Carbyne Complexes Research Articles

Total Synthesis of the Biphenyl Alkaloid (−)‐Lythranidine

A sequence comprising a ring-closing alkyne metathesis of a propargyl alcohol derivative, followed by a ruthenium-catalyzed redox isomerization of the derived cycloalkyne and a transannular aza-Michael addition allowed the formation of the distinguishing piperidine-metacyclophane framework of the Lythraceum alkaloid lythanidine in a few high-yielding steps. This application attests to the excellent functional-group tolerance of a molybdenum alkylidyne complex endowed with triphenylsilanolate ligands, which enabled the macrocyclization even in the presence of protic functionalities, and thus illustrates the power of contemporary catalytic acetylene chemistry for target-oriented synthesis.

Catalysis‐Based Total Synthesis of Putative Mandelalide A

A concise synthesis of the putative structure assigned to the highly cytotoxic marine macrolide mandelalide A (1) is disclosed. Specifically, an iridium-catalyzed two-directional Krische allylation and a cobalt-catalyzed carbonylative epoxide opening served as convenient entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal-acetylene metathesis into natural product synthesis, which is remarkable as this class of substrates was beyond reach until very recently; key to success was the use of the highly selective molybdenum alkylidyne complex 42 as the catalyst. Although the constitution and stereochemistry of the synthetic samples are unambiguous, the spectra of 1 as well as of 11-epi-1 deviate from those of the natural product, which implies a subtle but deep-seated error in the original structure assignment.

Catalysis‐Based Total Synthesis of Putative Mandelalide A

AbstractA concise synthesis of the putative structure assigned to the highly cytotoxic marine macrolide mandelalide A (1) is disclosed. Specifically, an iridium‐catalyzed two‐directional Krische allylation and a cobalt‐catalyzed carbonylative epoxide opening served as convenient entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal‐acetylene metathesis into natural product synthesis, which is remarkable as this class of substrates was beyond reach until very recently; key to success was the use of the highly selective molybdenum alkylidyne complex 42 as the catalyst. Although the constitution and stereochemistry of the synthetic samples are unambiguous, the spectra of 1 as well as of 11‐epi‐1 deviate from those of the natural product, which implies a subtle but deep‐seated error in the original structure assignment.

A Direct, Modular, and Efficient Construction of the PCP Structural Motif through Coupling of Manganese Carbyne Complexes with Phosphines

Easily available carbyne complexes of manganese were used as a source of carbyne fragments in an unconventional synthesis of backbone-substituted diphosphinomethanes and cyclic P-ylides upon coupling with secondary or tertiary phosphines, respectively, followed by demetalation under mild conditions.

Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO3– Pincer-Type Ligand

Synthetic protocols for a pyrrolide-centered ONO3– trianionic pincer-type ligand are presented. Treating (tBuO)3W≡CtBu with the proligand [pyr-ONO]H3 (2) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]W═CHtBu(OtBu) (3). Addition of a mild base to complex 3 provides the trianionic pincer alkylidyne complex {MePPh3}{[pyr-ONO]W≡CtBu(OtBu)} (4). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex 4, single-crystal X-ray crystallography. DFT calculations performed on 4 provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals.

Synthesis, Structure, and Reactivity of Tungsten Acetylide–Germylene Complexes

The novel acetylide–germylene complexes Cp*(CO)2W(GePh2)(CCR) (7a, R = SiMe3; 7b, R = CMe3) were synthesized by the reactions of Cp*(CO)2W(NCMe)Me with Ph2HGeC≡CR (R = SiMe3, CMe3). X-ray crystal analysis of 7a revealed significantly increased germylene–tungsten and decreased germylene–acetylide interactions in comparison to the corresponding interactions in the previously reported acetylide–silylene complex Cp*(CO)2W(SiPh2)(CCSiMe3) (1). Complexes 7a,b reacted with acetone to give the six-membered cyclic vinylidene complexes Cp*(CO)2W═C═C(R)CMe2OGePh2 (8a, R = SiMe3; 8b, R = CMe3) by acetone insertion reaction, similar to the case of 1 affording Cp*(CO)2W═C═C(SiMe3)CMe2OSiPh2 (2). In the presence of 4-(dimethylamino)pyridine (DMAP), complexes 8a,b gave trans- and cis-DMAP-stabilized germylene acetylide complexes Cp*(CO)2W(GePh2·DMAP)(CCR) (trans- and cis-9a, R = SiMe3; trans- and cis-9b, R = CMe3) and acetone, showing a reactivity different from that of the silicon analogue 2. Complexes cis-9a,b were isolated as crystals from the reaction of 7a,b with DMAP and formed mixtures with trans-9a,b in solutions, respectively. A mixture of cis- and trans-9a reacted with acetone to form an equilibrium mixture with 8a and DMAP. The reactivity of 7a,b toward Me3COH was also investigated to reveal the formation of the vinylidene complexes Cp*(CO)2W{GePh2(OCMe3)}═C═CHR (10a, R = SiMe3; 10b, R = CMe3); 10a is equilibrated with 7a and Me3COH, whereas 10b is further converted to the carbyne complex Cp*(CO)2W≡CCH(CMe3){GePh2(OCMe3)} (11).

Alkene to Carbyne: Tandem Lewis Acid Activation and Dehydrogenation of a Molybdenum Ethylene Complex

Carbyne formation: Treatment of a molybdenum ethylene complex with B(C6 F5 )3 induces ditopic activation of an ethylene ligand and acceptor-assisted ethane elimination to generate a novel type of zwitterionic carbyne complex.

ChemInform Abstract: Alkyne Metathesis by Molybdenum and Tungsten Alkylidyne Complexes

AbstractReview: 33 refs.

Alcohol‐Promoted Ring‐Opening Alkyne Metathesis Polymerization

Alcohol is the answer! An inactive, air-stable, dimeric molybdenum alkylidyne complex is activated toward ring-opening alkyne metathesis polymerization (ROAMP) by the addition of methanol. The ROAMP is compatible with water and phenol-containing substrates and with the in situ photochemical generation of alkyne monomers from cyclopropenones.

Tungsten alkylidyne complexes with ancillary imidazolin-2-iminato and imidazolidin-2-iminato ligands and their use in catalytic alkyne metathesis

The imines 1,3-di-tert-butylimidazolidin-2-imine (10) and 4,5-dimethyl-1,3-di-tert-butylimidazolin-2-imine (14) were prepared starting from the corresponding N-heterocyclic carbenes 8 and 11 and trimethylsilyl azide or tert-butyl azide, respectively. Deprotonation of the imines 10 and 14 with methyl lithium and reaction with an equimolar amount of the tungsten benzylidyne complex [PhCW{OCMe(CF3)2}3(DME)] (16) (DME = 1,2-dimethoxyethane) afforded the complexes [PhCW(L){OCMe(CF3)2}2] (L = 1,3-di-tert-butylimidazolidin-2-iminato, 6; L = 4,5-dimethyl-1,3-di-tert-butylimidazolin-2-iminato, 7), which were characterized by 1H, 13C{1H} and 19F{1H} NMR spectroscopy and by elemental and X-ray diffraction analysis. Complexes 6 and 7 are efficient catalysts for the homocoupling of 3-pentynyl benzyl ether (17) at room temperature in the presence of 5 Å molecular sieve and at low catalyst loadings (1 mol%).

FRET Sensitization of Tungsten–Alkylidyne Complexes by Zinc Porphyrins in Self-Assembled Dyads

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.

Formation of an alkyne during degradation of metal–alkylidyne complexes

The compound [(Ot-Bu)3W≡Ct-Bu] (1) (t-Bu = C(CH3)3) decomposes upon contact with water and several organic products are formed, including di-tert-butylacetylene, t-BuC≡Ct-Bu. This process is reminiscent of the degradation of trinuclear metal-alkylidyne complexes in which free carbynes are ejected into solution, couple and form alkynes along with many other products. The reactivity pattern of the resulting t-BuC carbynes that includes extensive hydrogen abstraction, cleavage of alkynes and lack of reactivity towards alkenes is indicative of a quartet (S = 3/2) spin state. A similar spin state was assigned to other RC (R = alkyl) species that were produced by degrading M3-alkylidyne (M = transition metal) complexes in water. t-BuC≡Ct-Bu is also produced during thermal decomposition of solid 1. In 1977 Fischer and co-workers reported a very similar process in which solids of Br(CO)4Cr≡CR(1) and Br(CO)4Cr≡CR(2) were co-thermolyzed to produce R(1)C≡CR(2), R(1)C≡CR(1), and R(2)C≡CR(2). Fischer had considered the involvement of free carbynes in the making of the alkynes but later resorted to other explanations. The current results suggest that his original proposal is indeed valid.

Understanding intermolecular C–F bond activation by a transient titanium neopentylidyne: experimental and theoretical studies on the competition between 1,2-CF bond addition and [2 + 2]-cycloaddition/β-fluoride elimination

Complex (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) (PNP(-) = N[2-P(CHMe(2))(2)-4-methylphenyl](2)) eliminates H(3)C(t)Bu to form transient (PNP)Ti≡C(t)Bu, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)Ti=C[(t)Bu(NC(5)H(3)F)](F) (1) and (PNP)Ti=C[(t)Bu(NC(5)H(4))](F) (2), respectively. When (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C((t)Bu)CC(4)H(3)-3-FNH) (3) is the only recognizable titanium metal complex formed. Theoretical studies reveal that pyridine binding disfavors 1,2-CF bond addition across the alkylidyne ligand in the case of ortho-fluoride pyridines, while sequential [2 + 2]-cycloaddition/β-fluoride elimination is a lower energy pathway. In the case of meta-fluoropyridine, [2 + 2]-cycloaddition and subsequent ring-opening metathesis is favored as opposed to C-H bond addition or sequential [2 + 2]-cycloaddition/β-hydride elimination. In all cases, C-H bond addition of ortho-fluoropyridines or meta-fluoropyridine is discouraged because such substrate must bind to titanium via its C-H bond, which is rather weak compared to the titanium-pyridine binding.

Two-step radical reactions that switch low multiplicity channels leading to the carbene and carbyne species detected for Ru(5F) + CH4−nFn (n = 2–4) interactions under matrix isolation conditions

The results obtained from a CASSCF-MRMP2 study of Ru(5F,3F) + CH4−nFn (n = 2–4) reactions are used to propose a two-reaction model that explains the high-oxidation state products, which primarily include carbene and carbyne complexes, detected for these interactions in infrared matrix-isolation determinations, without invoking intersystem crossings between electronic states of different multiplicities. For each of these reactions, it was determined that the channels emerging from the ground and first excited states of the reactants both evolve nearly degenerate radical asymptotes involving the species ·Ru–F and ·CH4−nFn−1 that differ only in the spin of the non-metal fragment. Therefore, under cryogenic-matrix conditions, the caged radicals produced from the ground state of the reactants can recombine and yield species that exhibit quintuplet and triplet multiplicities. The calculated energy profiles for the recombination reactions show that the low-spin channels evolve to the products under matrix conditions.

Effects of substituents on the formation of rhenium carbyne and η2-vinyl complexes from the reactions of ReH5(PMe2Ph)3 with terminal alkynes

The reactions of the rhenium polyhydride complex ReH5(PMe2Ph)3 with HCCAr (Ar = p-C6H4CF3, o-C6H4CF3, o-C6H4Me and o-C6H4Br) have been carried out. Treatment of ReH5(PMe2Ph)3 with HCC(p-C6H4CF3) in the presence of two equiv. of HCl produces a mixture of the carbyne complex Re(CCH2(p-C6H4CF3))Cl2(PMe2Ph)3 and the η2-vinyl complex Re(η2-CH2C(p-C6H4CF3))Cl2(PMe2Ph)3. Treatment of ReH5(PMe2Ph)3 with HCCAr (Ar = o-C6H4CF3, o-C6H4Me, o-C6H4Br) in the presence of two equiv. of HCl produces only the carbyne complexes Re(CCH2Ar)Cl2(PMe2Ph)3. The mechanism of the reactions of ReH5(PMe2Ph)3 with HCCR has also been studied with the aid of DFT calculations, which suggests that the reactions are kinetically controlled and the pathways leading to the carbyne and η2-vinyl complexes are influenced by the steric and electronic effects of the substituents of the alkynes.

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes is now ~45 years old. Progress in the practical aspects of alkyne metathesis reactions with well-defined complexes, as well as applications, in the last decade, guarantees that it is destined to become a useful method for the synthesis of organic molecules.

Dihydrogen activation by a tungsten–alkylidyne complex: toward photoredox chromophores that deliver renewable reducing equivalents

The d(1) tungsten-alkylidyne radical [W(CPh)(dppe)2Cl](+) reacts with H2 to give the d(0) hydride [W(CPh)(H)(dppe)2Cl](+), which on deprotonation yields the d(2) photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.

Reactions of d0 tungsten alkylidyne complexes with O2 or H2O. Formation of an oxo siloxy complex through unusual silyl migrations

(Me3SiCH2)3(Me3SiC≡)W←O=PMe3 (1), an adduct between (Me3SiCH2)3W≡CSiMe3 (2) and O=PMe3, reacts with O2 to give O=W(OSiMe3)(CH2SiMe3)3 (3) and CO2. Reaction of 2 with H2O yields 3 and the trimer [(μ-O)W(CH2SiMe3)2(=O)(THF)]3 (4). In the reaction of D2O with 2, 3-d(n) and methane isotopologues CH2D2, CHD3 and CD4 have been observed.

Electronic Structure and Reactivity of the Carbyne-Bridged Dimolybdenum Radical [Mo2(η5-C5H5)2(μ-CPh)(μ-PCy2)(μ-CO)]+

The unsaturated compound [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] (1, Cp = η5-C5H5) reacts with trace amounts of water in the presence of [FeCp2]BF4 to give a mixture of the hydroxycarbyne complex [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 (minor) and the hydroxo complex [Mo2Cp2(μ-CPh)(OH)(μ-PCy2)(CO)]BF4 (major product), with the latter rapidly rearranging to give the carbene isomer cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 (Mo–Mo = 2.9435(3) Å). An analogous reaction takes place with phenol, to give selectively the related phenoxo complex [Mo2Cp2(μ-CPh)(OPh)(μ-PCy2)(CO)]BF4. In contrast, the reactions of 1 with H2SiPh2 or H3BNH2tBu in the presence of [FeCp2]BF4 result in the selective H transfer to the O atom of the carbonyl ligand, to give the mentioned hydroxycarbyne complex. All the above reactions can be rationalized by assuming the initial formation of the radical cation [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)]+ (2), a molecule displaying a somewhat weakened intermetallic bonding (Mo–Mo = 2.537 Å vs 2.493 Å in 1) and a linear semibridging carbonyl, with both the LUMO and most of the unpaired electron density being located at a single molybdenum atom, with a much smaller distribution over the oxygen atom of the carbonyl ligand, according to density functional theory calculations. As expected, the radical 2 adds rapidly a molecule of nitric oxide to give a diamagnetic product, but spontaneous decarbonylation also takes place to eventually give the 30-electron nitrosyl complex [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-NO)]BF4. Deprotonation of cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 gives the neutral carbyne complex cis-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8024(5) Å), which upon protonation reverts to its carbene precursor, via the corresponding hydroxo complex. Related trans isomers can be prepared through protonation reactions of trans-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8206(6) Å), a complex easily prepared by reacting the dicarbonyl [Mo2Cp2(μ-CPh)(μ-PCy2)(CO)2] with air.

Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand

This report presents the synthesis of the first neutral trianionic ONO pincer-type tungsten alkylidyne complex, [CF(3)-ONO]W≡C((t)Bu)(OEt(2)) (5) {where CF(3)-ONO = (MeC(6)H(3)[C(CF(3))(2)O])(2)N(3-)}. Treating 5 with 1-phenylpropyne, 4,4-dimethyl-2-pentyne, and cyclooctyne yields the corresponding tungstenacyclobutadiene complexes [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C(Ph)] (6), [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C((t)Bu)] (7), and [CF(3)-ONO]W[κ(2)-C((t)Bu)C(CH(2))(6)C] (8). Complexes 6, 7, and 8 do not undergo retro-[2 + 2]-cycloaddition even at 200 °C or in the presence of PMe(3). DFT methods to elucidate the electronic structure of complexes 5 and 6 reveal important electronic factors that contribute to the lack of reactivity for the tungstenacyclobutadienes. An important bonding combination between the pincer N-atom lone pair and the W[triple bond, length as m-dash]C bond within 5, termed an inorganic enamine, provides an explanation for the lack of retro-[2 + 2]-cycloaddition from 6, 7, and 8. (15)N NMR spectroscopy was used to confirm the computational finding of an inorganic enamine bonding combination. Single crystal X-ray analysis of 5, 6, 7, and 8 provides insight into possible steric inadequacies within the CF(3)-ONO(3-) ligand to promote catalytic metathesis.