trans-trans diene ... 

The reaction of benzyne with cyclohexadiene has been known for some time 4>, but although a number of steroidal cis-dienes are readily available no reactions with arynes had been reported prior to our beginning such investigations 145>. This was somewhat surprising in view of the number of reports concerning the modification of steroids by means of reactions with carbenes 146 i49) and the known Diels-Alder reactions of steroidal dienes and trienes iso.isi). 

We have also investigated the reactions of benzyne and tetrachlorobenzyne with the rings A/C cis-diene (117), both of which gave the a-adducts (118, X = H or Cl)... 

In particular, the increase of tacticity and the possibility of achieving the synthesis of "all cis" diene copolymers of high molecular weight allows the possibilities of broadening the range of products attainable in conventional, solution polybutadiene plants,... 

Alderson (/) had observed that the amount of alcohol present strongly influences the ratio of the trans- to cis-diene formed. With little or no alcohol, the trans/cis ratio is less than 5 in the presence of a large excess of alcohol, the trans/cis ratio rises to 10 or 20. 

Mason and coworkers10 studied the chiral bicyclic derivatives 2, 3, 33 and 34, having known absolute configuration. These molecules possess a planar s-cis diene chromophore and formally their chirality is due to the presence of the D or CH3 substituents, which rule out all the symmetry planes. However, it is interesting to point out a peculiar structural... 

An X-ray crystal structure determination of calciferol (vitamin D-2,71) showed that steric crowding in the s-cis diene system resulted in a twisted conformation with a dihedral angle of 53° [59], On irradiation with a mercury lamp, it was partially converted into ergosterol (72) and tachysterol (73) [60, 61]. When a solution of calciferol in light petroleum containing a trace of iodine was exposed to diffuse daylight, the vitamin was photoisomerized to (74) [62],... 

The 13C NMR signals of a diene are shifted far upheld upon complexation to a transition metal33. The terminal carbons (C1/C4) for a complexed diene are ca 50 to 80 ppm more shielded than the free ligand, while the internal carbons (C2/C3) are ca 20 to 60 ppm more shielded (Tables 3 and 4). The a2,jt bonding mode (2b) which is found for Zr, Hf and Nb s-cis diene complexes is revealed in the diminished coupling to... 

In solution, certain complexes are observed to undergo an envelope-flip from one face of the diene ligand to the other. The XH NMR spectra of Cp2Zr(s-cis diene) complexes (11 R = R" = H) at ambient temperatures indicate a fluxional process which equilibrates the Cp signals as well as the terminal protons (equation 4). At lower temperature, signals for a... 

The ring complex shown would account for the low A factor, and further explains the formation of only cis-diene. This 1,5-hydrogen shift mechanism has also been postulated to account for the thermal isomerization of a number of 1,3-dienes (Wolinsky et al. 1962). This type of isomerization has been studied in detail as applied to the reversible interconversion of cis-2-methylpenta-l,3-diene and 4-methylpenta-l,3-diene,... 

Figure 12.5. (a) Orbital interaction diagram for an s-cis diene and an olefin in the orientation for maximum overlap. (b) Interaction frontier MOs of an X -substituted diene and a Z-substituted dienophile showing primary (I) and secondary (II) interactions. 

This transition can operate in both anionic and cationic polymerizations. The stereospecific transition state also requires a limited degree of freedom between the propagating polymer and the gegen ion like that with the isotactic polymerization systems. Cis polymerization occurs over a broad middle range of ionieity of the catalysts and there appear to be both anionic and cationic catalysts which produce the cis-diene structure. 

Monomeric alkyl lithium polymerizes isoprene through an anionic type propagating species. The transition between cis 1,4 and trans 1,4 polymerization is not clear since mono-ene polymerization also occurs in this region. Increased dielectric constant of the media, the addition of ethers, or the use of high lithium alkyl concentrations increased the character from that weakly anionic for the cis-diene polymerization to the slightly more anionic requirements for 3.4-monoolefinic polymerizations. 

Although all-trans-octadecatrienoate tended to elute with the cis-dienes, this was not a problem in practice with samples of hydrogenated fats containing dienoic and trienoic fatty acids. A gradient elution scheme was adapted from the isocratic mobile phase used for monoenes and polyenes for the separation of such mixtures. 

The photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

The stereochemistry of the reaction of 1122 with each of the three geometrical isomers of 2,4-hexadiene is shown in Scheme 2.22 There are only four products, the trans-trans diene yielding two, the cis-cis diene the other two, and the trans-cis diene all four. The numbers attached to the arrows show the... 

Sensitizers whose triplet energies are less than 60 kcal mole 1 are of lower energy than the butadiene r-trans triplet and they must therefore seek out an r-cis diene (triplet energy of r-cis butadiene is 53 kcal mole-1) before they can transfer their energy. The biradical produced by attack of an. f-cis triplet on an r-trans ground-state molecule can form a cyclohexene without bond rotation (see Scheme 2), and thus the relative amount of cyclohexene increases. When the... 

The fact that the reactions take place in the direction of ring-opening is determined by thermodynamics, but the stereochemistry is most certainly not, for the cyclobutene 4.36 gives the thermodynamically more strained product 4.37 with one of the double bonds cis. Thermodynamics affects the stereochemistry only with the opening of the cyclobutene 4.38, which shows a preference for one of the conrotatory modes, that giving the trans,trans diene 4.39, where the rules could have led to the cis,cis diene equally well. This type of selectivity is called torqueoselectivity. 

With a smaller ring in the bicycloheptene 4.60, a conrotatory reaction is virtually impossible, since it would put a trans double bond into a 7-mem-bered ring. No visible reaction occurs until the forbidden disrotatory reaction 4.62 gives the cis,cis-diene 4.61 at 400°, and even then only in low yield. 

When the products of these two reactions are heated, they give back isomers of the starting materials, the trans,cis diene 4.106 in place of 4.102, and the trafts-dimethylcyclohexadiene 4.107 in place of 4.104. In more constrained systems, one of the consequences of the photochemical and thermal rules being opposite is that a photochemical reaction can easily lead to a product thermodynamically much higher in energy than the starting material, but nevertheless thermally stable. The photochemical reactions... 

However, when the diene system is constrained to the s-cis conformation by a cyclic framework, the effective concentration of the s-cis diene is much higher than for acyclic dienes, which have the s-cis conformer as a minor component of the rotomeric equilibrium. Such conformationally constrained dienes react much more easily and are excellent Diels-Alder dienes. Examples are cyclopentadi-enes, 1,3-cyclohexadienes, and furans. 

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