Butadiene protonation

Fig. 8.5.3 Proton MAS images of solid discs (750 iJtm thick and 3.5 mm diameter) of poly(butadiene)-poly(styrene) blends, (a) Mechanical blend of both components, (b) Blend cast from solution in toluene. The image contrast is caused by the differences in the strengths of the dipole-dipole couplings among H for the two polymers. The signals from poly(styrene) are filtered out (white) so only poly(butadiene) protons contribute to the image (dark). The spatial resolution is better than 50 p-m at a spinning frequency of 5 kHz. Adapted from [Cor4]. Copyright 1989 American Chemical Society.

Such is not the case with 1,3-butadiene. Protonation of the diene is still regiospe-cific for the end carbon, but the two resonance forms of the resulting allylic carbocation are not equivalent. 

With 1-phenyl-1,3-butadiene, the addition is exclusively at the 3,4-double bond. This reflects the greater stability of this product, which retains styrene-type conjugation. Initial protonation at C-4 is favored by the feet that the resulting carbocation benefits from both allylic and ben2ylic stabilization. 

The FMOs of acrolein to the left in Fig. 8.2 are basically slightly perturbed butadiene orbitals, while the FMOs of protonated acrolein resemble those of an allyl cation mixed in with a lone-pair orbital on the oxygen atom (Fig. 8.2, right). Based on the FMOs of protonated acrolein, Houk et al. [2] argued that the predominant interaction in a normal electron-demand carbo-Diels-Alder reaction is between the dienophile LUMO and diene HOMO (Fig. 8.1, left). This interaction is greatly... 

Active Figure 14.4 An electrostatic potential map of the carbo-cation produced by protonation of 1.3-butadiene shows that the positive charge is shared by carbons 1 and 3. Reaction of Br-wit n the more positive carbon (C3 blue) gives predominantly the 1.2-addition product. Sign in St wwW.thornSOneuU.com to see a simulation based on this figure and to take a short quiz. 

Simple imines are poor dienophiles and must be activated by protonation or by attaching an electron-withdrawing group to the nitrogen atom. Scheme 6.10 illustrates the Diels-Alder reactions of benzyliminium ion 25, generated in situ from an aqueous solution of benzylamine hydrochloride and commercial aqueous formaldehyde, with methylsubstituted 1,3-butadienes [22]. This aqueous Diels-Alder reaction combines three components (an aldehyde, an amine... 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to form the more stable carbocation => a resonance stabilized allylic cation, i) Addition to one of the inner carbon atoms would have produced a much less 1 ° cation, one that could not be stabilized by resonance. 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

Linear oligomerization and telomerization of butadiene take place with nickel complexes in the presence of a proton source (7). In addition, cooligomerization of butadiene with functionalized olefins such as methacrylate is catalyzed by nickel complexes [Eq. (4)] (12, 13) ... 

The reactions covered in Scheme 2 are initiated by protonation but a hydride could form on the metal as intermediate. In some instances, cationic metal hydrides have been shown to be actually involved. See, for example, the addition of [HNi (POEt)3 4+] to butadiene (54) or of [HNi(Ph3P)3(7r-C3H5)] to olefins (10c, Vol. II, p. 25). Thus the reaction of olefins or dienes with acids in the presence of zero-valent nickel may be considered proton-promoted as well as hydride-promoted. 

Butadiene-ethylene dimerization (example 3, Table II) has been shown to proceed via a croty 1-nickel complex formed by protonation (54). It should be observed at this point that it cannot be excluded that linear cooligomerization of butadiene with ethylene to give 1,4,9-decatriene... 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

The rate equation for the dimerization of ethylene (5) can be used to describe the codimerization in the presence of large excesses of butadiene. The rate of the addition reaction as measured by the disappearance of ethylene is represented in Eq. (5). It is first order in ethylene, proton, chloride, and rhodium. 

Shida and Hamill23 found that the positive and negative molecular ions of 1,3-butadiene and its homologs have similar absorption spectra. Band maxima of the anions are not sensitive to substituent alkyl groups, whereas those of the cations are red-shifted as the number of substituent methyl groups increases. In alcoholic matrices the butadiene anions abstract the alcoholic proton to form an allylic radical (equation 23), as was proven by ESR spectroscopy. 

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

In this process the primary step is the formation of an anion, which is a synonym for a nucleophile, mostly by deprotonation using a base. It follows a reaction with an electrophile to give a new anion which in the anionic-anionic process again reacts with an electrophile The reaction is then completed either by addition of another electrophile as a proton or by elimination of an X group. Besides the anionic-anionic process there are several examples of anionic-pericydic domino reactions as the domino-Knoevenagel-hetero-Diels-Alder reaction in which after the first step an 1-oxa-l,3-butadiene is formed. 

Notes The uncertainty in the proton affinities of the alkylenes is probably 3 kcal/mole, in those of butadiene and styrene rather greater The proton affinities of multiply alkyl substituted ethylenes probably all lie within 3 kcal/mole of that of isobutene [4c] ... 

The proton affinities of 1,2- and 1,3-butadiene and of 2-butyne have been determined by Lias and Ausloos79 using equilibrium measurements in an Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Surprisingly, they were found to be almost identical. The bimolecular reactivity of the C4FL+ cations formed from the three isomers was also reported. 

---