New insights in the mechanism of amine catalyzed epoxidation: dual role of protonated ammonium salts as both phase transfer catalysts and activators of oxone

Amines have previously been reported to catalyze the epoxidation of alkenes using Oxone (2KHSO(5)+KHSO(4)+K(2)SO(4)), and significant levels of asymmetric induction were observed. From screening a series of amines based on 2-substituted pyrrolidines, it has now been found that more consistent and reproducible results are achieved with the HCl salt of the amine compared to the amine itself. Up to 66% ee was achieved in epoxidation of 1-phenylcyclohexene. The chiral amine could be reisolated in >90% yield when reactions were conducted at -10 degrees C, indicating that the integrity of the amine was maintained during the oxidation process. At -10 degrees C, (S)-2-(diphenylmethyl)pyrrolidine 1 reacted with Oxone to give a mixture of ammonium salts containing the peroxymonosulfate salt 6b. The enantioselectivity obtained with this salt was compared to the amine.HCl salt catalyzed process and identical results were observed, indicating that the true oxidant was the peroxymonosulfate salt 6b. The relative rates of oxidation of cis- and trans-beta-methylstyrenes together with the rho value of a series of 1-arylcyclohexenes were determined. These studies indicated that the amine catalyzed process involved electrophilic oxidation. On the basis of these findings, a new mechanism is advanced in which the protonated amine not only acts as a PTC but also activates Oxone, through hydrogen bonding, toward electrophilic attack.

Sulfanyl- and selanyldifluoromethylphosphonates as a source of phosphonodifluoromethyl radicals and their addition onto alkenes

[figure: see text] Two different strategies are shown to produce sulfanyl and selanyldifluoromethylphosphonates. Thus, treatment of sulfanyldichloromethylphosphonates by 3HF.NEt3 in the presence of zinc bromide produces the corresponding sulfanyldifluoromethylphosphonates. In addition, lithiation of difluoromethylphosphonates followed by quenching with phenylsulfanyl chloride, phenylselanyl chloride, or diphenyl diselenide yields the corresponding sulfanyl- and selanyldifluorophosphonates. Generation of phosphonodifluoromethyl radicals from such precursors in the presence of alkenes produces the expected adducts.

Phosphonyl, phosphonothioyl, phosphonodithioyl, and phosphonotrithioyl radicals: generation and study of their addition onto alkenes

The treatment of benzyl dialkyl phosphites and dithiophosphites with benzeneselanyl chloride generates an Arbuzov-type transformation leading to the dialkyl selenophosphates 19a and 19b and to selenophosphorodithioates 21a and 21b. Interaction of these substrates with Lawesson's reagent yields the corresponding selenophosphorothioates 20a and 20b and the selenophosphorotrithioates 22a and 22b. When treated with a radical initiator in the presence of a hydrogen donor and an alkene, all eight phosphorus(V) precursors undergo homolytic cleavage of the P-Se bond to generate the phosphonyl, phosphonothioyl, phosphonodithioyl, or phosphonotrithioyl radicals. Most of these are shown to add onto electron-rich and electron-poor alkenes to deliver the expected adducts in fair to excellent yields. Cyclic precursor 19b displays peculiar behavior and, under the reaction conditions, produces only the corresponding cyclic phosphite. Application of this radical chain process is carried out on furanosyl 3-exo-methylene derivative 37 to diastereoselectively furnish five new 3-phosphonomethyl-, 3-phosphonothiomethyl-, and 3-phosphonodithiomethyl-3-deoxofuranoses 38a-c and 38f,g. The possibility of conducting tandem processes is also discussed through experiments involving (1R)-(+)-alpha-pinene (39) and diallylamine 41.

First and efficient synthesis of phosphonodifluoromethylene analogues of nucleoside 3'-phosphates: crucial role played by sulfur in construction of the target molecules

Phosphoric esters of secondary alcohols are ubiquitous in biological systems. However, despite the obvious interest of the corresponding difluoromethylene phosphonates as isopolar mimics, a single example of such an analogue featuring this particular substitution pattern has so far been reported in the literature, due to synthetic problems associated with their preparation. The lithium salt of diethyl difluoromethylphosphonothioate 28d provides a solution to this problem, as demonstrated by an 8-step synthesis of all five fully protected analogues of nucleoside 3'-phosphates in 9-18% overall yield, from readily available ketones. Sulfur is shown to play a crucial role in the introduction of the phosphorus-substituted difluoromethylene unit onto the furanose ring. Complete diastereoselectivity is observed in the three steps of the process requiring stereocontrol. The key conversion of the P=S bond into its oxygenated analogue is simply achieved by use of m-chloroperoxybenzoic acid. It is noteworthy that the synthesis can be carried out on large scale: a 31-g batch of compound 26b has been prepared. The deprotected nucleoside 3'-phosphate analogues can be liberated from their precursors as exemplified by the conversion of 7b, 8b, and 9b into the corresponding difluorophosphonic acids, isolated in the form of their disodium salts.

Phosphonodifluoromethyl and Phosphonothiodifluoromethyl Radicals. Generation and Addition onto Alkenes and Alkynes

[reaction: see text] Selanylated difluoromethylphosphonates and difluoromethylphosphonothioates are good precursors to phosphonodifluoromethyl and phosphonothiodifluoromethyl radicals, respectively. When generated in the presence of alkenes and a hydrogen donor, the corresponding alpha,alpha-difluorinated alkylphosphonates or alkylphosphonothioates are produced in fair to good yields. The use of alkynes results in the formation of alpha,alpha-difluorinated allyl derivatives in useful yields. The presence of the sulfur atom in phosphonothiodifluoromethyl radicals usually translates into higher isolated yields.

The preparation of new phosphorus-centered functional groups for modified oligonucleotides and other natural phosphates

Efforts to develop synthetic methodologies allowing the preparation of alpha,alpha- difluorophosphonothioates, alpha,alpha-difluorophosphonodithioates, alpha,alpha-difluorophosphono- trithioates, and alpha,alpha-difluorophosphinates are reviewed in the light of applications in the field of modified oligonucleotides and cyclitol phosphates. Two successful approaches have been developed, based either on the addition of phosphorus-centered radicals onto gem-difluoroalkenes or on a process involving the addition of lithiodifluorophosphono- thioates 91 onto a ketone and the subsequent deoxygenation reaction of the adduct. The radical route successfully developed a practical route to alpha,alpha-difluoro-H-phosphinates which proved to be useful intermediates to a variety of phosphate isosters. The ionic route led to the first preparation of phosphonodifluoromethyl analogues of nucleoside- 3'-phosphates.

Nucleotides and nucleic acids: a source of inspiration for the development of new, phosphorus-centered functional groups

Synthetic methodologies aiming at the creation of new phosphorus-centered functional groups are reported, as well as applications to the field of nucleotide chemistry. Thus, difluorophosphonothioate-based, ionic reagents 3b and 3d are shown to allow the stereocontrolled and efficient synthesis of phosphonodifluoromethyl analogues of nucleoside-3'-phosphates. An alternate, radical approach describes the use of hypophosphorous acid to stereoselectively link two furanosyl units in positions 3 and 5, and to provide an access to alpha,alpha-difluoro-H-phosphinates. These intermediates are shown to be precursors to the corresponding fluorinated phosphonic acids, phosphonothioic acids and variously substituted phosphinates.