Bent triple bonds

The combination of probably the oldest synthetic procedure for formation of a triple bond, i.e., the dehydrobromination of a vinyl bromide, with modern crown ether chemistry has resulted in one of the simplest yet very powerful methods for making highly strained cycloal-kynes. Thus, 1,5-cyclooctadiyne (56) can be made by treating l,5-dibromo-l,5-cyclooctadiene (55) with potassium rerf-butanolate in nonpolar solvents in the presence of 18-crown-6 [3 b, 24]. The nonpolar solvent protects the bent triple bond from nucleophilic attack by tert-butanol (Scheme 8-5). 1,5-Cyclooctadiyne had previously been made in very low yield by dimerization of butatriene (57), which is not a readily accessible compound [25]. Other important 1,2-elimination reactions generating cyclic alkynes are the oxidative degradation of... 

The triple bonds in cyclic alkynes can, of course, be subjected to all known addition reactions of acetylenes. Here, we will discuss examples which either lead to particularly interesting addition products or demonstrate unusual reactivity of bent triple bonds. 

The high reactivity of bent triple bonds toward both nucleophiles and electrophiles is demonstrated by the reaction of cyclooctyne with lithium and iodine, both reactions affording a homonuclear addition product [3 b] (Scheme 8-20). 

Benzyne 1,2-dehydrobenzcne, a highly reactive intermediate, with a bent triple bond. 

X-ray structure analysis showed that macrocycle 57 was essentially planar, with the twist angle of the benzene rings from the plane of the macrocycle being less than 2°. Most of the strain was seemingly contained in the triple bonds, as these were bent from linearity by 10.1° to 12.3°. Despite its strained nature, the macrocycle showed remarkable stability. Decomposition occurred above 300°C on attempted melting. No reaction was observed between 57 and cyclopenta-diene at room temperature. 

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

Several vibrational modes in the excited singlet states of DPA have been identified from picosecond CARS measurements. The central C—C bond of DPA retains much of its triple-bond-like character in the S2 state however, the central C—C bond has double-bond-like character in the state. By analogy with the trans-bent form of S acetylene, it was proposed that S DPA has a trans-or cis-form bent structure that is consistent a previously proposed strucmre. It has been cautioned that, because of the conjugation between the phenyl groups and the central C—C bond, the S state may assume a structure different from the bent form. The interpretation of recent picosecond IR absorption measurements provide support for the trans-bent planar structure for S of DPA. The diphenylace-tylenic fluorophore has recently been incorporated into several chemosensor structures whose fluorescent signaling properties are controlled by the relative flexibilities of the molecules. ... 

The bent-bond model of the triple bond gives this bond a threefold symmetry axis and leads to the prediction that the two methyl groups in dimethylacetjdeno should be somewhat restricted in their mutual rotation, with the eclipsed configuration stable. Restriction of rotation about the single bonds in conjugated systems is also expected, and the nature of the stable configurations can be predicted from the theory described above. These systems are discussed in Chapters 6 and 8. 

Vinylacetylene,102 H C=CH—C=CH, and vinyl cyanide,103 have a planar bent structure with C=C—C bond angle 123° and C—C bond lengths 1.446 and 1.426 A, respectively, corresponding to 13 to 20 percent of double-bond character (Table 7-9 note that correction —0.06 A is made for adjacent double bond and triple bond). This agrees well with the value, 15 percent, found for butadiene. 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

Double and triple bonds are represented with bent bonds formed with flexible couplings. Substances that require models with bent bonds normally are found to be much less stable and, therefore, chemically more reactive than molecules which can be constructed with straight sticks. Figure 1-4 shows the double bond of ethylene, the triple bond of acetylene, and the distorted bonds of cyclopropane. 

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