Alkynylations C-H bonds

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

The 0 — H bonds of alcohols and the N—H bonds of amines are strong and stiff. The vibration frequencies of O—H and N—H bonds therefore occur at higher frequencies than those of most C—H bonds (except for alkynyl =C—H bonds). 

Alkynyl complexes contain metal-carbon bonds in which the metal is bound to the sp-hybridized carbon at the terminus of a metal-carbon triple bond. The materials properties of these complexes have been investigated extensively. The properties of these complexes include luminescence, optical nonlinearity, electrical conductivity, and liquid crystallinity. These properties derive largely from the extensive overlap of the metal orbitals with the ir-orbitals on the alkynyl ligand. The M-C bonds in alkynyl complexes appear to be considerably stronger than those in methyl, phenyl, or vinyl complexes. Alkynyl complexes are sometimes prepared from acetylide anions generated from terminal alkynes and lithium bases (e.g., method A in Equation 3.42), but the acidity of alkynyl C-H bonds, particularly after coordination of the alkyne to the transition metal, makes it possible to form alkynyl complexes from alkynes and relatively weak bases (e.g., method B in Equation 3.42). Alkynyl copper complexes are easily prepared and often used to make alkynylnickel, -palladium, or -platinum complexes by transmetallation (Equation 3.43). This reaction is a step in the preparation of Ni, Pd, or Pt alkynyl complexes from an alkyne, base, and a catalytic amoimt of Cul (Equation 3.44). This protocol for... 

The electronic structure of the triple bond reveals two ir bonds, perpendicular to each other, and a (T bond, formed by two overlapping sp hybrid orbitals. The strength of the triple bond is about 229 kcal mol that of the alkynyl C—H bond is 131 kcal mol . Triple bonds form linear structures with respect to other attached atoms, with short C—C (1.20 A) andC—H( 1.06 A) bonds. 

In general, the reaction can be performed only with organometallics of active metals such as lithium, sodium, and potassium, but Grignard reagents abstract protons from a sufficiently acidic C—H bond, as in R—C=C—H —> R—C=C—MgX. This method is best for the preparation of alkynyl Grignard reagents. ... 

Cyclization of 2-(l-alkynyl)XV-alkylidene anilines is catalyzed by palladium to give indoles (Equation (114)).471 Two mechanisms are proposed the regioselective insersion of an H-Pd-OAc species to the alkyne moiety (formation of a vinylpalladium species) followed by (i) carbopalladation of the imine moiety and /3-hydride elimination or (ii) oxidative addition to the imino C-H bond and reductive coupling. 

Scheme 4.5 shows several possible pathways from r -acetylene metal complexes RE to metal vinylidenes PR. In the first pathway (al + a2), metal vinylidenes PR can be obtained from an intermediate (INI) with a 1,2 hydrogen shift from C to Cp. The second pathway (bl + b2) is through an intermediate (IN2) with an r agostic interaction between the metal center and one C—H bond, which undergoes a 1,2 hydrogen shift to PR. The third pathway (bl + b3 + b4) also starts from IN2 but then goes into another intermediate, the hydrido-alkynyl IN3, which leads to PR with a 1,3 hydrogen shift from the metal center to Cp. 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

Other elfects besides mass and bond strength also affect infrared absorption frequencies. The structural environment of a bond is particularly important. Thus the absorption frequency of a C—H bond depends on whether it is an alkyl, alkenyl, alkynyl, or aryl C—H bond (see Table 9-2). 

Platinum complexes (continued) with aryls, thallium adducts, 3, 399 with bis(alkynyl), NLO properties, 12, 125 with bisalkynyl copper complexes, 2, 182-186 with bis(3,5-dichloro-2,4,6-trifluorophenyl), 8, 483 and C-F bond activation, 1, 743 in C-H bond alkenylations, 10, 225 in C-H bond electrophilic activation studies, 1, 707 with chromium, 5, 312 with copper, 2, 168 cyclometallated, for OLEDs, 12, 145 in diyne carbometallations, 10, 351-352 in ene-yne metathesis, 11, 273 in enyne skeletal reorganization, 11, 289 heteronuclear Pt isocyanides, 8, 431 inside metallodendrimers, 12, 400 kinetic studies, 1, 531 on metallodendrimer surfaces, 12, 391 mononuclear Pt(II) isocyanides, 8, 428 mononuclear Pt(0) isocyanides, 8, 424 overview, 8, 405-444 d -cP oxidative addition, PHIP, 1, 436 polynuclear Pt isocyanides, 8, 431 polynuclear Pt(0) isocyanides, 8, 425 Pt(I) isocyanides, 8, 425 Pt(IV) isocyanides, 8, 430... 

Hydrocarbons consist entirely of nonpolar C-H bonds with no unpaired electrons. These compounds are relatively unreactive. The substitution of one or more atoms with unpaired electrons into the hydrocarbon backbone creates a hydrocarbon derivative. The unpaired electrons result in polar or charged portions of these molecules. These atoms fall into categories known as functional groups, and they create local regions of reactivity. Alkyl, alkenyl, alkynyl, and aryl groups may also be considered functional groups in some circumstances as described in the previous skill. 

Unlike previous alkyne-aldehyde additions [23], the generation of an alkynyl carbanion is unlikely owing to the large pK, difference between the terminal acetylene and the solvent water [24]. A mechanism was proposed involving the simultaneous activation of the C-H bond of alkyne by the ruthenium catalyst and the aldehyde carbonyl by the indium ion. The ruthenium intermediate then underwent Grignard-type addition followed by an in situ hydrolysis in water to give the desired carbonyl addition product and regenerated the ruthenium and indium catalysts to catalyze further reactions (Fig. 3). 

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