Initiator phosphines

On heating, the neopentyl Pt(PEt3)2(CH2CMe3)2 undergoes an intramolecular metallation elimination [108a] (Figure 3.59), which appears to involve initial phosphine loss affording a platinum(IV) metallacycle. 

The initial phosphine products were converted to the phosphine sulfides to protect them from oxidation and readily reconverted to the free phosphines by Raney nickel desulfurization.77... 

A mechanistically similar, yet considerably less common, /3-methyl elimination occurs in the rearrangement of 3,3-dimethylruthenacyclobutane complex 117 (Equation 34) <1995JA3625, 1997JA11244>. The reaction proceeds via reversible initial phosphine dissociation. 

If acid was added there was a marked decrease in absorbance around 250 inn. The difference spectrum, a plot of fall in absorbance versus wavelength, was found to be identical to the spectrum of sodium butyl cyanoacetate (Na CH(CN)COOBut). If the initiator was triphenylphosphine the absorption remaining after addition of acid was identical to triphenyl-methylphosphonium bromide. Repetitive isolation and purification of the polymer did not reduce the intensity of this absorption. Assuming that the absorption coefficients of the model compounds and chain ions were identical it could be shown that molarities of polymer, cation, and anion were equal to the initiating phosphine concentration. 

Slurry Homogeneous Initiator Phosphine Low Reaction Rate Low Bulk Density Low Maximum Slurry Concentration... 

In 2006, Stephan introduced the concept of frustrated Lewis pairs , systems in which steric congestion precludes the formation of a Lewis acid-base adduct [25,26]. These combinations offer novel reactivity involving the activation of a variety of small molecules including amines, alkenes, alkynes, and molecular hydrogen [94]. While initially phosphines were the Lewis base of choice for FLP reactions, Al-heterocyclic carbenes were soon proven to be viable candidates as well. For instance, an FLP of ItBu and tris(pentafluorophenyl)borane was shown to react with molecular hydrogen in the usual FLP fashion, forming an imidazo-lium borate salt. This FLP was also used to activate ammonia, aniline, and diphe-nylamine, by addition of the NHC to the amine-borane adduct as shown in Scheme 15.5 [95]. 

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