ca01 only detailed ToC 1..24

104 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
tile for further transformations by ligand exchange. Exchange of the carbyne and an alkyl
ligand by intramolecular scrambling of the Æ-H atoms is possible. [65,66] Since the carbyne
function is polarized as M(ä + )”C(ä – ), these compounds are susceptible to electrophilic attack
at the carbyne ligand (e.g., protonation) and nucleophilic attack at the metal center
(e.g., ligand addition). The addition of acids (HX) converts carbyne complexes into carbene
complexes (see Section 2.6.1.4), [7] although, in many cases, the reagents attack other ancillary
ligands and the carbyne function remains intact. Examples are the reactions of trialkyl
derivatives with hydrochloric acid, ammonium chloride, or carboxylic acids. [67–69]
Synthesis of Product Subclass 2
2.6.2.1 Method 1:
By Æ,Æ-Hydrogen Elimination from Alkyl Complexes
The increase of steric bulk in a high oxidation state complex containing alkyl ligands, or
the in situ generation of encumbered complexes of this type by transmetalation reactions
(see Section 2.6.1.1), induces Æ,Æ-hydrogen elimination processes with formation of carbyne
products. The alkylation reaction works better from oxo or alkoxo derivatives than
from the corresponding chlorides, because these are less susceptible to competing reductive
processes. [70] This reaction appears to be the preferred entry into 2,2-dimethylpropylidyne
derivatives of molybdenum(VI) and tungsten(VI) through formation of the tris(2,2dimethylpropyl)
derivatives 38 and 39 (see Scheme 14). Other 2,2-dimethylpropylidyne
derivatives are then readily obtained by treating 38 and 39 with sufficiently strong acids,
e.g. hydrochloric or carboxylic acid, or by further ligand exchange from the trichloride
derivatives. This process is presumed to take place stepwise, via intermediate carbene
complexes (see Section 2.6.1.4), even when these are not observed. Some carbyne products
have been obtained by alkane elimination from carbene complexes, [15] although
this strategy does not appear to have general synthetic utility.
A particular case of Æ,Æ-hydrogen elimination from an alkyl ligand leads to the formation
of complex 40. [71] The two hydrogen atoms are eliminated as dihydrogen rather
than being transferred to alkyl ligands. Thus, the reaction involves a formal metal oxidation
and is so far limited to the system shown. The process is much faster for tungsten
than for molybdenum, this being ascribed to a more favorable pre-equilibrium yielding
the alkylidene–hydride intermediate. While the alkyl precursors can be isolated for molybdenum,
they may only be obtained in situ for tungsten by transmetalation from the
corresponding chloride. Other related rearrangements for cycloalkyl derivatives, leading
to carbyne products in some cases, have also been observed for this system (see Section
2.6.2.5). [72] The synthesis of 41 [73] may be mechanistically related to the synthesis of 40,
the dihydrogen byproduct being transferred to the vinyl group.
Scheme 14 Æ,Æ-Hydrogen Elimination Processes [67,68]
MoO 2Cl 2
WCl 3(OMe) 3
t-BuCH2MgCl (6 equiv)
Et2O, −78 oC 34%
t-BuCH2MgCl (6 equiv)
Et2O, −78 oC 50−70%
CBu t
Mo
But CH2Bu
H2C t
CH2But 38
Bu t H 2C
CBu t
W CH2But CH2But 39