Cyclobutenes metathesis

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Highly strained cycloolefins, such as norbornene and cyclobutene, have been reported (34) to undergo slow metathesis using (CO)5W=CPh2. 

This process is quite unexpected for another reason. The cyclobutene ring is highly strained, making this monomer one of the most easily polymerized of all the cycloolefins. Thus, the variety of catalysts effective for cyclobutene polymerization is much broader than that effective for metathesis of low-strained cycloolefins and acyclic olefins (73). Therefore, the recovery of monomeric cyclobutene rather than its respective polymer is remarkable and indicates the lack of substantial metathesis activity in the above retrocarbenation system. 

In 1995 the first examples of ring-opening cross-metathesis reactions for the preparation of functionalised monomeric products using the Grubbs ruthenium vinylalkylidene catalyst 4 were published by Snapper and co-workers [47]. Reaction of a variety of symmetrical cyclobutenes with simple terminal alkenes... 

Ring-opening cross-metathesis of unsymmetrical cyclobutenes was also accomplished, although an extra complication arises due to the possible formation of two regioisomers (for example 39 and 40) of the desired cross-metathesis product (for example Eq. 32). 

Reaction of this same cyclobutene substrate 48 with a terminal alkene (TB-DMS protected pent-4-en-l-ol) gave a good yield (84%) of the cross-metathesis products, but with very little regioselectivity (3 2 mixture of regioisomers). 

As expected, the metathesis polymerization of more strained cycloalkenes, such as cyclobutene, occurs more rapidly than less strained structures such as cyclopentene. 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

Ring expansion of vinylcyclopro-penes and cyclobutenes 8-34 Ring expansion of vinylcycloal-kanes cyclization of diynes 8-39 Metathesis of dienes 8-40 Metal-ion-catalyzed o-bond rearrangements... 

Cycloisomerization or metathesis also occurs, which can be understood as the formation of cyclobutene 326 by reductive elimination of 321. The metathesis product 327 is formed by isomerization of 326. The metatheses involving metal-carbene complexes are discussed in Section 7.2.6. They are closely related, but somewhat different from the metathesis explained here. Balance between the ene and the metathesis reactions seems to be delicate. 

Reaction of the carbene complex 148 with alkyne affords vinylcarbene 150 via metallacyclobutene 149. In the intramolecular reaction of enyne 152, catalysed by carbene complex 151, the triple bond is converted to vinylcarbene 153 which then reacts with the double bond to give the conjugated diene 154. Generation of 154 is expected by the formation and cleavage of cyclobutene 155 as a hypothetical intermediate. Based on this reaction, Ru-catalysed intramolecular metathesis of enyne 156 gave the N-containing cyclic diene 157, from which (—)-stemoamide (158) was synthesiszed. The reaction can be understood by assuming the formation of the hypothetical cyclobutene 159 from 156 [52],... 

There exists a wide variety of cyclic olefins capable of being polymerised via a ring-opening metathesis reaction From high-strained cycloolefins (cyclobutene and homologues, norbornene and homologues) up to low-strained (cyclopentene) and unstrained cycloolefins (cycloheptene, cyclooctene) [45]. 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

The policyclic triene monomer undergoes metathesis polymerisation exclusively by the cyclobutene double bond under mild conditions (in toluene solution at 20 °C) to give a soluble precursor polymer. This polyacetylene precursor can be purified and characterised prior to its conversion at elevated temperature, via thermally initiated symmetry-allowed elimination (retro Diels-Alder reaction), to polyacetylene (a heat treatment of the product also results in isomerisation of the initial cis form to a more stable turns form) [150],... 

Arrange the following monomers according to their polymerisability with metathesis catalysts cyclobutene, cyclopentene, cyclooctene, norbornene, dicyclopenta-diene. 

The ROM-CM of norbornenes, oxanorbornenes and cyclobutenes are among the most efficient and atom-economic of the metathesis reactions [75,76]. The proposed catalytic cycle for this transformation is shown in Fig. 5. 

The Cope rearrangement was used in the total synthesis of (-)-asterisca-nolide (14), a novel sesquiterpene natural product4 (Scheme 1.4e). Ring-opening metathesis of the cyclobutene 15 with ethylene in the presence of the ruthenium catalyst 165 proceeded smoothly to provide the cyclooctadiene 18 via Cope rearrangement of the intermediate dialkenyl cyclobutane (17). 

Cyclobutenes possessing an angular O-functionality, obtained from a Lewis acid-mediated [2+2] cycloaddition of cyclic silyl enol ethers to ethyl propynoate and subsequent reduction and butenylation, undergo a ring-opening metathesis that produces a substituted dihydropyran that forms part of a c -diene. After desilylation, an oxy-Cope rearrangement leads to the fused tetrahydropyran 4 <03JA14901>. 

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