Carbon allylation

Let us illustrate the meaning of F by the example of carbon atom 1 in the linear, three-carbon allyl anion C3Hg. There are two carbon atoms other than Ci, one adjacent and the other nonadjacent. Equation (8-44) has three temis, one for each carbon atom... 

Allylic carbocations are carbocations in which the positive charge is on an allylic carbon. Allyl cation is the simplest allylic carbocation. 

In mechanism (8.43) the bridgehead hydrogens of barrelene should be found at the a positions of semibullvalene (2a, 0/3,0y). Mechanism (8.44) can give three different hydrogen-label distributions. If the final bond formation is concerted with bond fission, and bond fission and formation take place at the same carbon atom [mechanism (8.44A)], the label distribution should be (la, 0/3, ly). If bond formation is concerted with bond fission but with a preference for bond formation at the carbon allylic to bond fission [mechanism (8.44B)], the label distribution should be (2a, 0/3, Oy). If there is a symmetric allylic biradical which has a finite existence [mechanism (8.44AB)], then the hydrogen-label distribution should be (1.5a, 0/3,0.5y). 

It was considered of interest to utilize dianion 26 <89PHC(1)1> as a potential three carbon allyl anion fragment to react with 6 with a view to develop a new synthesis of 2,3-substituted and fused carbazoies 28 involving protection, activation and deprotection of the... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

Nucleophilic Substitution at an Allylic Carbon. Allylic Rearrangements... 

A Pd(0)-catalyzed Claisen rearrangement with a subsequent intramolecular Heck vinylation has been demonstrated by Watson and coworkers [84], Upon treatment with Pd(PPh3)4 the allyl vinyl ether undergoes a Pd(0)-catalyzed 1,3 oxygen to carbon allyl shift to afford an a-allyl ketone that... 

The principal exception to this statement involves tertiary carbon, allylic carbon, and benzyl carbon atoms atl ached to X. In such cases it is often found that the second-order reaction with solvate ion such as RO (in ROH solvent) is much slower than apparent first-order reaction with solvent. These systems may be recognized as those giving quite stable carbonium ions, and the solvolysis has b( en ascribed to an Sifl mechanism. 

The preparation of tropidinyl transition metal complex 53 was achieved by reaction of 4-trimethylstannyltropidine 52 with M(CO)sBr, M = Mn, Re <2003ZFAC2408>. In complex 53, the tropidinyl ligand is coordinated through the nitrogen and the three-carbon allylic system which serve as a 2o /47t-electron donor. It has been characterized by single-crystal X-ray diffraction analysis (Equation II). 

An example of a neutral, chemoselective palladium-catalyzed carbon-allylation is the reaction of allyl ethyl carbonate with one equivalent of the nitroacetic ester shown below. 

Cyclic iodo carbonates. Allylic and homoallylic alcohols are converted regio-and stereoselectively into five- and six-membered cyclic iodo carbonates, respectively, by carboxylation of the alkoxide followed by reaction with iodine. These products can be converted into cis-1,2- or 1,3-diols as illustrated.2... 

These arguments point to a mechanism in which the rate-determining step is abstraction of the first a hydrogen to form a Ti-allyl surface species. The next step is the reversible formation of a a bond between a catalyst oxygen atom and either of the terminal carbon allyl atoms. The second hydrogen abstraction, which is facilitated by the presence of the C-O bond, follows with formation of the carbonyl bond and desorption of the product acrolein. The next question to be addressed is the source of oxygen, which appears in the product acrolein. 

Three p orbitals, all aligned parallel to one another, make up the allyl unit and produce three molecular orbitals. The MOs for the following systems differ from the completely carbon allylic system only in the previously discussed electronegativity effects (the bonding MOs will have a larger contrihution from the heteroatom p orbital and the antibonding MOs will have a lesser contribution. Section 2.3). 

The stereochemistry of a sigmatropic hydride shift depends on the highest occupied molecular orbital (HOMO) of the T framework. In the transition state, a three-centered bond is required, which involves overlap between the s orbital of the hydrogen and lobes of p orbitals of the two terminal carbons. From the HOMO configuration of a seven-carbon allylic radical in the ground state, we see that the symmetry-allowed shift is in an ahtarafacial manner (see Figure 1). 

The order of decreasing degradative chain-transfer constants for a series of allyl esters has been given as allyl ethyl carbonate > allyl acetate > allyl propionate > allyl laurate > allyl trimethylacetate [20]. 

New Synthetic Reactions ol Allyl Alkyl Carbonates, Allyl p-Keto Carboxylates and Allyl Vinylic Carbonates Catalyzed by Palladium Complexes"... 

The double bond is conjugated to the positively charged carbon. Instead of being localized on a single carbon, the positive charge is shared by the carbons at each end of the three-carbon allyl unit. Likewise, the electrons in the tt bond are delocalized over three carbons instead of two. The two resonance forms of allyl cation are equivalent, and the positive charge is shared equally by the carbons at each end. 

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