Hans-Werner Frühauf

Organotransition metal [3+2] cycloaddition reactions

Abstract The review, covering the literature of roughly the last three decades, describes the [3+2] cycloaddition reactions of metalla dipolarophiles MX with organic 1,3-dipoles, and of metalla 1,3-dipoles, MXY and XMY, with organic dipolarophiles. The resulting 5-membered metalla heterocycles can undergo consecutive insertion and/or reductive elimination reactions to give synthetically interesting organic heterocycles. The reactivity of the organometal 1,3-dipoles is explained by extensive series of isolobal transformations to classic organic 1,3-dipoles.

Comparing structures and reactivity in analogous Fe and Ru complexes. (iPr-DAB)Fe(CO)2I2 and (iPr-DAB)FeI2: a perfectly reversible CO-carrier system. (R-DAB=N,N′-R2-1,4-diaza-1,3-butadiene)

Abstract Fe( i Pr-DAB)(CO) 3 ( 1a ) oxidatively adds I 2 to give ( i Pr-DAB)Fe(CO) 2 - trans -I 2 ( 2a ). Photochemically or thermally, 2a readily dissociates both carbonyl ligands to give tetrahedral Fe( i Pr-DAB)I 2 ( 3a ). Under an atmosphere of CO, complex 2a is quantitatively regenerated from 3a . The X-ray crystal structures of 2a and 3a have been determined. 2a : triclinic, space group P 1 , a =8.7624(3), b =9.0550(4), c =10.6512(6) A, α =95.429(4), β =105.245(4), γ =95.209(3)°, Z =2, R =0.0251. 3a : orthorhombic, space group Aba 2, a =11.2693(10), b =15.933(2), c =7.5958(10) A, Z =4, R =0.036. Contrasting the behaviour of 2a , the analogous ruthenium complexes (R-DAB)Ru(CO) 2 trans -I 2 ( 4 ) are very stable, and the carbonyl ligands cannot be dissociated thermally. In the dichloro complexes, of which both the kinetic trans - ( 5 ) and the thermodynamic cis -compounds ( 6 ) have been isolated, both isomers undergo a photochemical CO mono-substitution to give for example the isolable trans -dichloro methanol complex ( 7 ).

Methyl- and acetylpalladium(II) complexes containing a P,N,O tridentate hydrazone ligand.

The hydrazone ligand 2-(diphenylphosphanyl)benzaldehyde benzoylhydrazone (HPNO) forms the methyl PdII complex [Pd(HPN)(Me)Cl] (1) in high yield when treated with [(COD)Pd(Me)Cl] in dichloromethane or diethyl ether at room temperature. In complex 1, the ligand remains neutral and is P,N bidentate. The deprotonation of the ligand by Et3N affords [Pd(PNO)(Me)] (2), in which the anionic ligand is P,N,O tridentate. When 1 is treated with AgCF3SO3, the cationic complex [Pd(HPNO)(Me)]+[CF3SO3]− (3) is formed; here the neutral ligand coordinates in a tridentate P,N,O fashion. All three methyl complexes were subjected to 30 atm of CO pressure, resulting in the isolation of the corresponding acetyl complexes [Pd(HPN)(MeCO)Cl] (4), [Pd(PNO)(MeCO)] (5) and [Pd(HPNO)(MeCO)]+[CF3SO3]− (6). Complex 4 is completely stable in solution in the absence of CO, whereas 5 and 6 quickly decarbonylate to regenerate the starting methyl complexes 2 and 3. Complexes 5 and 6 can also be obtained in high yields by treatment of 4 with Et3N and AgCF3SO3, respectively. The X-ray crystal structures of the complexes 2, 5 and 6 were determined.

Potentially tridentate hydrazonic ligands in the synthesis of methyl and acetyl palladium(II) complexes

Potentially tridentate hydrazonic ligands of the type HNNO have been used in the synthesis of some methyl palladium(II) complexes. Depending on the applied experimental conditions two different kinds of complexes are obtained. Thus, the reactions between HL1–HL5 and (COD)PdMeCl in diethyl ether led to the formation of bidentate methyl complexes of the type Pd(HNN)MeCl (1–5), where the ligands maintain a neutral character. However, in the presence of a base such as Et3N or NaOMe, the ligands are deprotonated with the consequent formation of tridentate methyl complexes of the type Pd(NNO)Me (7–10). In solution, complexes 1–5 tend to lose the hydrazonic proton with elimination of methane and formation of a tridentate chloride complex Pd(NNO)Cl (6); this tendency can be correlated with the acidity of the free ligands, which has been determined. On bubbling carbon monoxide through solutions of 1–5, the corresponding acetyl complexes Pd(HNN)[C(O)Me]Cl (11–15) are formed, in which both the cis and trans isomers are present. Their molar ratio is rationalised from the results of a molecular modelling study on the basis of electronic considerations. A remarkably different reactivity has been found in the carbonylation of the tridentate complexes 7–10: they decompose rapidly and quantitatively to palladium black and an organic product corresponding to the ligand with an acetyl group bonded to the hydrazonic nitrogen. The X-ray structures of a methyl complex (3) and its corresponding acetyl (13) derivative have been determined.

1,5-Dihydropyrrol-2-ones from (1,4-diaza-1,3-diene)tricarbonyliron, (dad)Fe(CO)3, and alkyne: IV. Electronic and steric effects along the reaction coordinate. Regioselectivities with unsymmetrical dad (methylglyoxal-bis-isopropylimine) or alkyne (methyl propynoate)

The [2.2.2]bicyclic intermediate observed in the title reaction in the case of dad ligands containing at least one unsubstituted imine carbon atom (aldimino group) were previously shown to be stabilized by introduction of a phosphite ligand, and on the basis of the bond distances revealed by X-ray analysis of the stabilized complex the stabilizing effect has been attributed to a strengthening of the FeC bond trans- to the P-donor atom, i.e., to one of the bonds broken in the final rearrangement reaction. By utilization of this stabilization, the heretofore unobservable [2.2.2] intermediate with a biacetyl derived dad (bis-ketimine) was observed, and identified by IR spectroscopy. Its rearrangement product, the first example of this type of species containing a phosphite ligand, was isolated. Its NMR (1H, 13C, 31P) spectra reveal that only the isomer with the phosphorus in the thermodynamically most favorable position is formed. With sterically demanding dad-substituents, products from the final rearrangement are obtained only in poor yields. A mechanistic sequence which accounts for the findings is proposed. The initial step in the reaction sequence, in which the dad and alkyne become CC connected, has been found to be very sensitive to electronic influences. In reactions with unsymmetrical dad (methylglyoxal-bis-isopropylimine) and alkyne (methyl propynoate), respectively, the formation of exclusively one of the possible regioisomers is observed. The products have been characterized, and their structures determined by MS, IR, 1H, and 13C NMR spectroscopy.

1,3-dipolar cycloaddition to the Fe-S=C fragment 20. Preparation and properties of carbonyliron complexes of di-thiooxamide. Reactivity of the mononuclear (di-thiooxamide)Fe(CO)(3) towards dimethyl acetylenedicarboxylate

Abstract Reaction of Fe2(CO)9 at room temperature in THF with the di-thiooxamides (L), SC{N(R,R′)}C{(R,R′)N}S [R=Me, R′–R′=(CH2)2 (a); R=H, R′=iPr (b); R=H, R′=iPr (c), R=H, R′=benzyl (d); R=H, R′=H (e)], results for ligands a–d initially in the formation of the mononuclear σ-S, σ-S′ chelate complexes Fe(CO)3(L) (7a–d), which could be isolated in case of 7a and 7d. Under the reaction conditions, complexes 7a–d react further with [Fe(CO)4] fragments to give three types of Fe2(CO)6(L) complexes (8a–d) in high yields, depending on the di-thiooxamide ligand used together with traces of the known complex S2Fe3(CO)9 (14). The molecular structures of these complexes have been established by the single crystal X-ray diffraction determinations of 8a, 8b and 8d. In the reaction with ligand e the corresponding complex 7e was not detected and the well-known complexes 14 and S2Fe3(CO)9 (15) were isolated in low yield. In situ prepared 7a reacts in a slow reaction with 1 equiv. of dimethyl acetylene dicarboxylate in a 1,3-dipolar cycloaddition reaction to give the stable initial ferra [2.2.1] bicyclic complex 10a in 60% yield. In complex 10a an additional Fe(CO)4 fragment is coordinated to the sulfido sulfur atom of the cycloadded FeSC fragment. When a toluene solution of 10a is heated to 50 °C it loses two terminal CO ligands to give the binuclear FeFe bonded complex 11a in almost quantitative yield. The molecular structures of 10a and 11a have been confirmed by single crystal X-ray diffraction. Reaction of 7d at room temperature with 2 equiv. of dimethyl acetylene dicarboxylate results in the mononuclear complex 12d in 5% yield. The molecular structure of 12b has been established by single crystal X-ray diffraction and comprises a tetra dentate ligand with two ferra-sulpha cyclobutene, and a ferra-disulpha cyclopentene moiety. When the reaction is performed at 60 °C a low yield of 2,3,4,5-thiophene tetramethyl tertracarboxylate is obtained besides complex 12d.

Alkene rotation in [Ru(η5-C5H5) (L2) (η2-alkene][CF3SO3] with L2 = iPr- or pTol-diazabutadiene. X-ray crystal structure of [Ru(η5-C5H5(pTol-DAB) (η2-ethene][CF3SO3]

Reaction of RuCl(η5-C5H5(pTol-DAB) with AgOTf (OTf = CF3SO3) in CH2Cl2 or THF and subsequent addition of L′ (L′ = ethene (a), dimethyl fumarate (b), fumaronitrile (c) or CO (d) led to the ionic complexes [Ru(η5-C5H5)(pTol-DAB)(L′)][OTf] 2a, 2b and 2d and [Ru(η5-C5H5)(pTol-DAB)(fumarontrile-N)][OTf] 5c. With the use of resonance Raman spectroscopy, the intense absorption bands of the complexes have been assigned to MLCT transitions to the iPr-DAB ligand. The X-ray structure determination of [Ru(η5-C5H5)(pTol-DAB)(η2-ethene)][CF3SO3] (2a) has been carried out. Crystal data for 2a: monoclinic, space group P21/n with a = 10.840(1), b = 16.639(1), c = 14.463(2) A, β = 109.6(1)°, V = 2465.6(5) A3, Z = 4. Complex 2a has a piano stool structure, with the Cp ring η5-bonded, the pTol-DAB ligand σN, σN′ bonded (Ru-N distances 2.052(4) and 2.055(4) A), and the ethene η2-bonded to the ruthenium center (Ru-C distances 2.217(9) and 2.206(8) A). The C = C bond of the ethene is almost coplanar with the plane of the Cp ring, and the angle between the plane of the Cp ring and the double of the ethene is 1.8(0.2)°. The reaction of [RuCl(η5-C5H5)(PPh)3 with AgOTf and ligands L′ = a and d led to [Ru(η5-C5H5)(PPh3)2(L′)]OTf] (3a) and (3d), respectively. By variable temperature NMR spectroscopy the rottional barrier of ethene (a), dimethyl fumarate (b and fumaronitrile (c) in complexes [Ru(η5-C5H5)(L2)(η2-alkene][OTf] with L2 = iPr-DAB (a, 1b, 1c), pTol-DAB (2a, 2b) and L = PPh3 (3a) was determined. For 1a, 1b and 2b the barrier is 41.5±0.5, 62±1 and 59±1 kJ mol−1, respectively. The intermediate exchange could not be reached for 1c, and the ΔG# was estimated to be at least 61 kJ mol−. For 2a and 3a the slow exchange could not be reached. The rotational barrier for 2a was estimated to be 40 kJ mol−. The rotational barier for methyl propiolate (HC≡CC(O)OCH3) (k) in complex [Ru(η5-C5H5)(iPr-DAB) η2-HC≡CC(O)OCH3)][OTf] (1k) is 45.3±0.2 kJ mol−1. The collected data show that the barrier of rotational of the alkene in complexes 1a, 2a, 1b, 2b and 1c does not correlate with the strength of the metal-alkene interaction in the ground state.

1,3-Dipolar cycloaddition to a FeN=C fragment XII °. Reactivity of Fe(2,6-xylyl-isocyanide)3 (iPr-α-diimine) towards heteroallenic C=S bonds

Abstract The reaction of Fe(2,6-xylylNC) 3 ( i Pr-DAB) ( 5 ) with carbon disulfide and carbonyl sulfide yields bicyclic structures. The reaction with carbon disulfide proceeds via a 1,3-dipolar cycloaddition of the C=S bond across the FeN=C unit, followed by an isocyanide insertion to form the ferra[2.2.2]bicyclic complex ( 8 ). A single crystal X-ray structure determination on ( 8 )showed it to contain three six-membered rings; the carbon disulfide is attached to the central metal atom through a sulphur atom and to the former imine carbon atom through its central carbon atom. In the 1,3-dipolar cycloaddition of carbonyl sulfide the presence of moisture resulted in a protonation of the amido function. This inhibits the isocyanide insertion and stabilizes the initially formed ferra[2.2.1]-bicyclic compound.

1,5-Dihydropyrrol-2-ones from (1,4-diaza-1,3-diene)tricarbonyliron and alkyne: III. Stereospecific ligand incorporation on formation of the [2.2.2]-Bicyclic intermediate☆

Abstract Examples of the [2.2.2.]-bicyclic intermediate formed in the reaction of (1,4-diaza-1,3-diene)tricarbonyliron complexes, (dad)Fe(CO) 3 , with dimethyl acetylenedicarboxylate and an additional ligand L [L = 13 CO, P(OMe) 3 ] have been isolated and characterized by elemental analysis, field desorption mass spectrometry and NMR ( 1 H, 13 C, 31 P) and IR spectroscopy. The ligand L which completes the octahedral coordination around the bridge head iron atom, is incorporated stereospecifically trans to the inserted carbonyl group of the [2.2.2] system. This is found to be the case with complexes of diazadienes of differing donor/acceptor abilities and also for the two electronically different ligands L. From the results it is concluded that a stereochemically rigid intermediate is initially formed, which for steric reasons and so in a kinetically controlled reaction, allows access of L to the iron from only one direction, consequently closing the [2.2.2] structure. Conclusive evidence that the reaction is kinetically controlled is provided by the isolation of two isomeric complexes ( 2b and 7 , respectively) from the reactions of (dad)Fe(CO) 3 in the presence of P(OMe) 3 , and of (dad)Fe[P(OMe) 3 ](CO) 2 from reaction in the presence of CO.

1,3-Dipolar cycloaddition to the CXM fragment 13. Regioselectivity in the reactions of mononuclear iminoketone complexes Fe(CO)3(tBu-NC(H)C(R)O) (R Ph, Me) with the asymmetric alkyne methyl propynoate

Abstract The reaction of complexes Fe(CO) 3 ( t Bu-NC(H)C(R)O) ( 6a : R  Ph, 6b : R  Me) with one equivalent of methyl propynoate (MP) under an atmosphere of CO, at −30°C (R  Ph) or −50°C (R  Me) results in formation of the Fe(CO) 3 (butenolide) complexes ( 9a,b ), which have been characterized spectroscopically (IR, 1 H, 13 C NMR) and by elemental analysis. The spectroscopic properties indicate that one of the possible regio-isomers is exclusively formed. INEPT and 2D heteronuclear correlation NMR techniques show that in the regio-isomer obtained the former ketone carbon atom is CC bonded to the carbon atom that in the alkyne bore the electron-withdrawing ester group. When the complexes Fe(CO) 3 ( t Bu-NC(H)C(R)O) ( 6a,b ) are treated with two equivalents of MP at −30°C in the absence of CO, the tricyclic complexes 12a,b are formed in moderate yield. The molecular structure of complex 12b has been determined by a single-crystal X-ray diffraction study. It contains three five-membered rings, with the two fused metallacycles sharing three common carbon atoms with the third, a γ-lactone ring, thus forming the tricyclic moiety. The coordination geometry around the central iron atom is distorted octahedral, with the dianionic tridentate ligand occupying three facial positions. The fact that complexes 12 can also be obtained in high yield by irradiation of the corresponding Fe(CO) 3 (butenolide) ( 9 ) in the presence of an excess of MP strongly supports the proposed reaction mechanism in which the second alkyne is initially π-coordinated and subsequently coupled with the π-coordinated double bond of the butenolide heterocycle.