Substitutions electrophilic, - 1,3-butadiene

Chiral electrophilic cyclopropanes (63) are prepared in high enantiomeric excess starting from butadiene-iron tricarbonyl complexes (60) containing a non-complexed double bond. Reaction with diazomethane and decomposition of the resulting pyrazolines (61) in the presence of Ce" gave the corresponding chiral cyclopropanes (62). Breakdown of the dienic substituent of electrophilic cyclopropane (62) by means of ozonization resulted in the formation of formyl-substituted electrophilic cyclopropane (63) still carrying the asymmetric centre (equation 10) " . ... 

Addition Reactions. 1,3-Butadiene reacts readily via 1,2- and 1,4-free radical or electrophilic addition reactions (31) to produce 1-butene or 2-butene substituted products, respectively. 

The SHMO orbitals of pyrrole, pyridine, and pyridinium are shown in Figure 11.6. The HOMO of pyrrole is the same as that of butadiene. Thus pyrrole is more reactive than benzene toward electrophilic attack. Attack, leading to substitution, occurs mainly at the... 

The question of aromaticity arises. Neither thiophenium salts nor thiophene sulfoxides are especially stable, making the classical reactivity test of electrophilic aromatic substitution difficult. The former dealkylate readily and the latter, at least for the case of thiophene sulfoxide, readily undergo self-dimerization (65CCC1158) (the bulky substituents of (57) impede this reaction). Aromaticity requires that the lone pair on sulfur participate in the aromatic sextet. If the lone pair, because of sp3 hybridization and improper symmetry, is not delocalized into the butadiene segment, the system will be antiaromatic. 

The SHMO orbitals of pyrrole, pyridine, and pyridinium are shown in Figure 11.6. The HOMO of pyrrole is the same as that of butadiene. Thus pyrrole is more reactive than benzene toward electrophilic attack. Attack, leading to substitution, occurs mainly at the 2- and 5-positions where the electron density of the HOMO is concentrated. In the case of pyridine (Figure 11.6b), the HOMO is not the n orbital, but the nonbonded MO, wn, which would be situated at approximately a - 0.5 //. Thus, it is not pyridine but pyridinium (Figure 11.6c) which undergoes electrophilic attack and substitution. The reactivity is much less than that of benzene, although this could not be deduced directly from the SHMO calculation. Neither does the calculation suggest the reason that electrophilic substitution occurs mainly at the 3- and 5-positions, since the n HOMO is... 

Electrophilic substitution of [Fe(diene)(CO)3] complexes was first described by Ecke who reported that acetylation of [Fe(butadiene)(CO)3], (1), gives 1- and 2-acetyl derivatives. Subsequent studies showed that acylation occurred only at the terminal carbons,9-12 to give the trans and cis isomers (2) and (3), respectively (equation 2). 

It is well-established that 7r-allylpalladium is electrophilic, and no reaction with electrophiles has been observed. However, there is an evidence that bis-7r-allylpalladium (172), generated in situ, could be amphiphilic. Typically, formation of the 2-substituted 3,6-divinylpyran (175) by the reaction of butadiene with aldehyde can be explained by the amphiphilic nature of the bis-7r-allylpalladium 173 generated in situ as an intermediate, which reacts with the electrophilic carbon and the nucleophilic oxygen in the aldehyde as shown by 174 [86]. As a similar reaction, piperidone is obtained by the reaction of butadiene with isocyanate [87]. The reaction of allyltributylstannane (176), allyl chloride and benzalmalononitrile (177) in the presence of PdCl2(Ph3P)2 (3 mol %) afforded the diallylated product 178 in high yield. 

Although the early examples of the 4ir participation of heterodienes in [4 + 2] cycloaddition reactions describe their reactions widi electron-deficient aJkenes, e.g. the thermal dimerization of a,3 unsaturated carbonyl compounds, the introduction of one or more heteroatoms into the 1,3-butadiene framewoiic does convey electrophilic character to the heterodiene. Consequently, such systems may be expected to participate preferentially in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, or simple alkene and alkyne dienophiles. The complementary substitution of the heterodiene with one or more electron-withdrawing substituents further lowers the heterodiene Elumo, accelerates the rate of heterodiene participation in the LUMOdioie-conn-olled Diels-Alder reaction, and enhances the observed regioselectivity of the [4 + 2] cycloaddition reaction. ... 

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. ... 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

Site selectivity is that selectivity shown by a reagent towards one site (or more) of a polyfunctional molecule, when several sites are, in principle, available. The preference for electrophilic attack at the ortho and para positions of X-substituted benzenes is just one of many examples discussed in Chapter 3. In cycloadditions, site selectivity always involves a pair of sites thus, butadiene reacts faster with the quinone (308) at C-2 and C-3 than at C-5 and C-6,256 and Diels-Alder reactions of anthracene (309) generally take place257 across the... 

Dienes are conjugated systems of two pi bonds. Above, the simplest diene, 13-butadiene, adds the electrophile to the end to produce a resonance stabilized allylic carbocation. As with alkenes, the more substituted the diene is, the more reactive. 

The l,3-bis(trimethylsililoxy)butadienes 130-132, as the equivalent of methyl acetoacetate dianion, constitute the three-carbon fragments with two nucleophilic sites (equation 110). Condensation of 130-132 with various equivalents of -dicarbonyl compounds and titanium(IV) chloride gives substituted methyl salicylates. The differential reactivity of the electrophiles which increases in the order conjugated position of enone > ketone > monothioacetal, acetal and of 130-132 (4-position > 2-position) ensures complete regioselectivity in this combination of two three-carbon units to form phenols such as 133 and 134 °° °. ... 

One of the subjects of our investigations involved the interaction of allenes with the P=E derivatives. This work provided some very interesting, unexpected results that may well be of use in synthetic organophosphorus chemistry. Thus, in attempting to study the (2+2)-cycloadditions of the phosphinimine and the (methylene)phosphine systems with the allenic C=C bond, we established that the reactions take a different and more interesting course — that of an ene" reaction (eq 1). In the first step of the reaction, the electrophilic phosphorus center apparently attacks the nucleophilic central carbon of the allene system. Then, instead of undergoing nucleophilic attack on the incipient carbonium ion, the anionic center (E) abstracts a proton from the terminal C-H bond, leading to the formation of a new double bond in the phosphorus-substituted 1,3-butadiene derivatives (3). 

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