Nitrile-alkyne metathesis

Terminal tungsten nitride complexes containing W N bonds have been reported to catalyze nitrile-alkyne cross-metathesis (NACM) by a mechanism that has parallels with the non-pairwise mechanism for olefin metathesis (A. M. Geyer, E. S. Weidner, J. B. Gary, R. L. Gdula, N. C. Kuhlmann, M. J. A. Johnson, B. D. Dunietz, J. W. Kampf, J. Am. Chem. Soc., 2008, 130, 8984.) When p-methoxybenzonitrile and 3-hexyne were mixed in the presence of catalyst candidate N=W(OC(Cp3)2Me)3(DME), two principal products attributable to metathesis were observed. 

Nitrile-Alkyne Cross Metathesis by the Reaction of W(N)Xj with 2-Butyne... 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

A route to alkylidynes containing metals with high oxidation states involves a metathesis exchange reaction that we have referred to briefly in earlier sections of Chapter 10. Scheme 10.10 shows the triply-bonded ditungsten complex 79 reacting with either alkynes or nitriles to give the corresponding metal carbyne (path a)90 or metal-carbyne plus nitride complex (path b).91 Scheme 10.10 also shows results of molecular orbital calculations at the DFT level.92 In the case... 

Recent advances on the synthesis of piperidines through ruthenium-catalyzed ring-closing metathesis reactions 12H(84)75. Rhodium-catalyzed [2+2+2] cycloaddition for the synthesis of substituted pyridines, pyridones, and thiopyranimines 12H(85)1017. Synthesis of 2,2 -bipyridines by transition metal-catalyzed alkyne/nitrile [2+2+2] cycloaddition reactions 12H(85)1579. 

Nitrido complexes have also been formed by metathesis and atom transfer processes. The reaction of dinitrogen with a molybdenum(III) species forms a molybdenum-nitrido complex, as shown in Equation 13.103. and described in more detail in the section of Chapter 5 on dinitrogen complexes. In a metathesis process involving related complexes, the reaction of a metal-alkylid)me complex with a nitrile extrudes an alkyne to form a metal nitride fliat adopts a dimeric structure (Equation 13.104). - Related nitrido complexes have been formed from an azabicydic compoimd that eliminates anthracene after forming the M-N bond (Equation 13.105). ... 

In the most recent work, the alkyne-nitrile metathesis reaction was examined by allowing [Fe(L)N] + (where L = 4) to undergo ion-molecule reactions with a series of nine different alkynes (Table 6.3) [104]. When unsymmetrical alkynes are used, two different metathesis products are observed (Eqs. (6.131) and (6.132)). 

The data shown in Table 6.3 show no obvious trends that may shed light on the mechanism(s) of the metathesis reactions. In terms of overall relative reactivity (krei of Table 6.3), phenylacetylene reacts fastest and pent-l-yne is the most sluggish alkyne, while ethyne and the parasubstituted phenylacetylenes are unreactive under the experimental conditions used. When the branching ratios and the relative reaction rates are combined, the alkyne - nitrile chaimel is the most productive for propargyl alcohol and phenylacetylene and least productive for pent-2-yne and pent-l-yne. The sterically congested alkyne exhibits modest reactivity for the metathesis reaction. No obvious relationship exists between the structure of the alkyne substrate and the propensity for a metathesis reaction. With regard to which metathesis reaction is favored for unsymmetrical alkynes, while no regioselectivity operates for pent-2-yne and phenylacetylene other terminal acetylenes favor the loss of the more substituted nitrile (Eq. (6.131)). 

In addition to 6, different ruthenium complexes have been investigated as catalysts for alkyne cyclotrimerizations and cyclocotrimerization of alkynes with nitriles. Among them, Grubbs-type carbene complexes are of particular importance since they catalyze inter- and intramolecular metathesis-cascade cyclotrimerizations under mild conditions. In addition, they have good tolerance toward a variety of functional groups. 

As an alternative, iridium complexes show exciting catalytic activities in various organic transformations for C-C bond formation. Iridium complexes have been known to be effective catalysts for hydrogenation [1—5] and hydrogen transfers [6-27], including in enantioselective synthesis [28-47]. The catalytic activity of iridium complexes also covers a wide range for dehydrogenation [48-54], metathesis [55], hydroamination [56-61], hydrosilylation [62], and hydroalkoxylation reactions [63] and has been employed in alkyne-alkyne and alkyne - alkene cyclizations and allylic substitution reactions [64-114]. In addition, Ir-catalyzed asymmetric 1,3-dipolar cycloaddition of a,P-unsaturated nitriles with nitrone was reported [115]. 

The requisite dihydroxyketones are commonly assembled via iterative aldol coupling reactions [1], but other methods including Nef reactions [17,18], acetylide additions [19, 20], 1,3-dipolar nitrile oxide cycloadditions [21], iterative alkylation of dithianes [22-28], hydrazones [29], oximes [30], nitriles [31], or dihalomethylene species [32-34], cross-metathesis/hydroboration/oxidation [35], iterative substitution of a xanthate [36], dihydroxylation/desymmetrization of alkenes [37], Homer-Wadsworth-Emmons olehnations [38, 39], allylmetallations [40], and alkyne-alkyne cross-coupling [41] have also been reported. 

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