1.3- dienes protonation

Furthermore, Morton and co-workers (59,69) examined the PMR spectra of 1,3-butadiene monomers both in the presence and absence of polybutadieny(lithium in order to determine whether the diene protons of the monomer are shifted by interaction with the polymer lithium. No shifts were observed. Their finding indicates that if any complex between monomer and C—Li exists, it must be of a shorter lifetime than the NMR time scale. However, such a complex has been observed in the presence of a modifier such as DPE (31). 

These are similar to those of the corresponding bis(ethylene) complex. The 100-MHz H nmr spectrum in C6D6 (internal TMS) shows a pair of multiplets at 6 2.41 and 4.41 due to the outer and inner coordinated diene protons, respectively, a multiplet at 5 1.71 due to the methylene protons, and a singlet at 5 1.95 due to the methyl protons of hexamethylbenzene. 

CpCo(n -penta-1,3-diene). Protonation of the n -trlene complex... 

Protonation at end of diene unit gives a carbocation that IS both secondary and allylic most stable carbocation product is formed from this carbocation... 

The major product corresponds to addition of a proton at C 1 and bromide at C 2 This mode of addition is called 1,2 addition, or direct addition The minor product has its proton and bromide at C 1 and C 4 respectively of the original diene system This mode of addition is called 1,4 addition, or conjugate addition The double bond that was... 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

Conjugate acid (Section 1 13) The species formed from a Brpnsted base after it has accepted a proton Conjugate addition (Sections 1010 and 1812) Addition reaction in which the reagent adds to the termini of the con jugated system with migration of the double bond synony mous with 1 4 addition The most common examples include conjugate addition to 1 3 dienes and to a 3 unsaturated car bonyl compounds... 

The addition of hydrogen halides to dienes can result in either 1,2- or 1,4-addition. The extra stability of the allylic cation formed by proton transfer to a diene makes the ion-... 

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

Reduction of linearly conjugated 4,6-dien-3-ones with lithium-ammonia yields either 5-en-3-ones or 4-en-3-ones depending upon the work-up procedure. Protonation of the dienyl carbanion intermediate (58) occurs at C-7 to give ultimately the enolate ion (59) kinetic protonation of (59) occurs largely at C-4 to give the 5-en-3-one (60). ... 

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

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... 

The coordination of the dienophile to a Lewis acid (in the calculations a proton was used as the Lewis acid) leads also to an increase in regioselectivity. The re-gioselectivity of reactions of electron-rich, or conjugated dienes, with electron-deficient dienophiles is also controlled hy the diene HOMO-dienophile LUMO interaction. From Fig. 8.2 it appears that the difference in magnitudes of the LUMO coefficients at carhon atoms 1 and 2 of acrolein (Ci -C2 = 0.20) is smaller than the same difference for protonated acrolein (Ci -C2 = 0.30-0.43) so that the reaction of the latter should he considerable more regioselective than the former in accordance with the experimental results [3]. 

Fig. 8.3 Diene HOMO-dienophile LUMO interaction of endo transition state for the reaction of cyclopen-tadiene with acrolein (a) and protonated acrolein (b)...

The characteristic property distinguishing macrocyclic polyamines from their linear counterparts is seen in successive protonation. One is the higher N basicity to the first proton and another is a sudden drop of N basicities in the later stages of protonation. Table 1 lists the protonation constants (Eq. 1) for the macrocyclic polyamines in comparison with the corresponding values for their linear homologues. When a linear triamine (e.g. dien) 36,37) is cyclized to, say, (9)aneN3, the basicity of the first amine increases (log Kt = 10.59 us 9.70), but the basicity of the second and especially the third amine diminish (log K2 = 6.88 vs 8.95, log K3 < 1 vs 4.25)36)... 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

The following model is that of an allylic carbocation intermediate formed by protonation of a conjugated diene with HBr. Show the structure of the diene and the structures of the final reaction products. 

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