Alkyne analogs

The most distinctive structural feature of the heavier alkyne analogs is that, unlike typical triple-bonded carbon atoms, the group 14 elements are not linearly coordinated. For the digermyne, the bent geometry suggests that the Ge-Ge interaction is not a true triple bond, but that the molecule might be better viewed as a bis(germylene)  

Like a diarylgermylene, the digermyne reacts readily with dihydrogen, yielding multiple products  

The Ge and Sn alkyne analogs also undergo cycloaddition reactions with carbon-carbon double bonds. The following reactions are common to both the digermyne and distannyne derivatives  

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The finding of preparatively available iminoboranes RB = NR some years ago opened exciting new possibilities not only in B—N chemistry, but also in coordination chemistry. The first examples of iminoborane-transition-metal complexes have now been published. The structurally completely characterized t-BuB = NBu-t adds, like its alkyne analog, to the 03(00)5 fragment as a bridging ligand. When Co2(CO)g and t-BuB = NBu-t are dissolved in pentane at 0°C, warming to RT and evaporation of unreacted iminoborane yields (t-BuBNBu-t)Co2(CO)5 (86%) as a black solid, which can be recrystallized from ether-nitromethane (1 3) ... 

This is an alkyne-analog of the Wittie reaction. Since organocopper chemistry finds utility in converting the C-Br bond to other alkyl groups, we show only the first part of the transformation in most examples. 

The catalyst was successfully recycled and reused five times without any loss in activity. It was also demonstrated that (phosphane)AuIcarboxylates and sulfonates are highly active catalysts for hydration of non-activated alkynes. Analogous Ag1 complexes are not active for these reactions due to the fact that the Ag1 cations are much stronger acceptors for their ligands and counterions compared with Au1 cations. 

No reaction was observed with internal alkynes, and carbonyl alkyne analogs undergo rapid alkyne substitution at much lower temperatures as mentioned above. Only these olefin alkyne derivatives are known to pro-... 

However, from the point of view of a synthetic chemical challenge it is the alkyne-analogous 13/15 compounds that are the most interesting. As shown below, there is a potentially rich variety of structural forms and bonding modes that includes alkyne-analogous (6), cyclobutadiene-analogous (7), and benzene-analogous... 

Alkyne cross-coupling reactions over the last 25 years have become one of the most valuable assets in the synthetic chemist s toolbox. The now famous Sonogashira coupling (50, 114) of terminal alkynes with aryl or vinyl halides is readily achieved with a palladium catalyst, a copper(l) cocatalyst, and amine base. In the catalytic cycle (Scheme 14a), copper-and palladium-alkyne complexes are the key intermediates that lead to coupling of R and R units via the alkyne. Analogously, the Stille coupling... 

Figure 27 Group 14 alkyne analogs (a) Ar GeGeAr (Ar = Dipp) and (b) Ar PbPbAr (Ar = Trip)...

Nitrile, Isonitrile, and Alkyne Analogs. Exploration of N2ase-catalyzed reductions of selected nitrile, isonitrile, and alkyne analogs provides examples of novel chemical reactions and yields information about the topography of the active sites of Noase. 

Ge2-alkyne analogs to give a mixture of Gc2 and primary germane products (Scheme 11.4) has been described by Power and coworkers [23]. It is also noteworthy that a unique metal-free hydrogenase has been identified from methanogenic archaea. This enzyme catalyzes reactions with H2 [24, 25], and it is suggested that a folate-Hke cofactor is important in the the reversible activation or liberation of H2 [26, 27]. 

The heavier carbene analogs provide an entry into other low—coordination-number heavy element derivatives such as alkene and alkyne analogs. 

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

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