Cyclobutadiene formation

Although numerous alkyne insertion products have been reported for Mo(II) and W(II), simple dimerization to form cyclobutadiene or tri-merization to form an arene ligand is rare. One brief report of cyclobutadiene formation from a bisalkyne complex (206) has been followed by a full paper which suggests that an rj2-vinyl complex may be the precursor to CpM(S2CNR2) [ 174-C4(CF3)4] (207). 

Thus, in tetrahydrofuran solution, indole 1093 (R =(CH2)2NHAc) was converted to tetracyclic lactam 1094 (in 46% yield as a single diastereomer) in the presence of CpCo(C2H4)2 and acetylene gas (the experiment was performed on a submillimolar scale). The primary by-products of this reaction, isolated in 20-30% yield, are the cis- and trans- (3 1) cinnamic amides 1095. Presumably, these arise from the cyclization of the terminal acetylene moiety of indole 1093 with two acetylene molecules and subsequent equilibration. Attempts to scale-up the procedure to 0.5 mmol or more of indole 1093 led to a significant decrease in yield (17-24%) of tetracycle 1094. The main product of the reaction was amide 1095, isolated in 50-60% yield as a mixture of cis- and /ra j-isomers. Utilizing the reactivity of CpCo(C2H4)2 at low temperatures to minimize cyclobutadiene formation, reaction of unsubstituted enynoylindole 1093 (R = H) with bis(trimethylsilyl)acetylene (R = TMS), which is resistant to... 

This type of Dewar compound was formed from a cyelobutadiene derivative 142). This process may be regarded as the retroreaction of cyclobutadiene formation (133). 

On irradiation in butane at -15°C, pyridine is converted into Dewar pyridine 96 (half-life 2.5 min at 25°C). Its structure has been spectroscopically confirmed and it was chemically characterized by reduction with NaBH4 to give the bicyclic azetidine 97, as well as by hydrolysis to the 5-amino-2,4-pentadienal 99 (via the hemiaminal 98). Matrix photolysis of pyridine at 8K leads to acetylene and HCN as the result of a [2+2] cycloreversion of the Dewar pyridine 96, resulting in cyclobutadiene formation. 

Irradiation of tricarbonyl(r(4-cyclobutadienc)iron(0) with ethyl 1//-azepine-l-carboxylate results in the formation of the novel sandwich compound (t 4-cyclobutadiene)[l-(ethoxycar-bonyl)-1 //-azepine]iron(0) (27).223... 

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]. 

Attempts to use in the alkynylation reaction not tin-substituted alkynes, but to couple 26 directly under the conditions developed by Heck, Cassar, Sono-gashira, and Hagihara, surprisingly enough gave rise to the formation of the corresponding amino-substituted cyclobutadiene complex 30 in good yields. 

Hogeveen and co-workers <82JOC1909 83JOC4275> also reported the synthesis of tricyclic sulfinamides by the reaction of cyclobutadiene aluminum halide o complexes (65 or 66) with 53 at low temperature. Treatment of 65 or 66 with 53 at -60 °C resulted in the formation of 67 or 68 in 52% and 55% yields, respectively (Scheme 18). 

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

The formation of cyclobutadiene derivatives from zirconacydopentadienes can be accomplished as follows (Eq. 2.76) [55]. 

Monoiodination of a zirconacyclopentadiene with one equivalent of iodine followed by the addition of one equivalent of CuCl gives the dimer of the cyclobutadiene and the Diels—Alder product in the presence of methyl maleate. This indicates the formation of a l-iodo-l,3-dienyl copper compound and the subsequent elimination of Cul to give a cyclobutadiene equivalent. Direct reductive elimination of zirconacydopentadienes affording cyclobutadienes has not yet been observed. 

Let us return to the thermal decomposition of Fe(CO)(l,3-C4H6)2. Once the calibration constant is known, the enthalpy of the net process 9.10 can be calculated as the product of s and the area (A + B). The next step is to correct this value to 298.15 K by using heat capacity data. This exercise is, however, complicated by the cyclobutadiene polymerization. Brown et al. analyzed the reaction products by mass spectrometry and found several oligomers, in particular the dimer (C4H6)2 and the trimer (C4H6)3 [163]. With such a mixture, it is difficult to ascribe the observed enthalpy change to a well-defined chemical reaction. This is discussed in the paper by Brown and colleagues, who were nevertheless able to recommend a value for the standard enthalpy of formation of the iron-olefin... 

A final, related example is the formation of tetrahedranes (84), from irradiation of tert-butyl substituted cyclobutadienes such as 83 (equation 31), a reaction which has been... 

Cobalt carbonyl complexes react with 1,3-diynes to give a variety of complexes in which two molecules of diyne have coupled to form y-cyclobutadiene ligands slightly different conditions result in formation of cluster complexes (see Section VILE.2). In the mixture of complexes obtained from the reaction... 

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