Cyclobutene catalyst

Later, J0rgensen and co-workers [71] and Vicario and co-workers [72] simultaneously reported a cooperative dienamine/hydrogen-bonding catalytic system for the enantioselective formal [2+2] cycloaddition of enals to nitroolefins catalyzed by the bifimctionalized cyclobutene catalyst 116 (Scheme 6.27). 

A cyclobutene ROCM sequence was also used in a synthesis of racemic sporochnol (410), a naturally occurring feeding deterrent toward herbivorous fish (Scheme 80) [170]. Exposing cyclobutene 406 (0.01 M in boiling 1,2-dichloroethane) in the presence of ethylene to second-generation catalyst C (8 mol%) led to 1,5-diene 407 in 73% yield, along with 9% of the homodimer derived from 407 by involving the less hindered double bond. Site-selective hy-... 

Scheme 10.2 gives some examples of ene and carbonyl-ene reactions. Entries 1 and 2 are thermal ene reactions. Entries 3 to 7 are intermolecular ene and carbonyl-ene reactions involving Lewis acid catalysts. Entry 3 is interesting in that it exhibits a significant preference for the terminal double bond. Entry 4 demonstrates the reactivity of methyl propynoate as an enophile. Nonterminal alkenes tend to give cyclobutenes with this reagent combination. The reaction in Entry 5 uses an acetal as the reactant, with an oxonium ion being the electrophilic intermediate. 

Polyether-type structures such as 6/3-38 are frequently found in bioactive compounds (e. g., maitansine). Nicolaou and coworkers [239] have developed a new, efficient approach to these compounds, which is based on a domino ROM/RCM using the second-generation Grubbs catalyst 6/3-15. Thus, the cyclobutene derivative 6/3-37 could be transformed into 6/3-38 in 80% yield (Scheme 6/3.10). 

Tributylstannyl)-3-cyclobutene-1,2-diones and 4-methyl-3-(tributylstan-nyl)-3-cyclobutene-l,2-dione 2-ethylene acetals undergo the palladium/copper-catalyzed cross coupling with acyl halides, and palladium-catalyzed carbon-ylative cross coupling with aryl/heteroaryl iodides [45]. The coupling reaction of alkenyl (phenyl )iodonium triflates is also performed by a palladium/copper catalyst [46],... 

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. 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

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... 

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

The optimum catalyst for a given reaction depends primarily on (a) the energetics of the reaction and (b) the functional groups present in the substrate. If, for instance, a strained cycloalkene such as norbomene or cyclobutene is to be polymerized, a catalyst of low activity will be sufficient to attain acceptable reaction rates. RCM... 

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)). 

Of the cyclobutene derivatives 76-87 all except 87 undergo clean ROMP. The polymer of 87, obtained in low yield using WClg/Me4Sn as catalyst, is a black insoluble powder of reduced chlorine content, caused by loss of HC1 and development of conjugation335. 

The behaviour of 3-methylcyclobutene (78) with various catalysts resembles that of cyclobutene. With 7 as initiator the propagating species gives a 1II NMR spectrum... 

The intermolecular [2 + 2]-cycloaddition of alkenes and alkynes utilizing an iron complex as a catalyst was reported by Rosenblum and Scheck [48]. The application of the [CpFe(CO)2]BF4 complex (Scheme 9.21) gave the desired cyclobutene derivatives 29 in up to 53% yield. 

The epoxidation of cyclobutene (1) to cyclobutane oxide (2) with nitrous oxide (Figure 2.20) was studied by Useless Polymers Inc. as a possible step in the route to 1,2-cyclobutadiol. Researchers Hannah and John chose two catalysts for kinetic studies, catalyst A and catalyst B. 

Hannah ran a reaction with catalyst A, using a 25 m solution of cyclobutene, and got the following results ... 

Repeating the reaction with catalyst B, using a 15 m solution of cyclobutene, he got ... 

Cycloaddition of aza[2]ladderane 107 with DMAD in the presence of a ruthenium catalyst RuH2CO(PAr3)3 (Ar = Ph, />-FC6H4) yielded the exo-fuscA aza[3]ladderane diester 108, which underwent further cycloaddition with cyclobutadiene through the dienophilic cyclobutene Jt-bonds of 108 to yield a mixture of stereoisomers 109 and 110 in a ratio of 3 2 (Scheme 5). The two isomers were separated by HPLC. The reaction of 107 with cyclopentadiene has also been studied both experimentally and theoretically <1997SL38>. 

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