Alkene Metathesis in Organic Synthesis
The tremendous progress in olefin metathesis achieved during the last decade can hardly be overestimated. Due to the development of a new generation of well defined and high-performance organometallic catalysts, this reaction is rapidly evolving into one
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efin metathesis is a catalytic process whose key step consists of a reaction between an olefin and a transition metal alkylidene complex. Some of the best understood alkylidene catalysts contain Mo or W in its highest possible oxidation state, along with “supporting” alkoxide ligands and an imido ligand. Recent advances in our understanding of these catalysts from a fundamental perspective, and in ring opening metathesis polymerization (ROMP) and ring closing metathesis (RCM), in particular, are reviewed. Keywords: Metathesis, Molybdenum, Tungsten Alkylidene, Ring-opening, Ring-closing.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of Synthesizing Mo and W Alkylidene Complexes. Oxo Alkylidene Complexes . . . . . . . . . . . . . . . . . . . Imido Alkylidene Complexes . . . . . . . . . . . . . . . . . . Variations and Other Types of Alkylidene Complexes . . . . Cross-Coupling Reactions . . . . . . . . . . . . . . . . . . . . Ring-Opening Metathesis Polymerization (ROMP) and Other Ring-Opening Reactions . . . . . . . . . . . . . . Conjugated Polymers. . . . . . . . . . . . . . . . . . . . . . . Ring-Closing Metathesis (RCM) . . . . . . . . . . . . . . . . Asymmetric RCM . . . . . . . . . . . . . . . . . . . . . . . . . Comments and Conclusions . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Introduction Olefin metathesis is a catalytic process whose key step is a reaction between an olefin and a transition metal alkylidene complex, usually M=CHR (Eq. 1) or M= CH2, in a 2+2 fashion to give an unstable intermediate metalacyclobutane ring [1]. All possible reactions of this general type are reversible, possibly nonproductive, and in competition with one another, so the overall result depends heavily on relative rates, and in the case of formation of volatile or insoluble products, displacement of equilibria as those products form.
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Richard R. Schrock
M=CHR1 + R1CH=CHR2
M
CHR1
M=CHR2 + R1CH=CHR1
R2HC
CHR1
cis or trans
(1)
Much of the proof for the mechanism of this reaction has been provided by studies that involve isolated alkylidene or metalacyclobutane complexes, first of Ti [2–4] and Ta [5], and then of Mo and W [6, 7]. In all such species the metal is in its highest possible oxidation state if the alkylidene ligand is viewed as a dianion. Alkylidene complexes are also called carbene complexes, especially for later transition metals in which the metal–carbon double bond is not as polarized (M(+)=C(–)) as it is in an early transition metal complex, and the metal is not in its highest possible oxidation state. Some later metal complexes are active metathesis catalysts, especially ruthenium complexes of the type Ru(CHR)Cl2 (PR'3)2 [8–11]. Later transition metal carbene complexes have a higher tolerance than “d0” alkylidene complexes toward oxygen and protic
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