Alkynes conjugation between

Although the B-H addition of borane etherates to simple alkenes occurs quite rapidly under ambient conditions, less electrophilic boranes are correspondingly less reactive. A classic example is given by catecholborane which reacts with alkynes only above 70 °C and with alkenes under somewhat more forcing conditions [1,85]. The reason for this is not hard to find, since the electrophilic character of the borane is substantially diminished by conjugation between boron and adjacent oxygens [86,87]. At the same time, the acidity of B-H is enhanced through an... 

Conjugation between the triple bond and the carbonyl function lowers the reduction potential considerably whereas alkyl substitution makes reduction more difficult (entries 1-5). A comparison between the half-wave potentials for reduction of PhC=CPh (1-69 V, vs. Hg pool) and // <7/t -PhCH=CHPh (1-65 V) substantiates the fact that, at least for this case, a likely product of reduction is more vulnerable to electroreduction than the starting material. In practice electrolyses in protic media aimed at producing alkene from alkyne usually proceed to give alkane. 

The only other alkyne the anodic oxidation of which has been studied in detail is diphenylacetylene. Again it is difficult to be certain about the mechanism involved the phenyl group and the triple bond may be electroactive but, because of the conjugation between them, it is probably more rigorous to consider electron transfer from the molecule as a whole. The relevant oxidation potential is apparently 2 V (vs. S.C.E.), the potential employed for anodic cyanation. ... 

The double bond may also be conjugated with another, and in the series propa-diene (allene), 1,3-butadiene, 1,4-pentadieneetc. the extent of interaction decreases as the number of single bonds between the points of unsaturation increases. So, as we shall see later, in catalysed reactions propadiene and 1,3-butadiene often resemble the corresponding alkynes. There is a conjugation between double and triple bonds in ethenylethyne, and between these bonds and the aromatic ring in, for example, styrene (phenylethene) and phenylethyne. 

Hyperconjugation in alkynes and its relation to the absence of conjugation between two triple bonds in 1 -diynes... 

From the conformational point of view, rotation aronnd the C-C bonds in alkynes is invisible due to the cylindrical symmetry of alkyne Jt-cloud. In bis-alkynes, the situation is analogous. However, the question of conjugation between two alkynes or between alkyne functionality and propargylic substituents has been the subject of a great deal of attention and recent debates that we will discuss in this section. 

The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can also proceed if the alkene is replaced by an alkyne moiety or even if some of the atoms... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Complexes in which the acceptor is a metal ion and the donor an olefin or an aromatic ring (n donors do not give EDA complexes with metal ions but form covalent bonds instead).41 Many metal ions form complexes, which are often stable solids, with olefins, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture of the bonding in these complexes,44 first proposed by Dewar,45 can be illustrated for the complex in which silver ion is bonded to an olefin. There are two bonds between the metal ion and the olefin. One is a a bond formed by overlap of the filled -it orbital of the olefin with the empty 5s orbital of the silver ion, and the other a -it bond... 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

Muller et al.332 have characterized a number of a-aryl-, (3,(3 -disilyl-substituted vinyl cations (130). Both 13C NMR data and calculations show the effect of both Tt-conjugation with the aromatic moiety and O-delocalization of the (3-Si—C bonds. Jt-Delocalization is manifested by the marked low-field shift of the ortho and para carbon atoms compared to those of the precursor alkynes, whereas o-delocalization is shown by the deshielding of the 29Si NMR resonance relative to the precursor silylalkyne (A829Si 29.5-41.4). Computed structures [B3LYP/6-31G(d) level] are all very similar, that is, calculation is not able to show the subtle interplay between O- and 7t-delocalization, which is evident from the NMR spectroscopic data. 

Pyridines have been formed by the cyclization of azadienes, themselves obtained by a conjugate addition between 24 and alkyne diesters (Scheme 24) <2000T4817>. 

---