Carboxyketenes from 4-hydroxy-1,3-oxazin-6-ones and Meldrum's acid derivatives

New 4-hydroxy-1,3-oxazin-6-ones 8 and 16 were prepared from chlorocarbonyl(phenyl)ketene and amides. The flash vacuum thermolysis (FVT) reactions of these compounds and the 4-methoxy derivative 17 were investigated by Ar matrix isolation IR spectroscopy and online mass spectrometry including MS/MS analysis. Carboxy(phenyl)ketene 10 is formed as the major product by thermal fragmentation of 4-hydroxy-1,3-oxazin-6-one 8. This takes place via the unstable 6-hydroxy tautomer 9. Another tautomer, the 5H-isomer 12, leads to the formation of benzoyl isocyanate 13 as a minor product together with phenylketene 14. Carboxy(phenyl)ketene 10 remains detectable at high FVT temperatures but undergoes thermal decarboxylation to phenylketene 14. The same carboxy(phenyl)ketene 10 is also produced in significant amounts by FVT of 5-phenyl-Meldrum's acid 18 via the unstable enol tautomer 19. A small amount of the unsubstituted carboxyketene 20 is observable on FVT of Meldrum's acid 1 itself.     

Amides as precursors of imidoyl radicals in cyclisation reactions

Amides have been successfully used as precursors of imidoyl radicals for radical cyclisation. The amides have been converted to imidoyl selanides via reaction with phosgene to yield imidoyl chlorides followed by reaction with potassium phenylselanide. Imidoyl selanides were reacted with tributyltin hydride (Bu₃SnH) as the radical mediator with triethylborane or AIBN as initiators to yield imidoyl radicals for cyclisation reactions. Imidoyl radicals have been cyclised onto alkenes to yield 2,3-substituted-indoles and -quinolines and also onto pyrroles and indoles to give bi- and tricyclic heteroarenes.     

Acylthioketene-thioacylketene-thiet-2-one rearrangements

Flash vacuum thermolysis (FVT) of 6-aryl-1,3-dioxine-4-thiones 9 leads to the formation of acylthioketenes 10, which are characterized by Ar matrix IR spectroscopy as well as on-line tandem mass spectrometry. The thioketenes 10 undergo a 1,3-shift of the aryl group to generate thioacylketenes 11. Ketenes 11 cyclize to 3-aryl-thiet-2-ones 12, which are also characterized by matrix IR spectroscopy and tandem mass spectrometry. The thiet-2-ones 12 undergo two kinds of reaction under the FVT conditions: (i) cheletropic CO extrusion with formation of arylthioketenes 13, and (ii) cycloreversion to COS and arylacetylene.     

Facile 1,3- and 1,5-Chlorine Migration

High-level ab initio molecular orbital calculations, using the G2(MP2,SVP) theory (and semiempirical methods) have been used to examine several 1,3- and 1,5-chlorine migrations. It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair-LUMO-mediated pericyclic reaction). The activation barriers for the 1,3-migration in chloro oxo ketene 1 (Cl(C=O)CH=C=O) and the 1,5-migration in (2-(chlorocarbonyl)vinyl)ketene 2 (Cl(C=O)CH=CHCH=C=O) are only 53 and 61 kJ mol(-)(1), respectively, compared to the 216 and 173 kJ mol(-)(1) barriers for the corresponding unassisted 1,3- and 1,5-sigmatropic shifts of Cl in 3-chloro-1-propene and 5-chloro-1,3-pentadiene. The transition structures for 1 and 2 reveal that migration of the chlorine atoms takes place in the molecular planes. The 1,5-chlorine shift in 6-chlorocyclohexa-2,4-dienone (3) has a significantly higher barrier due to a lack of appropriate orbital interaction. The related 1,3-shift in the (chlorocarbonyl)imine-alpha-chloro isocyanate system is also dramatically accelerated compared with conventional pericyclic 1,3-Cl migration.     

Energy profiles for ketene cyclizations. Interconversion of 1,3-oxazin-6-ones, mesoionic 1,3-oxazinium olates and acylketenes, imidoylketenes, oxoketenimines, and cyclization products

The energy surface connecting oxazinium olates 9, several possible conformers of ketenes 10 and 11, and the final cyclization products 12, 13 and 14, as well as the isomeric 1,3-oxazine-6-ones 15, ring opening of the latter to N-acylimidoylketenes 16, and subsequent rearrangement of 16 to oxoketenimines 17, azetinones 18, and the cyclization products 19 and 20 are evaluated computationally at the B3LYP/6-31G and B3LYP/6-311+G//B3LYP/6-31G levels. The cyclizations of ketenes to oxazinium olates 9 and oxazines 15 have the characteristics of pseudopericyclic reactions. Plots of the energy vs internal reaction coordinate for the cyclization of transoid acylketenes such as 10 to 9 (via TS1) and 16 to 15 (via TS7) feature two inflection points and indicate that the part of the energy surface above the lower inflection points describe internal rotation of the acyl function in the ketene moiety, and the part below this point describes the cyclization of the cisoid ketene to the planar mesoionic oxazinium olate 9 or oxazinone 15. The 1,3-shifts of the OR group that interconvert ketenes 16 and ketenimines 17 via four-membered cyclic transition states TS8 behave similarly, the first portion (from the ketenimine side) of the activation barrier being due largely to internal rotation of substituents, and the top part being due to the 1,3-shift proper.     

Imidoylketene-alpha-oxoketenimine and alpha-oxoketene-alpha-oxoketene rearrangements. 1,3-shifts of substituted phenyl groups

1,3-phenyl shifts interconvert imidoylketenes 1 and alpha-oxoketenimines 2 and, likewise, alpha-oxoketenes 3 automerize by this 1,3-shift. These rearrangements usually take place in the gas phase under conditions of flash vacuum thermolysis. Energy profiles calculated at the B3LYP/6-31G(d,p) and B3LYP/6311 + G(3df,2p)//B3LYP/6-31G(d,p) levels demonstrate that electron donating substituents (D) in the migrating phenyl group and electron withdrawing ones (W) in the non-migrating phenyl group, can stabilise the transition states TS1 and TS2 to the extent that activation barriers of ca. 100 kJ mol(-1) or less are obtained; i.e. enough to make these reactions potentially observable in solution at ordinary temperatures. The calculated transition state energies DeltaG(TS1) show an excellent correlation with the Hammett constants sigma(p)(W) and sigma(p+)(D).     

Tandem pseudopericyclic reactions: [1,5]-X sigmatropic shift/6pi-electrocyclic ring closure converting N-(2-X-carbonyl)phenyl ketenimines into 2-X-quinolin-4(3H)-ones

N-(2-X-Carbonyl)phenyl ketenimines undergo, under mild thermal conditions, [1,5]-migration of the X group from the carbonyl carbon to the electron-deficient central carbon atom of the ketenimine fragment, followed by a 6pi-electrocyclic ring closure of the resulting ketene to provide 2-X-substituted quinolin-4(3H)-ones in a sequential one-pot manner. The X groups tested are electron-donor groups, such as alkylthio, arylthio, arylseleno, aryloxy, and amino. When involving alkylthio, arylthio, and arylseleno groups, the complete transformation takes place in refluxing toluene, whereas for aryloxy and amino groups the starting ketenimines must be heated at 230 degrees C in a sealed tube in the absence of solvent. The mechanism for the conversion of these ketenimines into quinolin-4(3H)-ones has been studied by ab initio and DFT calculations, using as model compounds N-(2-X-carbonyl)vinyl ketenimines bearing different X groups (X = F, Cl, OH, SH, NH(2), and PH(2)) converting into 4(3H)-pyridones. This computational study afforded two general reaction pathways for the first step of the sequence, the [1,5]-X shift, depending on the nature of X. When X is F, Cl, OH, or SH, the migration occurs in a concerted mode, whereas when X is NH(2) or PH(2), it involves a two-step sequence. The order of migratory aptitudes of the X substituents at the acyl group is predicted to be PH(2) > Cl > SH > NH(2) > F> OH. The second step of the full transformation, the 6pi-electrocyclic ring closure, is calculated to be concerted and with low energy barriers in all the cases. We have included in the calculations an alternative mode of cyclization of the N-(2-X-carbonyl)vinyl ketenimines, the 6pi-electrocyclic ring closure leading to 1,3-oxazines that involves its 1-oxo-5-aza-1,3,5-hexatrienic system. Additionally, the pseudopericyclic topology of the transition states for some of the [1,5]-X migrations (X = F, Cl, OH, SH), for the 6pi-electrocyclization of the ketene intermediates to the 4(3H)-pyridones, and for the 6pi-electrocyclization of the starting ketenimines into 1,3-oxazines could be established on the basis of their geometries, natural bond orbital analyses, and magnetic properties. The calculations predict that the 4(3H)-pyridones are the thermodynamically controlled products and that the 1,3-oxazines should be the kinetically controlled ones.     

Ketene-acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio(G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH(2) and NMe(2)) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol(-1) for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for [small pi]-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol(-1) for X = NMe(2). In contrast, the 1,3-shifts of CH(3) and H have very high barriers.     

A one-pot reaction of (chlorocarbonyl) phenyl ketene with β-ketoamides and thiochromen-2-one

Readily available (chlorocarbonyl)phenyl ketene and a varied set of β-ketoamides were reacted in a one-step procedure to produce 2-pyrone derivatives. β-Ketoamide derivatives are versatile intermediates for the synthesis of heterocyclic compounds. For instance, 1-morpholino-3-phenyl-1,3-propanedione, 1-phenyl-3-piperidino-1,3-propanedione, 1-phenyl-3-pyrrolidino-1,3-propanedione, 1-piperidino-1,3-butanedione, 1-morpholino-2-phenyl ethanone were used in these reactions to produce 2-pyrone derivatives. In addition, the preparation of 4-hydroxy-3-phenylthiochromeno[4,3-b]pyran-2,5-dione derivatives and 4-hydroxy-7-methyl-3-phenyl pyrano[3,2-c]chromeno-2,5-dione were described. For the synthesis of these compounds 4-hydroxy-2H-thiochromen-2-ones and 4-hydroxy-8-methyl-2H-chromen-2-one were reacted with (chlorocarbonyl)phenyl ketene and the final products were isolated in good yields.     

Rearrangement of 2-quinolyl- and 1-isoquinolylcarbenes to naphthylnitrenes

2-quinolylcarbene 23 and 1-isoquinolylcarbene 33 are generated by flash vacuum thermolysis (FVT) of the corresponding triazolo[1,5-a]quinoline and triazolo[5,1-a]isoquinoline 19 and 29, as well as 2-(5-tetrazolyl)quinoline and 1-(5-tetrazolyl)isoquinoline 20 and 30, respectively. These carbenes rearrange to 1- and 2-naphthylnitrene 21 and 31, respectively, and the nitrenes are also generated by FVT of 1- and 2-naphthyl azides 18 and 28. The products of FVT of both the nitrene and carbene precursors are the 2- and 3-cyanoindenes 26 and 27 together with the nitrene dimers, viz. azonaphthalenes 25 and 35, and the H-abstraction products, aminonaphthalenes 24 and 34. All the azide, triazole, and tetrazole precursors yield 3-cyanoindene 26 as the principal ring contraction product under conditions of low FVT temperature (340-400 degrees C) and high pressure (1 Torr N(2) as carrier gas for the purpose of collisional deactivation). This ring contraction reaction is strongly subject to chemical activation, which caused extensive isomerization of 3-cyanoindene to 2-cyanoindene under conditions of low pressure (10(-3) Torr). 2-Cyanoindene is calculated to be ca. 1.7 kcal/mol below 3-cyanoindene in energy; accordingly, high-temperature FVT of these cyanoindenes always gives mixtures of the two compounds with the 2-cyano isomer dominating. Photolysis of trizolo[1,5-a]quinoline 19 and triazolo[5,1-a]isoquinoline 29 in Ar matrixes causes partial ring opening to the corresponding 2-diazomethylquinoline 19' and 1-diazomethylisoquinoline 29'. The photolysis of the former gives rise to a small amount of the cyclic ketenimine 22, the intermediate connecting 2-quinolylcarbene and 1-naphthylnitrene.     

Carboxyketenes, methyleneketenes, vinylketenes, oxetanediones, ynols, and ylidic ketenes from Meldrum's acid derivatives

It has been documented that 5-methylene-Meldrum's acid derivatives (1, 12 ) and their enols (2, 13) can undergo fragmentation to malonic anhydrides (4, 19 ), carboxyketenes (3, 16) and methyleneketene (5, 21 , 35 ), as well as cyclization to pyrrole-3-one and thiophene-3-one derivatives 11a,b (but not furan-3-ones 11c ) under the conditions of flash vacuum thermolysis (FVT). Here we report theoretical calculations at the B3LYP/6-311 + G(2d, p) and G3X(MP2) levels of theory, which allow a rationalization of these observations. The calculated activation barriers for these reactions are all of the order of 37-40 kcal mol(-1). Hydroxyacetylenes (alkynols) 7 are sometimes observed in FVT reactions of Meldrum's acid derivatives. Their formation is now explained as an FVT reaction of the carboxyketenes (e.g. 3-->7 and 32-->34) with a calculated activation barrier of ca. 39 kcal mol(-1). The cyclization of alkylamino- and alkylthio-substituted methyleneketenes 8a,b to pyrrolone and thiophenone derivatives 11a,b is found to be energetically very feasible under FVT conditions, and even in some cases in solution, with activation barriers of 33-39 kcal mol(-1). This cyclization takes place via the fleeting ylidic ketene intermediates 9a,b,25, and 37a,b, which exist in very shallow energy minima. Alkoxy-substituted methyleneketenes 8c do not cyclize in this manner due to the rather high, but in principle not impossible, activation barriers for the initial 1,4-H shifts to the ylidic ketenes 9c (ca. 47 kcal mol(-1)).     

Cationic polymerization of cyclic ketene acetals via zwitterion formation with cyanoallene

This article deals with the polymerization of the cyclic ketene acetals (CKAs) 2‐methylene‐4‐phenyl‐1,3‐dioxolane (2), 2‐methylene‐4‐phenyl‐1,3‐dioxane (3), 4,7‐dimethyl‐2‐methylene‐1,3‐dioxepane (4), 2‐ethylidene‐4‐phenyl‐1,3‐dioxolane (5), 2‐phenylmethylene‐1,3‐dioxolane (6), and 2‐isopropylidene‐4‐phenyl‐1,3‐dioxolane (7) in the presence of cyanoallene (1). For 2 and 3, the homopolymerization of the CKAs proceeded without ring opening, and the number‐average molecular weights of the obtained polymers depended on the feed ratio of 1. However, the reactions of 1 with 4–7 afforded no polymers but did afford spirocyclic 1 : 1 adducts possessing cyclobutane rings. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2075–2081, 2000     

The Behaviors of Metal Acetylides with Dinitrogen Tetroxide

Lithium phenylacetylide ( 1a ) and N₂O₄ ( 2 ) at −78° yield diphenylbutadiyne ( 6a ) by oxidative coupling, phenylacetylene ( 7a ) by oxidation and then solvent H‐abstraction, and benzoyl cyanide ( 8 ) by dimerizative‐rearrangement of nitroso(phenyl)acetylene ( 23 ). Nitro(phenyl)acetylene ( 3 , R=Ph) is not obtained. Benzonitrile ( 9 ), a further product, possibly results from hydrolytic decomposition of nitroso(phenyl)ketene ( 27 ) generated from phenylacetylenyl nitrite ( 26 ). Phenylacetylene ( 7a ) and 2 give, along with (E)‐ and (Z)‐1,2‐dinitrostyrenes ( 34 and 35 , resp.), 3‐benzoyl‐5‐phenylisoxazole ( 10 ), presumably as formed by cycloaddition of benzoyl nitrile oxide ( 40 ) to 7a . Further, 2 reacts with other lithium acetylides ( 1b – 1e ), and with sodium, magnesium, zinc, copper, and copper lithium phenylacetylides, 1f – 1l , to yield diacetylenes 6a – 6c and monoacetylenes 7a – 7c . Conversions of metallo acetylide aggregates to diacetylenes are proposed to involve generation and addition reactions of metallo acetylide radical cationic intermediates in cage, further oxidation, and total loss of metal ion. Loss of metal ions from metallo acetylide radical cations and H‐abstraction by non‐caged acetylenyl radicals will give terminal acetylenes. The principal reactions (75–100%) of heavy metal acetylides phenyl(trimethylstannyl)acetylene ( 44 ) and bis(phenylacetylenyl)mercury ( 47 ) with 2 are directed nitrosative additions (NO⁺) and loss of metal ions to give nitroso(phenyl)ketene ( 27 ), which converts to benzoyl cyanide ( 8 ).     

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3-dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3-dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m’-bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m’-bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m’-bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2-phenyl-1-azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol⁻¹ in the singlet state. In the bent triplet states, 1,3-hydrogen shifts interconverting nitrile ylides are potentially possible.     

1,3-噻唑烷衍生物的合成、结构及生物活性

为了寻找新的含噻唑杂环的先导化合物, 利用2-取代-1,3-噻唑烷1与氯甲酸取代苯基酯2及取代苯基甲酰氯4在三乙胺存在下发生缩合反应, 合成了6个2-取代-1,3-噻唑烷-3-甲酸酯类化合物3和5个N-苯甲酰基-2-取代-1,3-噻唑烷类化合物5, 并利用¹H NMR, IR, MS, 元素分析对其结构进行了表征. 用X-ray单晶衍射测定了2-硫代-1,3-噻唑烷-3-甲酸苯酯(3d)和N-苯甲酰基-2-氧代-1,3-噻唑烷(5a)的晶体结构, 证实了氯甲酸酯与2-硫代-1,3-噻唑烷的反应产物为硫酮式结构, 而非硫酯式结构. 初步生物活性试验结果表明, 在试验浓度下部分目标化合物3和5具有一定的杀菌和杀虫活性.     

Synthesis of ellipticine: a radical cascade protocol to aryl- and heteroaryl-annulated[b]carbazoles

[reaction: see text] Imidoyl selanides, synthesized from amides, have been used as radical precursors of imidoyl radicals in cascade reactions. The novel radical cascade has been developed for the simple synthesis of the medicinally important aryl-annulated[b]carbazoles. The protocol has been exemplified with the high-yielding total synthesis of the anticancer alkaloid ellipticine.     

Theoretical study on chlorine and hydrogen shift in cycloheptatriene and cyclopentadiene derivatives

The transition structures (TSs) for chlorine 1,7-shift and 1,5-shift in 1,7,7-trichlorocycloheptatriene (1) and those of chlorine 1,5-shifts in 1,5,5-trichlorocyclopentadiene (3) and 1,2,5-trichloro-1,3-pentadiene (5) derivatives have been located with density functional theory (DFT) at the Becke3LYP/6-311G [and Becke3LYP/6-311+G] level. The calculational results were compared with those for corresponding hydrogen shifts in nonsubstituted molecules (cycloheptatriene (2), cyclopentadiene (4), and 1,3-pentadiene (6)). The following points were clarified: (1) The activation energy (Delta E(++)) for chlorine 1,7-shift in 1 was evaluated to be only +50.1 [+49.2] kJ/mol, which is smaller than that (+69.9 [+68.3]) for a 1,5-shift, supporting the theory that the conversion between two equivalent A and A' proceeds through a TS for direct chlorine 1,7-shift (Figure 1), rather than through a TS for a 1,5-shift (Figure 2). (2) The considerable amount of charge separation between a migrating chlorine atom (Cl(m)) and a seven-membered ring (-0.53 and +0.47 for Merz-Singh-Kollman scheme) occurs in a chlorine 1,7-shift, which is in good contrast to the result that the migrating hydrogen atom (H(m)) for a 1,7-shift in cycloheptatriene (2) carries almost no charge (Figure 3). This large charge separation can stabilize the TS for the chlorine 1,7-shift pathway. (3) The Delta E(++) values for suprafacial hydrogen 1,7-shift in 2 are quite large (+288.0 [+284.8] kJ/mol), much larger than that (+166.8 [+167.0]) for a 1,5-shift in 4 which is orbital symmetrically allowed (Figure 3). The calculation suggests that the chlorine 1,7-shift in 1 occurs easily at room temperature (actually observed experimentally) by proceeding via concerted suprafacial 1,7-shift through the zwitterionic TS with the significant assistance of Coulomb interaction between charged fragments (negatively charged chlorine atom and positively charged tropylium ring), rather than via a suprafacial 1,5-sigmatropic pathway. Other cases studied in this paper showed usual results predicted by orbital symmetrical consideration.     

A simple synthesis of 5‐ethoxycarbonyl‐6‐phenyl‐1,3‐dioxin‐4‐ones and ethyl 3‐benzoyl‐4‐oxo‐2,6‐diphenylpyran‐5‐carboxylate

4‐Ethoxycarbonyl‐5‐phenyl‐2,3‐dihydrofuran‐2,3‐dione 1 reacts with aldehydes via the acylketene intermediate 2 giving the 1,3‐dioxin‐4‐ones 3a‐e and the 1,4‐bis(5‐ethoxycarbonyl‐4‐oxo‐6‐phenyl‐4H‐1,3‐dioxin‐2‐yl)benzene 4 , and a one step reaction between dibenzoylmethane and oxalylchloride gave 3,5‐dibenzoyl‐2,6‐diphenyl‐4‐pyrone 7 . The reaction of 1 with dibenzoylmethane, a dicarbonyl compound, provided ethyl 3‐benzoyl‐4‐oxo‐2,6‐diphenylpyran‐5‐carboxylate derivative 9 . Compound 9 was converted into the corresponding ethyl 3‐benzoyl‐4‐hydroxy‐2,6‐diphenylpyridine‐5‐carboxylate derivative 10 via its reaction with ammonium hydroxyde solution in 1 ‐butanol.     

Reissert compound studies. XXXVII. Attempts to form reissert compounds from quinazoline and cinnoline

Reaction of quinazoline with trimethylsilyl cyanide and benzoyl chloride gave 2,4-dicyano-1,3-dibenzoyl-1,2,3,4-tetrahydroquinazoline as the major product. This compound undergoes monoalkylation with methyl iodide in the presence of sodium hydride. Reaction of cinnoline with trimethylsilyl cyanide and benzoyl chloride gave 1,2-dibenzoyl-4-cyano-1,2-dihydrocinnoline.     

Double C-H activation during functionalization of phenyl(methyl)ketene on iridium(I) using alkynes. synthesis of 1,4-dien-3-ones

Under the influence of an Ir(I) metal fragment, the methyl group of phenyl(methyl)ketene undergoes two C-H activations in reacting with internal alkynes, giving metallacycles 3 in 86-94% yield. Treatment of 3 with CO liberates 1,4-dien-3-ones 5 in 81-93% yield, along with CO complex 4. A possible mechanism for the very selective double C-H activation-alkyne coupling is discussed.