Cyclobutadiene complexes reduction

Trimerization of 1-alkynes to substituted cyclobutadienes occurs in reactions of RhCl(l-alaninate)Cp with HC CR (R = Ph, tol), which afford Rh -C4HR2 (C=CR) Cp (310) possibly via intermediate dialkynylrhodium(III) complexes. Reductive coupling to an /j -diyne complex, which coordinates the third molecule of alkyne, is followed by further coupling to the rhodacyclopentadiene and reductive elimination of the cyclobutadiene (Scheme 72). ... 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

Intermediates with one coordinated alkene are often seen (e.g., Eq. 6.49), but the bis-alkcne species is probably the immediate precursor of the coupled product." The products from alkynes are often stable and are known as metalloles (Eq. 6.51) but they can also reductively eliminate to give cyclobutadiene complexes (Eq. 6.47). 

A rhodium-catalyzed cycloisomerization reaction of triyne 137 to 141 involves cleavage of the C=C triple bond (Scheme 7.49) [68]. The following reaction pathway is proposed initially, oxidative cyclization produces the rhodacycle 138, which then undergoes reductive elimination. The rhodium cyclobutadiene complex 139 is thus generated, and then undergoes oxidative addition to produce the rhodacycle 140. This isomerization from 138 to 140 would reduce the steric congestion of the heUcal structure. Subsequently, a cycloaddition reaction between the rhodacycle and the pendant alkyne moiety takes place to afford 141. 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Reduction of (312) has been found to afford the dimer (313) which upon heating rearranged to yield the unprecedented di(benzopentalene) complex (314). The regio- and stereo-specificity of the conversion (313) into (314) implies a metal-mediated pathway for the process (see Scheme 100). The first observable cis-bis(alkyne)cyclobutadiene rearrangement [see (315) to (316)] has been reported. 

Studies of rc-complexcs containing ligands other than the Cp family of ligands have also appeared in the literature in 20 08.23 25 Of particular note is the synthesis of Mg and Ca complexes of the tetrakis(trimethylsilyl)cyclo-butadiene dianion, [Mg(thf)3][(Me3Si)4C4] (19) and [Ca(thf)J[(Me3Si)4C4] (20), via the reduction of tetrakis(trimethylsilyl)cyclobutadiene [(Me3Si)4C4] with metallic Mg or Ca. The structure of the half-sandwich 19 shows the presence of an -bonded [(Me3Si)4C4]2 671-dianion to the Mg centre.23... 

Closely related to the dimerization of biphenylene (36) to tetraphen-ylene (37, Scheme XV) is the dimerization of an aryl-substituted cyclobutadiene to octadienes or cyclooctadienes by way of nickel complexes. A useful source of the cyclobutadiene group is its air-stable complex with NiBr2. Reduction of this complex with tert-butyllithium (electron-transfer agent) gives the tetraphenylcyclobutadiene-nickel(0)-triphenylphosphine complex (38), which isomerizes to the nickelole (39). The dimerization of 39 leads to 40, whose protonation yields the octadiene. Alternatively, at higher temperatures, 40 can extrude Ni(0) to produce 41 (26, Scheme XVI). 

Lithium aluminum hydride reduction established that the organic ligand was an acetylenic tail-to-tail dimer, while bromination gave cyclopentanones, cyclopentenones, and cyclopentadienones with incorporation of CO. On this basis the authors proposed the complexes to have cyclobutadienes as ligands, e.g., (XCI). 

Osmocene is a photoactive compound. Photolysis of osmocene in n-hexane led to metallic osmium by reductive elimination. In the LF excited triplet state, osmocene undergoes a distortion by bending, which facilitates the transfer of a CH group between the cyclopentadienyl ligands. The resulting intermediate is an osmium complex with benzene and cyclobutadiene ligands, that decomposes to osmium metal, benzene and cyclobutadiene. ... 

Liberation of excess cyclobutadiene from its iron complex in the presence of acetylenes leads to a double addition, affording the previously unknown tetracyclic system (831 R = Ph or OMe). Trapping with phenyl vinyl ketone affords (832X nhich has been converted by reduction, alkylation, and photolysis into the new tri-cyclo[3,1,1,0 ]heptanols (833). ... 

The ruthenium example probably involves oxidative addition of the di-halide to two Ru(CO)3 fragments derived from the photolysis of the cluster then the metals probably disproportionate, so that one becomes the observed product and the other carries away the halides in the form of undefined Ru(II) halo complexes. The reaction of Eq. 5.32 probably goes by an oxidative coupling to give 5.20, which is very favorable for alkynes, followed by a reductive elimination of the cyclobutadiene ligand. 

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