The ease with which simple starting materials can be transformed into highly functionalized products has made oxidative N‐heterocyclic carbene (NHC) catalysis an area of significant interest. ...However, the use of stoichiometric amounts of high molecular weight oxidants in most reactions generates an undesired equivalent amount of waste. To address this issue, the use of oxygen as the terminal oxidant in NHC catalysis has been developed. Oxygen is attractive due to its low cost, low molecular weight, and ability to generate water as the sole by‐product. However, molecular oxygen is challenging to use as a reagent in organic synthesis due to its unreactive ground state, which often requires reactions to be run at high temperatures and results in the formation of kinetic side‐products. This review covers the development of aerobic oxidative carbene catalysis, including NHC‐catalyzed reactions with oxygen, strategies for oxygen activation, and selectivity issues under aerobic conditions.
The ease with which simple starting materials can be transformed into highly functionalized products has made oxidative N‐heterocyclic carbene (NHC) catalysis an area of interest. The use of oxygen as the terminal oxidant has further developed the field. This review covers the development of aerobic oxidative NHC catalysis, strategies for oxygen activation and selectivity issues under aerobic conditions.
A simple and efficient alkylation of aromatic and heteroaromatic compounds via the direct SN1-type nucleophilic substitution of benzylic alcohols in the presence of catalytic amounts of the strong ...Brønsted acid o-benzenedisulfonimide under neat conditions is herein reported. A library of di- and triaryl (and heteroaryl) methanes was prepared in good yields and high regioselectivity. The observed reactivity was shown to be in agreement with Mayr's nucleophilicity and electrophilicity scales.
Display omitted A library of di- and triaryl (and heteroaryl) methanes was prepared in good yields and regioselectivity via a simple and efficient alkylation of aromatic compounds through direct SN1-type alcohol nucleophilic substitution in the presence of o-benzenedisulfonimide under neat conditions.
Cuprous halides, best described as (CuX)n (X = Cl−, Br−, and I−) in their solid state, catalyse selective aerobic oxidation of alcohols with the assistance of both NMI (N‐methylimidazole) and TEMPO ...(2,2,6,6‐tetramethylpiperidine‐1‐oxyl), and the iodide generally demonstrates the highest activity, for example, in the oxidation of 1‐octanol at ambient temperature under 24 h' reaction. However, in the aerobic oxidation of benzylic alcohols, the chloride showed superiority to the iodide in that the aerobic oxidation was quantitatively completed within 3 h at ambient temperature whereas the iodide showed only about half the activity of the chloride analogue. By probing the system using electrochemistry, electric conductivity, and 1H NMR titration, it was revealed that the surprising anomaly was due to the difference in the rate of forming active species, Cu (NMI)2X(MeCN), from the polymeric solid in a two‐stage process. Substrates expansion of 11 benzylic alcohols indicated that CuCl/NMI/TEMPO system demonstrated quantitative conversion of benzylic alcohols into corresponding aldehydes within 3 h and showed great tolerance to the substituents on the phenyl ring of the substrates. Furthermore, electron‐withdrawing substituent was beneficial to the oxidation and could offset the steric effect at orthro‐substituent. Such a behaviour suggested that in the catalysis, increasing the acidity of the hydroxyl group (OH) of the substrates could ease the oxidation, which implied that the deprotonation via an internal pathway might be one of the rate‐determining steps. Our results also showed that the anion halide participated actively in the catalysis by coordinating to Cu(I) in the active species.
The polarity of Cu–X bond as indicated by the difference in electronegativity Δχ dictates the activity (reaction yield) of CuX/NMI/TEMPO on the catalysis of aerobic oxidation of benzylic alcohols into benzylic aldehydes. Strong bond polarity facilitates the formation of active species, Cu (NMI)2X(MeCN) (A), from the solid, and hence cuprous chloride showed superiority to iodide in the catalysis within limited reaction time.
A tungsten (VI) complex,
WO(O
2
)L(CH
3
OH)
, L = (
E
)-
N'
-(2-hydroxy-3-methoxybenzylidene)isonicotinohydrazide has been synthesized by the action of WO
3
with a ONO donor tridentate Schiff base ...using H
2
O
2
. The tungsten(VI) complex has been characterized by CHN, FT-IR, multinuclear (
1
H &
13
C) NMR, and UV-Vis. spectroscopic methods. Theoretical calculations, using DFT at the B3LYP level of theory at the Def2-TZVP basis set, for the estimation of geometrical parameters like natural bond orbital (NBO), molecular electrostatic potential (MEP), and frontier molecular orbitals (FMOs) analysis of the synthesised compounds showed that the numerical results correspond to the actual findings. The catalytic activity of
WO(O
2
)L(CH
3
OH)
for the selective oxidation of benzylic alcohols to the corresponding aryl aldehydes was studied utilizing various factors, like the nature of the oxidant, the effect of solvent, and the amount of catalyst. The best results were obtained by using 2 mmol of urea hydrogen peroxide (UHP) as an oxidant in the presence of 1 mol% of the catalyst in CH
3
CN under reflux conditions.
A new protocol for the direct sulfonylation of benzylic, allylic and homoallylic alcohols with sodium arenesulfinates is described by using iron(III) chloride as a catalyst and chlorotrimethylsilane ...as an additive. This method requires no preactivation of alcohols. Surprisingly in the reaction with homoallyl alcohols nucleophilic addition of sulfinate anion, occurs at the terminal double bond instead of nucleophic substitution at the alcohol.
Investigations on the one-pot direct azidation of allylic/benzylic alcohols or their methyl ethers followed by the click reaction are reported. Two methods involving sequential reactions were ...developed for synthesizing substituted 1,2,3-triazoles starting from allylic/benzylic alcohols. The first method involves magnetically separable nano Fe3O4-catalyzed direct azidation of various allylic/benzylic alcohols with TMSN3 as the first step followed by the Cu-catalyzed click reaction of azides with alkynes as the second step. The second method involves Cu(OTf)2-catalyzed direct azidation of allylic/benzylic alcohols and their methyl ethers with TMSN3 as the first step followed by the click reaction of azides with alkynes as the second step. In this method, Cu(OTf)2 served as a single catalyst for both the azidation of alcohol and click reaction steps. Utility of this protocol has been revealed by synthesizing new classes of polyether systems and macrocyles embedded with the 1,2,3-triazole and 1,3-diyne units.
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A simple one-pot procedure has been developed to efficiently prepare 1,4,5-trisubstituted 1,2,3-triazoles from benzylic alcohols. The presence of diphenylphosphoryl azide (DPPA) and ...active ketones allows for an azide–enolate 3+2 cycloaddition by use of DBU.
The effect of fluorination on the conformational and hydrogen‐bond (HB)‐donating properties of a series of benzyl alcohols has been investigated experimentally by IR spectroscopy and theoretically ...with quantum chemical methods (ab initio (MP2) and DFT (MPWB1K)). It was found that o‐fluorination generally resulted in an increase in the HB acidity of the hydroxyl group, whereas a decrease was observed upon o,o′‐difluorination. Computational analysis showed that the conformational landscapes of the title compounds are strongly influenced by the presence of o‐fluorine atoms. Intramolecular interaction descriptors based on AIM, NCI and NBO analyses reveal that, in addition to an intramolecular OH⋅⋅⋅F interaction, secondary CH⋅⋅⋅F and/or CH⋅⋅⋅O interactions also occur, contributing to the stabilisation of the various conformations, and influencing the overall HB properties of the alcohol group. The benzyl alcohol HB‐donating capacity trends are properly described by an electrostatic potential based descriptor calculated at the MPWB1K/6‐31+G(d,p) level of theory, provided solvation effects are taken into account for these flexible HB donors.
Fluorination and hydrogen‐bond acidity: Fluorination of benzyl alcohols is shown to significantly modify their hydrogen‐bonding properties and their conformational landscapes through intramolecular OH⋅⋅⋅F, CH⋅⋅⋅F and/or CH⋅⋅⋅O interactions (see scheme). ortho‐Fluorination results in an increase in the hydrogen‐bond acidity of the hydroxyl group, whereas o,o′‐difluorination leads to a decrease in the hydrogen‐bond acidity.
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