Dienes transoid

Heteroannular diene (transoid or s-trans) More intense, e = 12,000-28,000 X shorter (234 nm)... 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

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]... 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

In case of a Z,Z-configurated diene 6, the transoid conformation is favored, because of unfavorable steric interactions of substituents at C-1 and C-4 in the... 

The comparison of rates of cycloaddition of maleic anhydride, tetracyanoethylene, and styrene to PPA shows that the latter, irrespective of the presence of electronegative groups, behaves in these reactions not as an electron-poor diene system. This fact, together with the composition of side products (giving evidence of PPA decarboxylation), allows the assumption to be made that the cycloaddition of dienophiles involves mainly decarboxylated polyene sections of cis-transoid structure213, 266. This is in agreement with the fact that PPA with predominant trans-transoid configuration interacts with these dienophiles at a substantially lower rate. The ultimate amounts of the dienophile combined with PPA of this structure is also considerably smaller. 

The diene must be in the cisoid conformation. If it is frozen into the transoid conformation, as in 84, the reaction does not take place. The diene either must be frozen into the cisoid conformation or must be able to achieve it during the reaction. 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

The sin0 proportionality of Rn n obtained here with two independent, however simplistic, approaches allows us to extend the DR, originally formulated for the cisoid dienes, even to the transoid chromophores, as shown in Scheme 5. 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

The use of UV-VIS spectra to analyse dienes and polyenes was historically the first method of choice. The spectra of isolated non-conjugated polyenes is actually the superposition of the spectrum of each one of the double bonds. For each double bond the spectrum depends on the various substituents and also on its location in the molecule. It also depends on the stereochemistry, since conjugated double bonds have either E or Z configuration around each jr-bond but also a cisoid and transoid conformer3 around the single bond marked as s-cis and s-trans4. 

The conformation of cyclic and polycyclic dienes is frozen either in the cisoid or in the transoid form (z). 

Nucleophilic addition to acyclic (diene)Mo+ cations has been examined. For (isoprene) Mo(CO)2L (L = Cp, Cp, In), the regioselectivity for nucleophilic attack has been found to depend on the nature of the nucleophile, the ligand L, the reaction solvent and the temperature21,813 833 193. The generation and in situ reactivity of transoid acyclic (diene)molybdenum and tungsten cations with nucleophiles has been previously mentioned (Section IV.C.2). 

A diene system with unsymmetrical 1,4-disubstitution is converted to the iron carbonyl complex 1 which is resolved into its enantiomers. The aldehyde function is conformationally locked in the transoid position and is diastereofacially shielded from the bottom face. Nucleophiles attack from the top face with high selectivity. Alternatively, chain elongation leads to the triene 2 which is reacted with diazomethane. Cerium(IV) oxidation removes the metal and furnishes the substituted cyclopropane 3. 

Dienes can be open-chain, inner-ring (e.g., 80), outer-ring856 (e.g., 81), across rings (e.g., 82). or inner-outer (e.g., 83), except that they may not be frozen into a transoid conformation... 

The Diels-Alder reaction is highly stereospecific. The diene reacts in an unfavorable conformation in which its double bonds lie in a plane on the same side (cis) of the single bond connecting them. This s-cis (or cisoid) conformation is required to give a stable product with a cis double bond. Addition of ethene to the alternate and more stable (transoid) conformation would give an impossibly strained Pimv-cyclohexene ring. Possible transition states for reaction in each conformation follow, and it will be seen that enormous mo-... 

A Transoid 1,3-Diene. In contrast to the considerable number of photochemical rearrangements of homoannular dienes only one photochemical rearrangement of a transoid heteroannular diene has been reported. Irradiation of 3,5-cholestadiene (Formula 400) in ethanol gives the pentacyclic ether (Formula 401) (176). 

---