The development of new ylidic substituents for stronger bispyridinylidene reducing agents
University of New Brunswick
The superb π-electron donating effect of iminophosphorano groups (R3P=N-) has been previously demonstrated, and the use of these groups led to the preparation of the strongest bispyridinylidene (BPY) organic reducing agents (ORAs). The sensitivity of the redox potential of these molecules to the electronic properties of the R group of the R3P=N- substituent allows for their tunability, which is important for their selective use as ORAs. Recently, we have established two important correlations. The first relates the Tolman electronic parameter (TEP) of the phosphine component of Ph3P=N- group to the reduction potential (E1/2) of the substituted BPYs. A new phosphine with the lowest TEP value ever reported has been prepared and will be incorporated onto the BPY scaffold. The synthesis of this BPY is anticipated to result in an ORA with the lowest reduction potential (E1/2= -1.88 V vs. SCE) to date, and is expected to validate the predictive use of the correlation over a wide range of potentials and TEPs. Additionally, we have explored the triphenylphosphonium ylidyl substituent (Ph3P=CH-) on a BPY scaffold and found that its reducing strength (E1/2= -1.55 V vs. SCE) exceeds that of its iminophosphorano counterpart (Ph3P=N-). The second correlation directly relates the Hammett substituent constant (σp+) to the E1/2 of the corresponding substituted-BPY. Extrapolation from the E1/2 allows for a Hammett-type substituent constant for the Ph3P=CHgroup to be reported for the first time (σp += -2.33), which is the most donating neutral substituent known. The successful preparation of these new BPYs will expand the library of ORAs and should provide greater substrate scope in organic reductions. The ylidic substituents should be useful in other areas of chemistry due to their strong π-electron donation, which provides a new option other than amino groups.