Phenyldioxaborolane promoted synthesis of bisphosphine compounds

Tin-free radical mediated C-C bond formation via the Xanthate transfer

“…Although the keto-form is generally accepted as the more stable conformation, electron-donating effects can also stabilize the enol-form as well. Keto-Enol Tautomerism of Acyl Phosphonates and Effect of Resonance byConjugation with Lone Pair Electrons of HeteroatomA report was found to observe that the enol form dominated the keto form of the acyl phosphonate when enol formation is favored by conjugation effects and the 31 P chemical shifts of both forms corresponded to those observed in our study 105. For example, the keto form of acylphosphonate(Figure 12, A)has a shift at δ -3.0 ppm105 which was closely similar to that of compounds3.16, 3.17, 3.20 and 3.21.…”

“…Keto-Enol Tautomerism of Acyl Phosphonates and Effect of Resonance byConjugation with Lone Pair Electrons of HeteroatomA report was found to observe that the enol form dominated the keto form of the acyl phosphonate when enol formation is favored by conjugation effects and the 31 P chemical shifts of both forms corresponded to those observed in our study 105. For example, the keto form of acylphosphonate(Figure 12, A)has a shift at δ -3.0 ppm105 which was closely similar to that of compounds3.16, 3.17, 3.20 and 3.21. The enol form(Figure 12, B) has a chemical shift at δ +12.9 ppm 105 which seem to closely correspond to the singlet observed at +11 ppm in both 31 P { 1 H} spectrums of 3.18 and 3.19.…”

“…Keto-enol tautomerism of the acyl phosphonate was favored towards enol formation due to the stablization effect brought about by conjugation of lone pair on the heteroatom to the enol. 104,105 This enol could then attack a diethyl phosphorochloridate which presumably may be formed from triethyl phosphite and excess oxalyl chloride 107 The crude acyl phosphonate 3.22 was then reacted with the keto-xanthates using our current protocol. We were delighted to isolate the desired dihydropyran-type 1,5diketones 3.30h in 65% yield and 3.30i in 54% yield respectively (Scheme 57).…”