Tungsten complexes reactive intermediates

Trialkoxy complexes of tungsten with terminal phosphido ligands could not yet be isolated. They were postulated to be very reactive intermediates in different transformation reactions, e.g., during the metathesis reaction of [W2(0R)6] with phosphaalkynes [6, 14]. However, we were able to characterize the complex [(t-BuO)3W=P] (3c) by P-NMR spectroscopy by monitoring the metathesis reaction of [W2(Ot-Bu)6] with MesC=P in the tempera-... 

Cowley and coworkers have reported two other reactions. One is the reaction of the silylphosphide 11a with dichloroborane, which afforded the diphosphadiboratane, resulting in the formation of the phosphaborene by flash-vacuum pyrolysis (equation 101)135. The other is the reaction of the silylphosphide 11a with ( j5-C5H4Me)2WCl2, which gave compounds 14 and 15 (equation 102)136. A phosphinidene tungsten complex was assumed to be a reactive intermediate. 

Phosphinidenes (R-P), short-lived, reactive intermediates that are isoelec-tronic with nitrenes and carbenes, are accessible by fragmentation of phos-phiranes, phosphol-3-enes, triazaphospholenes, and oxazaphospholenes and can be characterized by trapping reactions. The metal-complexed species [R-P-M, e.g., M = W(CO)5] are more stable (but also not isolable) and react more selectively. As a model example we describe the transformation of (3,4-dimethyl-l-phenylphosphole)(pentacarbonyl)tungsten 4 with dimethyl acetylenedicarboxylate to the 7-phosphanorbomadiene derivative 5.11 The (phosphinidene)(pentacarbonyl)tungsten complex 6 is generated from the latter by a [4+ 1]-cycloreversion and trapped with diphenylacetylene via a [2+1]-cycloaddition to furnish the triphenyl-2-phosphirene complex 7 (Scheme l).12... 

First, the sulfur atom from a thiirane is added to one of the carbon atoms of the thiirane ligand of the complex, with formation of a zwitterionic intermediate. The zwitterionic intermediate undergoes elimination of ethylene to yield a reactive alkene disulfide tungsten complex. The dithietane ligands from two or more molecules, then combine to form the cyclic polysulfides. If the macrocyclization of thiirane catalyzed by W(CO)s(SC2H4) is performed in the presence of DMAD, small quantities of polythioether macrocycles 156 and 157 are formed <1997OM1430>. 

In this reaction scheme, the formation of the pentacyano complex is a relatively fast reaction, with rate constants of about 116 and 2.9 Af" sec" for the molybdenum (20°C) and tungsten (25°C) complexes, respectively, whereas the formation of the octacyano complex from the pentacyano complex is a relative slow reaction, with a half-life of several minutes at a cyanide ion concentration of 1 Af for both the molybdenum and the tungsten complexes. The formation of the octacyano complex from the pentacyano complex is third order in the cyanide ion concentration 155,156). This suggests that the rate-determining step is the reaction of the heptacyano complex with cyanide ions. It seems, however, that the pentacyano complex is a necessary intermediate in the synthesis of the octacyano complex. This proposed reaction scheme makes it possible for the first time to explain why the octacyano complex of rhenium(V), which is also a d species, is still unknown in spite of several attempts (and claims of success) by different groups in the past (see Section IIA) to synthesize this complex The reactive complexes [Re0(H20)(CN)4]" and [ReO(OH)(CN)4] do not exist at a pH > 8, at which there are enough free cyanide ions since the values of [Re0(H20)(CN)4]" are only 1.4 and 4.2. The formation ofthe intermediate [ReOtCNlg] (see Scheme 6) is thus not possible. Thus one cannot proceed beyond the tetracyano complex in this way. 

Similar reactions proceed in the presence of molybdenum and tungsten complexes as catalysts. These catalysts are more reactive than chromium catalysts but the reaction selectivities are low. The induction period is usually one hour. The induction period is thought to be caused by the ligand exchange reaction of an aryl group and diene with the obvious formation of an intermediate as shown in eq. 

In 1999, Maeyama and Iwasawa reported an electrocyclization involving a tungsten-vinylidene complex as a reactive intermediate, where the cycloaroma-tization of o-ethynylstyrenes 14 proceeded smoothly to give the corresponding substimted naphthalenes 15 in high yields (Scheme 21.6) [12], At first, the mngsten-vinylidene complex is formed and then 6tt electrocyclization occurs to... 

Stereochemical data support the occurence of these intermediates, as also shown by Doyle et al. (1984 b) They compared reactivities and stereoselectivities of cyclopropanations of phenyldiazomethane and eleven different open-chain alkenes containing a terminal double bond or a double bond in the chain, and a cyclic alkene (cyclopentene) catalyzed by the binuclear complex Rh2(OCOCH3)4 (8.127, see later in this section), with the reactivities and stereoselectivities of cyclopropanations of the same alkenes with (benzylidene)(pentacarbonyl)tungsten [(CO)5W(CHC6H5)], i.e., a stable metal-carbene. An almost perfect linear relationship of the cyclopropane derivatives of the eleven alkenes with the two carbene sources was obtained. On this basis, Doyle and his coworkers concluded that the reaction starts with an initial association of the alkene 71-bond with the electrophilic center of the metal-carbene complex, followed by o-bond formation with backside displacement... 

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