Ethene reaction with 1,3-butadiene

Oxatungstacyclopropane (43) also undergoes a number of addition reactions. Reaction with C02 affords cyclic carbonate (42). Reaction with ethene gives oxatungstacyclopentanes which result from insertion into the metal-carbon bond. Reaction with 1,3-butadiene gives a n complex where the metallacycle is retained <86CC90>. 

The reaction between ethyne and phosgene has also been studied under photochemical conditions (>220 nm) [2185a]. As in the analogous reaction with ethene (see Section 10.1.2), the phosgene merely acts as a convenient source of chlorine radicals the principal products were CO, CHj=CHCl, 1-chloro-l, 3-butadiene, benzene and polymer trace amounts of HC Cl and CgHjCl were also detected [2185a]. 

There are many examples of the Diels-Alder reaction (a [4+2] cycloaddition). In the reaction of 1,3-butadiene (1) with ethene, high temperatures are required for the reaction to generate cyclohexene. If 1-methoxyethene (5) is heated with 1,3-butadiene, the reaction requires heating to >200°C in a reaction bomb for several hours however, the yield of product (6) is lower than the yield of cyclohexene observed in the reaction with ethene. This result suggests that 5 is even less reactive than ethene. As shown in Section 24.1, the reaction of 1 and maleic anhydride (2) occurs in benzene at 100°C to give 3 (see the experiment from Section 24.1). 

Briefiy explain why the reaction of acrolein and 1,3-butadiene requires much milder reaction conditions than the reaction with ethene. 

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

SCHEME 3. Comparison of van der Waals volumes of reaction and activation with the volumes of reaction and activation calculated for a pericyclic and stepwise Diels-Alder reaction of 1,3-butadiene with ethene... 

A gas-phase reaction between butadiene (A) and ethene (B) is conducted in a PFR, producing cyclohexene (C). The feed contains equimolar amounts of each reactant at 525°C (T,) and a total pressure of 101 kPa. The enthalpy of reaction is — 115 kj (mol A)- , and the reaction is first-order with respect to each reactant, with kA = 32,000 13,850/7 m3 moi-l S 1. Assuming the process is adiabatic and isobaric, determine the space time required for 25% conversion of butadiene. 

As is outlined above (Equation 1-3), with ethene in hand the way to propene and butene/butadiene is paved. Finally, two other base chemicals which can be obtained from methanol are isoprene and toluene - the first by the reaction of methanol with 1-butene and the second by alkylation of benzene with methanol. 

The use of lithium amides to metalate the a-position of the N-substituent of imines generates 2-azaallyl anions, typically stabilized by two or three aryl groups (Scheme 11.2) (48-62), a process pioneered by Kauffmann in 1970 (49). Although these reactive anionic species may be regarded as N-lithiated azomethine ylides if the lithium metal is covalently bonded to the imine nitrogen, they have consistently been discussed as 2-azaallyl anions. Their cyclization reactions are characterized by their enhanced reactivity toward relatively unactivated alkenes such as ethene, styrenes, stilbenes, acenaphtylene, 1,3-butadienes, diphenylacetylene, and related derivatives. Accordingly, these cycloaddition reactions are called anionic [3+2] cycloadditions. Reactions with the electron-poor alkenes are rare (54,57). Such reactivity makes a striking contrast with that of N-metalated azomethine ylides, which will be discussed below (Section 11.1.4). 

Problem 9.26 Which conformation of 1,3-butadiene participates in the Diels-Alder reaction with, e.g., ethene <... 

Fig. 3.11 Using molecular mechanics to get the (approximate) transition state for the Diels-Alder reaction of butadiene with ethene. This procedure gives a structure with the desirable Cs, rather than a lower, symmetry...

We dealt with [4+2]-cycloadditions very briefly in Section 3.3.1. As you saw there, a [4+2]-cycloaddition requires two different substrates one of these is an alkene—or an alkyne—and the other is 1,3-butadiene or a derivative thereof. The reaction product, in this context also called the cycloadduct, is a six-membered ring with one or two double bonds. Some hetero analogs of alkenes, alkynes, and 1,3-butadiene also undergo analogous [4+2]-cycloadditions. In a [2+2]-cycloaddition an alkene or an alkyne reacts with ethene or an ethene derivative to form a four-membered ring. Again, hetero analogs may be substrates in these cycloadditions allenes and some heterocumulenes also are suitable substrates. 

Why do the Diels-Alder reactions with both normal and inverse electron demand occur under relatively mild conditions And, in contrast, why can [4+2]-cycloadditions between ethene or acetylene, respectively, and butadiene be realized only under extremely harsh conditions (Figure 15.1) Equation 15.2 described the amount of transition state stabilization of [4+2]-cycloadditions as the result of HOMO/LUMO interactions between the 7T-MOs of the diene and the dienophile. Equation 15.3 is derived from Equation 15.2 and presents a simplified estimate of the magnitude of the stabilization. This equation features a sum of two simple terms, and it highlights the essence better than Equation 15.2. 

Silene Reactions with Organic Dienes. The second example of reactions wdll concern Diels-Alder reactions of silenes. Certainly, as was already mentioned, the diene additions to silaethenes occur much faster than those to ethenes. Over and above that, in most eases other reactions operate in addition. As is shown in Scheme 10, the [4+4] cycloaddition of silaetiienes with 2,3-dimethyl-l,3-butadiene (DMB) leads - independently of tiie direction of silaethene addition to DMB - to one and the same [4+2]... 

---