Allylic lone pairs

An allylic lone pair. Let s begin with some important terminology that we will use frequently throughout the remainder of the text. When a compound contains a carbon-carbon double bond, the two carbon atoms bearing the double bonds are called vinylic positions, while the atoms connected directly to the vinylic positions are called allylic positions ... 

In the last three cases above, the lone pairs are not next to a carbon-carbon double bond and are technically not allylic lone pairs (an allylic position is the position next to a... 

Allylic Allylic Lone pair adjacent TT bond between two Conjugated ir bonds... 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

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 stereochemical outcome of these reactions can be rationalized as arising from attack on a ground-state conformation in which the sulfoxide lone pair and the alkene double bond are syn coplanar2. Such a conformation would minimize 1,3-allylic strain. 

The argument of the directing effect of lone pairs on the substiment [92] easily extends to the alkyl cases. The orbital interaction (Scheme 20) [103] in the pere-poxide quasi-intermediate suggests the stabilization occurs by the simultaneous interaction of O with two allylic hydrogens on the same side of the alkene. Photooxygenation of trisubstituted olefins revealed a strong preference for H-abstraction from disubstituted side of the double bond [104, 105],... 

Thus the Sjvl reactivity of allyl and benzyl halides has already been referred to, and the particular effectiveness of the lone pair on the oxygen atom above is reflected in the fact that MeOCH2Cl is solvolysed at least 1014 times faster than CH3CI. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

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