Exo Face Research Articles

Stereodivergent C−C Bond Formation on Arene−Chromium Template: Endo-Selective Allylation by Hosomi−Sakurai Reaction

In arene-tricarbonylchromium complexes, excellent diastereocontrol at benzylic or homobenzylic sites can be achieved since the Cr(CO)3 group effectively blocks one face of the molecule and prevents approach of reagents from this direction.1 This feature has been widely utilized in designing useful synthetic strategies.2 Suitably structured substrates allow almost total diastereoselectivity even three carbons away from the complexed aromatic ring.3 In most of these instances, the reaction occurs from the exo face, i.e., the face opposite to the metal; endo-selective reactions are particularly rare.4 Herein we wish to report a completely endo-selective Hosomi-Sakurai allylation5 on the 2-arylidene-1-tetralone-Cr(CO)3 complex. The corresponding exo-adduct was obtained by an alternative sequence of reactions. In addition to displaying an unusual stereochemical preference, the TiCl4-catalyzed allylation reaction provides a prima facie evidence of out-of-plane coordination of titanium to the ketone dictating steric course of the reaction.

Bromination of exo- and endo-tricyclo[3.2.1.02,4]oct-6-ene

Bromination of exo- and endo-tricyclo[3.2.1.0 2,4]oct-6-ene is initiated at the double bond. This is in contrast to reaction with acid where reaction is initiated preferentially at the cyclopropane. The stationary points on the potential energy surfaces that result from bromine addition to the double bond have been identified by semi-empirical methods. The non-classical bromonium ions are predicted to be less stable than the classical structures. The exo-cyclopropylalkene gives cis and trans 1,2-dibromides along with products which result from reaction of bromine at the exo face of the alkene with subsequent rearrangement involving the C2C4 or C2C3 cyclopropyl bonds. The endo-cyclopropylalkene similarly reacts with bromine at the exo face of the alkene, but 1,2-addition does not compete with rearrangement.

π-Facial selectivities in the coordination of an optically pure fused cyclopentadienyl ligand to arene ruthenium moieties. NMR spectral and structural characterization of a single diastereomer of [(PCp)Ru(Me 6C 6)]PF 6

Optically pure thallium (1 R)-(−)-9,9-dimethyltricyclo[6.1.1.0 2′6]deca-2,5-dienide (TIPCp) reacts with [( n 6-arene)RuCl 2] 2 (arene = C 6H 6, MeC 6H 4, p-MeC 6H 4CHMe 2, Me 6C 6) and NH 4PF 6 to form two isomers of the complexes [( n 5-PCp)Ru( n 6-arene)]PF 6. The π-facial stereoselectivity for coordination of PCp is a function of the steric bulk of the arene increasing from a 1:1 (C 6H 6) to a 14:1 (Me 6C 6) exolendo isomer ratio. New complexes were characterized by elemental analyses, physical properties, differential pulse voltammetry and 1H and 13C{ 1H} one- and two-dimensional NMR spectroscopy. The structure of [( n 5-PCp)Ru( n 6-Me 6C 6)]PF 6 was confirmed by X-ray crystallography. This compound crystallized in the acentric space group C222 1 in a unit cell with the following dimensions: a = 11.451(4), b = 13.426(5), c = 31.249(3) Å, V= 4804(3) Å 3, Z = 8. Refinement converged to R(F) = 0.056 for 1723 independent observed ( I>3σ( I)) reflections. Ruthenium coordinates to the less sterically hindered exo face of the PCp ligand, anti to the gem-dimethyl group, and is closer to the arene (Ru-ring centroid = 1.73 Å) than to the PCp ligand (Ru-ring centroid = 1.79 Å).

Reactions of benzocyclobutene chromium complexes with carbon, nitrogen and oxygen nucleophiles: nucleophilic addition and unexpected proximal ring opening reactions

Tricarbonyl(η 6-1-oxobenzocyclobutene)chromium(0) (1) can be transformed to tricarbonyl(η 6-1- endo-hydroxybenzocyclobutene) chromium(0) derivatives with substituents R (RCH 3, CHCH 2, (CH 2) 4CHCH 2, (CH 2) 4OSi(Me) 2tBu) at Cl on the exo face of the complex. The relative configuration is proven by an X-ray crystal structure analysis of the trimethylsilyl ether 8 (C 16H 18CrO 4Si: a=8.693(1), b=9.490(1), c=11.063(1) Å, α=97.51(1), β=110.32(1), γ=95.38(1)°, triclinic, space group P 1 (No.2), R=0.037, R w=0.052 for 4609 observed reflections. Attempts directed at an intramolecular cycloaddition of the ortho-quinodimethane complex derived from 17 by anion promoted ring opening unexpectedly resulted in the formation of 12 as the product of an opening of the proximal bond of the anellated ring located between the hydroxy group and the coordinated aromatic ring in 16. The fact that the intermolecular cycloaddition reaction for 16 is possible in the presence of a dienophile is taken as evidence for an equilibrium between the alcoholate 17 and the two ring opened products 16 and 18. The proximal ring opening of 6 is not observed when the free organic ligand 21 is used as the educt. Ketone complexes 1 and 25 undergo proximal ring opening reaction when treated with alcoholate or primary amines.

Syntheses and reactions of iron and ruthenium complexes of an optically pure fused cyclopentadienyl ligand

Optically pure (1R)-(-)-9,9-dimethyltricyclo[6.1.1.0[sup 2,6]]deca- 2,5-diene (HPCp) reacts with Fe-(CO)[sub 5] and Ru[sub 3](CO)[sub 12], in the presence of the hydrogen acceptor norbornene, to form the isomorphic and isostructural complexes cis-[PCpM(CO)[sub 2]][sub 2] (M = Fe (1), Ru (8)), in which the metal is coordinated exclusively to the exo face of PCp. These complexes react with iodine to form PCpM(CO)[sub 2]I (M = Fe(2), Ru(9)). Complex 2 reacts with 1-phenyl-3,4-dimethylphosphole (DMPP), diphenylvinylphosphine (DPVP), and phenyldivinylphosphine (DVPP) to form two diastereomers of PCpFe(CO)(DMPP)I (3a,b), PCpFe(CO)(DPVP)I, (4a,b), and PCpFe(CO)-(DVPP)I (5a,b), respectively. Complex 9 reacts with DMPP to form two diastereomers of PCpRu(CO)(DMPP)I (10a,b). Complexes 4a and 4b react with DPVP and DVPP to undergo sequential ligand substitution and intramolecular [4 + 2] Diels-Alder cycloaddition forming four and five diastereomers of [PCpFe(CO)(DMPP)(DPVP)]I (6a-d) and [PCpFe(CO)(DMPP)-(DVPP)]I (7a-e), respectively. Complexes 10a and 10b similarly react with DPVP and AgBF[sub 4] to form four diastereomers of the [4 + 2] Diels-Alder adducts [PCpRu(CO)(DMPP)(DPVP)] BF[sub 4] (11a-d). Little asymmetric induction is observed in these metal-promoted intramolecular [4 + 2] Diels-Alder cycloadditions. 30 refs., 2 figs., 7 tabs.

Borole derivatives. 20. Three-center Fe-H-B bonding in (borole)(cyclopentadienyl)hydridoiron derivatives

(Borole)tricarbonyliron complexes Fe(CO)3(3,4-R12C4H2BPh) (3a, R1 = H; 3b, R1 = Me) react with cyclopentadiene under irradn. to give the hydrides CpFeH(3,4-R12C4H2BPh) (1a, b). Deprotonation by NaH in THF produces the borataferrocene anions [CpFe(3,4-R12C4H2BPh)]- (5a, b). These can be alkylated in the 2-/5-position by alkyl iodides in the presence of NaH to give 2,5-dialkyl hydride derivs.: e.g. CpFeH(2,5-R22C4H2BPh) [1c, R2 = Me, 1g, R2 = CH2:CH(CH2)4; 1h, R2 = (CH2)2CHCH2]. The structures of CpFeH(2,3,4,5-Me4C4BPh) (1d) and (C5H4Me)FeH(2,5-Me2C4H2BPh) (1f) as detd. by low-temp. x-ray diffraction show the presence of an Fe-H-B three-center bond with a weak B-H interaction. In soln. 1a establishes a mobile equil. between the ground state with Fe-H-B three-center bonding and the agostic isomer with Fe-H-C-2(C4H4B) three-center bonding with an estd. DG = 1.4 kcal/mol. The hydridic proton and the 2-/5-protons of the borole ring undergo fast intramol. exchange. A merry-go-round of the three H atoms involved with one hydrogen crossing the exo face of the borole ligand is proposed as an exchange mechanism. The bonding of CpFeH(C4H4BH) is analyzed by means of extended Hueckel calcns., which confirm the Fe-H-B three-center bonding. Exploratory extended Hueckel calcns. also support the existence of agostic isomers as well as the proposed exchange mechanism. Cyclic voltammetry revealed the existence of the 17e complex CpFe(C4H4BPh) (5a), which is characterized by its EPR and paramagnetic 1H NMR spectra. 1A reacts with CNBut to give the boracyclopentenyl complex CpFe(CNBut)(C4H5BPh) and, probably via the intermediate Fe(CNBut)(C4H4BPh)(C5H6), the borole complex Fe(CNBut)3(C4H4BPh) and 1 equiv of cyclopentadiene. Labeling expts. show that the shift of the hydride hydrogen to the borole and Cp ligands, resp., is an intramol. process. 1C reacts analogously.

Ring-opening olefin metathesis polymerization of spiro(bicyclo[2.2.1]hept-2-ene-7,1'-cyclopropane)

Spiro(bicyclo[2.2.1]hept-2-ene-7,1'-cyclopropane) was polymerized by ring-opening olefin metathesis polymerization (ROMP) to produce poly[2,1'-spirocyclopropane-1,3-cyclopentylenevinylene]. The polymerization reactions were carried out with conventional metathesis catalysts based on W and Ru. The steric bulk of the three-membered ring was not sufficient to prevent the approach of the catalytically active transition metal carbene species to the exo face of the monomer. However, steric hindrance had an important influence on the microstructure of the polymers formed. The resulting polymers contained a high proportion of trans carbon-carbon double bonds and a slight excess of racemic dyads. Models based on steric repulsion are proposed to describe trans/cis and racemic/meso selectivity

Dihedral angle-dependent orbital distortions arising from vicinal bonds in norbornene and 2-norbornanone

Exo reactivities of norbornene with respect to electrophilic addition and of 2-norbornanone with respect to nucleophilic addition are classical examples of the influence of distorted π faces of sp 2-trigonal centers of olefin and carbonyl groups. Intuitively, it seems likely that the asymmetry of π orbitals is an intrinsic feature of the nonequivalent environment around the trigonal centers arising from the unsymmetrical substituents, methano and ethano bridges in these cases. In this paper we will propose an orbital interaction motif (orbital diffusion effect) to describe a dihedral angle-dependent magnitude of interaction in the methano and ethano bridges. This divergent interaction provides an explanation for the origin of the unsymmetrical π face of norbornene and norbornanone. These two systems involve the mixing of CC σ (σ *) orbitals of the methano and ethano bridges into π or π * orbitals of the trigonal carbon centers. These interactions are primary interactions, although these mixing will not create by themselves a secondary hybridization or tilting of the π or π * orbitals to one face of the π bond. However these primary interactions involve intrinsically divergent orbital diffusion of the π (π *) orbitals which unsymmetrize the p face toward the attack of the reagents. In norbornene, out-of-phase interaction of the π orbital (π CC) of the olefin group with the less acute allylic σ CC orbital of the ethano bridge leads to greater depletion of the electron density of the π CC orbital on the endo face, favoring exo addition of electrophiles. In 2-norbornanone, the π * orbital (π * CO) of the carbonyl group is perturbed by the in-phase combination of the vacant σ * CC orbital of the less acute ethano bridge at the back-side lobe. This mixing builds up a larger virtual bonding region on the exo face which is to be attacked by a nucleophile. Ab initio calculations of 1-butene and 2-fluoroacetaldehyde as models for norbornene and 2-norbornanone supported the involvement of interactions with dihedral angle-dependent magnitude.

Stereoselective addition of nitromethane to levoglucosenone; formation and structure of 2 : 1 and 1 : 2 adducts

Tetramethylguanidine-catalysed addition of nitromethane to levoglucosenone affords in 98% yield a 10 : 1 mixture of 2 : 1 adducts 2 and 3, which result from initial Michael addition of nitromethanide exclusively from the exo face at C-4, followed by further reaction of the nitronate anion at both faces of the 1 : 1 adduct 4. In the presence of excess of levoglucosenone the major product (95%) is a pentacyclic 1 : 2 adduct 6, formed by reaction of 1 : 1 adduct 4 with further levoglucosenone. MeOH–levoglucosenone–MeNO2 adduct 12 and 2 : 2 adduct 7 were also formed as by-products in the corresponding diethylamine-catalysed reaction in methanol. The structure of compound 6 was established by X-ray crystallography.

Oxidative addition reactions of ω-alkenyl nitronate anions

Anions are oxidatively added to nitronate anions to yield α-substituted nitroalkanes when potassium hexacyanoferrate(III) is used as oxidant. Oxidation of nitronate anions yields intermediate anitroalkyl radicals, which undergo addition by anions to give intermediate α-substituted nitroalkane radical anions, which are further oxidised to yield the α-substituted nitroalkanes. The nitronate anions used are derived from 5-nitrohex-1-ene, 6-nitrohept-1-ene, endo-5-nitro-exo-6-phenylbicyclo[2.2.1 ]hept-2-ene, and 1-(bicyclo[2.2.1 ]hept-5-en-endo-2-yl)-2-nitropropane. The intermediate α-nitroalkyl radicals underwent addition of anions to form radical anions faster than intramolecular cyclisation onto the ω-alkenes. The α-nitroalkyl radical derived from 1-(bicyclo[2.2.1 ]hept-5-en-endo-2yl)-2-nitropropane underwent cyclisation when the oxidation was carried out in the absence of an added anion. The addition of anions (thiocyanate, benzenesulfinate, 4-chlorobenzenethiolate, and nitrite) to the α-nitroalkyl radicals derived from endo-5-nitro-exo-6-phenylbicyclo[2.2.1]hept-2-ene occurred stereoselectively from the less hindered exo face. Oxidative addition of 4-chlorobenzenethiolate to the nitronate from endo-5-nitro-exo-6-phenylbicyclo[2.2.1]hept-2-ene gave an unusual cyclisation via addition of a thiyl radical to the alkene.

Stereoselective deprotonation of tropinone and reactions of tropinone lithium enolate

Tropinone (6) was deprotonated with lithium diisopropylamide and with chiral lithium amides (18–24) and the resulting enolates (two enantiomers) were treated with electrophiles. The aldol reaction with benzaldehyde and deuteration were both diastereoselective. The former yielded only one isomer (exo, anti) of the aldol 8a; the latter proceeded from the exo face. This selectivity permitted us to probe the deprotonation of tropinone with lithium amides; it was concluded that the reaction involves predominantly the exo axial protons. The reaction of tropinone enolate with ethyl chloroformate led, via a ring opening, to the cycloheptenone derivative 9. The reaction with methyl cyanoformate yielded, in the presence of silver acetate and acetic acid, the β-ketoester 8b; however, in the absence of these additives, and especially when 12-crown-4 was added to the enolate, a ring opening leading to the pyrrolidine derivative 10 occurred instead. Deprotonation of tropinone with chiral lithium amides proceeded with modest enantioselectivity. A synthesis of non-racemic anhydroecgonine via this strategy allowed establishing the absolute stereochemistry of deprotonation.

Stereospecific solid state sodium borohydride reductions of cage dikotenes

Solid state reductions of carbonyl groups in three cage diketones, 1a-1c, have been performed by using sodium borohydride, and the results thereby obtained have been compared with the corresponding reductions performed in ethanol solution. In each case, the solid state reduction proceeds stereospecifically; hydride transfer occurs exclusively at the exo face of the carbonyl group. In contrast, the corresponding homogeneous (solution-phase) reductions display only moderate stereoselectivity.

A Convenient Solid-Support Procedure for the Synthesis of Symmetric Disulphides. The Configuration of Aromatic Carben Atoms in the 1,6-Methano[10]annulene Framework

Treatment of organic thiocyanates with aluminium oxide, at room temperature, is found to afford a convenient procedure for the synthesis of symmetric disulphides, under mild conditions. An X-ray structure analysis of meso-di-(1,6-methano[10]annulen-2-yl)-disulphide (2a) suggests an intermediate sp2—-sp3 hybridization state for the aromatic carbon atoms and a faceasymmetric electronic distribution in the annulene ring. Using reported atomic coordinates, a similar configuration is determined for the parent structure 7. A higher π-electron density on the exo face of the aromatic ring may account for the known syn,syn stereochemical course of electrophilic addition to 7, as well as for the high reactivity of this system towards electrophiles, in addition and in substitution reactions.

ChemInform Abstract: A Direct Synthesis of Hydroxysemperoside Deglucoside

A 1 0-step synthesis of hydroxysemperoside deglucoside has been achieved. The key step is an organopalladium-mediated cyclization that produces a highly functionalized oxabicyclooctane. The remaining functional groups are introduced by addition to the exo face of the fused bicyclic system.

Generation of homochiral quaternary carbon centres from (vinylketenimine)tricarbonyliron(0) complexes

The first homochiral (vinylketenimine)tricarbonyliron(0) complexes have been formed. Nucleophilic attack occurs exclusively on their exo face and after an oxidative work-up homochiral β,γ-unsaturated carboxylic acids containing an α quaternary carbon centre are obtained.

A direct synthesis of hydroxysemperoside deglucoside

A 1 0-step synthesis of hydroxysemperoside deglucoside has been achieved. The key step is an organopalladium-mediated cyclization that produces a highly functionalized oxabicyclooctane. The remaining functional groups are introduced by addition to the exo face of the fused bicyclic system.

A Diastereoselective Preparation of 7-Ethyl-7-methylbicyclo[3.2.0]HEPT-2-en-6-one(1)

Abstract Alkylation of 7-endo-ethylbicyclo[3.2.0]hept-2-en-6-one (2a) and 7-endo-methylbicyclo[3.2.0]hept-2-en-6-one (2b) with lithium diisopropylamide (LDA) and the appropriate alkyl iodide (methyl and ethyl iodide, respectively) afforded the title compound such that alkylation had occurred exclusively on the exo face of the bicyclo[3.2.0] system.

Studies on the reactivity of the halo-hydride complexes [M(η 5-C 5H 5) 2HX] (M = Mo, W; X = Cl, Br, I)

A convenient preparation of the cations [Mo(η 5-C 5H 5) 2HL] + and [Mo(η 5-C 5H 5) 2H(LL)] + from [Mo(η 5-C 5H 5) 2HI] ( 1), L and TlPF 6(BF 4) is described for L = CO, C 2H 4, PMe 2Ph, PEt 2,Ph, PPh 3, NH 3, NCMe and LL = Ph 2PCH 2PPh 2, (dppm), H 2NCH 2CH 2NH 2 (en). Complex 1 and butadiene give [Mo(η 5-C 5H 5) 2(η 3-CH 3C 3H 4)]BF 4. The chloro and bromo analogues of 1 are less reactive, and the W analogue of 1 does not react under the conditions used for 1. Reaction of 1 with Ph 2PCH 2CH 2PPh 2 (dppe) gives the cation [Mo(η 5-C 5H 5)(η 4-C 5H 6)dppe] + ( 18) in the presence or absence of TlPF 6. The deuterido analogue of 1, 1d gives 18d having the deuterium on the exo face of the C 5H 5D ring.

Chemo- and stereoselective functionalization of 7-oxabicyclo[2.2.1] hept-5-en-2-one derivatives with the system trichloroacetyl chloride/Zn(Cu) 1

Dichloroketene adds onto the exo face of the carbonyl group of 7-oxabicyclo [2.2.1] hept-5-en-2-one to yield a dichloro- β -lactone with complete chemo- and stereoselectivity. This dichlorolactonization has been extended to other oxanorbornenic systems and the resulting β-lactones were transformed into the corresponding 1,3-diols.

Microstructure and mechanism of formation of the ring‐opened polymers of anti‐7‐ethylbicycio[2.2.1]hept‐2‐ene initiated with metathesis catalysts

AbstractA 52:48 mixture of syn‐ and anti‐7‐ethylbicyclo[2.2.1]hept‐2‐ene was submitted to polymerization by five metathesis catalysts: RuCl3. 3H20, IrCl3.3H2O, (mes)W(CO)3/EtAlCl2/norbornene epoxide, WCl6/Me4Sn, and ReCl5. Only the anti isomer undergoes ring‐opening polymerization, from which it is concluded that propagation occurs by attack on the exo face. The fraction of cis double bonds and tacticities were determined from 13C n.m.r. spectra of the polymers and their hydrogenated products. For the unsaturated polymers these were respectively: 0 00 atactic, 0 21 atactic, 0 43 tactic (c/r, t/m), 059 atactic, 1 00 syndiotactic. The atactic polymers all show a bias towards m dyads for both cis and trans double bonds. This indicates an influence of the configuration of the polymer chain attached to the propagating metal‐carbene complex on the stereochemistry of addition of monomer.