Didentate phosphine ligands with alkenyl and alkynyl linker units as building blocks for dendrimer fixation

Abstract

Using diethyl-2-methylmalonate as a starting material, the two diphosphines HCCCH₂C(CH₃)(CH₂PPh₂)₂ (7) and H₂CCHCH₂C(CH₃)(CH₂PPh₂)₂ (13), containing alkynyl and alkenyl unit in the ligand backbone were prepared in multistep syntheses. These two ligands were employed in the synthesis of [{(CH₃)₃SiCCCH₂C(CH₃)(CH₂PPh₂)₂}PdCl₂] (8) and [{(CH₃)₃SiCCCH₂C(CH₃)(CH₂PPh₂)₂}PtCl₂] (9) as well as [{H₂CCHCH₂C(CH₃)(CH₂PPh₂)₂}PdI₂] (14b), all of which were characterized by X-ray diffraction. Reaction of Si[(CH₂)₃SiMe₂Cl]₄ (= “G[0]-[Cl]₄”) with four molar equivalents of the in situ lithiated alkynyl diphosphine 7 cleanly yielded the four-fold functionalized derivative G[0]-[CCCH₂C(CH₃)(CH₂PPh₂)₂]₄ (15) which was converted to the metallated derivative G[0]-[CCCH₂C(CH₃)(CH₂PPh₂)₂PdCl₂]₄ (16). Since the fixation of the alkenyl-functionalized diphosphine ligand 13 to the SiH-silane Si[(CH₂)₃SiMe₂H]₄ (= “G[0]-[H]₄”) by Pt-catalyzed hydrosilation proved to be unsuccessful due to the negative interference of the phosphine with Karstedt's catalyst, the chloro-substituted precursor of 13, H₂CCHCH₂C(CH₃)(CH₂Cl)₂ (12) was used. Hydrosilation of G[0]-[H]₄ with four molar equivalents of 12 readily gave the functionalized dendrimer G[0]-[CH₂CH₂CH₂C(CH₃)(CH₂Cl)₂]₄ (17) which was then reacted with eight equivalents of LiPPh₂ to yield the phosphinated derivative G[0]-[CH₂CH₂CH₂C(CH₃)(CH₂PPh₂)₂]₄ (18).