Electronic substituent effect of nitrogen ligands in catalytic asymmetric hydrosilylation of ketones: chiral 4-substituted bis(oxazolinyl)pyridines

Abstract

In this paper, we wish to report the high-yield single-step formation of alkynylphosphonium triflates 2 via the reaction of readily available3s4 alkynyl(pheny1)iodonium triflates 1 with Ph3P. CH&l*, -78 to 25 O C RCWIPh(0Tf) + Ph3P -RC=CPPh,(OTf) 2a: R = CH3 c: R = Me Si d: R = n-8u 30 min, b: R = t-Bu -PhI la: R = CH3 c: R = Me 8i d: R = n-8ue: R = P h e: R = Ph Reaction of 1 with a 5% excess of P h R in CH2C12 at -78 to 25 "C for 30 min affords the appropriate crude alkynyl phosphonium triflates 2 in essentially quantitative yield. Recrystallization from CH2C12 and ether affords the pure final product in 88-98% isolated yield as colorless microcrystalline solids for 2a-d and slight yellow for 20.The product phosphonium salts are characterized by multinuclear NMR and IR spectroscopy and by elemental analysis. Specifically, the singlet between 5.2 and 7 ppm in the 31P NMR spectrum is typical for the phosphorus signal in alkynylphosphonium The presence of the -OS02CF3 counter ion is indicated by the characteristic lgF signal at -78 ppm as well as the typical absorptionss for ionic triflates in the IR spectrum. The IR spectrum shows signals for the C=C bond between 2134 and 2212 cm-'. Most characteristic are the C = C signals in the 13C NMR spectrum, where the C, resonates between 60 and 79 ppm with a 'JCp = 162-192 Hz and the C, is at much lower field between 118 and 133 ppm with a 2 J~ = 12-33 Hz. The remainder of the 13C spectra as well as the 'H spectra are completely consistent with the proposed structures.Finally, unlike the reaction of the analogous alkynyl-(pheny1)iodonium tetrafluoroborate species,' the reaction of 1 with Ph3P does not require light.8 These reactions occur readily in the dark. Moreover, the reaction of 1 with Ph3P is not inhibited by radical traps such as O2 or 2,6di-tert-butyl-4-methylphenol (BHT). Hence, we believe that, unlike the proposed radical cation-like reactions' of the -BF4 salts, the triflate salts 1 react by the standard3 nucleophilic acetylenic substitution &A) process involving iodonium ylides and alkylidenecarbenes as intermediates. b: R t-B Experimental SectionMelting points were obtained in capillary tubes and are uncorrected. Infrared (IR) spectra were recorded as CCl, thin f i . NMR spectra were recorded on a Varian XL 300 spectrometer using CDC13 as solvent. Chemical shifts for 31P are reported in parts per million downfield from 85% H3P04, and '9 NMR shifta are relative to CFCIB. Commercially available Ph3P was recrystallized from hexanea prior to use. CH2C12 was distilled from CaH2 Alkynyl(pheny1)iodonium triflates 1 were prepared by standard method^.^^^ General Procedure for the Preparation of Alkynylphosphonium Triflates (2). Alkynyl(pheny1)iodonium triflate (3) Review: Stang, P. J. Angew. Chem., Int. Ed. Engl. 1992,31,274. (4) (a) Bachi, M. D.; Bar-Ner, N.; Crittell, C. M.; Stang, P. J.; Williamson, B. L. J. Org. Chem. 1991,56,3912. (b) Stang, P. J.; Zhdankin, V. V.; Williamson, B. L. (8) For a related reaction of R C d I P h ( 0 T s ) with P(OR)3 to give RC*P(O)(...

Convenient Enantioselective Hydrosilylation of Ketones Catalyzed by Zinc‐Macrocyclic Oligoamine Complexes

“…Chiral rhodium catalysts often use bidendate P,P, P,N or N,N ligands, such as PYTHIA, [10] PYBOX [11] or HETPHOX. [12] High enantioselectivities (over 90% ee) for the standard AHS of acetophenone were achieved with chiral diphosphine [13] or pyridine-phosphine [14] ligands having a ferrocene structural unit, as well as with a chiral P,S ligand reported by Evans.…”

Coordination Compounds Based on 1,2,3,4-Tetrahydro-isoquinoline-3-carboxylic Acid

“…The coordination compounds were prepared by a reaction of (S)- (2) is an tridentate ligand, as described for 2,6-bis-(oxazolyl)pyridines [4,16] or 2,6-bis(imidazolyl)pyridines [17,18], and it coordinates with Cu From the 13 C-NMR spectrum it is obvious that it also corresponds to a mixture of three compounds of the types AA, BB, AB+BA, which are present at roughly equimolecular ratios (the tetrad of signals for corresponding carbons in the spectrum). …”

A facile and efficient synthesis of bis(oxazoline)s

“…Chiral bis(oxazoline) ligands have widespread uses in asymmetric hydrosilylation [7], cyclopropantion reaction [8], Friedel-Crafts reaction [9], DielsAlder reaction [10], Aldol addition [11], Michael reaction [12], Henry reaction [13], allylic oxidation [14], 1,3-dipolar cycloaddition [15], and so on.…”