CC double bond

Since the olefmic CC double bond is trisubstituted, the relative configuration cannot be determined on the basis of the cis and trans couplings of vicinal alkene protons in the H NMR spectrum. What is the relative configuration given the C NMR spectra 19 ... 

The coupling constant of the aldehyde doublet 7.8 Hz) is repeated in the doublet of doublets signal at Sh = 6.3. Its larger splitting of 15.6 Hz is observed also in the doublet at Sh = 7.3 and indicates a CC double bond with a trans configuration of the vicinal protons. 

Protons of substrueture B and C are assigned by means of the mesomerie effeet of the aldehyde group whieh deshields the protons in o-position of the attaehed p-disubstituted benzenoid ring and in p-position of the eentral CC double bond ort/io-protons of the monosubstituted benzenoid ring D split into a doublet beeause of one ortho eoupling ( 7.5 Hz) while the meta-protons split into a triplet beeause of two ortho eouplings. 

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

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 position of the second CC double bond in the structural fragment E follows finally from the correlation of the C signals at 5c = 37.8 and 49.8 with the //signals at 3h = 4.47 and 4.65. Note that trans protons generate larger cross-sectional areas than cis protons as a result of larger scalar couplings. 

Starting from the five CC double bonds, three rings and a 3-methylfuran structural fragment, analysis of the CH COSY and CH COLOC diagrams leads to Table 49.1 and the identification of fragments B-J. 

Display and examine electrostatic potential maps for ethyl cation, 2-propyl cation and 2-methyl-2-propyl cation. Which cation shows the greatest localization of positive charge If you find that the methyl groups delocalize the positive charge, where does the charge go Write resonance contributors for the three cations to rationalize your conclusion. (Note You may need to draw resonance contributors that contain a CC double bond and are missing a CH bond see also Chapter 7, Problem 8.)... 

CC double-bond, 4 CC single-bond, 3 CH single-bond, 4 general, 1, 26, 27, 45 Heteroatomic, 2, 43... 

In the opinion of the present author, the tautomer with an OH group in the 5-position and a CC double bond in the 5,6-position should also be considered. 

I. 355(4) A, 1.358(3) A. The exocyclic CC double bonds in 8 appear to be significantly longer than in 7. This is, however, not surprising, as the exocyclic double bonds in 8 are cross-conjugated with the ethynyl substituents. The experimental results available for [3]radialenes are in good agreement with the calculated results for the parent compound (Table 20). 

These molecules also have large substituents, and it might seem surprising that the radialene rings avoid puckered conformations in these species. The nonbonded repulsions are, however, reduced in these molecules because of external ring closures (13) or because two of the exocylic CC double bonds involve cumulated double bond systems (14 and... 

MCQDPT/6-31G(d)).17 In the transition state of this inversion process, the orbitals at Cl at C2 are orthogonal. The low inversion barrier could be regarded as a measure of the olefinic bond energy of 15b, which seems to be reduced to a value slightly higher than 10% of the rr-part of the CC double bond energy... 

In the monocyclic aminoalkenes 36-39 with an exocyclic CC double bond, only for the eight-membered ring compounds 37 has sizeable un/ c=C interaction been detected by PES48,97. The through-space interaction of these orbitals in aminoalkenes was found to increase exponentially as their distance decreases97. 

The scope of this cycloaddition reaction was very promising. Subsequently, removal of the CC-double bond and stereoselective functionalizations at positions 4 and 5 (carbohydrate numbering) was investigated for the synthesis of carbohydrates and related natural products, to provide C-3 branched carbohydrate derivatives (or C-4 heteroatom substituted derivatives after carbon/he-teroatom exchange reactions). However, the desired hydrogenation of such systems with various hydrogen donors has mainly resulted in low yields and/or side reactions due to the inherent stability of the formal CC-double bond (12., 15). ... 

The CC and CH bonds of ethane (Example 10.1), and the final selection See = 69.633 and 8ch = 106.806 kcal/mol, are used to get the CC and CH bonds found in unsaturated hydrocarbons by retaining both the contribution of Fkh Eq. (11.12), and the effect of charge variations described by Eq. (10.37). The reference CC double bond of ethylene and the reference CC bonds of benzene, however, roughly estimated along the lines described in Example 10.1, are deduced from the appropriate CH bond energies and the energy of atomization of the corresponding molecule, AE, obtained from experimental data. 

The cycloaddition of an oxygen atom to the CN double bond, analogous to the epox-idation of CC double bonds, is rare. For example, when the imine 17 is oxidized with in-situ-generated dioxirane (equation 15), the oxaziridine 18 is obtained as the major... 

A convenient and quick way to prepare 1-bromoalkynes consists of adding an ethereal solution of cyanogen bromide to a strongly cooled solution of a metallated (Li or MgX) [119] acetylene in an organic solvent (EtjO or THF). The method seems very general and excellent yields can be obtained provided that the ethereal solution of BrC N (which is prepared from an aqueous solution) is dried well. The advantage of this method over the introduction with elemental bromine is that CC double bonds in the acetylenic compounds do not react... 

Benzylidene-3-dialkylidene-2,4-pyrrolidinediones can be selectively epoxidized at the exocyclic CC double bond [86ZN(B)640]. The products can then undergo further reactions, such as ring enlargement [80ZN(B)724 90AP381]. 

Carbon-13 shifts of representative phosphines [364], phosphonium salts [365], phospho-nium ylides [365, 366], diphosphines [367], phosphonates [368], phosphorous and phosphoric acid derivatives [369] are summarized in Table 4.49. I3C shift data of some group V organoelement compounds are compared in Table 4.50. It turns out that a sp3 carbon nuclei of phosphines and arsines are shielded (0-25 ppm) relative to those of amines (30-60 ppm), as expected from the heavy atom effect, sp2 carbons of CC double bonds behave correspondingly, as shown for the triphenyl derivatives in Table 4.50, with... 

Fig. 4.19. Eight cis-trans isomers relative to a central CC double bond may exist (Fig. 4.19(a)). Four resonances can be resolved (Fig. 4.19(b, c)) which are assigned to the most similiar pairs (Fig. 4.19(a)) [530], Signal intensities clearly respond to the different cis.trans ratios of both samples.

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