A ‘meta effect’ in the fragmentation reactions of ionised alkyl phenols and alkyl anisoles

The competition between benzylic cleavage (simple bond fission [SBF]) and retro‐ene rearrangement (RER) from ionised ortho, meta and para RC₆H₄OH and RC₆H₄OCH₃ (R = n‐C₃H₇, n‐C₄H₉, n‐C₅H₁₁, n‐C₇H₁₅, n‐C₉H₁₉, n‐C₁₅H₃₁) is examined. It is observed that the SBF/RER ratio is significantly influenced by the position of the substituent on the aromatic ring. As a rule, phenols and anisoles substituted by an alkyl group in meta position lead to more abundant methylene‐2,4‐cyclohexadiene cations (RER fragmentation) than their ortho and para homologues. This ‘meta effect’ is explained on the basis of energetic and kinetic of the two reaction channels. Quantum chemistry computations have been used to provide estimate of the thermochemistry associated with these two fragmentation routes. G3B3 calculation shows that a hydroxy or a methoxy group in the meta position destabilises the SBF and stabilises the RER product ions. Modelling of the SBF/RER intensities ratio has been performed assuming two single reaction rates for both fragmentation processes and computing them within the statistical RRKM formalism in the case of ortho, meta and para butyl phenols. It is clearly demonstrated that, combining thermochemistry and kinetics, the inequality (SBF/RER)_meta < (SBF/RER)_ortho < (SBF/RER)_para holds for the butyl phenols series. It is expected that the ‘meta effect’ described in this study enables unequivocal identification of meta isomers from ortho and para isomers not only of alkyl phenols and alkyl anisoles but also in other alkyl benzene series. Copyright © 2012 John Wiley & Sons, Ltd.     

Evidence for single electron transfer in metal-halogen exchange. The reaction of organolithium compounds with alkyl halides.

The reaction of $\text{t}$-BuLi with cyclizable 1° and 2° alkyl halide radical probes at low temperature produced stab]e cyclized and uncyclized organolithium products as well as cyclized hydrocarbons which clearly indicate the presence of radical intermediates during the course of these reactions.     

Aminonitrile rearrangement of s-triazolo[1,5-c]pyrimidines upon reaction with aryl (alkyl) halides

Aminonitrile cleavage of the cyclic system was observed in the reaction of s-triazolo[1,5-c]pyrimidine derivatives with aryl (alkyl) halides in an alkaline medium or in dimethylfonnamide. It is shown that this transformation proceeds through the formation of intermediate quaternary salts. The effect of electron-acceptor and electron-donor substituents on their stability was ascertained. The structures of the substances were established by means of IR, UV, PMR, and mass spectroscopy.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 421–425, March, 1985.     

Nickel-catalyzed cross-coupling of potassium aryl- and heteroaryltrifluoroborates with unactivated alkyl halides

A method for the cross-coupling of alkyl electrophiles with various potassium aryl- and heteroaryltrifluoroborates has been developed. Nearly stoichiometric amounts of organoboron species could be employed to cross-couple a large variety of challenging heteroaryl nucleophiles. Several functional groups were tolerated on both the electrophilic and the nucleophilic partners. Chemoselective reactivity of C(sp(3))-Br bonds in the presence of C(sp(2))-Br bonds was achieved.     

Nickel-catalyzed cross-coupling between functionalized primary or secondary alkylzinc halides and primary alkyl halides.

In the presence of Bu(4)NI (3 equiv) and 4-fluorostyrene (20 mol %), unreactive primary and secondary alkylzinc iodides undergo nickel-catalyzed cross-couplings with various primary alkyl iodides or bromides. More reactive secondary dialkylzincs and the mixed zinc organometallics RZnTMSM undergo the cross-coupling reaction in the absence of Bu(4)NI. The bicyclic secondary diorganozinc 6 prepared via boron-zinc exchange reacts with high retention of configuration. Free NH-groups are tolerated in the cross-coupling allowing the synthesis of aminated products.     

Nucleophilic identity substitution reactions. The reaction between hydrogen fluoride and protonated alkyl fluorides

The gas phase reactions between HF and the protonated alkyl fluorides MeFH+, EtFH+, Pr(i)FH+, and Bu(t)FH+ have been studied using ab initio methods. The potential energy profiles for both nucleophilic substitution (S(N)2) and elimination (E2) pathways have been investigated. Both backside Walden inversion and frontside nucleophilic substitution reaction profiles have been generated. Backside substitution is very favourable, but shows relatively little variation with the alkyl group. Frontside substitution reaction barriers are only slightly higher than the barrier for backside substitution for HF + MeFH+, and the difference in barrier heights for frontside and backside displacement seems negligible for the larger alkyl groups. Reaction barrier trends have been analysed and compared with the results of similar studies of the H2O/ROH2+ and NH3/RNH3+ systems (R = Me, Et, Pr(i), and Bu(t)). Compared to the two other classes, protonated fluorides have extreme structures which, with the exception of the Me substrate, are weakly bound complexes between an alkyl cation and HF. The results nourish the idea that nucleophilic substitution reactions are better understood in view of competition between frontside and backside substitution than from the traditional S(N)1/S(N)2 perspective.     

New data on the reaction of trialkyl trithiophosphites with alkyl halides

Conclusions The major pathway for the reaction of trialkyl trithiophosphites with alkyl halides is alkylation of the phosphorus atom with the formation of alkylthionephosphonates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1871–1873, August, 1987.     

Unprecedented reactivity in the Morita-Baylis-Hillman reaction; intramolecular alpha-alkylation of enones using saturated alkyl halides

sp3 Hybridized electrophiles, never before used in the organomediated Morita-Baylis-Hillman reaction, now facilitate the formation of five- and six-membered enone cycloalkylation products.     

Cross-couplings of unactivated secondary alkyl halides: room-temperature nickel-catalyzed Negishi reactions of alkyl bromides and iodides

The development of a nickel- or palladium-catalyzed method for cross-coupling unactivated secondary alkyl halides has been a long-standing challenge in synthetic chemistry. This communication describes a simple catalyst system-Ni(cod)2/s-Bu-Pybox-that achieves room-temperature Negishi reactions of an array of functionalized primary and secondary alkyl bromides and iodides.