A novel preparation of chlorophospholenium chlorides and their application in the synthesis of phospholene boranes

Preparation of 2-phospholene oxides by the isomerization of 3-phospholene oxides

“…The preparation of 2-phospholene oxides 4 was first attempted via the formation of chlorophospholenium salts 2 and 3. Ac-cording to our procedure [65], the corresponding 1-substituted-3-methyl-3-phospholene oxide 1 was reacted with oxalyl chloride to give chloro-3-phospholenium salts 2 with complete conversion. In our experience, a synthetic protocol involving the use of oxalyl chloride in a slight excess, followed by the removal of the unreacted reagent under reduced pressure gave the corresponding chloro-3-methyl-3-phospholenium chlorides 2 in the most reproducible manner.…”

“…The chloro-3-phospholenium salts 2 were then dissolved in chloroform, and were heated at 80 °C for 48 h. Upon this period, nearly complete isomerization of the chloro-3-phospholenium salts 2 to the corresponding chloro-2-phospholenium salts 3 occurred. The mechanism of this isomerization was investigated by quantumchemical calculations in our previous study [65]. However, the formation of the 2-phospholenium chlorides 3 was only postulated.…”

“…In contrast, nearly complete isomerization was observed via the formation of chlorophospholenium chlorides or in the presence of methanesulfonic acid, which may indicate that the intermediates formed over the course of the reaction may perturb the thermodynamic equilibrium of the 2-or 3-phospholene oxides 4 or 1. It was shown in our previous study [65] that the relative stability shifts towards the 2-phospholene isomer, when chlorophospholenium salts are involved, and it is the underlying reason why the 2-phospholene oxides 4 can be prepared effectively by this reaction. Under strong acidic conditions, the P=O moiety tends to be protonated, which may be responsible for the complete isomerization in the presence of methanesulfonic acid.…”

“…However, either the isomerization remained incomplete [58,59], or it was an undesired side-reaction caused by the high temperature and the excess of the reagent [62][63][64]. Our recent synthetic efforts were directed to the novel preparation of cyclic chlorophospholenium chlorides, and the utilization of these reactive derivatives in silane-free deoxygenations or dynamic resolutions [65][66][67]. In these studies, the isomerization of the 1-chloro-3-methyl-1-phenyl-3-phospholenium chloride to the corresponding 2-phospholenium salt was also observed as a side-reaction upon prolonged reaction times.…”

A Mild One-Pot Reduction of Phosphine(V) Oxides Affording Phosphines(III) and Their Metal Catalysts

“…Being readily afforded via inexpensive chlorinating reagents, CPSs have also been reduced with aluminum/metal salts, alkali metals, , LiAlH 4 , ,, thiols/Et 3 N, activated carbon, Hantzsch ester/Et 3 N, electrochemically, − ,, elemental aluminum , or silicon, and hydrogenolysis, which may be catalyzed by frustrated Lewis pairs (FLPs). , Harsh metal bases and Grignard reagents have even been used to deprotect certain CPSs . Alternatively, CPS can be converted to phosphine–boranes by either NaBH 4 , or LiBH 4 , − although ultimately the borane “protecting group” itself requires removal (Scheme b,d,e).…”