Butadiene selective, electronic effect

An electronic effect was also used to explain the difference in 1,3-butadiene hydrogenation selectivity observed over various types of nickel catalysts such as Ni(B), Raney nickel, nickel powder from the decomposition of nickel formate, Ni(P), and Ni(S). As discussed in Chapter 12, chemical shifts in XPS binding energies (Aq) for the various nickel species were compared with that of the decomposed nickel catalyst to determine the extent of 1-butene formation as related to the electron density on the metal. The higher the electron density, the more 1-butene formation was favored. 

Another important group of dienophiles of the a,p-unsaturated carbonyl class are quinones. 1,4-Benzoquinone reacts readily with butadiene at room temperature to give a high yield of the mono-adduct, tetrahydronaphthaquinone (3.8) under more vigorous conditions a bis-adduct is obtained which can be converted into anthraquinone by oxidation of an alkaline solution with atmospheric oxygen. As with other dienophiles, alkyl substitution on the double bond leads to a decrease in activity and cycloaddition of monoalkyl 1,4-benzoquinones with dienes occurs preferentially at the unsubstituted double bond. In addition to steric effects, electronic effects can play a part, such that cycloaddition occurs at the more electron-deficient double bond of the benzoquinone. The first step in an approach to the steroid ring system makes use of such selectivity (3.9). ... 

This model reaction has been used for interpreting geometrical and electronic effects [7, 16-19]. As an example, we used the same reaction system described above however, with a palladium-supported catalyst, fed with butadiene and hydrogen at T =215 K, WHSV = 800 h . Results of dispersions, activity, and selectivity are presented in Table 3.5 [16]. 

The reaction has been improved to a satisfactory process by modifying the reaction conditions. A remarkable effect of the addition of amines on the reaction was observed (49). For example, the reaction of butadiene (4 moles) and acetic acid (4 moles) in the presence of 2-(N,/V-dimethyl-amino)ethanol (4 moles) using Pd(acac)2 (3 mmoles) and PPh3 (3 mmoles) at 90°C gave complete conversion after 2 hours. The product was found to consist of 8-acetoxy-1,6-octadiene (47) (71%), 3-acetoxy-1,7-octadiene (48) (21%) and 1,3,7-octatriene (16) (8%). Various tertiary amines, such as triethylamine, )V-methylmorpholine, Af,Af,N, N -tetramethyl-1,3-bu-tanediamine, and triethylenediamine, showed the same favorable effect. Other basic salts, such as sodium and potassium acetate, accelerate the reaction, especially at high concentrations (50, 51). The selection of solvents is also important. Arakawa and Miyake found that electron-donating type solvents (e.g., THF and triethylamine) are good solvents... 

Significant improvement of the activity of selectivity of Pd on Si02 could be achieved in the hydrogenation of acetylene by adding Ti, Nd, or Ce oxides to the catalyst.395 The metal oxides modify both geometrically and electronically the Pd surface. They retard the sintering of the dispersed Pd particles, suppress the formation of multiply bound ethylene, and facilitate the desorption of ethylene. The beneficial effect of lead in the hydrogenation of 1,3-butadiene over a Pd-Pb-on-... 

Diels-Alder reactions of 3-chloro and 3-ethyl-2-p-tolylsulfinyl-1,4-benzoqui-nones (124 and 125) take place at the C5-C6 unsubstituted double bond with both cyclic and acyclic dienes [109]. This is not unexpected from the electronic and steric effects of the substituents at C-3, which makes more difficult the approach of the dienes (even the acyclic ones) to C2-C3 than to C5-C6. With cyclopentadi-ene, no 7r-facial selectivity is observed under thermal conditions, and it is moderate in the presence of ZnBr2 (Scheme 63). Slightly higher 7r-facial selectivity was observed in reactions with 2,3-dimethyl-l,3-butadiene, presumably due to the lower reactivity of the diene. 

The addition of a C-2 (equation 1 R = H > alkyl, aryl > OMe NR2), C-3, or C-4 electron-donating substituent to a 1 -oxa-1,3-butadiene electronically decreases its rate of 4ir participation in a LUMOdiene-controlled Diels-Alder reaction (c/. Table 5). Nonetheless, a useful set of C-3 substituted l-oxa-l,3-buta-dienes have proven to be effective dienes ° and have been employed in the preparation of carbohydrates (Table 6). The productive use of such dienes may be attributed to the relative increased stability of the cisoid versus transoid diene conformation that in turn may be responsible for the Diels-Alder reactivity of the dienes. Clear demonstrations of the anticipated [4 + 2] cycloaddition rate deceleration of 1-oxa-1,3-butadienes bearing a C-4 electron-donating substituent have been detailed (Table 6 entry 4). >> "3 In selected instances, the addition of a strong electron-donating substituent (OR, NR2) to the C-4 position provides sufficient nucleophilic character to the 1-oxa-1,3-butadiene to permit the observation of [4 + 2] cycloaddition reactions with reactive, electrophilic alkenes including ketenes and sul-fenes, often in competition with [2 + 2] cycloaddition reactions. ... 

One of the most effective approaches to implementing the Diels-Alder participation of 1-oxa-1,3-butadienes is through the use of an intramolecular [4 + 2] cycloaddition reaction.A select set of thermal and Lewis acid-catalyzed intramolecular cycloaddition reactions of unactivated and electron-rich alkenes with a,P-unsaturated aldehydes and ketones has been detailed. Two examples of the poorly matched intramolecular Diels-Alder reaction of an a,P-unsaturated aldehyde (4 ir component) with an a, 3-unsaturated amide (2ir component) have proven successful (190-160 °C) and may be attributed to the entropic assistance provided by the intramolecular reaction. These observations have been applied in... 

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