The Synthetic Potential of C-Halophosphaalkenes

Abstract.

Methodologies for the functionalization of phosphaalkenes Mes*P=CHal₂ were developed. Lithiation with n-butyllithium yielded carbenoids Mes*P=CLiHal which were reacted with various electrophiles such as acid chlorides, carbonyl compounds, and metal halides. The dihalophosphaalkenes were also converted to monohalophosphaalkenes; the latter proved to be suitable for Stille-type cross coupling reaction with Grignard reagents. New phosphaalkenes of the type (E)-Mes*P=C(H)Ar with a variety of functionalities were obtained in high yield and isomeric purity.     

Acetylene-expanded dendralene segments with exotopic phosphaalkene units

Bis-TMS protected C,C-diacetylenic phosphaalkene (A(2)PA) 1 (Mes*P=C(C≡CTMS)(2); Mes* = 2,4,6-tBu(3)Ph) has been used as a building block for the construction of butadiyne-expanded dendralene fragments in which phosphaalkenes feature as exotopic double bonds. Treatment of 1 with CuCl gives rise to a Cu(I) acetylide that is selectively formed at the acetylene trans to the Mes* group. The cis-TMS-acetylene engages in similar chemistry, albeit at higher temperatures and longer reaction times. The differentiation between the two acetylene termini of 1 allows for the controlled synthesis of the title compounds by a variety of different Cu- and Pd-catalyzed oxidative acetylene homo- and heterocoupling protocols. Crystallographic characterization of A(2)PA 1 and dimeric Mes*P=C(C≡CR(1))C(4)(R(2) C≡C)C=PMes* (3b, R(1) = R(2) = Ph; 6, R(1) = R(2) = TMS), and 10 (R(1) = R(2) = C≡CPh) verifies that the stereochemistry across the P=C bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis/trans isomerization in an iodo-substituted A(2)PA intermediate 4 (Mes*P=C(C≡CTMS)(C≡CI). UV/Vis spectroscopic and electrochemical studies reveal that efficient π conjugation operates through the entire acetylenic phosphaalkene framework, even in the cross-conjugated dimeric structures. The P centers contribute considerably to the frontier molecular orbitals of the compounds, thereby leading to smaller HOMO-LUMO gaps than in all-carbon-based congeners. Phenyl- and/or ethynylphenyl substituents at the A(2)PA framework influence the HOMO and LUMO to a varying degree depending on their relationship to the Mes* group, thus enabling a fine-tuning of the frontier molecular orbitals of the compounds.     

Coordination Chemistry of Highly Hemilabile Bidentate Sulfoxide N‐Heterocyclic Carbenes with Palladium(II)

Imidazolium salts, [RS(O)? CH₂(C₃H₃N₂)Mes]Cl (R=Me ( L1 a ), Ph ( L1 b )); Mes=mesityl), make convenient carbene precursors. Palladation of L1 a affords the monodentate dinuclear complex, [(PdCl₂{MeS(O)CH₂(C₃H₂N₂)Mes})₂] ( 2 a ), which is converted into trans‐[PdCl₂(NHC)₂] (trans‐ 4 a ; N‐heterocyclic carbene) with two rotamers in anti and syn configurations. Complex trans‐ 4 a can isomerize into cis‐ 4 a (anti) at reflux in acetonitrile. Abstraction of chlorides from 4 a or 4 b leads to the formation of a new dication: trans‐[Pd{RS(O)CH₂(C₃H₂N₂)Mes}₂](PF₆)₂ (R=Me ( 5 a ), Ph ( 5 b )). The X‐ray structure of 5 a provides evidence that the two bidentate SO? NHC ligands at palladium(II) are in square‐planar geometry. Two sulfoxides are sulfur‐ and oxygen‐bound, and constitute five‐ and six‐membered chelate rings with the metal center, respectively. In acetonitrile, complexes 5 a or 5 b spontaneously transform into cis‐[Pd(NHC)₂(NCMe)₂](PF₆)₂. Similar studies of thioether–NHCs have also been examined for comparison. The results indicate that sulfoxides are more labile than thioethers.     

Phosphorus Centers of Different Hybridization in Phosphaalkene‐Substituted Phospholes

Phosphole‐substituted phosphaalkenes (PPAs) of the general formula Mes*P?C(CH₃)?(C₄H₂P(Ph))?R 5 a – c (Mes*=2,4,6‐tBu₃Ph; R=2‐pyridyl ( a ), 2‐thienyl ( b ), phenyl ( c )) have been prepared from octa‐1,7‐diyne‐substituted phosphaalkenes by utilizing the Fagan–Nugent route. The presence of two differently hybridized phosphorus centers (σ²,λ³ and σ³,λ³) in 5 offers the possibility to selectively tune the HOMO–LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5 a – c by sulfur proceeds exclusively at the σ³,λ³‐phosphorus atom, thus giving rise to the corresponding thioxophospholes 6 a – c . Similarly, 5 a is selectively coordinated by AuCl at the σ³,λ³‐phosphorus atom. Subsequent second AuCl coordination at the σ²,λ³‐phosphorus heteroatom results in a dimetallic species that is characterized by a gold–gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO–LUMO gap relative to reference phosphole‐containing π systems that lack a P?C substituent.     

Darstellung,Charakterisierung und Struktur funktionalisierter Fluorphosphaalkene des Typs R3E–P=C(F)NEt2 (R/E = Me/Si,Me/Ge,CF3/Ge,Me/Sn)

Preparation, Characterization, and Structure of Functionalized Fluorophosphaalkenes of the Type R₃E–P=C(F)NEt₂ (R/E = Me/Si, Me/Ge, CF₃/Ge, Me/Sn) P‐functionalized 1‐diethylamino‐1‐fluoro‐2‐phosphaalkenes of the type R₃E–P=C(F)NEt₂ [R/E = Me/Si ( 2 ), Me/Ge ( 3 ), CF₃/Ge ( 4 ), Me/Sn ( 5 )] are prepared by reaction of HP=C(F)NEt₂ ( 1 , E/Z = 18/82) with R₃EX (X = I, Cl) in the presence of triethylamine as base, exclusively as Z‐Isomers. 2–5 are thermolabile, so that only the more stable representatives 2 and 4 can be isolated in pure form and fully characterized. 3 and 5 decompose already at temperatures above –10 °C, but are clearly identified by ¹⁹F and ³¹P NMR‐measurements. The Z configuration is established on the basis of typical NMR data, an X‐ray diffraction analysis of 4 and ab initio calculations for E and Z configurations of the model compound Me₃Si–P=C(F)NMe₂. The relatively stable derivative 2 is used as an educt for reactions with pivaloyl‐, adamantoyl‐, and benzoylchloride, respectively, which by cleavage of the Si–P bond yield the push/pull phosphaalkenes RC(O)–P=C(F)NEt₂ [R = tBu ( 6 ), Ad ( 7 ), Ph ( 8 )], in which π‐delocalization with the P=C double bond occurs both with the lone pair on nitrogen and with the carbonyl group.     

Combined Experimental and Computational Investigations of Rhodium‐Catalysed CH Functionalisation of Pyrazoles with Alkenes

Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C₂H₃Y (Y=CO₂Me ( a ), Ph ( b ), C(O)Me ( c )) with 3‐aryl‐5‐R‐pyrazoles (R=Me ( 1 a ), Ph ( 1 b ), CF₃ ( 1 c )) using a [Rh(MeCN)₃Cp*][PF₆]₂/Cu(OAc)₂ ? H₂O catalyst system. In the reaction of methyl acrylate with 1 a , up to five products ( 2 aa – 6 aa ) were formed, including the trans monovinyl product, either complexed within a novel Cu^I dimer ( 2 aa ) or as the free species ( 3 aa ), and a divinyl species ( 6 aa ); both 3 aa and 6 aa underwent cyclisation by an aza‐Michael reaction to give fused heterocycles 4 aa and 5 aa , respectively. With styrene, only trans mono‐ and divinylation products were observed, whereas with methyl vinyl ketone, a stronger Michael acceptor, only cyclised oxidative coupling products were formed. Density functional theory calculations were performed to characterise the different migratory insertion and β‐H transfer steps implicated in the reactions of 1 a with methyl acrylate and styrene. The calculations showed a clear kinetic preference for 2,1‐insertion and the formation of trans vinyl products, consistent with the experimental results.     

Disilene R*RSi=SiRR* (R* = SitBu3) mit siliciumgebundenen Me‐ und Ph‐Gruppen R: Bildung,Nachweis, Thermolyse,Struktur: Verbindungen des Siliciums. 154 [1]. Ungesättigte Siliciumverbindungen. 61 [1]

Compounds of Silicon. 154 [1]. Unsaturated Silicon Compounds. 61 [1] Disilenes R*RSi=SiRR* (R* = SitBu₃) with Silicon‐Bound Me and Ph Groups R: Formation, Identification, Thermolysis, Structure Dehalogenations of the 1, 2‐disupersilylsilanes R*MeBrSi—SiBrMeR* (gauche : trans 1.15 : 1.00) and R*PhClSi—SiBrPhR* (gauche : trans = 2.7 : 1.0) in THF with equimolar amounts of NaR* (R* = SitBu₃ = Supersilyl) lead at —78 °C under exchange of bromine for sodium to the disilanides R*MeBrSi—SiNaMeR* and R*PhClSi—SiNaPhR* which are identified by protonation and bromination (formation of R*RXSi—SiX′RR* with R = Me, X/X′ = Br/H, Br/Br: gauche : trans = 1.15 : 1.00, and R = Ph, X/X′ = Cl/H, Cl/Br: gauche : trans = 2.7 : 10, respectively). These eliminate at about —55 °C NaHal with formation of non‐isolable trans‐R*MeSi=SiMeR* and isolable trans‐R*PhSi=SiPhR*. The intermediate existence of the disilene R*MeSi=SiMeR* could be proved by trapping it with PhC≡CPh (formation of a [2+2] cycloadduct; X‐ray structure analysis). In the absence of trapping agents, R*MeSi=SiMeR* decomposes into a mixture of substances, the main product of which is R*MeHSi—SiMeR*—SiHMeR*. The light yellow disilene R*PhSi=SiPhR* has been characterized by spectroscopy (Raman: ν(Si=Si) = 592 cm^—1; UV/VIS: λ_max = 398 nm with ∈ = 1560; ²⁹Si‐NMR: δ(>Si=) = 128 ppm) and by X‐ray structure analysis (planar central framework >Si=Si<; Si=Si distance 2.182Å). R*PhSi=SiPhR* is reduced by lithium in THF with formation of a red radical anion which decomposes at room temperature into hitherto non‐identified products. At about 70 °C, R*PhSi=SiPhR* decomposes with intramolecular insertion of the Si=Si group into a C—H bond of a Ph group and with change of configuration of the R* groups, which at first are trans then cis‐positioned (X‐ray structure analysis of the thermolysis product).     

Thermal isomerization of ruthenium hydride compounds containing asymmetric bidentate pyrrole‐imine ligands

A series of ruthenium hydride compounds containing substituted bidentate pyrrole‐imine ligands were synthesized and characterized. Reacting RuHCl(CO)(PPh₃)₃ with one equivalent of [C₄H₃NH(2‐CH=NR)] in ethanol in the presence of KOH gave compounds {RuH(CO)(PPh₃)₂[C₄H₃N(2‐CH=NR)]} where trans‐Py‐Ru‐H 1, R = CH₂CH₂C₆H₉; cis‐Py‐Ru‐H 2, R = Ph‐2‐Me; and cis‐Py‐Ru‐H 3, R = C₆H₁₁. Heating trans‐Py‐Ru‐H 1 in toluene at 70°C for 12 hr resulted a thermal conversion of the trans‐Py‐Ru‐H 1 into its cis form, {RuH(CO)(PPh₃)₂[C₄H₃N(2‐CH=NCH₂CH₂C₆H₉)]} (cis‐Py‐Ru‐H 1) in very high yield. The ¹H NMR spectra of trans‐Py‐Ru‐H 1, cis‐Py‐Ru‐H 2, cis‐Py‐Ru‐H 3, and cis‐Py‐Ru‐H 1 all show a typical triplet at ca. δ–11 for the hydride. The trans and cis form indicate the relative positions of pyrrole ring and hydride. The geometries of trans‐Py‐Ru‐H 1, cis‐Py‐Ru‐H 1, and cis‐Py‐Ru‐H 3 are relatively similar showing typical octahedral geometries with two PPh₃ fragments arranged in trans positions.     

Gold(I) Complexation of Phosphanoxy-Substituted Phosphaalkenes for Activation-Free LAuCl Catalysis

The phosphanoxy-substituted phosphaalkene bearing the P=C−O−P skeleton can be prepared from diphosphene Mes*P=PMes* (Mes*=2,4,6-tBu₃C₆H₂), and their use for catalysis is of interest. In this paper, complexation of the phosphanoxy-substituted phosphaalkenes with gold are investigated, and the catalytic activity of the mono- and bis(chlorogold) complexes are subsequently evaluated. Reaction of the P=C−O−P compound with (tht)AuCl (tht=tetrahydrothiophene) showed dominant coordination on the sp³ phosphorus, and complete coordination on the sp² phosphorus required removal of tetrahydrothiophene. Atoms In Molecules (AIM) analysis based on the X-ray structure of the mono(chlorogold) complex indicated a pseudo coordinating interaction between the gold center and the P=C unit. The bis(chlorogold) complexes displayed conformational isomerism, and catalyzed the cycloisomerization/alkoxycyclization of 1,6-enyne and for hydration of terminal alkyne without activation treatment. Even the mono(chlorogold) complexes catalyzed the alkoxycyclization reactions without a silver co-catalyst, indicating that the alcohols were effective in activating the AuCl unit.     

Reactivity of TpMe2‐Supported Yttrium Alkyl Complexes toward Aromatic N‐Heterocycles: Ring‐Opening or CC Bond Formation Directed by CH Activation

Unusual chemical transformations such as three‐component combination and ring‐opening of N‐heterocycles or formation of a carbon–carbon double bond through multiple C–H activation were observed in the reactions of Tp^Me2‐supported yttrium alkyl complexes with aromatic N‐heterocycles. The scorpionate‐anchored yttrium dialkyl complex [Tp^Me2Y(CH₂Ph)₂(THF)] reacted with 1‐methylimidazole in 1:2 molar ratio to give a rare hexanuclear 24‐membered rare‐earth metallomacrocyclic compound [Tp^Me2Y(μ‐N,C‐Im)(η²‐N,C‐Im)]₆ ( 1 ; Im=1‐methylimidazolyl) through two kinds of C–H activations at the C2‐ and C5‐positions of the imidazole ring. However, [Tp^Me2Y(CH₂Ph)₂(THF)] reacted with two equivalents of 1‐methylbenzimidazole to afford a C–C coupling/ring‐opening/C–C coupling product [Tp^Me2Y{η³‐(N,N,N)‐N(CH₃)C₆H₄NHCH?C(Ph)CN(CH₃)C₆H₄NH}] ( 2 ). Further investigations indicated that [Tp^Me2Y(CH₂Ph)₂(THF)] reacted with benzothiazole in 1:1 or 1:2 molar ratio to produce a C–C coupling/ring‐opening product {(Tp^Me2)Y[μ‐η²:η¹‐SC₆H₄N(CH?CHPh)](THF)}₂ ( 3 ). Moreover, the mixed Tp^Me2/Cp yttrium monoalkyl complex [(Tp^Me2)CpYCH₂Ph(THF)] reacted with two equivalents of 1‐methylimidazole in THF at room temperature to afford a trinuclear yttrium complex [Tp^Me2CpY(μ‐N,C‐Im)]₃ ( 5 ), whereas when the above reaction was carried out at 55 °C for two days, two structurally characterized metal complexes [Tp^Me2Y(Im‐Tp^Me2)] ( 7 ; Im‐Tp^Me2=1‐methyl‐imidazolyl‐Tp^Me2) and [Cp₃Y(HIm)] ( 8 ; HIm=1‐methylimidazole) were obtained in 26 and 17 % isolated yields, respectively, accompanied by some unidentified materials. The formation of 7 reveals an uncommon example of construction of a C?C bond through multiple C–H activations.     

Direct,Sequential, and Stereoselective Alkynylation of C,C‐Dibromophosphaalkenes

The first direct alkynylation of C,C‐dibromophosphaalkenes by a reaction with sulfonylacetylenes is reported. Alkynylation proceeds selectively in the trans position relative to the P substituent to afford bromoethynylphosphaalkenes. Owing to the absence of transition metals in the procedure, the previously observed conversion of dibromophosphaalkenes into phosphaalkynes through the phosphorus analog of the Fritsch–Buttenberg–Wiechell rearrangement is thus suppressed. The bromoethynylphosphaalkenes can subsequently be converted to C,C‐diacetylenic, cross‐conjugated phosphaalkenes by following a Sonogashira coupling protocol in good overall yields. By using the newly described method, full control over the stereochemistry at the P=C double bond is achieved. The substrate scope of this reaction is demonstrated for different dibromophosphaalkenes as well as different sulfonylacetylenes.     

trans‐Chloridobis[(Z)‐1‐imino‐1‐methoxyethane‐κN](triphenylphosphane‐κP)platinum(II) chloride monohydrate

The title compound, [PtCl(C₃H₇NO)₂(C₁₈H₁₅P)]Cl·H₂O or trans‐[PtCl{Z‐HN=C(Me)OMe}₂(PPh₃)]Cl·H₂O, crystallizes from an acetone solution of isomeric trans‐[PtCl{E‐HN=C(Me)OMe}₂(PPh₃)]Cl. The two HN=C(Me)OMe ligands show typical π‐bond delocalization over the N—C—O group [Cini, Caputo, Intini & Natile (1995). Inorg. Chem. 34 , 1130–1137] and have the unprecedented Z–anti configuration. The relative orientation of the imino ether ligands is head‐to‐tail.     

Untersuchungen zum elektronischen Einfluß von Organoliganden. XIII. Synthese und Charakterisierung von 2-funktionalisierten Vinylrhodoximen

Studies on the Electronic Influence of Organoligands. XIII. Synthesis and Characterization of 2-Functionalized Vinyl Rhodoximes 2-Functionalized vinyl rhodoximes [Rh(dmgH)₂ (PPh₃)cis/trans-CH = CHZ] ([Rh]? CH = CHZ) ) ( 1 ) can be prepared with a wide variation of the substituent Z (cis: OEt ( 1 a ), OPh ( 1 b ), Cl ( 1 c ), Me ( 1 j ), Ph ( 1 k ), SMe ( 1 l ), SPh ( 1 m ); trans: SPh ( 1 d ), Me ( 1 e ), Ph ( 1 f ), CMe₃ ( 1 g ), SiMe₃ ( 1 h )) by oxidative addition of XCH = CHZ and/or by nucleophilic addition of HC?CZ and Me₃SiC?CZ, respectively, to [Rh]^?. 1 a is converted to [Rh]? CH₂CHO ( 2 ) already in a weakly acid medium. 1 l is isomerized to trans-[Rh]? CH = CHSMe ( 1 n ) in the presence of acids. The complexes 1 are characterized by microanalysis and by ¹H, ¹³C and ³¹P NMR spectroscopy. The magnitude of the coupling constants ¹J(¹⁰³Rh, ³¹P) reveals only a small effect of Z on the (NMR) trans influence of the vinyl ligands CH = CHZ. The molecular structures of cis-[Rh]? CH = CHSPh ( 1 m ) and trans-[Rh]? CH = CHSPh ( 1 d ) show a distorted octahedral coordination of Rh with a mutual trans position of triphenyl-phosphine and the 2-phenylmercaptovinyl ligands. Van der Waals interactions exist between the sulfur and the equatorial dimethylglyoximato ligands in the cis complex 1 m .     

Hydroalumination and Hydrogallation Reactions with Tri(ethynyl)silanes – Generation of Compounds with up to Three Coordinatively Unsaturated Aluminium Atoms

Hydroalumination or hydrogallation of tri(ethynyl)silanes, RSi(C≡C‐Ar)₃ ( 1a , R = Ph, Ar = Ph; 1b , R = Me, Ar = Ph; 1c , R = Me, Ar = C₆H₄Me), with the element hydrides H‐EtBu₂ (E = Al, Ga) in stoichiometric ratios of 1:1 to 1:3 at ambient temperature yielded the addition products (PhC≡C)₂(R)Si[(tBu₂E)C=C(H)Ph] ( 2 , R = Ph, E = Ga; 3a , R = Me, E = Al; 3b , R = Me, E = Ga), (PhC≡C)(Me)Si[(tBu₂E)C=C(H)Ph]₂ ( 4a , E = Al, 4b , E = Ga) and (Me)Si[(tBu₂Al)C=C(H)Ar]₃ ( 5 , Ar = Ph; 6 , Ar = C₆H₄Me). Compounds 2 – 4 show a relatively close interaction between the coordinatively unsaturated aluminium or gallium atoms and one of the C_α(≡C) atoms of unreacted alkyne substituents [245 (E = Al) and 266 pm (E = Ga)] that stabilises the kinetically favoured cis addition products with E and hydrogen on the same side of the resulting C=C double bonds. In the absence of these stabilising effects the compounds were found to isomerise to the thermodynamically favoured trans isomers.     

Synthesis of 2‐ and 2,7‐Functionalized Pyrene Derivatives: An Application of Selective C?H Borylation

An efficient synthetic route to 2‐ and 2,7‐substituted pyrenes is described. The regiospecific direct C? H borylation of pyrene with an iridium‐based catalyst, prepared in situ by the reaction of [{Ir(μ‐OMe)cod}₂] (cod=1,5‐cyclooctadiene) with 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine, gives 2,7‐bis(Bpin)pyrene ( 1 ) and 2‐(Bpin)pyrene ( 2 , pin=OCMe₂CMe₂O). From 1 , by simple derivatization strategies, we synthesized 2,7‐bis(R)‐pyrenes with R=BF₃K ( 3 ), Br ( 4 ), OH ( 5 ), B(OH)₂ ( 6 ), and OTf ( 7 ). Using these nominally nucleophilic and electrophilic derivatives as coupling partners in Suzuki–Miyaura, Sonogashira, and Buchwald–Hartwig cross‐coupling reactions, we obtained 2,7‐bis(R)‐pyrenes with R=(4‐CO₂C₈H₁₇)C₆H₄ ( 8 ), Ph ( 9 ), C≡CPh ( 10 ), C≡C[{4‐B(Mes)₂}C₆H₄] ( 11 ), C≡CTMS ( 12 ), C≡C[(4‐NMe₂)C₆H₄] ( 14 ), C≡CH ( 15 ), N(Ph)[(4‐OMe)C₆H₄] ( 16 ), and R=OTf, R′=C≡CTMS ( 13 ). Lithiation of 4 , followed by reaction with CO₂, yielded pyrene‐2,7‐dicarboxylic acid ( 17 ), whilst borylation of 2‐tBu‐pyrene gave 2‐tBu‐7‐Bpin‐pyrene ( 18 ) selectively. By similar routes (including Negishi cross‐coupling reactions), monosubstituted 2‐R‐pyrenes with R=BF₃K ( 19 ), Br ( 20 ), OH ( 21 ), B(OH)₂ ( 22 ), [4‐B(Mes)₂]C₆H₄ ( 23 ), B(Mes)₂ ( 24 ), OTf ( 25 ), C≡CPh ( 26 ), C≡CTMS ( 27 ), (4‐CO₂Me)C₆H₄ ( 28 ), C≡CH ( 29 ), C₃H₆CO₂Me ( 30 ), OC₃H₆CO₂Me ( 31 ), C₃H₆CO₂H ( 32 ), OC₃H₆CO₂H ( 33 ), and O(CH₂)₁₂Br ( 34 ) were obtained from 2 . These derivatives are of synthetic and photophysical interest because they contain donor, acceptor, and conjugated substituents. The crystal structures of compounds 4 , 5 , 7 , 12 , 18 , 19 , 21 , 23 , 26 , and 28 – 31 have also been obtained from single‐crystal X‐ray diffraction data, revealing a diversity of packing modes, which are described in the Supporting Information. A detailed discussion of the structures of 1 and 2 , their polymorphs, solvates, and co‐crystals is reported separately.     

cis-trans-and Intramolecular enol-enolic equilibrium of β-ketoaldehydes

Tautomerism of aromatic β-ketoaldehydes p-XPhCOCH₂CHO ( 1 , X = NMe₂, OMe, Me, H, Br, NO₂), aliphatic β-ketoaldehydes and benzoylacetaldehyde RCOCH₂CHO ( 2 , R = Me, i-Bu, t-Bu, Ph), RCOCH(Me)CHO ( 3 , R = Me, Et, i-Pr) and methyl 2-formylpropionate MeOCOCH(Me)CHO ( 4 ) has been studied by the ¹H NMR technique. In basic solvents both cis- and trans-enol forms of these compounds co-exist. trans-Enolisation, which occurs exclusively at the formyl group, is most favoured in compound ( 4 ) and least favoured in compounds ( 1 ) and ( 2 ). The increasing electron-attracting property of the substituent X in the aromatic β-ketoaldehydes ( 1 ), as well as increasing solvent basicity in the series propanediol-1, 2-carbonate, acetone < dimethylformamide < dimethylacetamide < pyridine, also shifts the equilibrium towards the trans-enol form. The trans-enol form is absent in aprotic solvents of low basicity such as CCl₄, C₂HCl₃ and toluene. The thermodynamic parameters of the cis-trans-enol (C ? T) and cis-enol-enolic (C ? C') equilibria have been estimated from the temperature dependences. The transition from the cis-to the trans-enol form is accompanied by an entropy decrease of about 10 cal mol^?1 degree^?1. Nevertheless the trans-enol form is stabilised due to its lower enthalpy. The cis-trans-enol equilibrium is determined by the relative strength of the intramolecular hydrogen bond in the cis-enol form and the intermolecular hydrogen bonds with basic solvent molecules of the trans-enol form. The enthalpy difference of the two cis-enolic forms does not exceed 1.0 kcal/mol, in rough agreement with the data calculated by the CNDO/2 approximation. Polar solvents favour the hydroxymethyleneketone form (C) for both groups of compounds 2 and 3 . The content of the hydroxymethyleneketone form is about the same within series 2 where R = Me, i-Bu, Ph and is a little higher for the t-Bu derivative. A decrease of temperature only slightly shifts the equilibrium of compounds 1 and 2 to the hydroxymethyleneketone form, while in the case of 2-methyl-β-ketoaldehydes (3) this effect is markedly pronounced.     

The β‐Agostic Structure in (C5Me5)2Sc(CH2CH3): Solid‐State NMR Studies of (C5Me5)2Sc−R (R=Me,Ph, Et)

Multinuclear solid‐state NMR studies of Cp*₂Sc?R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc?Et complex contains a β‐CH agostic interaction. The static central transition ⁴⁵Sc NMR spectra show that the quadrupolar coupling constants (C_q) follow the trend of Ph≈Me>Et, indicating that the Sc?R bond is different in Cp*₂Sc?Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cβ in Sc?CH₂CH₃ is related to coupling between the filled σ_C‐C orbital and the vacant orbital.     

The β‐Agostic Structure in (C5Me5)2Sc(CH2CH3): Solid‐State NMR Studies of (C5Me5)2Sc−R (R=Me,Ph, Et)

Multinuclear solid‐state NMR studies of Cp*₂Sc−R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc−Et complex contains a β‐CH agostic interaction. The static central transition ⁴⁵Sc NMR spectra show that the quadrupolar coupling constants (C_q) follow the trend of Ph≈Me>Et, indicating that the Sc−R bond is different in Cp*₂Sc−Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cβ in Sc−CH₂CH₃ is related to coupling between the filled σ_C‐C orbital and the vacant orbital.     

Polymerization of 1‐Phosphaisoprene: Synthesis and Characterization of a Chemically Functional Phosphorus Version of Natural Rubber

Macromolecules derived from 1,3‐dienes, such as polyisoprene (or natural rubber), are of considerable importance in polymer science. Given the parallels between P=C and C=C bonds, the prospect of polymerizing P‐containing 1,3‐dienes, such as 1‐phosphaisoprene, is intriguing due to the unique chemical functionality imparted by the heavier element combined with their structural relationship to natural rubber. Herein, we report the synthesis, characterization and coordination chemistry of the first polymers derived from Mes*P=CR−CH=CH₂ (Mes*=2,4,6‐t‐Bu₃C₆H₂; R=H, Me). In the case of 1‐phosphaisoprene (R=Me), the monomer is isolable and its anionic polymerization affords a polymer that retains P=C bonds in its microstructure. The chemical functionality of these novel materials is demonstrated by forming the macromolecular gold(I) complex where the P=C bond is retained for further chemical elaboration.     

Insertion Reactions of 1,2‐Disubstituted Olefins with an α‐Diimine Palladium(II) Complex

The migratory insertions of cis or trans olefins CH(X)?CH(Me) (X = Ph, Br, or Et) into the metal–acyl bond of the complex [Pd(Me)(CO)(ⁱPr₂dab)]⁺ [B{3,5‐(CF₃)₂C₆H₃}₄]^? ( 1 ) (ⁱPr₂dab = 1,4‐diisopropyl‐1,4‐diazabuta‐1,3‐diene = N,N′‐(ethane‐1,2‐diylidene)bis[1‐methylethanamine]) are described (Scheme 1). The resulting five‐membered palladacycles were characterized by NMR spectroscopy and X‐ray analysis. Experimental data reveal some important aspects concerning the regio‐ and stereochemistry of the insertion process. In particular, the presence of a Ph or Br substituent at the alkene leads to the formation of highly regiospecific products. Moreover, in all cases, the geometry of the substituents in the formed palladacycle was the same as in the starting olefin, as a consequence of a cis addition of the Pd–acyl fragment to the C?C bond. Reaction with CO and MeOH of the five‐membered complex derived from trans‐β‐methylstyrene (= [(1E)‐prop‐1‐enyl]benzene) insertion, yielded the 2,3‐substituted γ‐keto ester 9 with an (2RS,3SR)‐configuration (Scheme 3).