1,720,971 research outputs found
Gas-phase Heteroaromatic Substitution .14. Attack of Dimethylfluoronium Ion On 2-methyl-pyrroles and 3-methyl-pyrroles, 3-methyl-furans, and 3-methyl-thiophenes
No full textThe gas-phase methylation of 2- and 3-methyl-pyrroles (2P and 3P), -furans (2F and 3F), and -thiophenes (2T and 3T) by (CH3)2F+ ions, from gamma-radiolysis of CH3F, has been investigated at pressures ranging from 50 to 760 Torr, in the presence of a thermal radical scavenger (O2) and variable concentrations of an added base (NMe3: 0-10 Torr). The mechanism of the methylation process is discussed and the intrinsic positional selectivity of the (CH3)2F+ ions evaluated in the framework of the Charge and Frontier Orbital Control concept. Owing to the very large energy gap between the LUMO of (CH3)2F+ and the HOMOs of the selected heteroaromatic substrates, their gas-phase methylation is characterized by a distinct affinity of the ionic electrophile toward those substrate positions with the highest net negative charge, i.e., the C3 in 2P (100%), the C4 in 3P (100%), the heteroatom of 2F and 3F (> 80%), the C5 of 2T (32%), and the C2 of 3T (47%). Analysis of the methylated product distribution from 2F and 3F as a function of the experimental conditions reveals that the interaction of the (CH3)2F+ with the heteroatom of furans gives rise to the reversible formation of two sets of electrostatic adducts, i.e., a ''chelate'' adduct (III) and a single proton-bonded adduct (IV), the first rapidly evolving to the alpha-substituted heteroarenium intermediate by proximity effects and the latter slowly rearranging to the chelate structure III. Formation of these categories of electrostatic adducts from furans, which is much less extensive in the case of thiophenes and absent in pyrroles, accounts for the apparent pronounced affinity of gaseous alkylating electrophiles, irrespective of their LUMO energy, for the alpha-carbons of furans
GAS-PHASE ACID-INDUCED RING-OPENING IN SUBSTITUTED 1,2-EPOXYCYCLOHEXANES
No full textThe nature and stereoisomeric distribution of neutral products obtained from the gas-phase attack of radiolytically formed Broensted and Lewis acids on substituted 1,2-epoxycyclohexanes were studied under different exptl. conditions. Highly regio- and stereoselective cationic nucleophilic displacements, with complete inversion of configuration at the reaction center, were obsd. In some cases, extensive acid-induced isomerization became the only observable pathway. The relative extent of competing substitution and isomerization depend on several factors, including the structure features and internal-energy excess of the oxonium derivs. of the epoxy substrates
GAS-PHASE ACID-INDUCED RING-OPENING OF AN ALPHA,BETA-EPOXYSILANE - THE 1ST CASE OF COMPLETE REGIOSELECTIVITY AND STEREOSELECTIVITY IN AN ION-MOLECULE REACTION IN THE GAS-PHASE
No full textComplete regio- and stereoselectivity was obsd. in the gas phase acid-induced ring opening of the epoxy silane I by MeOH or H2O to give II (R = Me, R1 = H) and II (R = H, R1 = Me), resp. The product distribution closely resembled that of the same reaction performed in soln., showing that the intrinsic electronic properties of the oxonium ion intermediate are solely responsible for the regio- and stereoselectivity obsd
Evidence For A Concerted S(n)2' Mechanism In the Gas-phase Acid-induced Nucleophilic Substitutions On Allylic Substrates
No full textGas phase nucleophilic substitution on oxygen-protonated but-1-en-3-ol and trans-but-2-en-1-ol by methanol proceeds via the concerted S(N)2' mechanism in competition with the classical S(N)2 mechanism
Anomalous Stereochemistry of Gas-phase Acid-induced Ring-opening In 1-phenylcyclohexene Oxide
No full textThe nature and stereoisomeric distribution of neutral products obtained from the gas-phase attack of radiolytically formed Broensted and Lewis acids on substituted 1,2-epoxycyclohexanes were studied under different exptl. conditions. Highly regio- and stereoselective cationic nucleophilic displacements, with complete inversion of configuration at the reaction center, were obsd. In some cases, extensive acid-induced isomerization became the only observable pathway. The relative extent of competing substitution and isomerization depend on several factors, including the structure features and internal-energy excess of the oxonium derivs. of the epoxy substrates
Gas-phase acid-induced nucleophilic displacement reactions .10. Gas-phase acid-induced S(N)2' versus S(N)2 mechanism in allylic alcohols
No full textA first demonstration of the existence of the concerted S(N)2' mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA(+) acids (GA(+) = CnH5+ (n = 1, 2), iC(3)H(7)(+) and (CH3F+) with trans- (1) and cis-2-buten-1-ol (2) as well as with 1-buten-3-ol (3). Firm evidence in favor of the concerted S(N)2' pathway accompanying the classical S(N)2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA(+) attack on 1-3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3-butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive S(N)2' and S(N)2 pathways in the oxonium ions from 1 (S(N)2' (57+/-2%) and S(N)2 (43+/-2%)) and 3 (S(N)2' (54+/-2%) and S(N)2 (46+/-2%)), whereas, with 2, the S(N)2 (66+/-2%) reaction prevailed over the S(N)2' one (34+/-2%). The role of intrinsic structural factors in determining the S(N)2'/S(N)2 branching in the selected oxonium ions is discussed
Gas-phase acid-induced nucleophilic displacement reactions .11. Regioselectivity in the gas-phase nucleophilic attack on O-protonated 3-methyl-2-cyclohexen-1-ol and 1-methyl-2-cyclohexen-1-ol
No full textOur radiolytic study of the occurrence in the gas phase of concerted S(N)2' reactions on several open-chain allylic oxonium ions generated in the gas phase From the attack of gaseous GA(+) acids (GA(+) = CnH5+ (n = 1, 2), iC(3)H(7)(+) and (CH3F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3-methyl-2-cyclohexen-1-ol(1) and 1-methyl-2-cyclohexen-1-ol (2), with both MeOH and NMe(3) as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compound, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe(3), only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the sub stitution and elimination products allows definition of the corresponding reaction patterns. As for open-chain oxonium ions, the nucleophilic attack on O-protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the O-protonated 1 undergoes the concerted S(N)2 process with MeOH almost exclusively (greater than or equal to 99%). With O-prolonated 2, however, the concerted S(N)2' pathway (84-95%) prevails over the classical S(N)2 one (6-17%). Concomitant [1.2] (E2) and [1,4] elimination (E2') pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2'/E2:1.78-1.96 (1); 1.43-1.70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe(3)). and the leaving group (whether H2O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the S(N)2'/S(N)2 and E2'/E2 branchings in the selected oxonium ions is discussed and compared with related gas-phase data
STEREOCHEMICAL EFFECTS IN THE GAS-PHASE PINACOL REARRANGEMENT OF CIS-1-METHYLCYCLOHEXANE-1,2-DIOL AND TRANS-1-METHYLCYCLOHEXANE-1,2-DIOL
No full textThe gas-phase pinacol rearrangement of cis and trans-1-methyl-1,2-cyclohexanediols, promoted by D3+ and CnH5+ (n = 1,2), was studied by the radiolytic method in the pressure range 100-760 Torr. Under all conditions, 2-methylcyclohexanone is the predominant product, arising from both substrates via different pinacol rearrangements and successive fast isomerization of the corresponding primary intermediates, e.g., O-protonated 1-methyl-1-cyclopentanecarboxaldehyde. This conclusion is based on kinetic anal. of competition expts. with pinacol as ref. substrate, carried out at high pressure (760 Torr) with or without added base (NMe3, 3 Torr), showing that the pinacol rearrangement rates are markedly dependent on the stereochem. features of the diol. Accordingly, the trans diol rearranges more rapidly than the cis isomer, which in turn isomerizes faster than pinacol, indicating that anti-periplanar CH2 migration to the vicinal tertiary C-OH2+ center in trans diol is over five times faster than H migration in cis diol. Anal. of the relative migrating ability of the different CH2 moieties in trans diol allowed exclusion of appreciable anchimeric assistance in these gas-phase pinacol rearrangements. The results are compared with relevant gas-phase data and with those concerning the same substrates in acidic soln
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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