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Electron Deficiency at Carbon

Electron deficiency at a carbon causes drastic deshielding. This is observed for the sp2 carbons typical of carbocations [79], In such systems, the sp2 13C chemical shift range may approach 400 ppm relative to TMS. If the positive charge is dispersed in a carboca-tion, e.g. by resonance, the electron deficient carbon will be more shielded. The following comparison of f-butyl-, dimethylhydroxy- and dimethylphenyl-carbenium ion illustrates this  [Pg.113]

Downfield shifts due to electron deficiency are observed for carbene metal complexes [80 a] and for the sp carbons of metal carbonyls [80 b], [Pg.113]


The electrophilic OH reacts readily with C-C and C-N double bonds (in purines) but not with C-0 double bonds which are electron-deficient at carbon, the position where OH would prefer to add. Although OH reacts with C-C double bonds at close to diffusion-controlled rates, it is highly regioselective largely due to its... [Pg.49]

At this point the mechanistic problem was considered in light of Scheme 4. Immediately, paths B and C were excluded on the basis of the stereospecificity of the reaction. Later work on substituent effects (see later discussion) also strongly argued against such intermediates based on their electron deficiency at carbon. Because the strain effects were inconsistent with the concerted fragmentation, path A was ruled out. This left path D, intermediacy of the metallaoxetane, by process of elimination. However, positive evidence for the structure of the transition state was at this point limited. [Pg.152]

Because inductive and mesomeric effects work in the same sense in pyridine, there results a permanent dipole towards the nitrogen atom. It also means that there are fractional positive charges on the carbons of the ring, located mainly on the a- and 7-positions. It is because of this general electron-deficiency at carbon that pyridine and similar heterocycles are referred to as electron-poor , or sometimes 7r-deficient . A comparison with the dipole moment of piperidine, which is due wholly to the induced polarisation of the cr-skeleton, gives an idea of the additional polarisation associated with distortion of the 7r-electron system. [Pg.3]

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. [Pg.281]

An ab-initio calculation for (OC)5Cr = Si(OH)H shows parallels to the parent carbon compound. However, the electron deficiency at the silicon atom is significantly higher compared to the carbene complex. The LUMO of the Cr=Si... [Pg.23]

Photorearrangement of 4-/>-cyanophenyl-4-phenylcyclohexene (69) took place mainly byp-cyanophenyl migration/67,70,74 The conclusion could then be drawn that the rearranging excited state is not electron deficient at the /3-carbon atom, since one would not expect a cyanophenyl group to migrate to a positive carbon. The/3-carbon was proposed to have odd electron character ... [Pg.170]

It is necessary for the intermediate cation or complex to bear considerable car-bocationic character at the carbon center in order for effective hydride transfer to be possible. By carbocationic character it is meant that there must be a substantial deficiency of electron density at carbon or reduction will not occur. For example, the sesquixanthydryl cation l,26 dioxolenium ion 2,27 boron-complexed imines 3, and O-alkylated amide 4,28 are apparently all too stable to receive hydride from organosilicon hydrides and are reportedly not reduced (although the behavior of 1 is in dispute29). This lack of reactivity by very stable cations toward organosilicon hydrides can enhance selectivity in ionic reductions. [Pg.7]

C6 and C9 are at opposite ends of a four-carbon unit, but since one of these atoms (C7) is saturated and quaternary, a Diels-Alder reaction is unlikely (can t make diene). The combination of a diazo compound with Rh(II) generates a carbenoid at C9. The nucleophile 06 can add to the empty orbital at C9, generating the 06-C9 bond and a carbonyl ylide at C6-06-C9. Carbonyl ylides are 1,3-dipoles (negative charge on C9, formal positive charge on 06, electron deficiency at C6), so a 1,3-dipolar cycloaddition can now occur to join C2 to C6 and Cl to C9, giving the product. Note how a relatively simple tricyclic starting material is transformed into a complex hexacyclic product in just one step ... [Pg.116]

It has already been mentioned that thiocarbonyl compounds are much more reactive than their carbonyl congeners, but, at the same time, due to the low polarity of the C—S unit, they also react much less selectively. Thus, nucleophilic additions may occur either at the carbon (carbophilic addition, Section IV.C.l) or at the sulfur atom (thiophilic addition, Section IV.C.2), as shown in equation 111. This feature is in striking contrast with the behavior of the carbonyl group, which only undergoes nucleophilic attack on the electron-deficient carbonyl carbon, this being the cornerstone of the synthetic applications of oxo... [Pg.1439]

The observed rate constants for y-hydrogen abstraction are decreased in parallel fashion by electron-withdrawing substituents near the y-carbon for both the benzoyl and anisyl compounds 58 59). Such a result is expected for equilibration, as long as the substituent does not change the energy gap between n,n and 7i,7i triplets. However, it is difficult to rationalize how a 71,71 triplet, even with a small fraction of n,7t character, could become electron deficient at oxygen. [Pg.16]

LUMO of the conjugated system (and the HOMO of the nucleophile, of course) as the important frontier orbitals, as in Fig. 4.11, which shows electrophilic reactivity at the site where the arrow points for a range of carbon electrophiles. In each case, there is a high coefficient of the LUMO at the site of attack each of them also has a high total electron deficiency at this site and, with the possible exception of pyridine, the tetrahedral intermediate obtained from such attack is lower in energy than attack at the alternative sites. [Pg.138]

Cp(Ph2PCH2CH2PPh2)Fe=CH-CMe3]+ does not rearrange, however, probably because the increased back donation by the more electron-rich phosphine-substituted iron center decreases the electron deficiency at the carbene carbon. ... [Pg.5756]


See other pages where Electron Deficiency at Carbon is mentioned: [Pg.402]    [Pg.113]    [Pg.249]    [Pg.190]    [Pg.185]    [Pg.402]    [Pg.113]    [Pg.249]    [Pg.190]    [Pg.185]    [Pg.151]    [Pg.76]    [Pg.808]    [Pg.69]    [Pg.783]    [Pg.79]    [Pg.55]    [Pg.133]    [Pg.720]    [Pg.74]    [Pg.230]    [Pg.5]    [Pg.17]    [Pg.509]    [Pg.159]    [Pg.627]    [Pg.330]    [Pg.241]    [Pg.186]    [Pg.845]    [Pg.321]    [Pg.14]    [Pg.118]    [Pg.251]    [Pg.98]    [Pg.364]    [Pg.270]    [Pg.61]    [Pg.151]    [Pg.152]    [Pg.234]   


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At carbon

Carbon electrons

Electron deficiency

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