3 Organometallic chemistry Part (ii) Stoichiometric methods By GUY C. LLOYD-JONES School of Chemistry University of Bristol Cantock’s Close Bristol UK BS8 1TS 1 Introduction Despite continued growth in transition metal catalysis many areas of organic synthesis –particularly C–C bond forming reactions–are still underdeveloped and require stoichiometric methodology. A selection from the chemical literature of 1997 is reviewed below and is organised by transition metal group with further subdivisions by reaction or intermediates. 2 Applications Scandium Ytterbium and the Lanthanides The triflate salts of these metals show remarkable properties as catalysts for a variety of reactions but stoichiometric applications are scarce. Titanium Zirconium and Hafnium Olefination Despite the rapid rise of catalytic olefin metathesis the oxophilicity of titanium-based reagents currently assures that they remain a powerful methodology for carbon –carbon double bond forming reactions from carbonyl compounds.For example reaction of thioacetals with Cp 2 Ti[P(OEt 3 )] 2 generates reagents that react with ketones and aldehydes carboxylic acids and lactones to generate alkenes and enol ethers in good yield.1 Sequential treatment of lithiated N-benzyl or N-allyl benzotriazoles with ketones and then low-valent titanium gives olefins in good yield and with high trans selectivity.2 The titanium ylide (Me 2 N) 3 P CHTi(Pr*O)Cl 2 reacts with aldehydes and ketones to a§ord vinylphosphonium salts which can be deprotonated and reacted with a further aldehyde or ketone thereby generating an allene through a double olefination sequence.3 This was further extended to e§ect a one-pot ring closure of two tethered aromatic aldehydes.When the tether is a binaphthol ether the allene macrocycle is generated diastereoselectively (ca. 66% de).4 Intramolecular McMurry type coupling has been used to prepare photochromic cyclohexenes from the corresponding 1,6-diones5 and four configurational isomers of the intriguing macrocycle 1 whose dynamics were studied by NMR spectroscopy.6 Zirconium also 105 N Br N 3 2 i Scheme 1 Reagents i Cp 2 Zr(g2-butene) O O O O 1 facilitates olefination and active zirconium metal may be prepared simply by reduction of ZrCl 4 with lithium.7 Metallocenes Reaction of Me 3 Al with Cp 2 TiCl 2 a§ords a reagent which methyltitanates diphenylacetylene with [98% Z-selectivity; dec-5-yne reacts analogously but then undergoes b-H elimination to yield the allene 6-methyldeca-4,5-diene.Tebbe-type products (metallacyclobutenes) are not observed under these conditions.8 Treatment of b,c-unsaturated thioacetals with titanocene–alkene complexes yields vinylcyclopropanes. Usefully 1,2-dibromoalkenes serve as alkene precursors in this reaction thereby reducing problems associated with volatility of low molecular weight alkenes.9 Zirconocene(g2-butene) undergoes reductive cyclisation with bromodienes e.g. 2 and the unstable intermediate undergoes b-bromo elimination to a§ord pyrollidine derivative 3 bearing an exocyclic methylene (Scheme 1). Analogous carbocyclic products can also be formed from appropriate precursors.10 Diastereoselective skeletal rearrangement to carbocyclic 5 occurs on treatment of cis- or trans-2-vinylpiperidine derivative 4 with zirconocene,11 and reaction of triethyl orthoacrylate with zirconocene a§ords an a,b-unsaturated acyl anion equivalent which reacts with aldehydes to a§ord 2,2-diethoxybut-3-en-1-ol derivatives.12 Sequential addition of Cp 2 ZrBu 2 MeMgBr and then O 2 to enantiomerically pure allylic amine (S)-6 gave hexahydroindole 7 as a single diastereomer which was further transformed to ([)-mesembrane and ([)- mesembrine (Scheme 2).13 Zirconocene(g2-butene) induces regioselective ring opening of vinylcyclopropanes–this was applied to the stereocontrolled synthesis of steroidal side-chains14–and on reaction with 1,2-bis(TMS)acetylene and then vinyl bromide a§ords 2,3-bis(TMS)-buta-1,3-diene in 95% yield.This reaction which probably proceeds via a cyclobutene intermediate cleaves the vinyl bromide double bond. The reaction sequence is also successful with other acetylenes e.g. 1-phenyl-2-TMSacetylene and 1,2-diphenyl acetylene.15 Methylaluminoxane catalyses the regio- and stereocontrolled a-addition of allylzirconium species (prepared by hydrozirconation of allenes) to C2 of alk-1-ynes,16 and reaction of Cp 2 ZrCl 2 with two equivalent EtMgBr followed by an alkyne and then CO/I 2 yields cyclopentenone derivatives in analogy to 106 G. C. Lloyd-Jones N Boc TsO OMe OMe N OMe OMe N OH H N H Boc Ph Ph Ph Ph 7 (±)-4 (±)-5 6 i ii iii iv Scheme 2 Reagents i Cp 2 Zr; ii Cp 2 ZrBu 2 ; iii MeMgBr; iv O 2 Cp2Zr Ph Ph Zr(Cl)Cp2 8 9 i Scheme 3 Reagents i allyl chloride CuCl the Pauson–Khand reaction.17 Zirconacycle 8 reacts with allyl chloride in the presence of copper(I) chloride to give 9 as a result of highly regioselective insertion ([98% de Scheme 3).18 Miscellaneous Duthaler’s chiral titanium(IV) enolate 10 introduced the required anti stereochemistry at C6–C7 and C18–19 in a total synthesis of tautomycin 11.19 Cyclopropylamines are readily prepared from amides by a new reagent mixture formed from MeTi(Pr*O) 3 and one equivalent of Grignard reagent.Intramolecular examples a§orded bicyclo[n.1.0] alkylamines–e.g. 13 is formed from 12 (Scheme 4).20 The cyclic titanium chelate formed by transmetallation of a-methyl-b-siloxy ketones with TiCl 4 reacts with high diastereoselectivity with Grignard reagents to a§ord a-methyl-b-siloxy tertiary alcohols.21 Diisopropoxy Ti–TADDOLate complexes can be used to kinetically resolve racemic chiral 1,3-dioxolan-4-ones azalactones anhydrides and axially chiral biaryl lactones through isopropoxy ring-opening transfer esterification.22 Reaction of planar chiral all-1-en-7-ynes with (g2-propene)Ti(Pr*O) 2 occurs with complete chirality transfer to a§ord after quenching of the intermediate titanacycle 1-alkenyl-2-alkylidenecyclopentanes. 23 107 Organometallic chemistry Part (ii) Stoichiometric methods Ti O O O O O O O O O O OH O OMe OH O OH O O H H O 11 10 7 18 2 6 19 Reduction Nitro-aromatics are reduced under mild and neutral conditions to the corresponding anilines by use of a combination of Cp 2 TiCl 2 and samarium metal.24 Reaction of a-keto acids with Ti(Pr*O) 4 a§ords titanates which can then be reduced by LDA to the corresponding a-hydroxy acid.When the alkoxy ligands in the titanate are preexchanged with di-O-isopropylidine-D-mannitol the reduction proceeds with up to 44% ee.25 Lewis acids The addition of 1,1-diarylethenes to 1,2-naphthoquinones and derivatives proceeds e¶ciently in the presence of Ti(Pr*O)Cl 3 to a§ord the 4-(2,2-diarylethenyl) substituted products.26 Both zirconium(IV) and hafnium(IV) chlorides have been found to signifi- cantly extend the range of the sulfone based alkoxymethylene homologation procedure that converts ketones into their a-methoxylated homologues.27 Vanadium Niobium and Tantalum Of these metals thus far only vanadium has been used significantly in organic synthesis. Asymmetric quaternary carbons may be constructed directly from carbonyl compounds by sequential allylation with an allyl metal reagent (metal\Li Zn MgBr etc.) and then reaction with allyl propargyl or benzyl bromide and the strongly oxophilic vanadium(II) complex VCl 2 (tmeda) 2 .28 Dialkylzinc reagents react with vanadium(III) and (IV) chlorides to a§ord organovanadates which both alkylate and pinacol couple aldehydes.29 Chromium Molybdenum and Tungsten Carbene complexes Fischer carbene complexes (FCC) continue to be of great interest particularly those of chromium.Lithioacetylenes undergo [1,2] addition to FCC to give species that behave as propargyl metallics. Quenching a§ords enynes or enones–dependent upon 108 G. C. Lloyd-Jones NMe2 NMe2 Br O (OC)5Cr OMe OMe Bu Fe Fe ii i 12 13 14 15 Scheme 4 Reagents i MeTi(Pr*O) 3 EtMgBr; ii pent-1-ene the pH during work-up–whilst reaction with aldehydes imines orCO 2 a§ords furans pyrroles and butenolides respectively.30 a,b-Unsaturated chromium FCC react with BH 3 ·Me 2 S to a§ord on oxidative work-up 1,3-diols.31 2-Arylalkenyl chromium FCC cyclopropanate terminal alkenes with good diastereoselectivity for example 14 reacted with pentene to give 15 (97% de Scheme 4).32 Vinyl tungsten FCC undergo Diels–Alder addition with 2-amino-1,3-dienes to a§ord after hydrolysis cyclohexan- 4-one carbenes33 and chromium FCC undergo diastereoselective cycloaddition with diazomethane derivatives to yield dihydropyrazoles.34 A quite remarkable dimerisation with loss of one methylene occurs on treatment of a diene e.g.16 with FCC 17. The MeO2C OTBDMS CO2Me TBDMSO CO2Me MeO2C OTBDMS TBDMSO (OC)5Cr OMe Ph 17 16 18 resulting vinylcyclopentene 18 is obtained essentially quantitatively and as a single diastereomer.The carbene ligand of 17 is not incorporated in the product.35 Tungsten( II) FCC of type 19 react with aldehydes e.g. benzaldehyde in the presence of BF 3 ·Et 2 Oto give cycloalkenylated salts 20. These salts have a rich chemistry and react with nucleophiles as diverse as water cyanoborohydride Grignard reagents and organocuprates. The latter add 1,3- whilst the former three add 1,1-. Salt 20 is cyclopropanated diastereoselectively by diazomethane to give after hydrolysis 21 (Scheme 5).36 Enantiomerically pure ephedrine derived imidazolidinone FCC 22 exists as a pair of stable atropisomers which were separated. On cyclohexadienone annulation with alkynes the atropisomers generate a quaternary carbon centre in 23 with very high diastereoselectivity (Scheme 6).Opposite diastereomers with di§erent colours are formed from the two atropisomers.37 109 Organometallic chemistry Part (ii) Stoichiometric methods 19 20 (±)-21 O (CO)3CpW R Ph O O R Ph R HO (CO)3CpW H i ii Scheme 5 Reagents i PhCHO; ii CH 2 N 2 N N Ph N Cr(CO)5 O N N Ph O N O R' 22 23 24 (±)-25 R (CO)3CpW OHC O H n–2 i n ii,iii Scheme 6 Reagents i hex-1-ene; ii TsOH MeOH; iii NOBF 4 NaI p-Complexes Chromium tricarbonyl arene complexes that possess planar chirality remain an area of much endeavour–particularly in the development of methodology for their asymmetric synthesis rather than resolution. Asymmetric deprotonation of anisole derived (g6-arene)Cr(CO) 3 complexes with chiral lithium amide bases followed by electrophilic quench proceeded with up to 99% ee.38 The enantiomerically pure chromium tricarbonyl complexes of o-tolualdehyde and o-anisaldehyde underwent SmI 2 - mediated coupling with methyl acrylate to yield the corresponding c-butyrolactones as single disastereomers.39 Photochemical cycloaddition of 2-methyl- and 2,3- dimethyl-butadiene to Cr(CO) 3 (COT) followed by CAN oxidation a§ords bicyclotetraenes.40 Treatment of propargyl tungsten species 24 with acidic methanol a§ords 110 G. C. Lloyd-Jones O O O O H OSiPh2But O OTES MeO Br O O O OH OSiPh2But O OTES MeO 26 28 27 i Scheme 7 Reagents i CrCl 2 NiCl 2 (1% w/w) p-allyl tungsten complexes which generate fused a-methylenebutyrolactones of five six and seven membered rings 25 on intramolecular cyclisation triggered by addition of NO`–NaI (Scheme 6).41 Highly stereoselective methylation and ethylation of 2-ethyland 2-methyl-(g6-indan-1,3-dione)tricarbonylchromium respectively allowed access to 2,2-dialkylated(g6-indan-1,3-dione)tricarbonylchromium complexes which were not preparable by conventional means.42 The g6-chromiumtricarbonyl complex of chlorobenzene phenylates N,N-dimethyl hydrazone cuprates LiM[Me 2 NN––(R)CHR@] 2 CuN to a§ord the corresponding ketones RCOCHR@Ph on work-up.43 Miscellaneous The determination of the absolute configuration of chiral 1,3-diols by CD spectroscopy is greatly aided by complexation with [Mo 2 (OAc) 4 ].44 The polymeric complex derived from pyrazine and oxodiperoxochromium(VI) is a highly versatile reagent for oxidation of common organic functionality including alcohols diols thiols sulfides phosphines electron rich aromatics phenols and amines.45 An allylic chromium intermediate derived from 26 added smoothly to aldehyde 27 to give homoallylic alcohol 28 in a stereoselective synthesis of phorboxazole A (Scheme 7),46 and a vinylic chromium reagent was employed in a total synthesis of halicholactone.47 Manganese Technetium and Rhenium Of these metals only manganese appears in mainstream organic synthetic applications.Organomanganese r-compounds Lithium–halide exchange at low temperature followed by manganese–lithium exchange converts aryl and alkenyl halides to their corresponding aryl and alkenyl manganese halides. Usefully these species react cleanly with acyl halides to a§ord ketones48 and may be cross coupled with aryl halides or triflates through palladium catalysis.49 A range of organomanganese chlorides RMnCl (R\Bu dodecyl Pr* But Ph hept-1-ynyl) were reacted with the butyrolactone acyl chloride derived from L-glutamic acid to a§ord d-ketobutanolides in enantiomerically pure form.50 Organomanganese p-compounds Nucleophiles attack the allyl terminus of tetracarbonyl(p-allyl)manganese complex and the allylated products are obtained in moderate to excellent yield on oxidative 111 Organometallic chemistry Part (ii) Stoichiometric methods (OC)3Fe H2N n-Hept OMe OMe (OC)3Fe NH n-Hept OMe OMe H H NH n-Hept O O 29 31 32 30 i Scheme 8 Reagents i MeCN air decomplexation.Interestingly the pK! of the pro-nucleophile determines the degree of allylation non-stabilised nucleophiles are monoallylated whilst stabilised carbon nucleophiles are bis-allylated.51 Manganese ‘ate’ complexes The use of manganates is steadily increasing–particularly as transmetallating species in combination with catalysts such as Ni and Cu.Tributyl manganates readily generated by addition of butyllithium or magnesium species to manganese(II) salts induce cyclisation of allylic-2-halogenoaryl ethers to give 3-alkyl-2,3-dihydrobenzofurans in good yields. Analogous reactions with anilines a§orded indolines.52 Curiously manganate(II) complexes dehalogenate a-halogen bearing ketones esters and amides to a§ord the corresponding manganese enolate regiospecifically. a-Siloxy and a-acetoxy ketones behave analogously. An oxidative addition/reductive elimination type mechanism is suggested.53 Miscellaneous The BuLi-generated anions of manganese carbenes (Me-Cp)(CO) 2 Mn––C(OEt)CH 2 R react with a,b-unsaturated ketones to a§ord after decomplexation with CO or PPh 3 substituted cyclohexen-2-ones.54 Iron Ruthenium and Osmium p-Complexes Reaction of aniline 29 with g5-cyclohexadienyl iron cation 30 a§orded cycloadduct 31.This was further elaborated and demetallated to a§ord the potent lipid peroxidation inhibitor carbazoquinocin C 32 (Scheme 8).55 Chiral iron complexes continue to be of interest for the conduction of diastereoselective transformations prior to demetalla- 112 G. C. Lloyd-Jones Fe CO CO PMBO H H Fe CO CO O O Ph OCu Ph O CO2Me OH 34 33 35 36 i,ii iii iv Scheme 9 Reagents i LiMe 2 Cu; ii PMB-OH; iii CAN NaOMe CO; iv DDQ Fe H MeO2C H OC CO CO CO2Me MeO2C CO2Me H CO2Me MeO2C 38 37 i Scheme 10 Reagents i CAN tion.Enantiomerically enriched (g4-methylhexa-3,5-dienoate)Fe(CO) 3 (83% ee) was prepared by resolution with (R)-a-methylbenzylamine. Subsequent deprotonation and alkylation alpha to the carboxylate proceeded with 69–92% de.56 Optically active dicarbonyl cyclopentadienyl (vinyl ether) iron cations function as 3-hydroxypropionate 2,3-dication equivalents. Transetherification allows a range of optically active vinyl ether complexes to be obtained. For example complex 33 was transetherified to 34 and then reacted with enolate 35 to a§ord after deprotection optically active 3-hydroxyketo ester 36 (Scheme 9).57 Tricarbonyliron(pentenediyl) complexes e.g. 37 undergo reductive elimination (triggered by oxidation) to a§ord vinylcyclopropanecarboxylates e.g.38 in up to 70% yield. Deuterium labelling was employed to elucidate stereochemical aspects of the mechanism (Scheme 10).58 Intermediates in the stereoselective synthesis of all trans- and 9-cis-retinoic acids have been prepared by [1,2] addition of lithio acetonitrile and lithio ethyl acetate to the keto functionality of (g4-b-ionone)Fe(CO) 3 59 and addition of stannyl enolate 39 to C5 of g5-hexadienyl iron salt 40 generated two contiguous quaternary centers in 41 which were further manipulated in the construction of the A ring of racemic stemodinone (Scheme 11).60 Chelated allyl–iron carbene salts react with enolates at the allyl terminus61–this was applied to a synthesis of the terpene alcohol hotrienol.62 g2-Coordination of substituted anisoles to osmium(II) allows reaction at C4 with a range of carbon electrophiles.63 Chiral but racemic (g6-bromoarene)(g4-COD)ruthenium(0) complexes readily undergo lithium–halogen exchange with BuLi. The resulting anions are quenched with ([)-menthyl chloroformate to a§ord a 1 1 mixture of diastereomeric (g6- menthyl benzoate)(g4-COD)ruthenium(0) complexes which may be separated to afford enantiomerically pure compounds.64 Miscellaneous Dimethyldioxirane oxidation of the thioether complex of 4-phthalimidobutyl methyl sulfide with cyclopentadienyl ruthenium-(R,R)-chiraphos cation followed by cleavage of the phthalimido group with hydrazine and liberation of the sulfoxide with NaI 113 Organometallic chemistry Part (ii) Stoichiometric methods O O OSnBu3 MeO Fe OC OC CO 39 40 41 5 MeO Fe OC OC CO i O O O H Scheme 11 Reagents i MeCN,[78 °C Ph H CO2Et Ph H H H CO2Et O 42 43 i ii Scheme 12 Reagents i Co 2 (CO) 8 ; ii norbornene O O O O O O 45 44 i Scheme 13 Reagents i Co 2 (CO) 8 NMO a§ords (R)-sulforaphane MeS(O)(CH 2 ) 4 NCS in 80% ee.65 Polymer supported peruthenate provides a very convenient method for the oxidation of primary and secondary alcohols to aldehydes and ketones without conventional work-up.66 Cobalt Rhodium and Iridium Pauson–Khand Reactions Cyclopropane 42 undergoes a Pauson–Khand reaction (PKR) with norbornene to a§ord tricycle 43 with moderate regioselectivity (Scheme 12).67 The tandemPKR of 44 occurs with perfect regioselectivity to a§ord dicyclopenta[a,e]pentalene 45 (Scheme 13),68 and PKR reactions of alkynyl N-allyl FCC (Cr W) e.g.46 have been successfully carried out to give e.g.47.69 Near perfect regioselectivity ([99%) was observed in the PKR of[96% ee 48 to give cyclopentenone[96% ee 49 (Scheme 14).70 Miscellaneous Stereospecific intramolecular nucleophilic attack in p-allyl cobaloxime 50 proceeds with retention of configuration to generate intermediate 51 which underwent a photochemical cross coupling with styrene to give enantiomerically pure tetrahydrofuran derivative 52 (Scheme 15).71 Cobalt complexes bearing Ph 3 P or Me 3 P e¶ciently 114 G. C. Lloyd-Jones NH W(CO)5 NH W(CO)5 O H Ph Ph O H OBn HO OBn HO H 48 49 47 46 i i Scheme 14 Reagents i Co 2 (CO) 8 BzO OH Co(dmgH)2Py O Co(dmgH)2Py O Ph 51 50 52 iii i ii Scheme 15 Reagents i 1%NaOH MeOH; ii HCl PPTS; iii hl styrene mediate the equivalent of a Reformatsky reaction between a-haloketones and ketones or aldehydes to produce b-hydroxyketones in fair yield.72 Cobalt(I)(salen) anions undergo S N 2@ addition to allenic electrophiles to a§ord cobalt(III)–buta-1,3-diene complexes which react with a range of dienophiles.The resulting cycloadducts are readily demetallated to a§ord Diels–Alder type products in good yield.73 Reaction of Co 2 (CO) 8 with 1-allenylcyclopropan-1-ols followed by acetic anhydride yields derivatives of 1,4-diacetoxy-2-methylbenzene.74 Nickel Palladium and Platinum Palladium mediated reaction of 3-haloalkenoates with thiostannyl reagents Bu 3 SnSR generates 3-arylthio or alkylthio propenoates. Reactions with alkoxystannyl reagents proceed analogously.75 The intriguing tricyclobutyl 53 and radialene 54 were prepared in 16 and 24% yields via a one-pot procedure in which hexakis(dibromomethyl) benzene was heated with [(Bu 3 P) 2 Ni(COD)].76 o-Palladated complex 55 undergoes a highly diastereoselective Diels–Alder reaction with vinyldiphenylphosphine to a§ord after decomplexation P-chiral diphosphine ligand 56 in which four 115 Organometallic chemistry Part (ii) Stoichiometric methods Br Br Br Br Br Br Br Br Br Br Br Br 53 54 N Pd P Ph Cl P Ph i Ph2P 56 55 Scheme 16 Reagents i CH 2 ––CHPPh 2 i i Et S p-Tol OMs Ph O Et S p-Tol OMs Ph S p-Tol Et Me O Ph S p-Tol Et Me 57 58 Ph 59 60 Scheme 17 Reagents i MeCuCNMgBr asymmetric centres are generated (Scheme 16).77 (Dppe)PtCl 2 mediates the cyclocoupling of diazoesters and alk-3-yn-1-ols to give tetrahydrofuran-2-ylidene esters.78 Copper Silver and Gold Organocuprates Not surprisingly organocuprates continue to be used extensively in organic synthesis.For example stereoselective methylcuprate addition to allylic mesylates 57 and 59 occurs in an S N 2@ manner and the stereochemical outcome may be controlled by the oxidation state at the vinylic sulfur centre–57 gives 58 whilst 59 gives 60 (Scheme 17).79 S N 2@ anti displacement of allylic phosphate 61 by Grignard derived methylcuprate was employed in a synthesis of vitamin D 2 analogue 62 (Scheme 18).80 A 116 G. C. Lloyd-Jones OTBDMS TBDMSO H O H OH HO H HO H OP(O)(OEt)2 O 62 61 i Scheme 18 Reagents i MeMgBr CuCN LiCl methylcuprate was reacted with enol triflate 63 with ultrasonication to generate the required trisubstituted olefin 64 in a total synthesis of FK-506 (Scheme 19).81 Interestingly in toluene solution and in the absence of ethers LiBu 2 Cu adds [1,2] to a,b- unsaturated ketone 65.On addition of two molar equivalents of diethyl ether a near-complete switch to [1,4] addition occurs. The ether is proposed to stabilise the formally copper(III) intermediate 66.82 A zinc-modified cuprate derived from serine reacts with enantiomerically pure 1-methyl-3-(phenylsulfonyl)allylirontetracarbonyl cation to a§ord vinyl sulfones stereospecifically.83 The combination of tin with cuprates is also becoming popular–for example the higher order cuprate formed from bis-stannyl reagent 67 undergoes [1,4] addition to 4-siloxycyclohexenone 68 with perfect stereoselectivity to a§ord vinyl stannane 69 which is a useful substrate for Stille type cross coupling in a synthetic approach to TaxolTM (Scheme 20).84 Also Z-stannyl diene 71 a key intermediate in the synthesis of multidrug resistance reversal agent 6,7-dehydrostipiamide was prepared by carbocupration of acetylene 70 with a vinyl tin cuprate (Scheme 21).85 Cyclodimerization of iodostannane 72 mediated by copper( I) thiophene-2-carboxylate provided a novel synthesis of elaiophylin 73 (Scheme 22).86 a-Stannyl epoxides may be coupling with electrophiles by employing Cu 2 S87 and vinyltributyl tin compounds homocoupled to produce dienes by treatment with CuCl.88 Cuprates generated in situ by reaction of Grignard reagents with CuI react smoothly with tosylates.This process is employed for the enantioselective synthesis of threonine analogues 75 from 74 (Scheme 23).89 (a-Aminoalkyl)cuprates prepared from N-tert-BOC-pyrollidine or N-tert-BOC-dimethylamine undergo smooth conjugate addition to acyclic a,b-unsaturated esters thiol esters imides and a,b-ynoates,90 and couple stereospecifically with dialkylvinyl iodides and with 1-iodoalkynes.91 Cuprates derived from primary organolithium species attack aziridine tosylates derived from 117 Organometallic chemistry Part (ii) Stoichiometric methods OTf OTES O O O H TESO BDPSO MeO Me OTES O O O H TESO BDPSO MeO OEE OEE 63 64 i Scheme 19 Reagents i,Me 2 CuLi ultrasound O O Cu Li O Et Et Bu Bu 65 66 serines ([97% ee) exclusively at the least substituted aziridine carbon thereby allowing access to b,c-disubstituted 2-aminopropan-1-ol derivatives in high enantiomeric purity.92 Analogous regioselectivity is reported for N-phosphorylated aziridines.93 Although TMSCl is often used as an accelerant for cuprate additions a combination of Bu 2 BOTf–TMEDA was found to be much more e§ective in the conjugate addition of benzylic and allylic organocuprates to enantiomerically pure ephedrine-derived imidazolidinone 76.94 At [78 °C in the presence of Bu 3 P dialkyl cyanocuprates R 2 Cu(CN)Mg readily insert CO to a§ord a-hydroxy ketones RCOCH(OH)R in good yield.95 Diastereoselective conjugate addition of the silylcuprate Me(PhMe 2 Si)CuLi to enantiomerically pure 77 generates 78 which on protonation and stereoselective oxidation [mercury(II)/AcOOH] a§ords 79 ([99% ee Scheme 24).96 On lithiumbromine exchange and then transmetallation with CuCN (2-bromoaryl) allyl ethers rearrange to the corresponding 2-allylphenolates with the allyl terminus being delivered to the ortho position.97 118 G.C. Lloyd-Jones i Bu3Sn SnBu3 O OTES 67 O OTES 68 69 Bu3Sn Scheme 20 Reagents i,Me 2 CuCNLi 2 LiCl SnBu3 OEt i 71 70 O O OEt Scheme 21 Reagents i Bu 3 Sn(Me)CuCNLi 2 acetylene O OH O O O OH OH O O I SnMe3 OH O O I Me3Sn i 73 72 72 Scheme 22 Reagents i Cu(I) thiophene-2-carboxylate Halide abstraction and addition AgOTf abstracts chloride from a-ketoimido chlorides e.g. 80 and thereby induces cyclisation to a§ord pyrroline derivatives e.g. 81 (Scheme 25).98 Treatment of hydrazones with copper(II) halides and triethylamine in methanol proves a convenient method for preparing geminal dihalides from ketones.99 119 Organometallic chemistry Part (ii) Stoichiometric methods N O O O Ph OTs N O O O Ph R 74 i 75 Scheme 23 Reagents i RMgBr–CuI N N O Ph O R 76 O O O Ph O OLi O Ph PhMe2Si O O O Ph OH i 77 78 79 ii Scheme 24 Reagents i Me(PhMe 2 Si)CuLi; ii Hg(OAc) 2 AcO 2 H Miscellaneous Ullmann-like reductive coupling of aryl hetero-aryl and alkenyl halides occurs at ambient temperature by employing copper(I) 2-thiophenecarboxylate,100 and an asymmetric Ullmann coupling of oxazoline 82 was employed in the synthesis of (S)-gossypol.101 Treatment of 1,2-disubstituted cyclohexane-1,2-diols with CuBr 2 and LiOBut results in oxidative cleavage to a§ord the corresponding 1,6-diketones.102 Copper(I) was used to template the preparation of catenane 86 from 83 84 and 85 (Scheme 26).103 Zinc Cadmium and Mercury Addition of dialkylzinc reagents to aldehydes The enantioselective addition of dialkylzinc to aldehydes catalysed by chiral ligands is 120 G.C. Lloyd-Jones i N Cl O TMS TMS 80 N O TMS 81 Scheme 25 Reagents i AgOTf Br N O OMe O MeO MeO 82 now ubiquitous–particularly the diethylzinc–benzaldehyde reaction. However the following asymmetric amplification reaction is remarkable a trace amount of 2- methyl-1-(2-methyl-5-pyrimidyl)propan-1-ol of ca. 0.2–0.3% ee autocatalyses the addition of diisopropylzinc to 2-methylpyrimidone-5-carbaldehyde. The ee of the product which is itself the catalyst for its further generation rises rapidly throughout the reaction reaching a terminal value of up to 90% ee.104 Organozinc and mercury nucleophiles Transmetallation of lithium enolate 87 with ZnBr 2 at [90 °C gave 88 (after quenching) as a 97 3 ratio of diastereomers via a 5-exo trig cyclisation onto a non-activated double bond (Scheme 27).105 Lithiation (at the propargylic site) of homoallyl propargyl ethers followed by transmetallation with ZnBr 2 allows the preparation of substituted tetrahydrofuran derivatives via a zinca–ene–allene reaction.106 Nucleophilic attack by a range of organozinc species on enones with tethered alkynes in the presence of a nickel catalyst led to carbocyclisation.For example 89 gave 90 on reaction with MeZnCl (Scheme 27).107 Cyclopentenylzinc intermediate 91 underwent Pd-catalysed cross coupling with dienyl iodide 92 to a§ord after deprotection and oxidation nakienone A 93 (Scheme 28).108 The diazo substituted mercury compound 94 reacted cleanly with acyl halides RCOX to a§ord 1,3-dicarbonyl diazo reagents of type 95 (Scheme 29).109 Hydroboration of phenylcyclopentene 96 with Et 2 BH and then transmetallation with Pr* 2 Zn then CuCN and finally reaction with 2-bromo-1- phenylacetylene gave (^)-97 with 96% anti selectivity (Scheme 29).110 Secondary alkyl iodides undergo smooth I–Zn exchange with Pr* 2 Zn allowing access to functionalised secondary alkylzincs.111 Homoallylic hydroxylamines can be prepared by addition of allylzinc bromides to nitrones,112 and reaction of aldehydes with TMS–dialkylamine in the presence of LiClO 4 to generate an intermediate O-silyl aminol followed by addition of organozinc bromide a§ords b-dialkylamino esters in good yield and high purity.113 The a-zinciohydrazone 98 reacts with vinylmagnesium bromide to form 121 Organometallic chemistry Part (ii) Stoichiometric methods N N O O O N N O O O O O O O O O N N O O O O O O N N O O O i ii iii HO OH O I I 83 86 85 84 Scheme 26 Reagents i Cu(MeCN) 4 PF 6 ; ii Cs 2 CO 3 ; iii KCN bimetallic 99.This bimetallic may then be trapped with two electrophiles. For example trapping with Me 2 S 2 then allyl bromide gives 100 in what is a four-component sequential one-pot synthesis of polyfunctional ketones (Scheme 30).114 Ketones are readily prepared by addition of dialkylzincs to acyl chlorides in the presence of one equivalent AlCl 3 as Lewis acid promoter.115 Diethylzinc can be added to chalcone in a conjugate manner with up to 83% ee by use of camphor-derived cobalt catalysts.116 Zinc carbenoids Enantioselective cyclopropanation of allylic alcohols with bis(iodomethyl)zinc occurs with high selectivity in the presence of a catalytic quantity of enantiomerically pure 1,2-trans-N,N@-bismethylsulfonamidocyclohexane.The pre-formation of the zinc alkoxide of the allylic alcohol prior to reaction is essential for high selectivity.117 A 122 G. C. Lloyd-Jones N OLi OEt Ph Me N O OEt Ph Me 87 O Ph 88 O Ph 90 89 i ii Scheme 27 Reagents i ZnBr 2 ; ii MeZnCl Ni cat O OH OH TMSO OTBDMS OTBDMS ZnI I 93 92 91 i ii iii iv Scheme 28 Reagents i 5mol% PdCl 2 [P(furyl) 3 ] 2 10 mol% BuLi; ii K 2 CO 3 ; iii PCC; iv Bun 4 NF Hg N2 N2 O EtO O OEt N2 O OEt O R Ph Ph Ph 94 95 96 97 i ii iii iv v Scheme 29 Reagents i RCOX; ii Et 2 BH; iii Pr* 2 Zn; iv CuCN; v 2-bromo-1- phenylacetylene tartramide-derived dioxaborolane ligand is used to e§ect analogous reactions with functionalised iodoalkylzinc reagents.118 Repeated asymmetric cyclopropanation of allylic alcohols was used to build the multicyclopropane antifungal agent FR- 900848.119 b-Keto esters are homologatively transformed to c-keto esters on reaction with CH 2 I 2 –Et 2 Zn.120 123 Organometallic chemistry Part (ii) Stoichiometric methods N N R ZnBr N N R ZnBr MgBr N N R SMe 98 100 99 i ii iii Scheme 30 Reagents i CH 2 ––CHMgBr; ii Me 2 S 2 ; iii allyl bromide O O OH 101 102 i ii Scheme 31 Reagents i Hg(O 2 CCF 3 ) 2 ; ii LiAlH 4 MeO2C CO2 Me CO2Me SePh TMS CO2Me CO2Me H CO2Me H PhSe TMS H 103 104 105 i Scheme 32 Reagents i ZnBr 2 Lewis acids Homo Diels–Alder cycloadducts of type 101 allow access to diquinane precursors e.g.102 (Scheme 31) on reaction with mercury(II).121 These electrophilic ring opening reactions are usually concerted.In contrast bi- and tetra-cyclopropane arrays open via stabilised carbocations. Attack by tethered hydroxy proceeds with high stereoselectivity. 122 Zinc(II) bromide activates tricarbonyl olefins (e.g. 103) su¶ciently for attack by 1-seleno-2-silylethenes (e.g. 104) to a§ord cyclopropanes (e.g. 105) with near-perfect cis-selectivity (Scheme 32).123 A range of enantiomerically pure bisoxazoline zinc triflate complexes act as enantioselective Lewis acid promoters for the allylation of oxazolidinone imides with allylsilanes as allylating reagents.124 Miscellaneous (DABCO)–Zn(BH 4 ) 2 is a stable and useful reducing agent for the selective reduction of aldehydes ketones a,b-unsaturated ketones a-diketones and acyl chlorides at room temperature.125 Diethylzinc in the presence of a Pd catalyst induces reductive homocoupling of alkynyl or phenyl iodonium salts to a§ord biaryls or diynes.126 HMPA accelerated reaction of Zn(N 3 ) 2 ·2Py–2,4,6-tetrabromocyclohexa-2,5- dienone–PPh 3 with primary and secondary alcohols yields the primary and secondary azides (with inversion of stereochemistry) in excellent yield.127 Radical chain aromatic 124 G.C. 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