In recent years, the olefin metathesis reaction has attracted widespread attention as a versatile carbon−carbon bond-forming method. Many new applications have become possible because of major advances in catalyst design. State-of-the-art ruthenium catalysts are not only highly active but also compatible with most functional groups and easy to use. This Account traces the ideas and discoveries that were instrumental in the development of these catalysts, with particular emphasis on (PCy3)2Cl2RuCHPh and its derivatives. The discussion includes an analysis of trends in catalyst activity, a description of catalysts coordinated with N-heterocyclic carbene ligands, and an overview of ongoing work to improve the activity, stability, and selectivity of this family of L2X2RuCHR complexes.

http://web4.uwindsor.ca/users/j/jlichaa/reference.nsf/0/10ff8b04ff3a317885256d88005720f6/$FILE/acr-2001-34-18-metath-grubbs.pdf

The Development of L2X2RuCHR Olefin Metathesis Catalysts: An Organometallic Success Story

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3773%2820020503%2941%3A9%3C1460%3A%3AAID-ANIE11111460%3E3.0.CO%3B2-N

Dynamic covalent chemistry.

Recent advances in olefin metathesis and its application in organic synthesis

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

The fascinating story of olefin (or alkene) metathesis (eq
1) began almost five decades ago, when Anderson and
Merckling reported the first carbon-carbon double-bond
rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μeτάθeση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia.

https://core.ac.uk/download/pdf/4884240.pdf

Ruthenium-based heterocyclic carbene-coordinated olefin metathesis catalysts.

The reaction between Mo(C-t-Bu)(dme)Cl{sub 3} (dme = 1,2-dimethoxyethane) and Me{sub 3}SiNHAr (Ar = 2,6-diisopropylphenyl) yields Mo(C-t-Bu)(NHAr)Cl{sub 2}(dme) (1), which upon treatment with a catalytic amount of NEt{sub 3} is transformed into Mo(CH-t-Bu)(NAr)Cl{sub 2}(dme) (2). Complexes of the type Mo(CH-t-Bu)(NAr)(OR){sub 2} (OR = OCMe(CF{sub 3}){sub 2}, OCMe{sub 2}(CF{sub 3}), O-t-Bu, or OAr) have been prepared from 2. Complexes of the type Mo(C-t-Bu)(NHAr)(OR){sub 2} (OR = OCMe(CF{sub 3}){sub 2} or OAr) have been prepared from 1, but they cannot be transformed into Mo(CH-t-Bu)(NAr)(OR){sub 2} complexes. A precursor to imido alkylidene complexes that is related to 2 has been prepared by the sequence MoO{sub 2} {yields} MoO{sub 2}Cl{sub 2} {yields} Mo(NAr){sub 2}Cl{sub 2} {yields} Mo(NAr){sub 2}(CH{sub 2}R{prime}){sub 2} {yields} Mo(CHR{prime})(NAr)(OTf){sub 2}(dme) (R{prime} = t-Bu or CMe{sub 2}Ph; OTf = OSO{sub 2}CF{sub 3}). Mo(CH-t-Bu)(NAr)(OTf){sub 2}(dme) crystallizes in the space group P{anti 1} with a = 17.543 {angstrom}, b = 19.008 {angstrom}, c = 9.711 {angstrom}, {alpha} = 91.91{degree}, {beta} = 99.30{degree}, {gamma} = 87.27{degree}, Z = 4, M{sub r} = 729.60, V = 3,191.1 {angstrom}{sup 3}, {rho}(calcd) = 1.518 g cm{sup {minus}3}.

Synthesis of molybdenum imido alkylidene complexes and some reactions involving acyclic olefins

Well-characterized olefin metathesis catalysts that contain molybdenum

Reduction of molybdenum imido-alkylidene complexes in the presence of olefins to give molybdenum(IV) complexes

: Addition of 50-200 eq of norbornene to Mo(CH Superscript t Bu) (NAr) (O Superscript t Bu)2 yields living polymers, Mo?CH(C5H8)CHxCH Superscript t Bu (NAr)(O Superscript t Bu)2 (55% trans), that are stable for days in the absence of water and oxygen with essentially no isomerization of the double bonds in the chain. Addition of benzaldehyde cleaves off the polymer in a Wittig-like reaction to yield polynorbornenens with polydispersities in the range 1.04-1.11. Analogous reactions involving 50-200 eq of endo, endo-5,6-dicarbomethoxynorbornene yields homopolymers with polydispersities in the range 1.11-1.22. Block copolymers containing 50 eq of endo,endo-5,6-dicarbomethoxynorbornene and 200 eq of norbornene prepared by adding one or the other monomer first are virtually identical (polydispersities 1.06 and 1.09 with M sub n 57200 and 59900, respectively). These results suggest that the molybdenum catalyst will not react significantly with up to 100 eq of ester functionality during the time of a typical polymerization reaction (approx. 15 m at 25 degs.).

Low polydispersity homopolymers and block copolymers by ring opening of 5,6-dicarbomethoxynorbornene

Multiple metal-carbon bonds. 39. Preparation of molybdenum and tungsten neopentylidyne complexes of the type M(CCMe3)(O2CR)3, their reactions with acetylenes, and the x-ray structure of the .eta.3-cyclopropenyl complex W[C3(CMe3)Et2](O2CCH3)3

John S. Murdzek

Papers

Synthesis of molybdenum imido alkylidene complexes and some reactions involving acyclic olefins

Well-characterized olefin metathesis catalysts that contain molybdenum

Reduction of molybdenum imido-alkylidene complexes in the presence of olefins to give molybdenum(IV) complexes

Low polydispersity homopolymers and block copolymers by ring opening of 5,6-dicarbomethoxynorbornene

Multiple metal-carbon bonds. 39. Preparation of molybdenum and tungsten neopentylidyne complexes of the type M(CCMe3)(O2CR)3, their reactions with acetylenes, and the x-ray structure of the .eta.3-cyclopropenyl complex W[C3(CMe3)Et2](O2CCH3)3