Correspondence cyclobutadiene

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

The reaction of cyclooctyne (14) with NiBr2 and Nil2 gave the corresponding cyclobutadiene-complexes in low yield, while with Ni(CO)4 the nickel(O) cyclopentadienone complex (76) was formed, whose thermal decomposition yielded the cyclopentadienone (77). In this case again a cyclooctyne Ni(0) complex (78) was postulated as an intermediate, but could not be isolated due to its instability l93). 

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. 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

Different polystyrene-bound biscyclopentadienyl complexes have been prepared. One example is shown as 69 in Eq. 5-9 [180]. Treatment of soluble copolymers of styrene and methacrylic acid with dichlorotitanocene resulted in soluble titanium-containing copolymers 70 (Eq. 5-10] [181]. Finally polymeric organometallic mesogens 71 with [l,3-(diethynyl)cyclobutadiene]cyclopentadi-enyl Co(II) moieties were prepared by Heck coupling of a corresponding cyclobutadiene monomer with 2,5-diiodothiophene [182],... 

Txi-tert-hvXy trimethylsilyl tetrahedrane " and tetra-trimethylsilyltetrahedrane have also been prepared. The former gives the cyclobutadiene at 160°C, and the latter is stable up to 300°C. Interestingly, B3LYP calculations indicate that the latter tetrahedrane is 9 kcal/mol more stable than the corresponding cyclobutadiene. These same calculations provide an energy difference of 24.3 kcal/mol in the parent case favoring the cyclobutadiene which is close to the G2 value so the value with the tetra TMS derivative is probably believable. 

Instances in which cyclobutadiene complexes are the major products from the reactions of acetylene complexes with additional alkyne are uncommon. These generally have been found to be significant products with sterically hindered alkynes and with palladium and platinum metals. For example, phenyl tert-butyl acetylene was converted to the corresponding cyclobutadiene complex (one isomer) upon treatment with (PhCN)2 PdCl2 (Hos-okawa and Moritani, 1969) [Eq. (71)]. With sterically less demanding tolane,... 

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... 

The total 7t-eIectron energy of benzene is 6a -I- 8)3, corresponding to a DE of 2)3. Cyclobutadiene is predicted to have a triplet ground state (for a square geometry) and zero... 

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. 

All of the ethynylated cyclobutadienes are completely stable and can be easily manipulated under ambient conditions, as long as the alkyne arms carry substituents other than H. For the deprotected alkynylated cyclobutadiene complexes, obtainable by treatment of the silylated precursors with potassium carbonate in methanol or tetrabutylammonium fluoride in THF, the stability is strongly dependent upon the number of alkyne substitutents on the cyclobutadiene core and the nature of the stabilizing fragment. In the tricarbonyUron series, 27b, 27c, 29 b, and 28b are isolable at ambient temperature and can be purified by sublimation or distillation under reduced pressure. The corresponding tetraethynylated complex 63 e, however, is not stable under ambient conditions as a pure substance but can be stored as a dilute solution in dichloro-methane. It can be isolated at 0°C and kept for short periods of time with only... 

In contrast to 69 Li, which was formulated above as the bji-electron aromatic compound, other representatives of this class of cyclic compounds, the heavy analogs of cyclobutadiene dianion, were found to be nonaromatic. The two compounds of this type, disiladigermacyclobutadiene dianion 71 K2 and tetrasilacyclobutadiene dianion 72 K2, were synthesized by the reductive dehalogenation of the corresponding precursors 73 and 74 " with KCg (Scheme 2.61)... 

A rather special case are unsaturated, cyclic hydrocarbons undergoing second-order double-bond localization14, e.g. cyclobutadiene or pentalene. Although equivalent pairs of Kekule structures can be written for these molecules, they assume a structure with alternant single and double bonds, corresponding to only one of these structures. These molecules will be dealt with later, in a separate section. 

Square. Bi42-, Se42+, and Te42+ are isoelectronic and isolobal with the delocalized planar cyclobutadiene dianions. There are 14 skeletal electrons [e.g., for Bi42- (4)(3) + 2 = 14] corresponding to 8 electrons for the 4 c-bonds and 6 electrons for the 71-bonding (see Chapter 2.7.2.1). 

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