Alkene metathesis, catalytic selective

A first evaluation of complex 71a by Blechert et al. revealed that its catalytic activity differs significantly from that of the monophosphine complex 56d [49b]. In particular, 71a appears to have a much stronger tendency to promote cross metathesis rather than RCM. Follow-up studies by the same group demonstrate that 71a allows the cross metathesis of electron-deficient alkenes with excellent yields and chemoselectivities [50]. For instance, alkene 72 undergoes selective cross metathesis with 3,3,3-trifluoropropene to give 73 in excellent yield and selectivity. Precatalyst 56d, under identical conditions, furnishes a mixture of 73 and the homodimer of 72 (Scheme 17) [50a]. While 56d was found to be active in the cross metathesis involving acrylates, it failed with acrylonitrile [51]. With 71a, this problem can be overcome, as illustrated for the conversion of 72—>74 (Scheme 17) [50b]. 

Alkene metathesis has grown from a niche technique to a common component of the synthetic organic chemistry toolbox, driven in part by the development of more active catalyst systems, or those optimized for particular purposes. While the range of synthetic chemistry achieved has been exciting, the effects of structure on reactivity have not always been particularly clear, and rarely quantified. Understanding these relationships is important when designing new catalysts, reactions, and syntheses. Here, we examine what is known about the effect of structure on reactivity from two perspectives the catalyst, and the substrate. The initiation of the precatalyst determines the rate at which active catalyst enters the catalytic cycle the rate and selectivity of the alkene metathesis reaction is dependent on how the substrate and active catalyst Interact. The tools deployed in modern studies of mechanism and structure/activity relationships in alkene metathesis are discussed. 

We have witnessed the remarkable advance of selective alkene metathesis reactions over the last a few years. Many synthetic chemists have utilized this reactiOTi as a very practical, versatile, and selective synthetic tool to prepare complex molecules including natural products. Selective alkene metathesis has helped to elevate the art and science of natural product total synthesis to its present high level. However, many critical discoveries in catalytic alkene metathesis, particularly the development of more effective catalysts that are easily obtained and able to provide excellent selectivities, remain to be made. It has been delightful to review this field and highlight some of the most significant and exciting examples of recent applications of selective alkene metathesis in the total synthesis of complex natural... 

Olefin metathesis enables the catalytic formation of C=C double bonds under mild conditions.1 After the development of well-defined catalysts,1 2 selective cross-couplings between functionalized terminal alkenes (CM) have been noted.2 A general problem... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

The makeup of Mo-based complexes, represented by 1 [3], offers an attractive opportunity for the design, synthesis, and development of effective chiral metathesis catalysts. This claim is based on several factors 1) Mo-based catalysts such as 1 possess a modular structure [4] involving imido and alkoxide moieties that do not disassociate from the metal center in the course of the catalytic cycle. Any structural alteration of these ligands may thus lead to a notable effect on the reaction outcome and could be employed to control selectivity and reactivity. 2) Alkoxide moieties offer an excellent opportunity for incorporation of chirality within the catalyst structure through installment of non-racemic tethered chiral bis(hydroxy) ligands. 3) Mo-based complexes provide appreciable levels of activity and may be utilized to prepare highly substituted alkenes. 

On the other hand, although well-defined or in-situ initiated metallacarbenes are inactive for selfmetathesis of vinyl-substituted silanes and siloxanes, we revealed recently a high catalytic activity of Grubbs catalyst in cross-metathesis of vinyltrialkoxysilanes and vinyltrisiloxanes with styrene, 1-alkenes and selected allyl ethers and other derivatives [10-12]. 

The metallo-square hauvin s mechanism is still operating, as demonstrated by the isolation of metallacyclobutadiene complexes formed by cycloaddition of alkynes to alkylidyne complexes. The metallacyclobutadiene complexes themselves can also serve as alkyne metathesis catalysts confirming their intermediacy in the catalytic reactions starting from the alkylidyne complexes. Interestingly, they do not react readily with alkenes, rendering alkyne metathesis selective in the presence of olefmic bonds. ... 

Thus, the treatment of metathesis precursor 142 with alkene 143 in the presence of catalyst [Ru]-ll (10mol%) furnished desired compound 144 in 61% yield with Z-selectivity Z/E = 5 1). Carbene 147 is proven to be a cata-lytically viable intermediate in the catalytic cycle. 

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