Dialkylaminodifluorosulfinium Tetrafluoroborate Salts
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Process optimization Dialkylaminodifluorosulfinium Marc-Olivier Turcotte-Savard tetrafluoroborate salts: synthesis and applications MARC-OLIVIER TURCOTTE-SAVARD1*, OLIVIER MAHÉ2, JEAN-FRANÇOIS PAQUIN2 *Corresponding author 1. OmegaChem, 480, Rue Perreault, Lévis, QC, G6W 7V6, Canada 2. Université Laval, Département de chimie, Canada Research Chair in Organic and Medicinal Chemistry, CCVC, PROTEO, 1045 avenue de la Médecine, Québec, QC, G1V 0A6, Canada various fluoride ion acceptors (PF , SeF , SbF and AsF ) along KEyWORDS 5 4 5 5 with DAST-type reagents (7 - 10). However, the reactivity of these Organofluorine chemistry; deoxyfluorination; salts has been scarcely studied until 2009. C-F bond; XtalFluor; dialkylaminodifluorosulfinium Recently, a one-pot preparation of diethylaminodifluorosulfinium tetrafluoroborate salts tetrafluoroborate has been developed. Hence, the addition of THF•BF3 adduct to crude DAST, previously formed from SF4 in ABSTRACT CH2Cl2 and N,N-diethyltrimethylsilylamine, resulted in 90% yield of diethylaminodifluorosulfinium tetrafluoroborate 1 (4). The solid The syntheses of dialkylaminodifluorosulfinium obtained was under a different polymorphic form with higher tetrafluoroborate salts are described. These reagents, melting point (89.8°C vs. 74-76°C) and lower moisture sensitivity first used for the deoxyfluorination of alcohols, compared to the first polymorph reported by Markovskii (5). were also shown to fluorinate aldehydes, ketones, carboxylic acids and glycosyl groups. Alternatively, dialkylaminodifluorosulfinium tetrafluoroborate salts were 14 recently shown to act as activating agents for hydroxyl, carbonyl and carboxyl groups in a variety of transformations leading to non-fluorinated products. Scheme 1. INTRODUCTION Similarly, addition of boron trifluoride etherate to an ice cold solution of crude morpholinosulfur trifluoride gave an excellent ith 30 to 40% of agrochemicals and 20 to 25% of yield of the corresponding tetrafluoroborate salt 2 (4). pharmaceutical compounds bearing at least one atom Wof fluorine (1), the development of efficient fluorination methods is a highly active field of research within organic synthesis. Deoxyfluorination represents a valuable strategy to insert fluorine atoms on organic compounds (2). However, deoxyfluorination agents are often hygroscopic, unstable, toxic, hazardous and/or non-selective towards fluorination. Dialkylaminodifluorosulfinium Scheme 2. tetrafluoroborate salts (3) were developed as cost effective, safer and more efficient deoxyfluorination reagent when used in DAST was found to react exothermically with tetrafluoroboric conjunction with an appropriate base or fluoride additive (4, 5). acid to form diethylaminodifluorosulfinium tetrafluoroborate Since their first uses, researchers have discovered a number of 1 with elimination of HF. Triflic acid was also reacted with alternate applications for these reagents, such as DAST, thus giving rise to the first dialkylaminodifluorosulfinium cyclodehydration, hydroxyl, carboxyl and glycosyl derivatives triflate salt 3 (5). activation towards substitution, thus broadening the field of action of these practical and inexpensive salts. Preparation of Dialkylaminodifluorosulfinium TETRAFLUOROBORATE SALTS Markovskii et al. first reported the formation of dialkylaminodifluorosulfinium tetrafluoroborate salts by reacting dialkylaminosulfur trifluoride and its analogues with boron trifluoride etherate (6). Since then, other Scheme 3. dialkylaminodifluorosulfinium salts have been synthesized using Chimica Oggi - Chemistry Today - vol. 31(4) July/August 2013 Process optimization A variety of other dialkylaminodifluorosulfinium salts were also Cyclooctanol was converted in good yield to cyclooctyl prepared, analyzed and their reactivity was studied (11, 12). fluoride with a high fluoro/alkene ratio, using a mixture of However, 1 and 2 were preferred and commercialized. diethylaminodifluorosulfinium tetrafluoroborate and triethylamine dihydrogen fluoride at a low temperature (5). Scheme 7. (R)-benzyl-3-hydroxypyrrolidine-1-carboxylate 9 was converted to 10 with inversion of configuration and minimal loss of Scheme 4. enantiomeric excess by a combination of diethylaminodifluorosulfinium tetrafluoroborate and DBU (5). Although dialkylaminodifluorosulfinium salts remain Other combinations of reagents gave lower yields, poorer somewhat hygroscopic and moisture sensitive (13), they are fluoro/alkene selectivities, but gave complete inversion and relatively stable solids, as opposed to most of their minimal loss of enantiomeric excess. On the basis of these dialkylaminodifluorosulfur trifluoride counterparts, which are results, both DBU and triethylamine xHF promoted fluorinations fuming liquids that degrade over time and display significant proceeded in an SN2 fashion. moisture sensitivity (14 - 17). Dialkylaminodifluorosulfinium salts are relatively inexpensive and are not subject to the stringent shipping restrictions that apply to DAST and SF4, which makes them a reagent of choice available worldwide, even in remote areas. DEOXyfluorinatinG AGENT Dialkylaminodifluorosulfinium tetrafluoroborate salts were initially Scheme 8. developed as a suitable replacement for previous 15 deoxyfluorination reagent. They were found to react appropriately as nucleophilic fluorination reagents with a variety The deoxyfluorination of androstenolone was achieved in 77% of oxygen-containing molecules when used in conjunction with yield using a combination of diethylaminodifluorosulfinium proper additives. tetrafluoroborate and 4 equivalents of triethylamine trihydrogen fluoride (5). No elimination byproducts were observed under Alcohols these conditions. Hydrocinnamyl alcohol 4 was fluorinated in a high yield (93%) using a combination of diethylaminodifluorosulfinium tetrafluoroborate and triethylamine trihydrogen fluoride (5). Scheme 9. Diethylaminodifluorosulfinium tetrafluoroborate, with DBU as an additive, were the optimal reagents for the deoxyfluorination of Scheme 5. 2-hydroxy-2-methyl-1-phenylpropan-1-one, giving a moderate fluoro/alkene product ratio of 4.1:1 in high yield (93%) (5). Deoxyfluorination of secondary alcohol 6 using diethylaminodifluorosulfinium tetrafluoroborate and DBU gave a complete conversion of 6, 72% of admixed 7 and 8 in a moderate 6.25:1 ratio (5). Scheme 10. Scheme 6. Morpholinodifluorosulfinium tetrafluoroborate enabled the deoxyfluorination of 2-hydroxy-2-methyl-1-phenylpropan-1-one Chimica Oggi - Chemistry Today - vol. 31(4) July/August 2013 Process optimization in better yield (72%) and higher selectivity (21:1) than derivatives in generally good to excellent yields (5). diethylaminodifluorosulfinium tetrafluoroborate under the same Monofluoroalkenes were sometime observed as a minor conditions (64%, 12:1) (5). The two previous examples, along with elimination byproduct. this one, demonstrate that alcohols were successfully fluorinated in the presence of a remote carbonyl group in high yields. However, several secondary and tertiary alcohols were shown to form alkenes as minor elimination byproducts when submitted to deoxyfluorination conditions using dialkylaminodifluorosulfinium tetrafluoroborate salts. Scheme 11. Deoxyfluorination of geraniol 11, an allylic alcohol, using Scheme 14. morpholinodifluorosulfinium tetrafluoroborate, exclusively formed the SN2’ type fluorinated product in an 88% yield (5). The same product was obtained (90%) when using A one-pot procedure using dialkylaminodifluorosulfinium diethylaminodifluorosulfinium tetrafluoroborate under similar tetrafluoroborate salts and triethylamine trihydrogen fluoride on reaction conditions. an in situ generated O-trimethylsilyl cyanohydrin afforded α-fluoronitrile 12 in excellent yield (5). 16 Scheme 12. The mechanism for the deoxyfluorination reaction using dialkylaminodifluorosulfinium tetrafluoroborate salts requires an exogenous fluoride source or a base to generate the desired Scheme 15. fluorinated products (2, 4, 5). This clearly contrasts with DAST, which generates a free fluoride upon addition of the oxygen The deoxyfluorination of 13 using diethylaminodifluorosulfinium nucleophile to the sulfur trifluoride moiety. The use of tetrafluoroborate resulted in a rearrangement to bridged triethylamine trihydrogen fluoride as the exogenous fluoride biphenyls 14 and 15. Among the deoxyfluorination reagents and source was preferred to other fluoride sources since it is soluble conditions screened for this transformation leading to in most organic solvents, much less corrosive than free HF and it fluorinated analogues of bioactive bridged biphenyls 14, can be handled in borosilicate glassware without etching at diethylaminodifluorosulfinium tetrafluoroborate, used in temperatures up to 150°C (18). conjunction with Et3N•HF, was preferred (19). Although 14 and 15 are obtained as a mixture, they were separable by flash chromatography. Scheme 13. Scheme 16. Aldehydes, Ketones and Carbonyl Derivatives Methylphenyl sulfoxide was shown to undergo quantitative Dialkylaminodifluorosulfinium salts, when used in conjunction fluoro-Pummerer type rearrangement (20) when reacted with a with triethylamine di-, or trihydrogen fluoride allowed the combination of dialkylaminodifluorosulfinium tetrafluoroborate difluorination of a variety of carbonyl groups or related salts and triethylamine trihydrogen fluoride (5). Chimica Oggi - Chemistry Today - vol. 31(4) July/August 2013 Process optimization Scheme 21. Scheme 17. NBS, as an additive, allowed for a