σ-Cyclopropyl to π-Allyl Rearrangement at AuIII

The possibility for Au^III σ-cyclopropyl complexes to undergo ring-opening and give π-allyl complexes was interrogated. The transformation was first evidenced within (P,C)-cyclometalated complexes, it occurs within hours at −50 °C. It was then generalized to other ancillary ligands. With (N,C)-cyclometalated complexes, the rearrangement occurs at room temperature while it proceeds already at −80 °C with a dicationic (P,N)-chelated complex. Density Functional Theory (DFT) calculations shed light on the mechanism of the transformation, a disrotatory electrocyclic ring-opening. Intrinsic Bond Orbital (IBO) analysis along the reaction profile shows the cleavage of the distal σ(CC) bond to give a π-bonded allyl moiety. Careful inspection of the structure and bonding of cationic σ-cyclopropyl complexes support the possible existence of C−C agostic interactions at Au^III.     

Cationic polymerization of cyclocarbonates

Trimethylenecarbonate (TMC) and neopentane diol carbonate (NPC) were polymerized with two groups of initiators, proton and carbenium ion donors or Lewis acids. Initiation with methyltriflate, triflic acid or triethyloxonium tetrafluoroborate in solution gave satisfactory yields (up to 90%) but only low molecular weights (M_n < 5000), due to rapid back-biting degradation. IR- and NMR-spectroscopy demonstrate that the propagation steps involve alkylation of the carbonyl oxygen and cleavage of the alkyl-0 bond by analogy with lactones. Whereas borontribromide and trichloride form solid complexes with NPC or TMC, but do not initiate a polymerization, boron trifluoride is a good initiator. High yields (up to 99,5%) and high molecular weights (M_w > 10⁵) were obtained. However, in analogy to triflic acid initiated polymerizations all polycarbonates contain ether groups. The molar fraction of the ether groups increases with the reaction temperature. High molecular-weight polycarbonates containing ether groups were also obtained with other strong Lewis acids such as SnCl₄, SnBr₄ and TiCl₄. In contrast, weak Lewis acids such as Bu₂SnBr₂ Bu₃SnOMe and Sn(II)2-ethylhexanoate yield polycarbonates free of ether groups. This finding and the NMR-spectroscopically identified endgroups suggest that these weak Lewis acids initiate an insertion mechanism.     

Cationic polymerization of aliphatic cyclocarbonates

Numerous polymerizations of trimethylene carbonate (TMC) and neo-pentylene carbonate (NPC) were conducted in solution using methyl triflate as initiator. The polymerization mechanism was elucidated and rapid backbiting degradation along with the formation of ether groups was detected. When strong Lewis acids such as BF₃, SnCl₄ or SnBn₄ were used as initiators, much higher molecular weights were obtained, but the resulting polycarbonates still contained ether groups. BuSnCl₃ was found to be the most useful initiator yielding high molecular weight polycarbonates free of ether groups. However, in this case it is not clear, if a cationic mechanism or a coordination-insertion mechanism is involved.