Transition triple bonds

Triple bonds in the P position (in propargyl systems) have about the same effect as double bonds. Alkyl, aryl, halo, and cyano groups, among others, in the 3 position of allylic substrates increase Sn2 rates, owing to increased resonance in the transition state, but alkyl and halo groups in the 1 position decrease the rates because of steric hindrance. 

In others, there is a five- or a six-membered transition state. In these cases, the addition to the double or triple bond must be syn. The most important reaction of this type is the Diels-Alder reaction (15-58). 

However, the observations of Ward and Sherman need not rule out triple-bond participation and vinyl cations in the systems studied by Hanack and co-workers (75-79). Presumably, the enol formate 61 itself arises via a transition state involving a rate-determining protonation and vinyl cation 62 (see previous section). A vinyl cation such as 62 with an adjacent phenyl group is considerably more stable and hence more accessible than a vinyl cation such as 63, stabilized only by a neighboring alkyl group. Hence, formation of enol formate 61 and its... 

Carter, E. A., Goddard III, W. A., 1988, Relationships between Bond Energies in Coordinatively Unsaturated and Coordinatively Saturated Transition Metal Complexes A Quantitative Guide for Single, Double, and Triple Bonds , J. Phys. Chem., 92, 5679. 

PHAs containing carbon-carbon triple bonds synthesized by P. oleovorans and P. putida grown with 10-undecynoic acid (10-UND=) have been reported [64]. The amount of carbon-carbon triple bond could be controlled between 0% and 100%, but the yield of the PHA containing 100% carbon-carbon triple bond was very low. The repeating units formed from 10-UND= were 3-hydroxy-8-nonynoate (3HN=) and 3-hydroxy-6-heptynoate (3HHp=) units in the amounts of 26 mol% and 74 mol%, respectively. The glass transition temperatures of PHAs synthesized from mixtures of NA and UND= increased from -30°C to -20°C as the content of carbon-carbon triple bond increased from 0% to 100%. These polymers were crosslinked when cations such as Co2+ and Pt2+ were added. 

The similarity between the bonding models for transition metal carbene and carbyne complexes was noted in Section II. That the reactivity of the metal-carbon double and triple bonds in isoelectronic carbene and carbyne complexes should be comparable, then, is not surprising. In this section, the familiar relationship between metal-carbon bond reactivity and metal electron density is examined for Ru and Os carbyne complexes. 

The various transitions of triafulvenes to pentafulvenes achieved by addition of electron-rich double bonds is complemented by the reaction of triafulvenes with ynamines and yndiamines299, which gives rise to 3-amino fulvenes 539. This penta-fulvene type deserves some interest for its merocyanine-like inverse polarization of the fulvene system and its formation is reasonably rationalized by (2 + 2) cycloaddition of the electron-rich triple bond to the triafulvene C /C2 bond (probably via the dipolar intermediate 538) ... 

The cycloaddition-isomerization procedure can be accomplished in the presence of a catalytic amount of a transition metal salt. The reactions proceed at room temperature, neither air nor water needed to be excluded. The presence of an electron-withdrawing group is not necessary to activate the dienophile as the example below shows that gold coordination increases the electrophilicity of the triple bond. The presence of a terminal alkyne should also be important. In the case of a disubstituted alkyne no reaction can be observed <00JA11553>. 

Silver belongs to the late transition metals and, like gold, favors coordination to C=C triple bonds. A lot of silver-containing organometallic complexes, where silver-alkyne interactions assist the assembly of the complexes, are known. None of these complexes, however, was applied to efficient carbon-carbon bond formation in organic synthesis. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

Heterocyclic compounds are frequently used as hydrogen donors in the reduction of C-C double and triple bonds catalyzed by complexes of transition metals. Cyclic ethers such as [l,4]dioxane (39) and 2,3-dihydrofuran are known to donate a pair of hydrogen atoms to this type of compound. 2,3-Dihydro-[l,4]di-oxine (41), the product of dioxane (39), is not able to donate another pair of hydrogen atoms [46, 60, 73, 74]. These heterocyclic compounds are in general also very good solvents for both the catalyst and the substrates. 

Transition metal bisadduct 138 Transition metal complexes 121,157,158 Transport through cell membranes 43,46 Triazolylboranes 8-9 Triple bonding, group 13 element compounds 75,76... 

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

The N=N Triple Bond Activation by Transition Metal Complexes... 

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