Cycloheptadiene complexes

Cycloheptadiene complexes are useful synthetic building blocks in the preparation of complex organic and natural products. A variety of substituted cycloheptadiene complexes can be prepared via catalytic hydrogenation of the respective cycloheptatriene-Fe(CO)3 complex. Subsequent oxidation liberates the iron carbonyl moiety affording the uncomplexed substituted cycloheptadiene complex (Scheme 34). 

Opening of Cyclopropane Rings. Ring enlargement of compounds such as bicyclo[4.1.0]hept-2-ene (36) or its derivatives (+)-2-carene (37) or (+)-3-carene (38) by carbonylation results in the formation of bicycloheptenones or cycloheptadiene complexes, depending on the reaction conditions. ... 

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

A number of zinc-bound bidentate bis(thioether) donor ligands were characterized as distorted tetrahedral complexes of zinc dichloride. The complexes 2,3-bis(methylthio)hexane,l,2-bis (methylthio) cyclohexane, and m-[5,6-bis(methylthio)-l,3-cycloheptadiene showed a variation in S—Zn—S angles.539... 

Metal-mediated and -catalyzed [3 + 2 + 2]-higher-order cycloaddition reactions have also proved to be viable and mechanistically novel methods for the synthesis of seven-membered rings. The reported [3 + 2 + 2]-cycloadditions of allyliridium (Equation (30)),139 -allylcobalt (Scheme 47),140 and allylmanganese (Equation (31 ))141 complexes with alkynes involve the reaction of preformed allylmetal complexes with two separate alkynes, leading to a cycloheptadiene-metal complex. 

If 1,3-butadienes are cyclopropanated by use of vinylcarbene complexes, the divinylcyclopropanes which result can rearrange to cycloheptadienes [71,241,384 -387] (Figure 2.37). 

Heteroatom-substituted vinylcarbene complexes react particularly well with donor-substituted 1,3-butadienes to yield the corresponding cycloheptadienes [264]. Some of these reactions proceed at room temperature. The intermediate divinylcyclopropanes have occasionally been isolated [264]. As indicated in Figure 2.37, for donor-substituted dienes in particular the formation of zwitterionic intermediates... 

Fig. 2.37, Formation of cycloheptadienes by reaction of vinylcarbene complexes with 1,3-dienes.

Non-heteroatom-substituted vinylcarbene complexes are readily available from alkynes and Fischer-type carbene complexes. These intermediates can undergo the inter- or intramolecular cyclopropanation reactions of non-activated alkenes. Cyclopropanation of 1,3-butadienes with these intermediates also leads to the formation of cycloheptadienes (Entry 4, Table 2.24). 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

CM of propynyl benzoate and cyclopentene in excess was carried out by the same group (Equation (15)). Ring opening of cyclopentene with ruthenium carbene complex Ig followed by intramolecular RCM takes place to give cycloheptadiene in good yield." ... 

A combinatorial approach was applied to evaluate various catalysts for the animation of 1,3-dienes.594 Complexes formed from [(T 3-C3H5)PdCl]2 and PPh3 were the most active to induce the reaction of a broad range of primary and secondary ary-laminesand 1,3-cyclohexadiene, 1,3-cycloheptadiene, or 2,3-dimetyl-1,3-butadiene to give allylamines. The enantioselective version of the transformation is also very effective ... 

Given that nucleophile addition to (dienyl)Fe(CO)3 complexes proceeds stereospecifically trans to the metal, the question arises as to whether this can be used to control relative stereochemistry during multiple functionalization of cyclohexadienes and cycloheptadienes. A hypothetical example is shown in Scheme 29, where nucleophile addition is followed by a second hydride abstraction, or its equivalent, to generate a substituted dienyl complex. Addition of a second nucleophile, assuming steieocontrol from the metal, would generate a disubstituted derivative with defined relative stereochemistry. 

Double functionalization appears to be quite general, since a variety of 5-substituted cycloheptadiene-iron complexes are readily converted to 6-substituted cycloheptadienyliron systems, which in turn undergo regio- and stereo-controlled nucleophile addition.23... 

The nickel-catalysed 3 + 1 + 1-cycloaddition of alkenyl Fischer carbene complexes with methylenecyclopropanes produced the methylenecyclopentanone in high yields.169 The nickel-catalysed three-component 3 + 2 + 2-cocyclization of ethyl cyclopropylide-neacetate with two alkynes gave cycloheptadienes in good yields and high chemo- and regio-selectivity.170... 

Aclohcpuidiciiyl complexes, with vanadium, 5, 50 Cycloheptadien-l-yne, in Pd(0) it-complexes, 8, 355 tj6-Cyclohepta-l,3,5-triene complex, as catalyst precursor,... 

Very stable diene complexes can be prepared by the reaction of conjugated dienes with Fe(CO)5, Fe2(CO)9 or Fe3(CO)12 by heating or under irradiation. For synthetic purposes, the complexes of acyclic 1,3-dienes, 1,3-cyclohexadienes and 1,3-cycloheptadienes are useful. 

Loss of the cycloheptadiene ring occurs preferentially, giving as the base peak C7H7Cr+, and no peak is observed for C7Hl0Cr+ (75). The mass spectra of the hexafluorobutyne complexes [(CF3)2C2]3MNCMe (M = MoW), show that the acetonitrile ligand is readily eliminated from the molecular ion, and that transfer of fluorine atoms from the ligand to the metal occurs. 

P(CH3)3, P(OCH3)3], For each of the [Cr(CO)3(j/4 CH-diene)] complexes 39g, 37n, and 37o two enantiomers are expected. As a consequence of the C—H—Cr bridge from C-5 or C-6 of the 1,3-cyclohexadiene, and from C-5 or C-7 of the 1,3-cycloheptadiene, both ligands lose their inherent mirror planes. The situation is somewhat different with the 2,4-dimethyl-1,3-pentadiene complex (37n). Here only the 4Z methyl group is able to form the C—H—Cr bridge. Due to the two coordination modes of the diene, R or S, a pair of enantiomers is present for 37n. Finally, 37r forms two diastereomeric pairs of enantiomers, because the C—H—Cr bridge can be formed from C-5 or C-6 and the diene can be coordinated in the R or S mode. 

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