Cyclobutadiene, preparation

Colourless liquid with alkenic properties. Many substituted derivatives are known, the preferred method of preparation being the addition of an alkyne to a cyclobutadiene. 

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

A number of alkyl-substituted cyclobutadienes have been prepared by related methods Increasing alkyl substitution enhances the stability of the compounds. The tetra-/-butyl derivative is stable up to at least 150°C but is very reactive toward oxygen. This reactivity reflects the high energy of the HOMO. The chemical behavior of the cyclobutadienes as a group is in excellent accord with that expected from the theoretical picture of the structure of these compounds. 

Cyclo butadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at —78 °C, however, cyclobutadiene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. 

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... 

In the bisdecarboxylation of the cyclobutenedicarboxylic acid 52, products are obtained whose formation possibly involves a cyclobutadiene intermediate [322], A case of a 1,3-bisdecarboxylation has been reported in the preparation of a bicyclobutane (Table 11, No. 26). An elimination, that involves the cleavage of an carbon-oxygen bond after the decarboxylation, has been observed with the carboxylic acid 53 (Eq. 33) [282]. 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (52). Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4 + 2. There is a long history of attempts to prepare this... 

Cyclobutadiene owes its observed instability much to the kinetic property. Cyclobutadiene dimerizes in the argon matrix above 35 K [69] and only exists for 2-10 ms under low pressure [70, 71], However, cyclobutadiene is stabilized by bulky substituents (Scheme 30). The ferf-butyl derivative was quantitatively prepared even at a high temperature (130 °C) [72]. Yellow crystals of the cyclobutadiene fused by two seven-membered rings did not decompose below 240 °C [73]. 

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. 

The same procedure was later effectively used by Masamune1Sb 18 to prepare tert-butylated cyclobutadienes. Regitz19 widely employed the Masamune cyclopmpenylcarbene mute in his studies on the properties of the tert-butylester of tri-rert-butylcyclobutadiene carboxylic acid. We also used several differently... 

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. 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

Heteroatom-containing (cyclobutadiene)Co complexes (e.g. 237, 238 and 239) have been prepared by the reaction of heteroatom containing cobalt precursors with dipheny-lacetylene or by the reaction of cobalt precursors with phospha-alkynes228. 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

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... 

One question remained to be resolved regarding the structure of cyclobutadiene. Chapman reported in 1973 that in an attempt to separately prepare the two dideuterio cyclobutadienes ([1,2- H2]-1 and [1,4- H2]-1) from the two dideuterio 2-pyrones ([3,6- H2]-2 and [5,6- H2]-2), both yielded the same IR spectrum. Of course if cyclobutadiene were square only a single IR spectrum would have been expected, while if it were rectangular one would have expected different IR spectra for the two dideuterio cyclobutadienes. The IR spectra of the two dideuterio cyclobutadienes were computed, and indeed they were found to be quite different from each other. When a 1 1 mixture of the two calculated spectra was compared with Chapman s spectrum (see Figure 8) with inclusion of additional bands later observed by Michl, it was immediately obvious that Chapman... 

Some years after the successful isolation and characterization of cyclobutadiene, another C4H4 isomer, methylenecyclopropene (3), was prepared by Billups and... 

Organic ligands with a cyclic array of four carbon atoms have been of particular interest in connection with the chemistry of cyclobutadiene. Organometallic compounds containing cyclobutadiene as a ligand were first prepared in 1965.183 184 185 186 187 The carbocyclic ring... 

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 ... 

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