Hydrophosphination/cyclization

Scheme 5-16 Proposed mechanism of organolanthanide-cat-alyzed hydrophosphination/cyclization...

In the most recent developments in this area, organometallic lanthanide complexes have been utilized for hydrophosphination/cyclization reactions of phosphinoalkenes.202,203... 

Douglass, M.R. and Marks, T.J., Organolanthanide-catalyzed intramolecular hydrophosphination/cyclization of phosphinoalkenes and phosphi-noalkynes, /. Am. Chem. Soc., 122, 1824, 2000. 

Lanthanide metals catalyze intramolecular hydrophosphination-cyclization of alkenyl- and alkynylphosphines rapidly at room temperature [19]. As summarized in Table 3, the reaction is quite general. 

Scheme 12 Diastereoselective hydrophosphination/cyclization of chiral phosphinoalkenes [147,179]...

Scheme 19 Proposed mechanism for organolanthanide-catalyzed hydrophosphination/ cyclization...

Other catalytic hydroelementations, E-H (E = P, B, Si, H) addition to C C multiple bonds, are efficiently mediated by organolanthanide complexes, and many are highly diastere-oselective. Hydrophosphination/cyclization appears to follow the catalytic patiiway similar to hydroamination/cyclization, whereas hydroboration, hydrosilylation, and hydrogenation proceed via different catalytic cycles, in which a lanthanide-hydride is the intermediate that is generated from protonolysis of the precatalyst. The proposed catalytic cycle of hydrosilylation of alkene using Cp LnCH(TMS)2 is presented in Scheme 5. 

Douglass MR, Marks TJ (2000) Organolanthanide-catalyzed intramolecular hydrophosphination/ cyclization of phosphinoalkenes and phosphinoalkynes. J Am Chem Soc 122 1824—1825... 

Douglass MR, Stem ChL, Marks TJ (2001) Intramolecular hydrophosphination/cyclization of phosphinoalkenes and phosphinoalkynes catalyzed by organolanthanides scope, selectivity, and mechanism. J Am Chem Soc 123 10221-10238... 

Homoleptic lanthanide alkyls of the form Ln[GH(SiMe3)2]3 (Ln = Y, La, Nd, Sm, Lu) serve as efficient precatalysts for intramolecular homogeneous hydrophosphination. Both phosphinoalkenes and phosphinoalkynes undergo cyclization to the corresponding heterocyclic structures.1055 The catalytic intramolecular hydrophosphination/cycli-zation of phosphinoalkenes and phosphinoalkynes using organolanthanide precatalysts of the type... 

While the highly versatile rare-earth metal-catalyzed hydrogenation, hydroboration, hydrosilylation, hydroamination, and hydrophosphination has been studied over the last two decades, the rare-earth metal-mediated activation of the hydroxyl 0-H bond for an analogous hydroalkoxylation process [87,190] has been reported only recently. Homoleptic trisamides were reported to catalyze the smooth cyclization of allenyl alcohols [191] and alkynyl alcohols [191,192] to form the corresponding unsaturated ethers. 

The reaction is believed to proceed via a mechanism analogous to hydroamina-tion and hydrophosphination. There is experimental evidence for a rate-determining insertion step (Fig. 23). The high oxophilicity of the lanthanide ion results in a high barrier for the olefin insertion and therefore, diminished reactivity of alkenyl alcohols. Rare-earth metal triflates are also capable to catalyze cyclization of alkenyl alcohols in ionic liquids [193], although the mechanism is unlikely to be similar to the o-bond metathesis mechanism discussed above. 

If the alkene is connected to the phosphine, hydrophosphination results in cyclization, as observed for a series of lanthanocene catalysts. DPT and experimental studies were consistent with the mechanism of Scheme 19, where P-C bond formation occurs by the approximately thermoneutral insertion of an alkene into a... 

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