Highly Active Trialkoxymolybdenum(VI) Alkylidyne Catalysts Synthesized by a Reductive Recycle Strategy

Recent advances in the development of alkyne metathesis catalysts

The number of well-defined molybdenum and tungsten alkylidyne complexes that are able to catalyze alkyne metathesis reactions efficiently has been significantly expanded in recent years.The latest developments in this field featuring highly active imidazolin-2-iminato- and silanolate-alkylidyne complexes are outlined in this review.

Lewis acid activation of molybdenum nitrides for alkyne metathesis

The substantial kinetic barrier to molybdenum nitride-alkyne metathesis is facilitated by precomplexation of the borane Lewis acid B(C(6)F(5))(3), providing convenient access to metathesis-active molybdenum alkylidynes. Spectroscopic and X-ray structural analysis suggest Mo≡N bond weakening upon borane complexation.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

Highly active molybdenum-alkylidyne catalysts for alkyne metathesis: synthesis from the nitrides by metathesis with alkynes

Terminal nitrido complexes NMo(OC(CF3)2Me)3 (4), NMo(OC(CF3)2Me)3(NCMe) (4-NCMe), and NMo(OC(CF3)3)3(NCMe) (5-NCMe) react irreversibly with 3-hexyne at elevated temperature in hydrocarbon solution to form the corresponding propylidyne complexes EtCMo(OC(CF3)2Me)3 (3) and EtCMo(OC(CF3)3)3 (6), long known as exceptionally active catalysts for alkyne metathesis. The propylidyne complexes are isolated as the more readily crystallized 1,2-dimethoxyethane (DME) adducts for convenience; 3-DME is isolated in 61% yield on a multigram scale.

Terminal, anionic carbide, nitride, and phosphide transition-metal complexes as synthetic entries to low-coordinate phosphorus derivatives

Anionic terminal one-atom nitride, phosphide, and carbide complexes are excellent starting materials for the synthesis of ligands containing low-coordinate phosphorus centers in the protecting coordination sphere of the metal complex. Salt-elimination reactions with chlorophosphanes lead to phosphaisocyanide, iminophosphinimide, and diorganophosphanylphosphinidene complexes in which the unusual phosphorus ligands are stabilized by coordination. X-ray structure analyses and density-functional calculations illuminate the bonding in these compounds.

Total Syntheses of the Actin-Binding Macrolides Latrunculin A, B, C, M, S and 16-epi-Latrunculin B

The latrunculins are highly selective actin-binding marine natural products and as such play an important role as probe molecules for chemical biology. A short, concise and largely catalysis-based approach to this family of bioactive macrolides is presented. Specifically, the macrocyclic skeletons of the targets were forged by ring-closing alkyne metathesis (RCAM) or enyne-yne metathesis of suitable diyne or enyne-yne precursors, respectively. This transformation was best achieved with the aid of [(tBu)(Me(2)C(6)H(3))N](3)Mo (37) as precatalyst activated in situ with CH(2)Cl(2), as previously described. This catalyst system is strictly chemoselective for the triple bond and does not affect the olefinic sites of the substrates. Moreover, the molybdenum-based catalyst turned out to be broader in scope than the Schrock alkylidyne complex [(tBuO)(3)W[triple chemical bond]CCMe(3)] (38), which afforded cycloalkyne 35 in good yield but failed in closely related cases. The required metathesis precursors were assembled in a highly convergent fashion from three building blocks derived from acetoacetate, cysteine, and (+)-citronellene. The key fragment coupling can either be performed via a titanium aldol reaction or, preferentially, by a sequence involving a Horner-Wadsworth-Emmons olefination followed by a protonation/cyclization/diastereoselective hydration cascade. Iron-catalyzed C--C-bond formations were used to prepare the basic building blocks in an efficient manner. This synthesis blueprint gave access to latrunculin B (2), its naturally occurring 16-epimer 3, as well as the even more potent actin binder latrunculin A (1) in excellent overall yields. Because of the sensitivity of the 1,3-diene motif of the latter, however, the judicious choice of protecting groups and the proper phasing of their cleavage was decisive for the success of the total synthesis. Since latrunculin A and B had previously been converted into latrunculin S, C and M, respectively, formal total syntheses of these congeners have also been achieved. Finally, a previously unknown acid-catalyzed degradation pathway of these bioactive natural products is described. The cysteine-derived ketone 18, the tetrahydropyranyl segment 31 serving as the common synthesis platform for the preparation of all naturally occurring latrunculins, as well as the somewhat strained cycloalkyne 35 formed by the RCAM reaction en route to 2 were characterized by X-ray crystallography.

Diphenylamido Precursors to Bisalkoxide Molybdenum Olefin Metathesis Catalysts

We have found that Mo(NAr)(CHR')(NPh(2))(2) (R' = t-Bu or CMe(2)Ph) and Mo(NAr')(CHCMe(2)Ph)(NPh(2))(2) (Ar = 2,6-i-Pr(2)C(6)H(3); Ar' = 2,6-Me(2)C(6)H(3)) can be prepared through addition of two equivalents of LiNPh(2) to Mo(NR″)(CHR')(OTf)(2)(dme) species (R″ = Ar or Ar' dme = 1,2-dimethoxyethane), although yields are low. A high yield route consists of addition of LiNPh(2) to bishexafluro-t-butoxide species. An X-ray structure of Mo(NAr)(CHCMe(2)Ph)(NPh(2))(2) reveals that the two diphenylamido groups are oriented in a manner that allows an 18 electron count to be achieved. The diphenylamido complexes react readily with t-BuOH and (CF(3))(2)MeCOH, but not readily with the sterically demanding biphenol H(2)[Biphen] (Biphen(2-) = 3,3'-Di-t-butyl-5,5',6,6'-tetramethyl-1,1'-Biphenyl-2,2'-diolate). The diphenylamido complexes do react with various 3,3'-disubstituted binaphthols to yield binaphtholate catalysts that can be prepared in situ and employed for a simple asymmetric ring-closing metathesis reaction. In several cases conversions and enantioselectivities were comparable to reactions in which isolated catalysts were employed.

A Mo(VI) Alkylidyne Complex with Polyhedral Oligomeric Silsesquioxane Ligands: Homogeneous Analogue of a Silica-Supported Alkyne Metathesis Catalyst

A highly active alkyne metathesis catalyst is realized by replacing the amide ligands of a molybdenum(VI) trisamide alkylidyne complex with silanol groups from incompletely condensed POSS (polyhedral oligomeric silsesquioxane) ligands. This catalyst serves as an effective homogeneous mimic of an amorphous silica-supported catalyst. Reactivities of various catalytic mixtures are reported along with an X-ray structure of the aniline-coordinated amidodisiloxymolybdenum(VI) alkylidyne complex.

Shape-Persistent Macrocycles: Structures and Synthetic Approaches from Arylene and Ethynylene Building Blocks

Shape-persistent arylene ethynylene macrocycles have attracted much attention in supramolecular chemistry and materials science because of their unique structures and novel properties. In this Review we describe recent examples of macrocycle synthesis by cross-coupling (Sonogashira: aryl acetylene macrocycle or Glaser: aryl diacetylene macrocycle) and dynamic covalent chemistry. The primary disadvantage of the coupling methods is the kinetically determined product distribution, since a significant portion of oligomers grow beyond the length of the cyclic targets ("overshooting"). Better results have been obtained recently by a dynamic covalent approach involving reversible metathesis reactions that afford macrocycles in one step. Mechanistic studies demonstrate that macrocycle formation is thermodynamically controlled by this route. Remaining synthetic challenges include the efficient preparation of site-specifically functionalized structures and larger, more complex two- and three-dimensional molecules.

Donor-Substituted 1,1,4,4-Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge-Transfer Interactions by Atom-Economic Synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.