Methods for the Synthesis of Tungsten Alkylidenes: Ring-Opening of Cyclopropenes and Alkylidene Transfer from Phosphoranes

Citation

Johnson, Lynda K. (1992) Methods for the Synthesis of Tungsten Alkylidenes: Ring-Opening of Cyclopropenes and Alkylidene Transfer from Phosphoranes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/scx7-0w89. https://resolver.caltech.edu/CaltechTHESIS:09132011-105236506

Abstract

In the research reported in this thesis, two methods for synthesizing alkylidene complexes were investigated: (1) ring-opening of cyclopropenes to give vinyl alkylidene complexes and (2) alkylidene transfer from phosphorus ylides to metal centers. Tungsten(IV) imido precursors of the form WX_2(NAr)L_n (X = Cl or OR; Ar = Ph, 2,6-C_6H_3-Me_2, 2,6-C_6H_3-(i-PR)_2, L_n = PR3, P(OR)_3 or ether donor ligands) were used throughout the investigation.

A brief overview of the syntheses and uses of high-valent alkylidene complexes is given in Chapter 1. The reactions of WCl_2(NAr)(PX_3)_3 (X = R or OR) precursors with 3,3-diphenylcyclopropene and 4,8-dioxaspiro[2.5]oct-1-ene are reported in Chapter 2. η^2-Cyclopropene complexes [W(η2-cyclopropene)Cl_2(NAr)(PX_3)_2] were synthesized from precursors containing the smaller imido ligands; increasing the steric bulk of the imido ligand favored the ring-opening of the cyclopropenes to yield the vinyl alkylidene compounds [W(=CH-CH=CR'_2)Cl_2(NAr)(PX_3)_2]. Conversion of thermally stable η^2-cyclopropene complexes to give the corresponding vinyl alkylidene compounds was observed upon photolysis or addition of catalytic amounts of H_gCl_2.

The transfer of alkylidenes from Ph_3P=CHAr' and Ph_3P=CH-CH=CMe_2 to WCI_2(NPh)(PMePh_2)_3 to give W(=CHR')CI_2(NPh)(PMePh_2)_2 is reported in the first half of Chapter 3, and the effects of varying the solvent, the ylides, and the tungsten precursors are discussed. The remainder of Chapter 3 deals with the in situ reduction and trapping of WCl_2(NAr)[OCMe(CF3_ 3) _ 2] _ 2(THF) precursors by Ph_3P=CHAr' to give W(=CHAr')(NAr)[OCMe(CF_3)_2]_2(PPh_3). The use of the chelating o-methoxybenzylidene was especially effective here, as coordination by the o-methoxy group greatly aided the transfer reaction and, in addition, stabilized the resulting product.

Chapter 4 documents initial studies involving the reactions of WCI_2(NAr)(PX_3) _3 precursors with exo-5,6-dimethoxymethyl-7-oxanorbornene. For reactions involving tungsten precursors with the smaller imido ligands, fonnation of η2-olefin complexes W[η_2-(7-oxanorbomene]Cl2(NAr)(PX3h was observed. Oxygen abstraction to give 5,6- dimethoxymethylcyclohexadiene occurred upon reaction of this olefin with WCl_2[N-2,6-C_6H_3-(i-Pr) _2] [P(OMe) _3] _3.

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