Density Functional Investigation of the Mitsunobu Reaction

Electrochemical Azo-free Mitsunobu-type Reaction

The classic chemical Mitsunobu reaction suffers from the need of excess alcohol activation reagents and the generation of significant by-products. Efforts to overcome these limitations have resulted in numerous creative solutions, but the substrate scope of these catalytic processes remains limited. Here we report an electrochemical Mitsunobu-type reaction, which features azo-free alcohol activation and broad substrate scope. This user-friendly technology allows a vast collection of heterocycles as the nucleophile, which can couple with a series of chiral cyclic and acyclic alcohols in moderate to high yields and excellent ee's. This practical reaction is scalable, chemoselective, uses simple Electrasyn setup with inexpensive electrodes and requires no precaution to exclude air and moisture. The synthetic utility is further demonstrated on the structural modification of diverse bioactive natural products and pharmaceutical derivatives and its straightforward application in a multiple-step synthesis of a drug candidate.

Alkoxyphosphorane/Borane Cooperative Alkylations: A Frustrated Lewis Pair Version of the Mitsunobu Reaction

The combination of the alkoxyphosphoranes, Ph₂ P(OR)(O₂ C₆ Cl₄ ) and the borane B(C₆ F₅ )₃ generates the zwitterions 3 which act as FLP to effect the alkylation of several nucleophiles affording C-C, C-N, C-H and C-Cl coupling products. A DFT study shows the reaction proceeds via an FLP activation pathway generating an alkoxyphosphonium intermediate which effects the alkylation of the nucleophiles, akin to the Mitsunobu reaction.

Recent advances in PIII-assisted deoxygenative reactions under photochemical or electrochemical conditions

The nucleophilic substitution reactions of hydroxyl groups are one of the most fundamental and widely spread transformations in organic chemistry. Among them, PIII-mediated deoxygenative nucleophilic substitution reactions, such as the Mitsunobu reaction, are frequently used strategies and often require stoichiometric oxidants to activate PIII reagents to induce the desired reactions. It has been illustrated that PIII reagents can be oxidized into the corresponding radical cations through single-electron oxidation by photocatalysis or electro-oxidation. These phosphine radical cations can react with alcohols or carboxylic acids to form the corresponding alkoxyphosphonium or acyloxyphosphonium intermediates, which are very reactive and easily get decomposed. The release of tri-substituted phosphine oxides as a driving force triggers the following nucleophilic substitution. This strategy does not require the use of stoichiometric oxidants and it eludes safety and stability problems. In this review, we summarise the recent advances and discoveries in PIII-assisted direct deoxygenative reactions under photochemical or electrochemical conditions.

Synthesis of tricyclic quinazolinone-iminosugars as potential glycosidase inhibitors via a Mitsunobu reaction

A series of novel tricyclic quinazolinone-iminosugars 1 (a-c) were synthesized from the benzyl protected sugars through three steps. Firstly, the benzyl protected sugar (aldehyde) 5 reacted with o-aminobenzamide by the iodine-induced oxidative condensation to afford the corresponding aldo-quizanolinone 6. Secondly, through the intramolecular cyclization of the unprotected OH and the amide NH in 6, the tricyclic compounds 7 and 8 were constructed by the key Mitsunobu reaction. Finally, removal of the benzyl group gave the target tricyclic quinazolinone-iminosugars 1. The protocol was effective for the preparation of the tricyclic iminosugars in satisfactory yield. Interestingly, an unusual C-2 epimerization was observed with d-mannose and d-ribose compounds under the conditions of the Mitsunobu reaction that generated the products having the trans configuration at the C-2 and C-3 positions. Unfortunately, such tricyclic quinazolinone-iminosugars showed no inhibitory effects on the tested five glycosidases.

Nitrosobenzene: Reagent for the Mitsunobu Esterification Reaction

Nitrosobenzene has been demonstrated to participate in the Mitsunobu reaction in an analogous manner to dialkyl azodicarboxylates. The protocol using nitrosobenzene and triphenylphosphine (1:1) under mild conditions (0 °C) provides the ester derivatives of aliphatic and aromatic acids using various alcohols in moderate yield and with good enantioselectivity, giving the desired products predominantly with an inversion of configuration. The proposed mechanism, which is analogous to that observed using dialkyl azodicarboxylates, involves a nitrosobenzene-triphenylphosphine adduct and an alkoxytriphenylphosphonium ion and was supported by density functional theory calculations, ³¹P NMR spectroscopy, and experiments conducted with isotopically labeled substrates.

The catalytic Mitsunobu reaction: a critical analysis of the current state-of-the-art

The Mitsunobu reaction is widely regarded as the pre-eminent method for performing nucleophilic substitutions of alcohols with inversion of configuration. However, its applicability to large-scale synthesis is undermined by the fact that alcohol activation occurs at the expense of two stoichiometric reagents - a phosphine and an azodicarboxylate. The ideal Mitsunobu reaction would be sub-stoichiometric in the phosphine and azodicarboxylate species and employ innocuous terminal oxidants and reductants to achieve recycling. This Review article provides a summary and analysis of recent advances towards the development of such catalytic Mitsunobu reactions.

Azodicarboxylate-free esterification with triphenylphosphine mediated by flavin and visible light: method development and stereoselectivity control

Triphenylphosphine (Ph3P) activated by various electrophiles (e.g., alkyl diazocarboxylates) represents an effective mediator of esterification and other nucleophilic substitution reactions. We report herein an aza-reagent-free procedure using flavin catalyst (3-methyl riboflavin tetraacetate), triphenylphosphine, and visible light (448 nm), which allows effective esterification of aromatic and aliphatic carboxylic acids with alcohols. Mechanistic study confirmed that photoinduced electron transfer from triphenylphosphine to excited flavin with the formation of Ph3P˙+ is a crucial step in the catalytic cycle. This allows reactive alkoxyphosphonium species to be generated by reaction of an alcohol with Ph3P˙+ followed by single-electron oxidation. Unexpected stereoselectivity control by the solvent was observed, allowing switching from inversion to retention of configuration during esterification of (S)- or (R)-1-phenylethanol; for example with phenylacetic acid, the ratio shifting from 10 : 90 (retention : inversion) in trifluoromethylbenzene to 99.9 : 0.1 in acetonitrile. Our method uses nitrobenzene to regenerate the flavin photocatalyst. This new approach to flavin re-oxidation has also been successfully proved in benzyl alcohol oxidation, which is a "standard" process among flavin-mediated photooxidations.

Anomeric modification of carbohydrates using the Mitsunobu reaction

The Mitsunobu reaction basically consists in the conversion of an alcohol into an ester under inversion of configuration, employing a carboxylic acid and a pair of two auxiliary reagents, mostly triphenylphosphine and a dialkyl azodicarboxylate. This reaction has been frequently used in carbohydrate chemistry for the modification of sugar hydroxy groups. Modification at the anomeric position, leading mainly to anomeric esters or glycosides, is of particular importance in the glycosciences. Therefore, this review focuses on the use of the Mitsunobu reaction for modifications of sugar hemiacetals. Strikingly, unprotected sugars can often be converted regioselectively at the anomeric center, whereas in other cases, the other hydroxy groups in reducing sugars have to be protected to achieve good results in the Mitsunobu procedure. We have reviewed on the one hand the literature on anomeric esterification, including glycosyl phosphates, and on the other hand glycoside synthesis, including S- and N-glycosides. The mechanistic details of the Mitsunobu reaction are discussed as well as this is important to explain and predict the stereoselectivity of anomeric modifications under Mitsunobu conditions. Though the Mitsunobu reaction is often not the first choice for the anomeric modification of carbohydrates, this review shows the high value of the reaction in many different circumstances.

Systematic Evaluation of 2-Arylazocarboxylates and 2-Arylazocarboxamides as Mitsunobu Reagents

2-Arylazocarboxylate and 2-arylazocarboxamide derivatives can serve as replacements of typical Mitsunobu reagents such as diethyl azodicarboxylate. A systematic investigation of the reactivity and physical properties of those azo compounds has revealed that they have an excellent ability as Mitsunobu reagents. These reagents show similar or superior reactivity as compared to the known azo reagents and are applicable to the broad scope of substrates. p K_a and steric effects of substrates have been investigated, and the limitation of the Mitsunobu reaction can be overcome by choosing suitable reagents from the library of 2-arylazocarboxylate and 2-aryl azocarboxamide derivatives. Convenient recovery of azo reagents is available by one-pot iron-catalyzed aerobic oxidation, for example. SC-DSC analysis of representative 2-arylazocarboxylate and 2-arylazocarboxamide derivatives has shown high thermal stability, indicating that these azo reagents possess lower chemical hazard compared with typical azo reagents.

Photocatalytic esterification under Mitsunobu reaction conditions mediated by flavin and visible light

Excited flavin facilitates stereoselective coupling of alcohols and acids to esters in the presence of triphenylphosphine and catalytic amount of dialkyl hydrazinedicarboxylate.

Metal-free amidation of carboxylic acids with tertiary amines

Combination of Ph₃P and I₂ with an appropriate reagent addition sequence could enable effective amidation of carboxylic acids with tertiary amines under mild conditions.

Mitsunobu mischief: neighbor-directed histidine N(τ)-alkylation provides access to peptides containing selectively functionalized imidazolium heterocycles

There are few methodologies that yield peptides containing His residues with selective N(τ), N(π)-bis-alkylated imidazole rings. We have found that, under certain conditions, on-resin Mitsunobu coupling of alcohols with peptides having a N(π)-alkylated His residue results in selective and high-yield alkylation of the imidazole N(τ) nitrogen. The reaction requires the presence of a proximal phosphoric, carboxylic or sulfonic acid, and proceeds through an apparent intramolecular mechanism involving Mitsunobu intermediates. These transformations have particular application to phosphopeptides, where "charge masking" of one phosphoryl anionic charge by the cationic histidine imidazolium ion is now possible. This chemistry opens selective access to peptides containing differentially functionalized imidazolium heterocycles, which provide access to new classes of peptides and peptide mimetics.

Synthesis of N,N-Diethylbenzamides via a Non-Classical Mitsunobu Reaction

The use of the Mitsunobu reaction for the synthesis of N,N-diethylbenzamides affords ortho-, meta-, and para-substituted benzamides, containing both electron-donating and electron-withdrawing groups. While the preparation of numerous functional groups has been efficiently demonstrated employing the Mitsunobu reaction, our methodology represents the first application of the Mitsunobu reaction for the construction of benzamides using benzoic acid and amine starting materials. Moreover, this synthetic transformation is believed to proceed via a non-classical mechanism involving the existence of an acyloxyphosphonium ion.

Mechanistic study of the synthesis of CdSe nanocrystals: release of selenium

We outline a reaction pathway for the cleavage of the P═Se bond in trialkylphosphine selenide during the synthesis of CdSe nanocrystals. The reaction between cadmium carboxylate and trimethylphosphine selenide in the presence of an alcohol produces alkoxytrimethylphosphonium (2). Control experiments and density functional theory calculations suggested that the cleavage of the P═Se bond is initiated by nucleophilic attack of carboxylate on a Cd(2+)-activated phosphine selenide to produce an acyloxytrialkylphosphonium intermediate (1), which is converted to 2 in the presence of an alcohol.

Towards the development of synthetic routes using theoretical calculations: an application of in silico screening to 2,6-dimethylchroman-4-one

This study describes an attempt to develop a synthetic route using theoretical calculations, i.e., in silico synthesis route development. The KOSP program created four potential synthetic routes for generating 2,6-dimethylchroman-4-one. In silico screening of these four synthetic routes was then performed. In silico screening involves theoretical analysis of synthetic routes prior to actual experimental work. A synthetic route using the Mitsunobu reaction had already been reported by Hoddgets et al. Theoretical investigations were also conducted on two S(N)Ar reactions as well as a Michael reaction before they were examined experimentally. In silico screening using DFT calculations indicated that only the Michael reaction was likely to produce the target. Experimental work confirmed that the target was obtained in a yield of 76.4% using the Michael reaction. The other two routes, except for the Mitsunobu reaction, failed to generate the target. Our results demonstrate that theoretical calculations can be used to narrow down the number of experiments that need to be conducted when developing novel synthetic routes.

The first enantioselective synthesis of palinurin

The first enantioselective synthesis of palinurin has been accomplished starting from commercially available furaldehyde and (R)-methyl-3-hydroxy-2-methylpropionate; the key steps of the synthesis include the use of a chiral pyrrolidine to create the chiral tetronic moiety, and Horner-Wadsworth-Emmons, Wittig and Wittig-Horner reactions to construct the alkene units.

Triphenylphosphine Dibromide: A Simple One-pot Esterification Reagent

We report a one-pot, expedient protocol for the conversion of carboxylic acids to their esters using excess triphenylphosphine dibromide, base, and the alcohol. The reaction gave the esterified product in moderate-to-high yields (30-95%). For chiral acids, the reaction proceeded with little or no racemization. Use of a chiral alcohol in this transformation gave the ester with retention of configuration of the stereogenic center. Information is presented indicating that esterification proceeds through the intermediate generation of an acyloxyalkoxyphosphorane and where steric interactions play an important role in the energetics of the reaction.

The Mitsunobu Reaction: Origin, Mechanism, Improvements, and Applications

The Mitsunobu reaction is a widely used and versatile method for the dehydrative oxidation-reduction condensation of an acid/pronucleophile usually with a primary or secondary alcohol that requires the combination of a reducing phosphine reagent together with an oxidizing azo reagent. The utility of this reaction stems from the fact that it is generally highly stereoselective and occurs with inversion of the stereochemical configuration of the alcohol starting material. Furthermore, as carboxylic acids, phenols, imides, sulfonamides, and other compounds can be used as the acid/pronucleophile, this reaction is useful for the preparation of a wide variety of functional groups. This Focus Review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications.

Phosphine-dependent stereoselectivity in the mitsunobu cyclodehydration of 1,2-diols: stereodivergent approach to triaryl-substituted epoxides

Triaryl-1,2-ethanediols, readily available from natural mandelic acid, can be stereospecifically converted into their corresponding chiral nonracemic epoxides by means of a Mitsunobu cyclodehydration reaction. Upon selection of the phosphine component in the reaction, the two enantiomers of the final epoxides are accessible in high enantiomeric excess. In view of this surprising phosphine-dependent stereoselectivity, here we examine the influence of the steric and electronic nature of both the phosphine and the substrate. [reaction: see text].

Further Characterization of Mitsunobu-Type Intermediates in the Reaction of Dialkyl Azodicarboxylates with P(III) Compounds

Structural characterization of compounds analogous to the proposed intermediates in the Mitsunobu esterification process is achieved by the combined use of NMR spectroscopy and X-ray diffractometric studies. The results show that compounds (t-BuNH)P(mu-N-t-Bu)(2)P[(N-t-Bu)(N-(CO(2)R)-N(H)(CO(2)R))] [R = Et (11), i-Pr (12)], obtained by treating [(t-Bu-NH)P-mu-N-t-Bu](2) (10) with diethylazodicarboxylate (DEAD) or diisopropylazodicarboxylate (DIAD), respectively, have a structure with the NH proton residing between the two nitrogen atoms ((P)N(t-Bu) and (P)N-N(CO(2)Et)); this is the tautomeric form of the expected betaine (t-BuNH)P(mu-N-t-Bu)(2)P(+)[(NH-t-Bu)(N-(CO(2)R)-N(-)(CO(2)R)]. Treatment of ClP(mu-N-t-Bu)(2)P[(N-t-Bu){N-(CO(2)-i-Pr)-N(H)(CO(2)-i-Pr)] (6) with 2,6-dicholorophenol affords (2,6-Cl(2)-C(6)H(3)-O)P(mu-N-t-Bu)(2)P(+)[(NH-t-Bu){N[(CO(2)i-Pr)(HNCO(2)i-Pr)]}](Cl(-))(2,6-Cl(2)-C(6)H(3)-OH) (14) that has a structure similar to that of (CF(3)CH(2)O)P(mu-N-t-Bu)(2)P(+)[(NH-t-Bu){N[(CO(2)i-Pr)(HNCO(2)i-Pr)]}](Cl(-)) (13), but with an additional hydrogen bonded phenol. Both of these have the protonated betaine structure analogous to that of Ph(3)P(+)N(CO(2)R)NH(CO(2)R)(R'CO(2))(-) (2) proposed in the Mitsunobu esterification. Two other compounds, (ArO)P(mu-N-t-Bu)(2)P(+)(NH-t-Bu){N(CO(2)i-Pr)(HNCO(2)i-Pr)}(Cl(-)) [Ar = 2,6-Me(2)C(6)H(3)O- (15) and 2-Me-6-t-Bu-C(6)H(3)-O- (16)], are also prepared by the same route. Although NMR tube reactions of 11 or 12 with tetrachlorocatechol, catechol, 2,2'-biphenol, and phenol revealed significant changes in the (31)P NMR spectra, attempted isolation of these products was not successful. On the basis of (31)P NMR spectra, the phosphonium salt structure (t-BuNH)P(mu-N-t-Bu)(2)P(+)[(HN-t-Bu){N-(CO(2)R)-N(H)(CO(2)R)](ArO(-)) is proposed for these. The weakly acidic propan-2-ol or water did not react with 11 or 12. Treatment of 12 with carboxylic acids/ p-toluenesulfonic acid gave the products (t-BuNH)P(mu-N-t-Bu)(2)P(+)[(HN-t-Bu){N-(CO(2)-i-Pr)-N(H)(CO(2)-i-Pr)](ArCO(2)(-)) [Ar = Ph (18), 4-Cl-C(6)H(4)CH(2) (19), 4-Br-C(6)H(4) (20), 4-NO(2)-C(6)H(4) (21)] and (t-BuNH)P(mu-N-t-Bu)(2)P(+)[(HN-t-Bu){N-(CO(2)-i-Pr)-N(H)(CO(2)-i-Pr)](4-CH(3)-C(6)H(4)SO(3)(-)) (22) that have essentially the same structure as 2. Compound 18 has additional stabilization by hydrogen bonding, as revealed by X-ray structure determination. Finally it is shown that the in situ generated (t-BuNH)P(mu-N-t-Bu)(2)P(+)[(HN-t-Bu){N-(CO(2)Et)-N(H)(CO(2)Et)](4-NO(2)-C(6)H(4)CO(2)(-)) can also effect Mitsunobu esterification. A comparison of the Ph(3)P-DIAD system with the analogous synthetically useful Ph(3)P-dimethyl acetylenedicarboxylate (DMAD) system is made.