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92 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands<br />
CH 2CH 2CH 3]; 13 C NMR (benzene-d 6, ä): 272 [d, WC(Pr)H, 1 J CW = 180 Hz, 1 J CH = 142 Hz], 41.6<br />
(WCCH 2CH 2CH 3), 29.5 (WCCH 2CH 2CH 3), 14.5 (WCCH 2CH 2CH 3).<br />
2.6.1.1.2 Variation 2:<br />
Ligand Addition<br />
When di- or polyalkyl complexes do not spontaneously give rise to Æ-hydrogen elimination,<br />
this process may often be accomplished by addition of two-electron ligands (e.g.,<br />
phosphines). [14,15] Thus, the tris(2,2-dimethylpropyl)(2,2-dimethylpropylidyne)tungsten<br />
complex 5 transforms into the (2,2-dimethylpropyl)(2,2-dimethylpropylidene)(2,2-dimethylpropylidyne)<br />
product 6 upon addition of trimethylphosphine (Scheme 2). The conditions<br />
required to induce this process depend on the system, from –788C for dibromotetrakis[(trimethylsilyl)methyl]dimolybdenum(III)(Mo”Mo)<br />
[15] to room temperature for cyclopentadienylbis(2,2-dimethylpropyl)nitrosylmolybdenum(II).<br />
[16]<br />
In the synthesis of compound 7, substitution of two alkoxide ligands with the less<br />
sterically encumbering (and also poorer ð-donor) chlorides opens up the coordination<br />
sphere to the coordination of a bidentate 1,2-dimethoxyethane molecule, inducing carbene<br />
formation. The dialkoxo precursor, albeit five coordinate, does not spontaneously<br />
undergo the alkane elimination process. [17] The increase of the coordination sphere can<br />
also be achieved by replacement of a monodentate ligand (e.g., chloride) with a polyfunctional<br />
ligand, e.g. hydrotris(pyrazolyl)borate [18] or 2-[(dimethylamino)methyl]phenyl (see<br />
synthesis of 8) (Scheme 2). [19] This strategy has also been utilized in other cases via replacement<br />
of a bulky alkyl with chloride by protonation with LH + Cl – in the presence of excess<br />
ligand L. [10]<br />
Scheme 2 Æ-Hydrogen Elimination Induced by Ligand Addition [14,17,19]<br />
ButH2C W CBut But Bu<br />
H2C<br />
tH2C 5<br />
OBu<br />
W N<br />
t<br />
OBut ButH2C But Pr<br />
H2C i<br />
Pri CH2TMS<br />
NPh<br />
Cl W<br />
CH2TMS<br />
CH2TMS Me3P (neat)<br />
100 oC, 5 min<br />
quant<br />
PCl5, DME<br />
−35 oC 90%<br />
2-Me 2NCH 2C 6H 4Li<br />
Et 2O, −78 o C<br />
70%<br />
But Me3P t CBu<br />
H2C W<br />
CHBu<br />
Me3P<br />
t<br />
6<br />
Pr i<br />
Cl<br />
N<br />
W<br />
Cl<br />
CHBut O<br />
O<br />
Pri Me<br />
Me<br />
8<br />
7<br />
NMe2<br />
NPh<br />
W<br />
CHTMS<br />
CH2TMS Dichloro[(2,6-diisopropylphenyl)imido](1,2-dimethoxyethane-O,O¢)(2,2-dimethylpropylidene)tungsten(VI)<br />
(7); Typical Procedure: [17]<br />
Finely ground PCl 5 (2.25 g, 10.8 mmol) was added to a chilled (–358C) soln of [W(CH 2t-<br />
Bu) 2(Ot-Bu 2) 2(=NC 6H 3-2,6-iPr 2) (7.0 g, 10.8 mmol) in DME (120 mL). The mixture was<br />
warmed to rt and stirred for an additional 1 h after all the solids had disappeared. The<br />
mixture was then concentrated in vacuo until an orange powder formed. This material<br />
was washed with cold pentane to give the product as a yellow-orange powder. This synthesis<br />
can fail virtually completely if the DME is not scrupulously dried and the PCl 5 not