61076-90-2Relevant academic research and scientific papers
The Role of Single-Electron Transfer in SN2-Type Substitution Reactions of Anions with Alkyl Halides
Bordwell, F.G.,Harrelson, John A.
, p. 4893 - 4898 (2007/10/02)
Rate constants (kobsd) for reactions of electrophiles with: (a) various carbanions, including 9-substituted fluorenide ions (9-GFl-), 9-substituted xanthenide ions, α-cyano carbanions, anb β-diketo enolate ions, (b) phenothiazinide (PTZ-) and carbazole (Cb-) nitranions, (c) 4-substituted phenoxide ions, and (d) thiphenoxide ions, have been campared with rate constants for single-electron transfer (kSET) calculated using a Marcus-type equation.For both 9-GFl- carbanion and 2-GPTZ- nitranion families reacting with SET acceptors, such as 1,1-dinitrocyclohexane, the kobsd/kSET ratio is near unity.For 9-GFl- carbanions reacting with Ph2CHCl the kobsd/kSET ratios range from 2 to 105, those for 2-PTZ- nitranions from 370 to 1.3*104, and those for Cb- nitranions from 108 to 1010.For reactions of 4-GC6H4O- oxanions with Ph2CHCl the ratios range from 103 for G = Me2N to 1013 for G = CN; for PhS- ion reacting with n-BuBr the ratio is 1017.The significance of these results with respect to the role of an outer sphere SET mechanism in substitution reactions of anions with alkyl halides is discussed.
Carbon-Skeletal Anionic Rearrangements and the ?-Orbital Overlap Constraint: The Question of Nucleophilic Attack versus Electron Transfer
Eisch, John J.,Kovacs, Csaba A.,Chobe, Prabohd
, p. 1275 - 1284 (2007/10/02)
In order to evaluate the geometrical requirements and the actual electronic nature of apparent anionic rearrangements of metalated aromatic hydrocarbons, amines, and ethers, cyclic structural types of such anions were generated as lithium salts by proton abstraction from C-H bonds by RLi or by C-Cl bond cleavage by Li.The cyclic systems examined were anions of 9,9-dimethyl-, 9-methyl-9-benzyl-, and 9-benzyl-9-phenylfluorenes; 9-methyl-9-phenyl-, 9,9-diphenyl-, and 9,9-(2,2'-biphenylene)-9,10-dihydrophenanthrenes; 5-methyl- and 5-phenyl-5,6-dihydrophenathridines;and 9H-dibenzopyran.The anions generated from 9-methyl-9-benzylfluorene, 9-benzyl-9-phenylfluorene, 5-methyl-5,6-dihydrophenanthridine, and 9H-dibenzopyran, as well as 9-methyl-9-(lithiomethyl)fluorene, did not undergo skeletal rearrangement when heated between 40 and 120 deg C for protracted periods.However, the anions derived from the 9-methyl-9-phenyl-, 9,9-diphenyl-, and 9,9-(2,2'-biphenylene)-9,10-dihydrophenanthrenes did undergo rearrangement with a shift of the 9-aryl group.With the anion of 5-phenyl-5,6-dihydrophenanthridine, some shift of the 5-phenyl was observed, but the principal rearrangement was ring contraction with the formation of N-phenyl-9-fluorenylamine.By noting which anions underwent skeletal rearrangement and which competing migrating groups in a given anion underwent a shift preferentially, we have formulated an appropriate geometrical view of the transition states involved.Furthermore, by generating the 2,2,2-triphenylethyl anion (as its lithium salt) from (a) 2-chloro-1,1,1-triphenylethane and Li, (b) 2-bromo-1,1,1-triphenylethane and n-BuLi, and (c) bis(2,2,2-triphenylethyl)mercury and n-BuLi, we attempted to learn whether such shifts were truly nucleophilic or whether SET processes were involved.Evidence for SET processes was obtained for the generation of (2,2,2-triphenylethyl)lithium by method a, but no ESR or CIDNP evidence for radical intermediates was observable when (2,2,2-triphenylethyl)lithium was produced by method c.
Reactions of 9-Substituted Fluorenide Carbanions with Allyl Chlorides by SN2 and SN2' Mechanism
Bordwell, Frederick G.,Clemens, Anthony H.,Cheng, Jin-Pei
, p. 1773 - 1782 (2007/10/02)
The average ratio of SN2 rate constants for benzyl chloride vs. allyl chloride reacting with 9-substituted fluorenide carbanions, 9-G-Fl- (G = Me, PhS, Ph o-tol, or t-Bu), in Me2SO solution is 2.3, which is close to the ratio previously observed for reactions of smaller nucleophiles with more localized charges, such as iodide ion, in various solvents.Broensted plots of log k vs. pKHA (both in Me2SO) are used to calculate reactivity values (r) for 9-G-Fl- ions of the same basicity reacting with allyl chloride.The r values reveal the same order of steric effects for G in these SN2 reactions as was found previously for benzyl chloride, i.e., Me - ions were found to increase progressively and appreciably with γ-methyl substitution, i.e., CH2=CHCH2Cl (1) , MeCH=CHCH2Cl (4) , Me2C=CHCH2Cl (5), whereas the Broensted β values decreased progressively.The second-order rate constants for reactions with families of 9-G-Fl- ions were found to follow a somewhat different pattern for α-methyl substitution, i.e., CH2=CHCH2Cl (1) > CH2=CHCH(Me)Cl (2) CH2=CHC(Me)2Cl (3); the rate differences were small, but the β values again decreased progressively.Product studies indicated that 1 reacts with 9-G-Fl- ions by an SN2 mechanism but that 2 gives SN2' products in a amounts increasing along the series G = Me, Ph, o-tol, t-Bu; 3 appears to give only SN2' products.The rates of reactions of 9-CN-Fl- and 9-CO2MeFl- ions with 3 to form SN2' and SN2 products are independent of the carbanion concentration.The first-order reaction of 9-CN-Fl- ion with t-BuCl in MeOH gives 17percent yield of 9-CN-9-t-BuFl.Entalpies of activation for SN2 reactions of 9-G-Fl- ions reacting with 5 are 7-9 kcal/mol lower, and entropies of activation are 18, or more, eu units less positive than for SN2' reactions of 9-G-Fl- ions with 3.Mechanism for SN2' reactions are discussed in light of these findings.
Rate-Equilibrium Relationships for Reactions of Families of Carbanion Nucleophiles with N-Benzyl-N,N-dimethylanilinium Cations and with Alkyl Chlorides, Bromides, and Iodides
Bordwell, Frederick G.,Hughes, David L.
, p. 7300 - 7309 (2007/10/02)
Rates of reaction of a family of 9-methylfluorenide anions, 9-MeFl-, with 15 substituted N-benzyl-N,N-dimethylanilinium cations, GC6H4CH2N(Me)2C6H4Y+, were measured in a 0.8 M KNO3 solution of Me2SO at 25 deg C.Substitution in the benzyl moiety produced a U-shaped Hammett plot, with m-CN reacting 9- to 15-fold faster than H and p-MeO reacting 1.5- to 3-fold faster than H, depending on the basicity of the 9-RFl- ion.Broensted ΒNu values obtained by varying nucleophile basicity in the 9-MeFl- ion family were 0.28 for N-(p-methoxybenzyl)-N,N-dimethylanilinium ion, 0.33 for N-benzyl-N,N-dimethylanilinium ion, and 0.39 for N-(m-cyanobenzyl)-N,N-dimethylanilinium ion.These βNu values were unaffected by substitution in the aniline portion of the substrate.Hammett pLg values were obtained by substitution of m-Cl, p-Br, H, p-Me and p-MeO groups (Y) in the dimethylaniline leaving group; pLg values were all near 2.0 and were insensitive to the basicity of the attacking nucleophile and the substituent G in the benzyl group of the substrate. βNu values for families of 9-RFl- ions reacting with ArCH2X, n-BuX, and CNCH2X also remained constant as the leaving group, X, was changed (Cl, Br, and I) for a given substrate and when the donor atom was changed along the series C-, N-, O-, and S-.These results indicate that there is no direct relationship between the size of βNu and the extent of bond making or bond breaking in the transition state for SN2 reactions, as has been generally assumed in theoretical attempts to rationalize the effects of structural variations on rate.It is suggested that SN2 reactions occur in two stages.The size of βNu is determined in the first stage where an ion-dipole complex is formed.The rate is determined largely in the second stage where bonds are formed and cleaved and where molecular and solvent reorganization occurs.
Steric and Electronic Effects in SN2 reactions of 9-Substituted Fluorenyl and α-Cyano Carbanions with Benzyl Chloride in Dimethyl Sulfoxide Solution
Bordwell, Frederick G.,Hughes, David L.
, p. 2206 - 2215 (2007/10/02)
Rates of reactions of PhCH2Cl with carbanions derived from six α-cyano carbon acids of varied structure with pKa values ranging over 16 units have been measured.When placed on a log k vs. pKa plot the points fit near to the extended
A Direct Relationship between Nucleophilicity and Basicity in SN2 Reactions of Fluorenyl Anions with Benzyl Chloride in Dimethyl Sulfoxide Solution
Bordwell, Frederick G.,Hughes, David L.
, p. 3314 - 3320 (2007/10/02)
The rates of reactions for nine 9-substituted fluorenyl anions, 9-G-Fl- (G=CN, CO2Me, PhSO2, PhS, Ph, PhO, PhCH2, Me, t-Bu), reacting with benzyl chloride in Me2SO solution have been measured.For 9-CN-Fl- the rates increased with sol
