Cyclobutadiene complexes preparation

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

Until 1992, the only ethynylated cyclobutadiene complexes pertinent in the literature were 22 - 24, prepared by Fritch and Vollhardt using [2 -i- 2]-cycloaddition of suitable polyynes over CpCo(CO)2 [24]. No alkynylated derivatives of 25, however, had been prepared. 

Thermal cyclization of alkynes with Fe(CO)5 proceeds predominantly with CO incorporation to afford (cyclopentadienone)Fe(CO)3 complexes, however small amounts of cyclobutadiene complexes can be isolated (see Section VI.B.)15. 1,6-FIeptadiyne and 1,7-octadiyne substrates 107 have been utilized to prepare bicyclo[3.3.0] and bicyclo[4.3.0] complexes 108 in excellent yield (equation 12)115, while 1,8-nonadiynes gave bicyclo [5.3.0] complexes in low yield. 

Bunz et al. explored the possibility of doping PPE chains covalently with small amounts of fluorescence-quenching cyclobutadiene complexes, in order to endow their optical properties to the base polymer, PPE [80]. Due to their extensive experience of cyclobutadiene complexes in polymer synthesis [81], the authors prepared several polymers PAE-CoCpl-5 (Table 4) containing different contents of CoCp complexes. The quantum yields were determined by simple comparison of the intensities of the emitted light to that of a standard... 

The cyclobutadiene complex 1 can be prepared in enantiomerically pure form. When the complex is reacted with an oxidizing agent and a compound capable of trapping cyclobutadienes, the products are racemic. When the reaction is carried only to partial completion, the recovered complex remains enantiomerically pure. Discuss the relevance of these results to the following questions In oxidative decomposition of cyclobutadiene complexes, is the cyclobutadiene liberated from the complex before or after it has reacted with the trapping reagent ... 

The double-addition products of type XVI are surprisingly stable. Complex 36 i, e.g., can be crystallized from CH2Cl2/MeOH in the presence of hydrochloric acid without substantial decomposition. The most striking property of 36i is the conversion into the cyclobutadiene complex 33 (R = Ph), which is more conveniently prepared from 28a and PhC2Ph at 150 °C. As already mentioned, the reaction proceeds by a stepwise mechanism through complexes XIV—XVI. This reaction offers a new, facile preparation of 33. The cyclo-dimerisation of alkynes other than diphenylacetylene could not been substantiated, however. 

Pentacarbonyliron(O) with butadiene gives the tricarbonyl derivative Fe(CO)3(C4H6). Cyclobutadienetricarbonyltron(O), Fe(CO)3(C4H4), is prepared by dehalogenation of m-3,4-dichlorobutene in the presence of Fe2(CO)9. This was the first cyclobutadiene complex to be reported in the hterature (see equation 30). ... 

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. 

Cyclobutadiene complexes are prepared by the method described in equation (8) some members of the series can also be synthesized by dimerization of an acetylene on an iron tricarbonyl fragment (eq (16)) [50]. 

Some highly substituted phospholium salts, e.g., (180), and phosphole oxides have been prepared by a ring-expansion reaction of aluminium chloride-cyclobutadiene complexes with dichlorophosphines. The [4 - - 2] dimer (181)... 

Although a large number of aryne-metal complexes have been prepared and characterized, and they have an interesting chemistry of their own, that chemistry is in general quite different from the types of reactions usually associated with arynes. One might have hoped that aryne-metal complexes could act as shelf-stable storehouses for arynes, that could then be released on demand to do their usual chemistry (as in the manner, for example, of cyclobutadiene complexes), but to date this has not proved to be the case. The synthesis and chemistry of aryne-metal complexes has been reviewed, and the interested reader is also provided with a few references to work that has appeared since those reviews. ... 

As iron forms cyclobutadiene complexes preferentially in the (0) oxidation state, the best starting materials for these complexes are Fe(0) compounds such as the carbonyls. Cobalt, however, tends to form Co(I) (also cyclobutadiene complexes. The best known one (cyclopentadienyl)(tetra-phenylcyclobutadiene)cobalt (XXVIII), has been prepared from cyclo-pentadienylcobalt(I) derivatives and also from cobaltocene, a cobalt(II)... 

While sodium cyclopentadienide attacks tetramethylcyclobutadiene-nickel chloride both at carbon and nickel (Section VI, F), the discovery of a novel cyclopentadienylation reaction which is in effect a ligand-transfer reaction involving attack at the metal only has allowed other types of cyclobutadiene complexes to be prepared. Thus on reaction of tetra-phenylcyclobutadienenickel and -palladium bromides (LXXXVI) with cyclopentadienyliron dicarbonyl bromide, the paramagnetic (cyclopentadi-enyl)(tetraphenylcyclobutadiene)nickel and palladium tetrabromoferrates (LXXXVII M=Ni, Pd) are obtained 64, <55). 

The C-C distances in cyclobutadiene complexes are the same generally the substituents are located away from the metal with respect to the plane of the ligand. Exceptions are encountered in some complexes possessing substituents which force their tilting toward the metal. The dinuclear compound which is prepared by reaction (8.3) may... 

The most important methods for the preparation of cyclobutadiene complexes are oxidative addition reactions of dihalogenocyclobutenes to low-valent transition metal compounds [equations (8.27) and (8.28)], as well as reactions of acetylenes with coordination compounds. 

Iron(III) and Ce(IV) compounds readily oxidize cyclobutadiene complexes. In this way it is possible to obtain many organic compounds which are otherwise difficult to prepare. 

The first cyclobutadiene complexes were obtained either from alkynes (p. 246) or by reaction of cis-dichlorocyclobutenes with metal carbonyls. Many more derivatives were then prepared by transfer of the cyclobutadiene ligand from one transition metal to another, especially from palladium in [(Ph4C4)PdCl2]2. In their complexes cyclobutadienes adopt a planar square structure. 

In support of this mechanism is the fact that stable cobaltacyclopentadienes 11.23, with an additional PPhs ligand can be prepared (Scheme 11.12). These cobaltacycles undergo further reactions with alkynes, alkenes, azides and diazo compounds to give a variety of cyclic products. Cyclobutadiene complexes 11.26, cyclopentadienone complexes 11.27 and arenes 11.28 can also be formed (Scheme 11.13). ... 

Variously substituted tricarbonyliron-cyclobutadiene complexes are readily prepared (Fitzpatrick et ah, 1965 Roberts et al., 1969 Agar et al., 1974) and cyclobutadiene complexes with other transition metals are known (Maitlis, 1966), but few of these have been used in organic synthesis. 

Endocyclic dienes ranging in ring size from four to seven have also been used in the intramolecular Diels-Alder reaction. Cyclobutadienes are prepared from the corresponding metal complexes [37]. Cyclopentadienes are most readily prepared by alkylation of cyclopentadienyl anion [38, 39]. They are also available by Michael addition to fulvene derivatives and by aminal exchange [40, 41]. 

The tetrachlorodiester (431), prepared from the photoaddition product of dichloro-ethylene and dichloromaleic anhydride, reacts directly with sodium tetracarbonyl-ferrate to give the functionalized cyclobutadiene complex (432) in 30—40% yield. The vicinal diester groups have been modified to give a wide variety of 1,2-disubsti-tuted cyclo butadiene complexes. 

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