Phosphane Groups Research Articles

Syntheses and Conformations of a Trinuclear Gold(I) System Based on a Triethinyl‐trisilacyclohexane Scaffold

AbstractTrisilacyclohexanes and bissilylmethanes with acetylene units attached to silicon sites have been converted into tri‐ and di‐nuclear gold(I)phosphane complexes and characterized by NMR spectroscopy and X‐ray diffraction. The bissilylmethane‐based gold complexes were used as a test system to work out the synthetic routes before transferring them to the trisilacyclohexane scaffolds, because the latter are more difficult to access. The occurrence of aurophilic interactions (short Au ⋅ ⋅ ⋅ Au contacts) depends on the steric bulk of the phosphorus‐bonded alkyl and aryl groups. DFT studies show that dispersion effects play an important role in the ability to form intramolecular Au ⋅ ⋅ ⋅ Au interactions. QTAIM analyses show that a subtle change of the steric bulk of the phosphane groups alters the relative strength of the dispersion and aurophilic interactions. The combination of DFT studies and X‐ray diffraction experiments shows that the conformation of the trisilacyclohexane backbone is also phase dependent.

Synthesis and Some Coordination Chemistry of Phosphane-Difunctionalized Bis(amidinato)-Heavier Tetrylenes: A Previously Unknown Class of PEP Tetrylenes (E = Ge and Sn).

The bis(amidinato)-heavier tetrylenes E(bzamP)2 (E = Ge (2a) and Sn (2b); bzamP = N-isopropyl-N'-(diphenylphosphanylethyl)benzamidinate), which are equipped with one heavier tetrylene (germylene or stannylene) and two phosphane fragments (one on each amidinate moiety) as coordinable groups, have been synthesized from the benzamidinum salt [H2bzamP]Cl and GeCl2(dioxane) or SnCl2 in 2:1 mol ratio. A preliminary inspection of their coordination chemistry has shown that their amidinate group can also be involved in the bonding with the metal atoms as tridentate ENP and tetradentate PENP' coordination modes have been observed for the ECl(bzamP)2 ligand of [Ir{κ3E,N,P-ECl(bzamP)2}(cod)] (E = Ge (3a) and Sn (3b); cod = η4-1,5-cyclooctadiene) and the E(bzamP)2 ligand of [Ni{κ4E,N,P,P'-E(bzamP)2}] (E = Ge (4a) and Sn (4b)), which are products of reactions of 2a and 2b with [IrCl(cod)]2 (1:0.5 mol ratio) and [Ni(cod)2] (1:1 mol ratio), respectively. These products contain a 5-membered NCNEM ring that results from the insertion of the metal M atom into an E-N bond of 2a and 2b. Additionally, while iridium(I) complexes 3a and 3b are chloridotetryl derivatives (insertion of the tetrylene E atom into the Ir-Cl bond has also occurred) that have an uncoordinated phosphane group, nickel(0) complexes 4a and 4b contain a tetrylene fragment that, maintaining the lone pair, behaves as a σ-acceptor (Z-type) ligand.

Mono- and Dinuclear Gold(I) Coumarin Complexes: Luminescence Studies and Singlet Oxygen Production.

The 4-(thiolmethyl)-7-(diethylamino)-2H-chromen-2-one ligand has been synthesized and used as chromophore in several mono- and dinuclear gold(I) compounds that contain a phosphane at the second coordination position. Four final products were able to obtain in pure form containing one coumarin and one phosphane ligand in the case of PTA (1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane) and PPh3 (triphenylphosphine); one coumarin and two gold(I)-phosphane groups in the case of phosphane=DAPTA (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) and two coumarin and two gold(I) atoms in the case of phosphane=DPEphos (bis[(2-diphenylphosphino)phenyl]ether), when it was used a diphosphane. Other diphosphane ligands used were not able to give the desired products in pure form. The luminescent properties of the compounds are governed by the fluorescence of the coumarin moiety in all compounds both for measurements carried out in solution and also immobilized in PMMA organic matrix. Phosphorescence emission can be detected in all cases at 77 K both for the uncoordinated coumarin ligand and the gold(I) derivatives, being more favoured in the presence of the gold(I) heavy atom. The compounds have been used as photosensitizers to generate 1 O2 with moderate quantum yields values.

Transition-metal complexes of group 12 with 1,1'-bis(phosphanyl)ferrocene ligands.

The syntheses of four new cadmium and zinc complexes with 1,1'-bis(phosphanyl)ferrocene ligands and their phosphine chalcogenide derivatives are reported. The complexes were characterized by elemental analyses and IR, 1H NMR, 31P NMR and electronic absorption spectroscopy. The crystal structures of dichlorido[1-diphenylphosphinoyl-1'-(di-tert-butylphosphanyl)ferrocene-κ2O,P]cadmium(II), [CdCl2{(C17H14OP)(C13H22P)Fe}] or CdCl2(κ2P,O-dppOdtbpf) (1), bis[μ-(tert-butyl)(1'-diphenylphosphinoylferrocen-1-yl)phosphinato-κ3O,O':O'']bis[chloridozinc(II)], [Zn2{(C9H13O2P)(C17H14OP)Fe}2Cl2] or [ZnOCl{κ2O,O'-Ph2POFcPO2(t-Bu)}]2 (2), 1,1'-bis(di-tert-butylthiophosphinoyl)ferrocene, [Fe(C13H22PS)2] or dtbpfS2 (3), and [1,1'-bis(dicyclohexylphosphanyl)ferrocene-κ2P,P'][chlorido/cyanido(0.25/1.75)]zinc(II), [Zn(CN)1.75Cl0.25{(C17H26P)2Fe}] or Zn(CN)2(κ2-dcpf) (4), were determined crystallographically. Compound 1 has tetrahedral geometry in which the CdII centre is coordinated by one dppOdtbpf ligand in a κ2-manner and by two Cl atoms, while compound 2 displays a centrosymmetric dimeric unit in which two oxide atoms bridge the two Zn atoms to generate an eight-membered ring. Compound 3 revealed a sandwich structure with both phosphane groups sulfurized. In compound 4, the ZnII atom adopts a tetrahedral geometry by coordinating to the 1,1'-bis(dicyclohexylphosphanyl)ferrocene ligand in a κ2-manner and to two cyanide ligands.

The Transition Metal Chemistry of PGeP and PSnP Pincer Heavier Tetrylenes

This review article collects and discusses the syntheses of the currently known metal‐free heavier tetrylenes having a PEP pincer topology (E = tetrel atom) as well as their transition metal derivative chemistry. To date, only five PGeP germylenes and two PSnP stannylenes have been isolated. These compounds have been successfully synthesized by treating GeCl2(diox) (diox = 1,4‐dioxane), GeCl2(NHC) (NHC = N‐heterocyclic carbene) or SnCl2 with two equivalents of an appropriate lithiated phosphine. Their transition metal chemistry is dominated by processes in which, in addition to the coordination of one or both phosphane groups, the E atom ends inserted into M–M or M–X (X = anionic group) bonds. Only in a few occasions a simple coordination of the tetrylene fragment (as a neutral two‐electron‐donor ligand) has been observed. The structural and bonding characteristics of the metal‐free PEP tetrylenes and of relevant examples of their transition metal derivatives are also surveyed.

New selective thiolate gold(i) complexes inhibit the proliferation of different human cancer cells and induce apoptosis in primary cultures of mouse colon tumors.

New thiolate gold(i) complexes with P(NMe2)3 (HMPT) as a phosphane group [Au(SR)(HMPT)] (SR = Spy, Spyrim, SMe2pyrim, Sbenzothiazole, Sthiazoline, Sbenzimidazole and 2-thiouracil) have been synthesized. All of them have been characterized, including X-ray studies of complexes with SMe2pyrim, Sbenzothiazole and 2-thiouracil moieties. In addition, their potential application as anticancer drugs has been analyzed by determining their pharmacokinetic activities (water solubility, cell permeability and BSA transport protein affinity). Based on the good results of these experiments, we carried out the studies of cell viability with our compounds on different cell lines (A2780, A2780R and Caco-2/TC7 cells), showing higher cytotoxic activity than cisplatin in all cases. Besides, two of the synthesized complexes with Sbenzimidazole and 2-thiouracil groups exhibit specific selectivity for cancerous Caco-2 cells and are considered as potential candidates for anticancer drugs. These complexes were able to induce a strong inhibition of the thioredoxin reductase (TrxR) protein and oxidative damage in membrane lipids. Additional studies in primary cultures of mouse colon tumors showed that these two complexes are proapoptotic upon exposure to phosphatidylserine. Based on our results, we conclude that two of our thiolate gold(i) complexes are good and effective candidates for use in chemotherapy.

A Z-type PGeP pincer germylene ligand in a T-shaped palladium(0) complex.

A dipyrromethane-based germylene decorated with two phosphane groups has been used to prepare an unusual T-shaped palladium(0) containing a PGeP pincer germylene that acts as a Z-type ligand. This compound is a strong reducing reagent, as it has been easily oxidized to germyl-palladium(ii) derivatives with a gold(i) complex, HCl and Ph2S2 through processes that involve formal addition of a bond of the oxidant across the Ge-Pd bond.

A Germylene Supported by Two 2-Pyrrolylphosphane Groups as Precursor to PGeP Pincer Square-Planar Group 10 Metal(II) and T-Shaped Gold(I) Complexes.

An efficient synthesis of 2-di-tert-butylphosphanylmethylpyrrole (HpyrmPtBu2 ), by treating 2-dimethylaminomethylpyrrole (HpyrmNMe2 ) with tBu2 PH at 135 °C in the absence of any solvent, has allowed the preparation of the new PGeP germylene Ge(pyrmPtBu2 )2 (1), by treating [GeCl2 (dioxane)] with LipyrmPtBu2 , in which the Ge atom is stabilized by intramolecular interactions with one (solid state) or both (solution) of its phosphane groups. Reactions of germylene 1 with Group 10 metal dichlorido complexes containing easily displaceable ligands have led to [MCl{κ3 P,Ge,P-GeCl(pyrmPtBu2 )2 }] [M=Ni (2), Pd (3), Pt (4)], which have an unflawed square-planar metal environment. Treatment of germylene 1 with [AuCl(tht)] (tht=tetrahydrothiophene) rendered [Au{κ3 P,Ge,P-GeCl(pyrmPtBu2 )2 }] (5), which is a rare case of a T-shaped gold(I) complex. The hydrolysis of 5 gave the linear gold(I) derivative [Au(κP-HpyrmPtBu2 )2 ]Cl (6). Complexes 2-5 contain a PGeP pincer chloridogermyl ligand that arises from the insertion of the Ge atom of germylene 1 into a M-Cl bond of the corresponding metal reagent. The bonding in these molecules has been studied by DFT/NBO/QTAIM calculations. These results demonstrate that the great flexibility of germylene 1 makes it a better precursor to PGeP pincer complexes than the previously known germylenes of this type.

From a PGeP Pincer-Type Germylene to Metal Complexes Featuring Chelating (Ir) and Tripodal (Ir) PGeP Germyl and Bridging (Mn2) and Chelating (Ru) PGeP Germylene Ligands

Different coordination modes of a PGeP chloridogermyl ligand (Ge,P-chelating and P,Ge,P-tripodal) and a PGeP germylene ligand (P,Ge,P-bridging and Ge,P-chelating) have been identified in coordination compounds resulting from reactions of the PGeP pincer-type diphosphane-germylene Ge(NCH2PtBu2)2C6H4 (1) with iridium(I), manganese(0), and ruthenium(II) complex reagents. Germylene 1 reacted with [Ir2(μ-Cl)2(η4-cod)2] (cod = 1,5-cyclooctadiene) to give [Ir{κ2Ge,P-GeCl(NCH2PtBu2)2C6H4}(η4-cod)] (2), which contains a Ge,P-chelating PGeP chloridogermyl ligand and an uncoordinated phosphane group that weakly interacts with the Ge atom. Carbon monoxide readily displaced the cod ligand of 2 to give the dicarbonyl derivative [Ir{κ3P,Ge,P-GeCl(NCH2PtBu2)2C6H4}(CO)2] (3), in which the PGeP chloridogermyl ligand displays a P,Ge,P-tripodal coordination mode. A bridging germylene moiety has been identified in the binuclear derivative [Mn2{μ-κ3P,Ge,P-Ge(NCH2PtBu2)2C6H4}(CO)8] (4), which resulted from the treatment of [Mn2...

Synthesis of 6,6′‐Bis(dimethylamino)‐ and 6,6′‐Dibromo‐Substituted 2,2′‐Diphosphanylbiphenyls and Their Palladium Complexes

AbstractNew 6,6′‐dibromo‐ and 6,6′‐bis(dimethylamino)‐substituted 2,2′‐diphosphanylbiphenyl ligands 11–14 were prepared starting from 2,2′‐dibromo‐4,4′‐dimethyl‐6,6′‐dinitro‐1,1′‐biphenyl (4). Depending on the phosphane groups [diphenylphosphanyl (11, 13) or diisopropylphosphanyl (12, 14)] the palladium dichloride complexes show different coordination symmetry. Whereas the smaller diphenylphosphanyl groups lead to C2‐symmetric complexes, the respective bis(diisopropyl)phosphanes 12 and 14 form C1‐symmetric complexes that show fluxional behavior due to the restricted rotation of the isopropyl groups as well as the exchange of atom positions within the C1‐symmetric conformer. All complexes have been tested as precatalysts in the Suzuki–Miyaura cross coupling reaction of 2‐bromotoluene and phenylboronic acid. The activity of the catalytic system increases with the size of the diphosphanes and the donating ability of the ligand. In contrast to C2‐symmetric palladium complex 15, platinum complex 19 was found to be C1‐symmetric in the solid state despite the fact that both complexes have the small bis(diphenylphosphanyl)‐substituted diphosphane ligand 11 in common. NiBr2 adduct 20 with a similar diphosphane 13 exists as a mixture of distorted square‐planar and tetrahedral coordination sphere geometries in equilibrium with each other.

Synthesis, Characterization, and Dynamic Behaviour of Triosmium Clusters Containing the Tridentate Ligand {Ph2PCH2CH2}2S (PSP)

AbstractReaction of [Os3(CO)11(NCMe)] with bis‐diphenylphosphanylethylene sulfide, {Ph2PCH2CH2}2S (PSP), leads to the formation of [Os3(CO)11(PSP)] and [{Os3(CO)11}2(μ‐PSP)] in good yield. Similarly, treatment of [Os3(CO)10(NCMe)2] with PSP affords the cluster [Os3(CO)10(μ‐PSP)], in which the two phosphanes of the PSP ligand coordinate to different osmium atoms of the same triosmium unit. Reaction of [Os3(CO)11(PSP)] with [Os3(CO)10(NCMe)2] yields the compound 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] in which the thioether moiety and one of the phosphane groups of the PSP ligand are coordinated equatorially to the {Os3(CO)10} subunit. The cluster 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] is also formed when [Os3(CO)11(PSP)] is oxidatively decarbonylated by reaction with trimethylamine N‐oxide. The metastable cluster 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] undergoes slow isomerisation at room temperature to form 1,1‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] in which the thioether and phosphane moieties coordinate in a chelating mode to one of the {Os3(CO)10} subunits with the thioether moiety in an axial position. The dynamic behaviour of these clusters has been investigated by variable‐temperature 13C{1H} and 13P{1H} NMR spectroscopy. The solid‐state structures of [{Os3(CO)11}2(μ‐PSP)] and [Os3(CO)10(μ‐PSP)] are reported.

Oxidation of anticancer Pt(ii) complexes with monodentate phosphane ligands: towards stable but active Pt(iv) prodrugs

The synthesis, characterization and cytotoxicity studies of two novel platinum(IV) complexes, trans-PtCl4(dma)(PPh3), 1, and trans-PtCl4(ipa)(PPh3), 2, where dma is dimethylamine and ipa is isopropylamine, have been carried out. Both complexes contain aliphatic amines trans to phosphane ligands as a good alternative to take advantage of the phosphane group lipophilicity and the stability of platinum(IV) to obtain more effective drugs. Moreover, the complexes are stable in solution and such stability allowed their antitumoral action and DNA interaction to be checked and proved.

Decacarbon-yl[μ(4)-(ethane-1,2-diyl-dinitrilo)-tetra-kis-(methane-thiol-ato)]bis(triphenyl-phosphane)tetra-iron(2 Fe-Fe).

In the title compound, [Fe4(C6H12N2S4)(C18H15P)2(CO)10], the unit cell contains one mol­ecule, which exhibits a crystallographically imposed center of symmetry. The independent Fe2S2 fragment [Fe—Fe = 2.527 (1) Å] is in a butterfly conformation, and each Fe atom displays a pseudo-square-pyramidal coordination geometry. The phosphane group occupies an apical position [Fe—P = 2.2670 (14) Å]. In the crystal, weak inter­molecular C—H⋯O hydrogen bonds link the mol­ecules into chains along [110].

Etherification of Functionalized Phenols with Chloroheteroarenes at Low Palladium Loading: Theoretical Assessment of the Role of Triphosphane Ligands in CO Reductive Elimination

AbstractThe present study highlights the potential of robust tridentate ferrocenylphosphanes with controlled conformation as catalytic auxiliaries in CO bond formation reactions. Air‐stable palladium triphosphane systems are efficient for selective heteroaryl ether synthesis by using as little as 0.2 mol% of catalyst. These findings represent an economically attractive and clean etherification of functionalized phenols, electron‐rich, electron‐poor and para‐, meta‐ or ortho‐substituted substrates, with heteroaryl chlorides, including pyridines, hydroxylated pyridine, pyrimidines and thiazole. The etherification tolerates very important functions in various positions, such as cyano, methoxy, amino, and fluoro groups, which is useful to synthesize bioactive molecules. DFT studies furthermore demonstrate that triphosphane ligands open up various new pathways for the CO reductive elimination involving the third phosphane group. In particular, the rate for one of these new pathways is calculated to be about 1000 times faster than for reductive elimination from a complex with a similar ferrocenyl ligand, but without a phosphane group on the bottom Cp‐ring. Coordination of the third phosphane group to the palladium(II) center is calculated to stabilize the transition state in this new pathway, thereby enhancing the reductive elimination rate.

Anchoring of Ru–Pt and Ru–Au Clusters onto a Phosphane‐Functionalized Carbon Support

AbstractTwo mixed‐metal clusters, [Ru5PtC(CO)14(COD)] (1) and [Ru6Au2C(CO)16(PPh3)2] (2), were anchored onto a prefunctionalized active carbon support (C) with chelating phosphane groups on its surface. These clusters were also deposited onto the unmodified support (CSX+) for comparison. The incorporation of 1 and 2 on both supports was studied by a combination of SIMS and XPS. When the clusters were anchored onto the functionalized carbon support, SIMS spectra displayed characteristic patterns that were different from those obtained in the case of their deposition on the unmodified support. In the latter case, spectra corresponded to the results obtained with pure unsupported clusters. XPS analyses of the supported species seemed to indicate that the stoichiometry of the clusters was retained after anchoring and that their dispersion was better on C than on CSX+. This indicates that the phosphanes act as anchors for noble metal compounds through a ligand exchange mechanism. The supported samples were then thermally activated and characterized by SIMS, XPS, TEM/EDXS and XRD. Analyses by SIMS showed that the cluster ligand shell was removed during thermal treatment. XPS measurements indicated that the composition of the supported particles corresponded to that of the starting clusters and that the dispersion remained higher in the case of C. Well‐dispersed bimetallic‐supported nanoparticles (1–2 nm) were obtained when cluster 1 was used as the precursor, whereas anchoring of cluster 2 resulted in its fragmentation leading to supported nanoparticles of variable stoichiometries. Reactions with soluble model molecules were carried out to prove the chemical bonding of 1. Crystallization of the new cluster [Ru5PtC(CO)14{(PPh2CH2)2NC3H7}] confirmed the chemical bonding of cluster 1 onto the C support through a ligand exchange mechanism involving COD.

FeFe]‐Hydrogenase Models: Unexpected Variation in Protonation Rate between Dithiolate Bridge Analogues

AbstractThe model [FeFe]‐hydrogenase subsite Fe2(μ‐odt)(CO)4(PMe3)2 (odt = 2‐oxapropane‐1,3‐dithiolate) has been crystallized for the first time, revealing an apical–basal arrangement of the two phosphane groups. Protonation of this species has been studied by a combination of stopped‐flow ultraviolet and infrared techniques along with time‐resolved NMR spectroscopy. The kinetics of the protonation are similar to those for Fe2(μ‐edt)(CO)4(PMe3)2 (edt = ethane‐1,2‐dithiolate) and are much slower than those for the protonation of Fe2(μ‐pdt)(CO)4(PMe3)2 (pdt = propane‐1,3‐dithiolate). The dithiolate bridge length is therefore not the key determinant of reactivity in these simple model systems.

The first example of stereoselective self-assembly of a cryptand containing four asymmetric intracyclic phosphane groups

The rac stereoisomer of a novel cryptand containing two bridgehead nitrogen and four asymmetric phosphorus atoms in the 16-membered core cycle was obtained stereoselectively via the reaction of bis(mesitylphosphino)propane, formaldehyde, and meta-xylylenediamine in the course of a covalent self-assembly process.

3]Ferrocenophane Ligands with an Inserted Methylene Group

AbstractNew planar chiral [3]ferrocenophane aminosulfane and aminophosphane ligands displayed interesting results in model Pd‐catalyzed allylic substitution reactions. The phosphane derivative having a methylene group between the cyclopentadienyl ring and the phosphane group showed enantioselectivities up to 86 % ee, whereas the ligand without the methylene group afforded almost racemic allylation products. Analogous sulfane ligands showed the opposite trend. Tentative catalytic complexes were studied by 31P NMR spectroscopy and DFT computational methods. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Dynamic Behaviour of the [(Triphos)Rh(η1:η2‐P4RR′)]n+ Complexes [Triphos = MeC(CH2PPh2)3; R = H, Alkyl, Aryl; R′ = Lone Pair, H, Me; n = 0, 1]: NMR and Computational Studies

AbstractSolution multinuclear and multidimensional NMR analyses of the [(triphos)Rh(η1:η2‐P4RR′)]n+ complex cations [triphos = MeC(CH2PPh2)3; R = H, Me, Ph; R′ = lone pair, H, Me; n = 0, 1] confirm the same primary structure determined by X‐rays in the solid state. In addition, 2D 1H NOESY and 31P{1H} exchange NMR spectroscopy show that these complexes are nonrigid on the NMR time‐scale over the 253–318 K temperature range. A dynamic process that involves the terminal phosphane groups of the triphos ligand is displayed by each compound. The NMR spectroscopic data indicate a slow scrambling motion in which the P4R unit tumbles with respect to the (triphos)Rh moiety. DFT calculations outline a possible turnstile mechanism involving the threefold and twofold rotors into which the complex is subdivided. The process goes through a transition state in which the axial and equatorial dispositions of the PRR′ and P=P donating groups of the P4RR′ ligand are inverted with respect to the ground state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The Ditertiary‐Phosphane‐Bridged Heteronuclear Cluster RuOs3(μ‐H)2(CO)9(μ‐CO)2(μ‐dppm): Synthesis and Reactivity with Alkynes

AbstractA high‐yielding alternative synthesis of the dppm‐bridged heteronuclear cluster RuOs3(μ‐H)2(CO)9(μ‐CO)2(μ‐dppm) (1) [dppm = bis(diphenylphosphanyl)methane] has been developed. Thermolysis of 1 resulted in dephenylation of both phosphane groups to afford the butterfly cluster RuOs3(CO)12(μ4,κ2:κ2‐PhPCH2PPh) (2). The reaction of 1 with selected alkynes afforded the products RuOs3(μ‐H)(CO)8(μ‐CO)(μ3,κ2:κ1‐PhPCH2PPh2)(μ3,η1:η2:η1‐L) (3) (L = alkyne) in which the alkyne is aligned parallel to an Os–Os bond(// Os‐Os isomer). The clusters incorporating internal alkynes underwent isomerization to the //Ru‐Os isomer by alkyne and hydride migration. Several other cluster products of these reactions were also identified, including RuOs3(μ‐H)(CO)8(μ‐CO)(μ3,κ2:κ1‐PhPCH2PPh2)(μ3,η1:η2:η1‐tBuC2CHO)(6) in which the methyl group of the alkyne has been oxidized to an aldehyde. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)