Reactivity of dienes

A few years after the first articles of Breslow had appeared, Grieco elegantly demonstrated that the astonishing rate and selectivity enhancements of Diels-Alder reactions in water can be exploited sirccessfully in organic synthesis. He extensively studied the reactivity of dienes containing... 

In the Diels-Alder reaction a double bond adds 1,4 to a conjugated diene (a 2 + 4 cycloaddition), so the product is always a six-membered ring. The double-bond compound is called a dienophile. The reaction is easy and rapid and of very broad scope and reactivity of dienes and dienophiles can be predicted based on analysis of the HOMOs and LUMOs of these species. Ethylene and simple alkenes make poor dienophiles, although the reaction has been carried out with these compounds. 

The relative reactivity of dienes has been estimated on the basis of competition experiments performed using two kinds of dienes (2 mmol each) and... 

Scheme 10 Relative reactivity of dienes toward homoallylation of benzaldehyde under the Ni-EtjB catalysis...

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

The aim of this chapter is to present the most recent theoretical contributions to the study of structure, properties and reactivity of dienes and polyenes. Earlier stages in these areas are covered in the above-mentioned reports1-5. 

The reactivity of dienes in Diels-Alder reactions is also controlled by the diene conformation. The two planar conformations of 1,3-butadiene are referred to as s-trans and s-cis (equation 16). Calculations have shown the s-trans conformation to be 2-5 kcalmol-1 more stable than the s-cis conformation. Open-chain dienes can only react in their cisoid conformation. Thus, 2-substituted dienes are generally more reactive than 1,3-butadiene due to their stronger preference for the s-cis conformation. 1 -Cis substituted 1,3-butadienes are almost exclusively in the s-trans conformation and are not reactive in Diels-Alder reactions. Highly substituted dienes may, however, be present in the s-cis conformation during a sufficient amount of time to participate in Diels-Alder reactions, even if a 1 -cis substituent is present62. 

The reactivities of dienes in the Diels-Alder reaction depend on the number and kind of substituents they possess. The larger the substituents are, or the more of them, at the ends of the conjugated system, the slower the reaction is likely to be. There also is a marked difference in reactivity with diene configuration. Thus trans- i, 3-pentadiene is substantially less reactive toward a given dienophile (such as maleic anhydride) than is cis-1,3-pentadiene. In fact,... 

Investigations into the synthesis and reactivity of -diene iron tricarbonyl complexes are among the most active areas of research in iron organometalhc chemistry. A nnmber of excellent reviews are available for the reader seeking greater detail than can be provided here. ... 

Dienes were copolymerized with vinyl monomers such as p-chlorostyrene 17), acrylic esters18,19), vinyl carborane, isoprenylborane 20,2)), and ferrocenyl derivatives 21). The reaction conditions were similar (65 °C, dioxane, 72 h, 3 mole % of initiator). Liquid low-molecular-weight (Mn < 7000) copolymers were obtained. High concentration of p-chlorostyrene 17), acrylates, or methacrylates18,19) in the initial solution leads to higher copolymerization yields compared with systems rich in diene. Molecular weight and polydispersity vary in the same manner 15). The reactivity of dienes decreases in the series chloroprene (85-98%) > butadiene (64-83%) > isoprene (43-73%). 

Konovalov, A. I., Kiselev, V. D. Diels-Alder reaction. Effect of internal and external factors on the reactivity of diene-dienophile systems. Russ. Chem. Bull. 2003, 52, 293-311. 

In view of the widespread interest in the Diels-Alder reaction, the results of a kinetic study of the order of reactivity of dienes toward maleic anhydride is of particular interest. The figures under the formulas give values of rate constants, 10 x/ j (liters/mole/sec.) in dioxane at 30°. Where cisoid and transoid forms are both possible, the transoid form is shown, although there is evidence that a bulky group at the 2-position favors the cisoid form required for reaction. ... 

Continuing the research on the reactivity of dienes with biphasic catalytic systems, Morel and Mignani discovered that the Rh/TPPTS functionalization of dienes in the 4-position has beneficial efforts [22]. With other asymmetrical dienes such as myrcene and the addition of, for example, ethyl acetoacetate, interesting regioselectivities of 99% for the desired isoprenic compounds were achieved [23, 24], A couple of subsequent reaction steps, based on TPPTS from Ruhrchemie, thus convert geranylacetone to vitamin E [25]. 

An important development in the hetero Diels-Alder reaction came when Jurczak used high pressure to help improve the reactivity of dienes with aldehydes. 1-Methoxybutadiene, for example, was shown to react at 15-25 kbar (I bar = l(X) kPa) with a variety of unactivated aldehydes. - These reactions give predominantly endo adducts and allow for the use of simple alkyl and aromatic aldehydes as dienophiles. Jurczak also realized the importance of this reaction in the synthesis of carbohydrates and applied it to the construction of simple monosaccharide derivatives (see Section 2.5.2. ). " ... 

This chapter will mainly describe new developments in the area of theoretical methods and experimental structural methods, and in the rapidly growing field of inter- and intramolecular Diels-Alder reactions of these electron-poor azadienes. Quantitative data for the reactivity of dienes and dienophiles in these (4 -1- 2) cycloadditions with inverse electron demand will be discussed. The synthesis and reactions of dihydro, tetrahydro, and hexahydro tetrazines cannot be discussed broadly beyond the scope of <84CHEC-I(3)53l>. Verdazyls, a well-known class of compound, cannot be treated in detail within the frame of this contribution (see Section 6.21.5.9). 

The [4 + 2] cycloaddition of azadienes with alkynes is a poor reaction. However, the reactivity of dienes with Fisher carbenes has been applied to the azadiene 11 providing the 1,4-... 

Conjugation usually reduces the reactivity of dienes to hydroboration. Symmetrical acyclic 1,3-dienes can often be monohydroborated successfully with 9-BBN (equation III) unsymmctrical acyclic 1,3-dienes usually undergo mono-hydroboration without difficulty (equations IV and V). 

More details on this explanation of the reactivity of dienes can be found on pp. 146-8 of the textbook. 

Diethyl azodicarboxylate will participate in the ene reaction with both cyclo-nona-l,2-dienes and acyclic allenes to give the corresponding 2-ylbicarbamate-1,3-dienes under mild conditions and in good yields. A study of the reactivity of diene (37) with activated olefins indicates that the percentage of ene adduct (38) over the Diels-Alder product (39) increases with a more highly substituted olefin. ... 

Figure 6.114 Combining multiple p-donors for increasing the Diels-Alder reactivity of dienes.

Relative Reactivities of Styrene and Dienes. The relative reactivities of dienes versus styrenes depend on the chain-end concentrations because of the differences in kinetic order dependencies on chain-end concentration. The relative rates of propagation at [PLi] = 10 M are in the order styrene > isoprene > butadiene. However at [PLi] < 10 M, isoprene propagates faster than styrene (53). 

The hydroboration of symmetrical nonconjugated dienes, such as 1,5-hexa-diene with 9-BBN, in a 1 1 molar ratio proceeds in an essentially statistical manner, giving approximately 25% residual diene, 50% monohydroboration product, and 25% dihydroboration product. On the other hand, conjugated dienes, for example, 1,3-butadiene, behaves differently and affords equal amounts of residual diene and 1,4-dihydroboration product. Conjugation markedly decreases the reactivity of diene toward hydroboration. Consequently, nonconjugated dienes like 1,4-hexadiene are selectively hydroborated with 9-BBN in the presence of 1,3-pentadiene (Scheme 5.6) [3]. 

Fig. 3.9 The figure illustrates the HOMO-LUMO eneigy gap in terms of FMO theory on the reactivity of diene and dienophile in normal electron demand Diels-Aldts reaction. The narrower the gap the higher will be the TS stability and faster will be the reactivity...

In this study the reactivity of dienes will be considered under three headings ... 

---