Steric crowding phosphorus

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

First order steric crowding is due to the steric demands of the coordinating atom, e.g. the oxygen atom of the phosphine oxide or a chlorine atom on its own. Second order steric crowding is due to the atoms or groups attached to the coordinating atom, e.g. the substituents on the phosphorus atom in the phosphine oxide above. 

The use of bulky phosphorus ligands does not necessarily push the reaction towards the more phosphinated cycles. This is because too much steric crowd-... 

Several reports have appeared of the application of unusual organometallic reagents in phosphine synthesis. Treatment of 2-lithiopyridine with anhydrous zinc chloride results in the formation of a 2-pyridylzinc reagent which can be used to introduce the 2-pyridyl group at phosphorus in a controlled manner. Thus, e.g., in its reaction with phenyldichlorophosphine, the 2-pyridyl-(phenyl)chlorophosphine (15) is formed. This has then been converted via the phosphide route into a new class of binucleating ligands (16). The sterically crowded dichlorophosphine (17) (accessible from the reaction of phosphorus trichloride with lithium diphenyl(2-pyridyl)methanide) is converted into the thermally stable phosphirane (18) on treatment with calcium, strontium or barium cyclooctatetraenide.The reaction of phenyldichlorophosphine with the readily accessible titanacycle (19) affords a convenient route to the phosphetene (20). ... 

Figure 7.8. Bisphosphine ligands and the common structural feature that affects steric crowding of ligands bound trans to phosphorus [97,125]. Note vacant upper left and lower right quadrants in the generalized structure on the left.

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