Resonance alkenes

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

All lation. Maleic anhydride reacts with alkene and aromatic substrates having a C—H bond activated by a,P-unsaturation or an adjacent aromatic resonance (31,32) to produce the following succinic anhydride derivatives. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

In a similar way, the linking of the earboxy function with a CC double bond follows from the correlation of the earboxy resonance (5c = 170.4) with the alkene protons (d/y = J.Ji and 6.] 8)-, the latter give correlation signals with the C atom at 5c = 38.5, as do the protons at Sff = ].33 and 1.53, so that taking into account the molecular unit B which is already known, an additional substructure D is established. 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

Mass Spectra of Alkenes and Arenes. Resonance Stabilized... 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Carbon atoms of an aromatic ring absorb in the range 110 to 140 8 in the 13C NMR spectrum, as indicated by the examples in Figure 15.16. These resonances are easily distinguished from those of alkane carbons but occur in the same range as alkene carbons. Thus, the presence of l3C absorptions at 110 to 140 8 does not in itself establish the presence of an aromatic ring. Confirming evidence from infrared, ultraviolet, or 1H NMR is needed. 

You knowr the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates. 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

Apart from type 62, which is only slowly convergent to the optimised geometry, the other centres are well described by the ROHF method. Polyhedral views of the three type a structures are shown in Fig. 6. These all illustrate the change of hybridisation at the point of muonium attachment and at the adjacent carbon atom where the unpaired electron is effectively localised as expected from addition to an alkene. The bi and c defects (Fig. 7) are quite different. The expected hybridisation change to sp is clearly present for the atom bonded to muonium, but other significant distortions are not obvious. This is consistent with the prediction from resonance theory (Fig. 8) that the unpaired electron for these structures is delocalised over a large number of centres. 

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