[603-32-7] · C18H15As · Triphenylarsine · (MW 306.25)
(nucleophilic agent for the synthesis of arsonium salts, which undergo epoxidation or alkenation reactions; can be used as a ligand)
Physical Data: plates (EtOH), mp 60.5 °C; bp 360 °C, 232-234 °C/14 mmHg.
Solubility: insol H2O, ethanol; sol ether, THF, acetonitrile, etc.
Form Supplied in: commercially available.
Preparative Method: prepared from arsenic trichloride, chlorobenzene, and powdered sodium in benzene under reflux.2
Handling, Storage, and Precautions: use in a fume hood.
Arsonium ylides are more reactive than the corresponding phosphonium ylides since the
covalent canonical form (ylene form) makes a smaller contribution to the overall structure of arsonium ylides (eq 1) than to that of phosphonium ylides.1 This has been supported by X-ray crystallography.3
Reaction of triphenylarsine with halo compounds forms arsonium salts which are converted to ylides on treatment with base. These perform alkenation reactions under PTC conditions.4 A general procedure for the synthesis of unsaturated aldehydes, ketones, esters, and amides directly via arsonium salts in the presence of a weak base (solid Potassium Carbonate) under PTC conditions at rt has been devised (eq 2).5 All the arsonium salts are stable and can be stored for a long time. The Ph3AsO byproduct can be easily reconverted to Ph3As by reduction.6
Formylmethyltriphenylarsonium bromide (1) (eq 3) reacts with aldehydes to give (E)-a,b-enals exclusively (eq 4).7,8
Formylallyltriphenylarsonium bromide (3) reacts with aldehydes to give mixtures comprising mostly (2E,4E)- with some (2E,4Z)-dienals. The latter can be isomerized to the former by treating with a catalytic amount of Iodine in daylight (eq 5).9
2-(Oxoamido)triphenylarsonium bromides (5)10 react with saturated and unsaturated aldehydes at rt in the presence of K2CO3(s) to afford (2E)-unsaturated amides (6) or (2E,4E)-dienamides in excellent yields. No (Z) stereoisomer is detected (eq 6).10 A vinylog (7)11 reacts with aromatic aldehydes to give exclusively the (2E,4E)-products in 80-98% yield (eq 7); with aliphatic aldehydes, (2E,4E/2E,4Z) products are formed in a ratio of 85/15. The (2E,4Z) product can be isomerized to the (2E,4E) product.11 The reactions of aldehydes with arsonium salts are listed in Table 1.
Triphenylarsonium salts have been used to synthesize a variety of natural products under very mild conditions: (EZ)-,diene sex pheromones,12 pellitorine,10b trichonine,13 Achillea amide,10b Otanthus maritima amide,10a (+)-yingzhaosu A,14 LTA4 methyl ester,15 19-hydroxy-LTB4,16 and lipoxins A4 and B417 have all been successfully synthesized.
Since tributylarsine is more reactive towards halides than triphenylarsine, Wittig-type alkenation of carbonyl compounds can be performed catalytically. Thus the reaction of various aldehydes with Methyl Bromoacetate (or Bromoacetone) in the presence of Triphenyl Phosphite and Bu3As (0.2 equiv) provides a,b-unsaturated esters (or a,b-unsaturated ketones) in 60-87% yields and with (E)/(Z) ratios of 97:3-99:1 (eq 8).18
An efficient one-pot synthesis of a-iodo- a,b-unsaturated esters, ketones, and nitriles via arsonium salts has been reported (eq 9).19 A one-pot synthesis of trans-fluorovinylic epoxides has also been achieved (eq 10).20
Silylated enynyl carboxylic esters can be synthesized with high stereoselectivity by the reaction of trimethylsilyl-2-propynylidenetriphenylarsorane, prepared in situ from the corresponding arsonium salt with n-Butyllithium, and BrCH2CO2R (eq 11).21 Triphenylarsoranylideneketene reacts with 2-benzoylpyrrole in methylene chloride to give 1-phenylpyrrolizin-3-one in 85% yield (eq 12).22
Methylenetriphenylarsorane (Ph3As=CH2), which is thermally unstable both in the solid state and in solution, has been isolated and characterized by analytical and spectroscopic methods.23 Triphenylarsonium ethylide (Ph3As=CHCH3), prepared from triphenylethylarsonium tetrafluoroborate with Potassium Hexamethyldisilazide in THF/HMPT at -40 °C, reacts with aliphatic aldehydes to give trans-epoxides with high selectivity (eq 13). Stereoselection is lower with aromatic aldehydes (83% (E) for benzaldehyde). The reagent also reacts with ketones to form trisubstituted epoxides.24
The synthesis and reactivity of (3,3-diisopropoxypropyl)triphenylarsonium ylide have been reported. This reagent can be considered as a b-formylvinyl anion equivalent (eq 14), as shown by the conversion of aldehydes to 4-hydroxy-2(E)-enals under very mild conditions (eq 15).25 This route has been successfully applied to the total synthesis of (±)-hepoxilin A3.26
Homologation of aldehydes using (phenylthiomethylene)triphenylarsorane has been reported. Reaction with aldehydes gives exclusively a-phenylthio epoxides in THF and enol phenol thioethers in THF/HMPA. The former adducts are readily transformed to a-thiophenoxy carbonyl compounds and the latter to one-carbon homologated aldehydes (eq 16).27
Large rate enhancement in Stille cross-coupling reactions is observed with triphenylarsine (a factor of 70 over the triphenyl phosphine-based catalyst) (eq 17).28
Yao-Zeng Huang, Li-Lan Shi & Zhang-Lin Zhou
Shanghai Institute of Organic Chemistry, Academia Sinica, China