Triple-bond compounds

In triple-bond compounds (e.g., acetylene), carbon is connected to only two other atoms, and hence uses sp hybridization, which means that the four atoms are in a straight line (Fig. 1.6). Each carbon has two p orbitals remaining, with one electron in each. These orbitals are perpendicular to each other and to the C—C axis. They overlap in the manner shown in Figure 1.7 to form two n orbitals. A triple bond is thus composed of one a and two n orbitals. Triple bonds between carbon and nitrogen can be represented in a similar manner. 

The product in this case is an allene, ° but such shifts can also give triple-bond compounds or, if Y=OH, an enol will be obtained that tautomerizes to an a,P-unsaturated aldehyde or ketone. 

Similarly, a-hydroxy triple-bond compounds have given a,p-unsaturated ketones. See 15-1 for related reactions in which double bonds migrate or isomerize. 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

Carbon-carbon multiple bonds are not the only units that can participate in Diels-Alder reactions. Other double- and triple-bond compounds can be dienophiles and they give rise to heterocyclic compounds. ... 

Triple-bond compounds react with carbenes to give cyclopropenes, except that in the case of acetylene itself, the cyclopropenes first formed cannot be isolated because they rearrange to allenes. Cyclopropenones (p. 58) are obtained by hydrolysis of dihalocyclopropenes. ... 

The mechanism of oxidation probably involves in most cases the initial formation of a glycol (15-35) or cyclic ester,and then further oxidation as in 19-7. In line with the electrophilic attack on the alkene, triple-bonds are more resistant to oxidation than double bonds. Terminal triple-bond compounds can be cleaved to carboxylic acids (RC=CHRCOOH) with thallium(III) nitrate or with [bis(trifluoroacetoxy)iodo]pentafluorobenzene, that is, C6F5l(OCOCF3)2, among other reagents. 

Alkylation of enamines Addition of amines to triple-bond compounds Addition of amines to aldehydes or ketones Reaction between Grignard reagents and formamides Reaction of phosphonates with aldehydes or ketones... 

Borst et al. <2005CEJ3631> conducted a study on the synthesis of strained bicyclic phosphirane and phosphirene iron-tetracarbonyl complexes (Scheme 11). It was shown that, depending on the ring size of the resulting heterocycle, electrophilic phosphinidene [Ri-PrNP=Fe(CO)4] could be trapped intramolecularly with both double and triple bonds (compounds 146-150). The phosphinidene addition was found to be reversible at room temperature and when using phenylacetylene as solvent, exchange between phenylacetylene and the phosphinidene group took place. Compound 151 was isolated as the dimer, compound 152. 

The triple bond compounds can also be ozonised but the reaction is much slower. 

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. 

Elemental silicon is relatively stable in most substances at ordinary temperatures. Silicon shows similarity with other elements of its group, especially with germanium in many chemical properties. It forms tetravalent compounds with tetrahedral geometry almost exclusively. However, only in silicon monoxide, SiO, is its valence +2. Also, unlike carbon, silicon does not form unsaturated double or triple bond compounds. Silicon dissolves in germanium... 

The thin control of the energetic characteristics and the electronic transition nature may be carried out by chemical structure alteration. For example the appearance of CH2 group between phenyl ring and triple bond (compounds 3 and 5 in Table 4) interrupts the interaction between it-ekctrons of the both elements. This leads to the short wavelength shift in the absorption and photoconductivity spectra. The insertion of the ethynyl group instead of a CH2 group restores the conjugation. The increase in the numbers and sizes of the... 

Evans Andrews Buckwalter ]. Am. Chem. Soc. 1974, 96. 5560 Still Macdonald J. Am. Chem. Soc. 1974, 96, 5561 Ref. 1519. For a similar reaction with triple-bond compounds, see Horames Verkruijssc Brandsma Reel. Trav. Chim. Pays-Bas 1980, 99, 113, and references cited therein. 

Catalytic hydrogenation is a standard method for cleaving benzylic ethers. The resulting secondary alcohol constitutes the nucleophilic reaction center in a subsequent Michael addition to the acceptor-substituted triple bond. Compound 5 is obtained quantitatively with a double bond in the trans configuration. 

Tungsten-coordinated phosphetenes reacted with double- or triple-bond compounds, like iV-phcnylmalcimidc, dimethyl acetylenedicarboxylate, and benzaldehyde, to give formal [4+2] cycloaddition products <1988TL3077>. Several years later, non-metal-coordinated phosphetenes were found to be reactive toward multiple bond compounds after prolonged heating (at 150 °C for several weeks) <1998HAC9>. 

The above ab initio and related calculations clearly reveal that unsaturated silicon compounds are less stable than their carbon analogs (156,170), but as gas-phase studies and the chemistry of sterically crowded silicon derivatives have proved (153,154,156,158,162,172-175) such compounds can no longer be labeled as nonexistent (176,177). A number of isolable double-bonded silicon derivatives have recently been synthesized and characterized in agreement with theoretical predictions (Table III) (169,173,178-195). The data illustrate well that the kinetic stability of multiply bonded silicon species can be greatly improved by substitution of sufficiently bulky groups across the p -pw bond. The ease of addition reactions is then severely restricted. The new target for organosilicon chemists appears to be silicon triple-bonded compounds (196,197). 

This understanding that the MoMo bond order in 2 and similar species should be taken as rather less than three does not detract from the usefulness of writing a triple bond. Compound 2 and relatives display many chemical characteristics of multiple bonding. Their nature is to react as unsaturated molecules. 

Triple-Bond Compounds between Group 14 Elements 21... 

C=CH2) is very unstable and not a minima in calculations. For the purpose of synthesis and isolation of these heavier triple-bond compounds, theoretical calculations suggest that bulky substituents should be necessary to prevent the 1,2-substituent-shift of the triply bonded heavier group 14 atoms. 

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