Alpha position

It is also possible to iaterfere with the polymerization by attaching at the alpha positions either too many groups, or groups which are too bulky. Four chlorine atoms (12) or four methyl groups (13) seem to be sufficient to hinder the production of polymer. These crowded -xylylene monomers can be polymerized, but not through a VDP process. 

Methylene derivatives ate readily formed in the vapor phase with compounds having a hydrogen in the alpha position to an electron-withdrawing group. Acryhc (40,41) and methacrylic (42) acids (or esters) are produced at 300—425°C from acetic and propionic acids (or esters), respectively, using alkafl and... 

An alternative chain-terminating decomposition of the tetroxide, known as the Russell mechanism (29), can occur when there is at least one hydrogen atom in an alpha position the products are a ketone, an alcohol and oxygen (eq. 15). This mechanism is troubling on theoretical grounds (1). Questions about its vaUdity remain (30), but it has received some recent support (31). 

Ahphatic ketones (qv) are readily brominated in the alpha position. Mixtures are usually obtained (24). 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Substitution Reactions. The chemistry at alpha positions hinges on the fact that an aHyUc hydrogen is easy to abstract because of the resonance stmctures that can be estabUshed with the neighboring double bond. The aHyUc proton is easier to abstract than one on a tertiary carbon these reactions are important in the formation of alkoxybutenes (ethers). 

Replacement of hydrogen with halogen can be carried out in the alpha position of fluorinated ethers, amines, aldehydes, or nitriles In 2,2,3,4,4,4 hexafluoro-bulyl methyl ether, chlorination occurs predominantly at the methyl, however, bromination occurs mostly at the internal position of the fluorobutyl group 133] (equation 20)... 

An alpha hydroxy acid is an organic carboxylic acid in which an additional hydroxyl functional group (-OH) is present at the alpha position, i.e., on the carbon adjacent to the carboxyl functionality, -COOH. Figure 13.10.1 presents the struc-... 

The lack of clear-cut hallucinogen-type activity for the 2-aminotetralins could be explained in several ways. The known deleterious effect of molecular bulk in the alpha-position would seem to direct attention to the steric effect of the reduced ring of the tetralins as detrimental to activity. In 18b, however, it has been noted (156) that the 5-methoxy group is forced out of plane by the adjacent 6-methyl and 4-methylene groups. The importance to activity of maintaining the methoxy groups coplanar with the aromatic ring has been emphasized earlier. Both substituent orientation and N-alkylation must also be important to activity, and it may not be realistic to make direct comparisons between the phenethyl-amines and the 2-aminotetralins. 

Friedlander synthesis org chem A synthesis of quinolines the method is usually catalyzed by bases and consists of condensation of an aromatic p-amino-carbonyl derivative with a compound containing a methylene group in the alpha position to the carbonyl. fred.lan dar. sin-tha-sas ... 

Shell Chemical has a process that does both the Oxo reaction and hydroformyiation in one step in the same reactor. They use a special catalyst, thought to be cobalt modified with a trialkyl or triaryl phosphine ligand— but they are holding this one pretty close to the vest. Overall yields are 70-80%, with straight-chain alcohols representing greater than 80%. Major by-products are paraffins that are recovered and used to make olefins and then recycled back as feed. This process can also use internal olefins (with the double-bond somewhere besides the alpha position) and yield similar normakiso alcohol ratios. ... 

Alpha olefins are straight-chain hydrocarbons having a double bond in the number one carbon-carbon position. That s called the alpha position, and hence the name alpha olefin, (There are beta, gamma, etc., compounds around, too.) The chains can have as few as four carbons (butene-1) or more... 

Alpha olefins. Straight-chain olefins with the double bond in the alpha position, i.e., in the first carbon-carbon bond of the chain. Alpha olefins start with the butenes and go up to the C30 s and higher. 

The favoured dihedral angles for protein main chains were derived from energy considerations of steric clashes in peptides giving the well known Ramachandran plot (Ramachandran and Sasisekharan, 1968). These phi/psi combinations characterize the elements of secondary structure. Accurate main chain models can be constructed from spare parts, that is short pieces of helices, sheets, turns, and random coils taken from highly refined structures, provided a series of C-alpha positions can be established from the electron density map... 

Rule B. A secondary olefin will tend to rearrange to a tertiary olefin when a tertiary carbon atom is in alpha position to a double-bonded carbon atom. The rearrangement of 3-methyl-l-butene to 2-methyl-2-butene is well known (Norris and Reuter, 66). 

Rule D. When a quarternary carbon atom is the alpha position to a secondary or tertiary double-bonded carbon atom, a pinacol type of rearrangement involving migration of a methyl group may occur. Examples of this type of rearrangement include isomerization of... 

COSY type H NMR experiments for H2O samples are established primarily to reveal coupling connectivities between exchangeable N// protons and those in the alpha position. The chemical shift difference of these pro-... 

Y. Seimbille, F. Benard, J. Rousseau, E. Pepin, A. Aliaga, G. Tessier, J.E. van Lier, Impact on estrogen receptor binding and target tissue uptake of [F-18]fluorine substitution at the 16-alpha-position of fulvestrant, Nucl. Med. Biol. 31 (2004) 691-698. 

Hie mechanism by which the mixed function oxidase converts a nitrosamine into a carcinogenic compound is not clearly understood (]5). Hiese enzymic oxidations can occur at the alpha, beta, gamma, or omega position of an alkyl chain attached to the nitrosamine function. Regardless of whether or not these other oxidations are significant in carcinogenicity, it has been dorvon-strated that hydroxylation at the alpha position does indeed produce a carcinogen (33). Hie discussion which follows will be limited to those factors relative to oxidation at the a-position. 

A similar mechanism was invoked by Ohshima and Kawabata (2) to account for their results in the nitrosation of tertiary amines and amine oxides. In applying these concepts to the nitrosative dealkylation of tetraalkyltetrazenes, Michejda al. 5) introduced an interesting variant by suggesting that immonium ions could be formed in two successive one-electron oxidation steps (for example by ferric ion oxidation of tertiary amine to the radical cation followed by radical abstraction of a hydrogen atom from the alpha position), rather than exclusively through the one-step removal of a hydride ion as nitroxyl. The resulting immonium ion was again considered to react directly with nitrite to produce the N-nitroso derivative. These reactions are summarized in Fig. 2b. 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

An additional complication and opportunity is provided by the fact that the placement of the methyl group on the alpha-position introduces a chiral carbon. The R- and S-isomers have been compared (see in the Qualitative Comments section) and the S-isomer is clearly three or four times more potent that the R-isomer. These were assayed completely blind, with the code having been broken only after the completion of the study. The dramatic differences in potency let the assignment of the more active isomer be made without hesitation. This S-isomer is the d-or dextrorotary one, and has the absolute configuration of the active member of the isomer... 

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