Professor Abel is an Emeritus Professor at the University of Exeter.

Organometallic Chemistry Volume 8

A Review of the Literature Published during 1978

By E. W. Abel, F. G. A. Stone

The Royal Society of Chemistry

Copyright © 1980 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-690-1

Contents

Chapter 1 Group I: The Alkali and Coinage Metals By J. L Wardell, 1,
Chapter 2 Group II: Alkaline Earth Metals and Zinc and its Congeners By J. L. Wardell, 18,
Chapter 3 Group III: Boron By J. D. Odom, 34,
Chapter 4 The Carbaboranes, including their Metal Complexes By J. B. Leach, 60,
Chapter 5 Group III: Aluminium, Gallium, Indium, and Thallium By J. P. Maher, 84,
Chapter 6 Group IV: The Silicon Group By D. A. Armitage, 110,
Chapter 7 Arsenic, Antimony, and Bismuth By J. L Wardell, 176,
Chapter 8 Metal Carbonyls By E. W. Abel and F. G. A. Sione, 181,
Chapter 9 Organometallic Compounds containing Metal–Metal Bonds By N. G. Connelly, 193,
Chapter 10 Substitution Reactions of Metal and Organometal Carbonyls with Group V and VI Ligands By D. A. Edwards, 228,
Chapter 11 Sigma-Bonded Organometallic Compounds of Transition Elements of Groups III A. to VIIA By D. J. Cardin and R. J. Norton, 260,
Chapter 12 Complexes containing Metal–Carbon σ-Bonds of the Groups Iron, Cobalt, and Nickel By S. D. Robinson, 296,
Chapter 13 Hydrocarbon–Metal π-complexes By J. A. S. Howell, 324,
Chapter 14 π-Cyclopentadienyl, π-Arene, and Related Complexes By W. E. Watts, 369,
Chapter 15 Homogeneous Catalysis by Transition-metal Complexes By C. White, 400,
Chapter 16 Organometallic Compounds in Biological Chemistry By D. J. Cardin, 434,
Chapter 17 Diffraction Studies of Organometallic Compounds By A. D. Redhouse, 447,
Author Index, 512,

CHAPTER 1

Group I: The Alkali and Coinage Metals

BY J. L. WARDELL

1 Alkali-metal compounds

Hydrocarbon Radical Anion and Dianion Alkali-Metal Compounds. — The crystal structure of (Ph-Ph)-·,K+(tetraglyme)2 is essentially the same as that of the Rb analogue; the cations are surrounded by ten oxygens of the two tetraglyme molecules. The effect of pressure on the ion-pair equilibria, solvent-separated ion-pair (s.s.i.p.) [??] contact ion-pair (c.i.p.), for Naph-·,Na+ in THF was studied as was the volume change for the conversion c.i.p. -> s.s.i.p. Disproportionation equilibria [equation (l)] were variously studied, e.g. for ArH = perylene (Pe),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

tetracene (Te), cis– and trans-stilbene (C and T respectively), and azobenzene. To account for the values of KDisp, ΔH, and ΔS for the Pe and Te equilibria (M = Li, Nay K, or Cs) in DME and THF, substantial differences in solvation of the cations were invoked; (i) Li+ was always highly solvated and Cs+ always poorly so, (5) Na+ highly solvated in ArH-·,Na+ but poorly so in ArH2-,2Na+ (both solvents), and (iii) K+ only well solvated in ArH-·,K+ (by DME). The crowding encountered in C and C-·,Na+ is at least partially alleviated in C2-, 2Na+. Hence a higher value results for KDisp(= 2 in THF at room temperature) for C-·,Na+ than for either the trans isomer (T-·,Na+) or (1) (KDisp = 0.03), for which strain is considered consistent in all the three species involved in equilibrium (1). Szwarc also studied the laser induced photoconversion of T2-,2Na+ into C2-,2Na+ and the dimerization of Ph2C=[bar.C]CH2-·,M+ (-> Ph2[bar.C]H2CH2[bar.C]Ph2, 2M+), a process dependent on M+.

Reactions of ArH-·, M+ with alkyl halides and proton sources have attracted further attention. The same isomeric mixture of RLi was produced from Naph-·,-Li+ in THF at room temperature and 1-Br-cis,cis-2,3-Me2-cyclopropan(c,c-RX) or its trans,trans-isomer (t,t-Rx). Hence trapping of RX-·,Li+ was not encountered because of its very rapid decomposition (to R·), or alternatively dissociative electron transfer takes place to give R· directly. Furthermore, equilibration of the s-cyclopropyl radicals, R· must occur at a faster rate than their reaction with Naph-·,Li+ to give the configurationally stable RLi. Complex separated ion-pairs, Anth-·,D, K+ (e.g. D = dicyclohexano-18-crown-6) are protonated by EtOH in a pseudo first-order process, which is several orders of magnitude slower than the pseudo first-order protonation of the c.i.p. form. A claim by Stevenson et al. that reaction of Naph-·,Na+ and H2O provides Naph, OH-, and H2 has been disputed; the reaction followed by Stevenson was argued to have been that of Na rather than Naph-·,Na+.

Alkyl- and Aryl-metal Compounds. — Structures. Three theoretical calculations made on methyl-lithium used (i) a semi-empirical and non-empirical method, (ii) a floating spherical Gaussian orbital model, and (iii) an electrostatic model (for the tetramer) involving two interpenetrating tetrahedra of positive and negative charges subject only to coulombic forces. The most stable arrangement had C — C and C — Li bond lengths in good agreement with experimentally determined values. Hence it was stated that no unusual multicentre bonding was required to account for the structure of (MeLi)4. The following crystal structures were determined by Weiss: (i) (MeLi)4(TMED)2 [MeLi tetramers linked through Li — TMED — Li bridges: Li — C 2.23 — 2.27 Å and Li — Li 2.56 — 2.57 Å compared with the less refined values 2.3(1) and 2.6(8) Å obtained for unco-ordinated (MeLi)4 in an earlier X-ray powder structure determination], (ii) (PhLi,TMED)2 [Li atoms are linked via bridging phenyls and each in coordination to TMED: Li — C 2.208(6) and 2.278(6) Å], and (iii) (TMEDLi)2 [di-μ-phenyl-bis(dipheny1magnesate)l (2), formed from PhLi, Ph2Mg, and TMED (1 : 1 : 1) in toluene.

The structure of [(η5-C5H4)2Fe(PMDT)Li2] was also determined; it was obtained from ferrocene and BuLi in the presence of PMDT [pentamethyldiethylene triamine]. All three nitrogens of PMDT are co-ordinated to one Li while the other Li bridges between a carbon of a C5H4 unit from each ferrocene monomer to form a dimer.

The most stable geometry for (CH2Li2)2 was calculated [RHF/STO-3G; RHF/4-31G] to be (3), in which the four Li bridge two perpendicular CH2 units.

Preparations. Irradiation of C2Li2 in liquid NH3 at -45 °C provides a C4Li4 species, which from a combination of spectral data (mass, i.r., and n.m.r.) and MO calculations was assumed to have the lithium face-bridged tetrahedrane structure (4); confirmation is eagerly awaited. Lithium vapour reacts with benzene to give a variety of polylithiated compounds, such as C2Li2, C2Li4, C2Lis6, C3Li4, C3Li6, and C3Li6 with only small quantities of C6 products, e.g. > 1% C6H5Li at 25 °C.

Orthometallation by ButLi as well as α-addition to PhN=C [->o-LiC6H4N=C-(Li)But] occurs at -78 °C in the presence of TMED. The kinetic product of lithiation of 1,4-phenylenedi(1′-pyrazole) (BuLi2, Et2O, -60 °C) is 2-Li-1,4-(1′-pyrazole)2C6H3, while the thermodynamic product (using BuLi, THF, 20 °C) is 1,4-(5′-Li-1′-pyrazole)2C6H4. PhCH2HMe2 is metallated by BuLi-hexane to give o-LiC6H4CH2NMe2 and by ButO-K+-BuLi-hexane to Ph[bar.C]HNMe2,K+ (5); n.m.r. spectra were recorded. Compound (5) contains a trigonal benzylic anion. MeCH(C6H4CH2Li-p)2 is the kinetic product of lithiation (excess ButLi,-THF, -70 °C) of MeCH(C6H4Me-p)2 while the thermodynamic product is LiCMe(C6H4Me-p)2 (using PhCH2Li, THF, -70 °C). Ease of metallation by BuLi,THF decreases in the sequence: Ph3CH > Ph2CH2 > (p-MeC6H4)2-CH2 [??] p-PhC6H4Me > PhMe. While Ph[bar.C]HMe,Na+ (6) is the exclusive and almost quantitative product of the reaction of n-C6H11Na–TMED with PhEt after 1 h, a number of other species (o-, m-, and p-EtC6H4Na and PhCNa2Me) are initially formed. These subsequently undergo reaction to the thermodynamically more stable compound (6).

Treatment of RCI (R = 1-diadamantyl, 1-twistyl, 1-triptycyl, and 2-adamantyl) with a 2% Na-Li alloy in pentane provides high yields of RLi. The radical nature of the reaction was indicated by the concurrent formation of RR and RH. Selective halogeno-lithium exchange in 2-X-4-C1-C6H3Br (X = Cl or F) (by BuLi, Et2O, -80 °C) results in the formation of 2-Li-4-ClC6H3Br. As Naph-·,Li+ or lithium itself reacts with RCH2CH2SPh (7) to give high yields of RCH2CH2Li [R = PhCH2O(CH2)n, PhS(CH2)n, etc.], a useful route to alkyl-lithiums from alkenes, RCH=CH2, is available [cf. RCH=CH2 + PhSH -> (7)]. β-Hydroxy- and -amino-ethyl-lithiums may be synthesized from corresponding and readily available mercurials (Scheme 1). Cleavage of the C — Hg bonds in

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[(CO)3Cr(η6-C6H5)]2Hg by BuLi provided (η6-phenyl-lithium)tricarbonyl-chromium.

Properties. The observed inversion at the α-carbon in (RCH2Li)n aggregates is indicated by an ab initio MO calculation to involve RCH2Li2 fragments in which two Li atoms stabilize a planar RCH2 group. In either Et2O or THF solution butyl-lithium exists as an unreactive tetramer and a reactive monomer in equilibra with K ~ 10-4 mol3 l-3 (THF) and 10-16 mol3 l-3 (Et2O); the tetramer–hexamer equilibrium for isopropyl-lithium in iso-octane was also studied (K = 3.41 mol-1 at -47 °C; -ΔH = 4.9 ± 1.2 kcal mol-1).

The s.s.i.p. [??] c.i.p. equilibria in THF were followed spectroscopically for Ph2CH-,Li+ and (p-MeC6H4)2CH-,Li+ (8). For (8), ΔH and ΔS were calculated to be -5.9 kcal mol-1 and -18 e.u., respectively. Changes in s.s.i.p.–c.i.p. equilibrium and u.v. spectra on altering the solvent and the cation may be generalized as follows (i) s.s.i.p. show bathochromic shifts compared with c.i.p., (ii) more s.s.i.p. for Li+ than for K+, (iii) for s.s.i.p., λmax varies only slightly with M+ and solvent, and (iv) for c.i.p., λmax suffers a bathochromic shift from M = Li to M = K. Different rates of ring rotation occur in (p-MeC6H4) [bar.C]H(C6H4D-p),Li+; at 223 K, activation parameters were ΔH* (kcal mol-1) 12.1 ± 0.4 and 11.2 ± 0.4 and ΔS* (ex.) 1.9 ± 1.0 and 1.8 ± 1.0 for rotation of p-DC6H4 and p-MeC6H4 rings, respectively.

In toluene 9-Pr-fluorenyl-lithium, 9-PrFl-Li+ (9) exists as a monomeric ion-pair (λmax 353 nm) and a dimeric ion-pair aggregate (λmax 370 nm) in equilibrium; while Li+,Fl-(CH2)nFl-,Li+(n = 2,4, or 6) (10) exist as intramolecular aggregates (370 nm). Addition of small amounts of THP to (9) and (10) produces THP solvated c.i.p. (λmax 361 nm) and s.s.i.p. (λmax 386 nm). The ratio of the formation constants of the s.s.i.p. for the two terminal sites K1/K2 = 4 (for 10, n = 6) and 0.3 (for 10, n = 2). U.v.–vis absorption emission and excitation spectra of Fl-,M+ were recorded in ethereal solvents. The fraction of s.s.i.p. in the first excited state increases in the solvent order, DME > THF > THP, similar to the sequence found for the ground state. Furthermore, the proportion of s.s.i.p. was higher in the first excited state.

The stereoselectivity of deuterolysis of 9-But-10-Li-9,10-dihydroanthracene (11) by MeOD is governed by the type of ion-pairing present; thus in Et2O or dioxane (c.i.p. of 11), the trans product results, while in HMPT (s.s.i.p.), the cis product is obtained. The reactivity of carbanion–cation ion-pairs has been generally reviewed.

Combined results of i.r. spectra and CNDO/2 and normal co-ordinate calculations indicate that in MCH2CN from MeCN and Naph2-,2M+ (M = Li, K, or Na) (i) the C — M bond is ionic, (ii) the carbanion is preferentially planar, and (iii) M+ is situated close to the methylene carbon; for reactions of MCH2CN and MCH(Ph)CN with α-enones, see ref. 32b. The i.r. spectra of Me3CCOCH2M (M = Li, Na, or K) in strongly solvating media indicate enolate structures; greater amounts of the keto forms, however, exist in heptane; for are actions of related metal derivatives, see ref. 33b. Further studies (using 7Li, 13C, and 31P n.m.r.) of the barrier of phenyl rotation in 2-Li-2-Ph-1,3-dithianes (12a and 12b) have confirmed such barriers to be lower in THF in which c.i.p. forms exist, [e.g. for 12b), ΔH = 13.0 kcal mol-1] than in THF–HMPT mixtures (s.s.i.p. with attendant delocalization into the phenyl ring) [for (12b), ΔH = 16.4 kcal mol-1]. For (12c) the barriers are independent of the solvent and are nearly equal to the barrier for ring reversal in HMPT (ca. 12.7 kcal mol-1). The rate determining step for phenyl rotations may be the ring reversal. The 1J(13C — 1H) coupling constants, in particular the change in J values on metallation, are useful probes of the geometry of the anionic carbon in the metallated compounds. Some results for sulphur stabilized species are summarized thus (i) PhSCH2-,M+ (M = Li or K): pyramidal and solvent independent (ΔJ<-13), (ii) PhSOCH2-,-M+ (13) and 4-But-2-Li-thiacyclohexane-1-oxide (14): planar (ΔJ > 14), and (iii) PhSO2CH2-,M+ intermediate geometry (and intermediate values of ΔJ): M and solvent dependent. For sulphoxides, four-centred chelate structures (15) were proposed which are not broken by HMPT, DABCO, and Li salts but are by cryptates. Such chelates, as shown with (13; M = Li) and (14), play vital roles in the stereochemistry of reactions with electrophiles. An electrophile [e.g. D2O and to a lesser extent (MeO)3PO] able to co-ordinate strongly or interact with Li+ in the chelate, provides a product with retention, while an electrophile [e.g. MeI or (MeO)3PO + LiClO4], reluctant to do this, produces an inverted product. However, Fava has reported MeI and oxirane reactions of α-lithio-thiepane- and -thiocane-1-oxides which give, as the major product, the cis isomer – the product not predicted from the above considerations. Further work on these larger ring compounds thus appears necessary. Analogous chelate structures to (15) were suggested for (EtO)2P(O)[bar.C]HR,Li+ (16; R = Ph) in THF. In Me2SO (16; R = CN) are s.s.i.p., while in THF, c.i.p. exist. N.m.r spectra (1H, 13C, 31P, and 7Li) of (16) indicated planar anionic centres, carrying high negative charges, i.e. little delocalization into the R groups; for reactions of these and related α-lithiophosphonates, see ref. 36b.

Other reactions. A report has been made on the products from carbonation–hydrolysis of isomeric CF3C6H4Li under different conditions. The extent of the by-products [(CF3C6H4)2CO and (CF3C6H4)3COH] should act as a caution in the use of carbonation as the sole means of determining relative metallation. The thermal decomposition of (PhS)3CLi (17) is illustrated in Scheme 2; (17) has been used as a formyl anion equivalent.

Various products arise from reactions of CH2=CHSePh with BuLi (e.g. CH2=CLiSePh, CH2CH=SeBu, and BuCH2 — CHLiSePh). Alkyl radicals (R·) have been detected by e.s.r. spectroscopy approximately 80 ms after mixing benzene solutions of (RLi)n and TiCl4 (R = Me, Et, Pr1, Bu, Bus, But, or cyclo-C5H9). Methyl radicals react to give a second paramagnetic species, probably Me3Li4CH2·. 1-Lithiomethylisoquinolines (as do lithio-cyanohydrins, -phenylacetonitrile, -carboxylic acid esters, and -dithianes) add to αβ-enones in reversible 1,2-kineticalIy controlled reactions and by 1,4-thermodynamically controlled additions.

(Continues…)Excerpted from Organometallic Chemistry Volume 8 by E. W. Abel, F. G. A. Stone. Copyright © 1980 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Professor Abel is an Emeritus Professor at the University of Exeter.

Organometallic Chemistry Volume 2

A Review of the Literature Published during 1972

By E. W. Abel, F. G. A. Stone

The Royal Society of Chemistry

Copyright © 1973 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-511-9

Contents

Chapter 1 Group I: The Alkali and Coinage Metals By J. L. Wardell, 1,
Chapter 2 Group II: The Alkaline Earths and Zinc and its Congeners By J. L. Wardell, 16,
Chapter 3 Group III: Boron By S. K. Gupta, 38,
Chapter 4 Group III: The Carbaboranes By T. Onak, 75,
Chapter 5 Group III: Aluminium, Gallium, Indium, and Thallium By J. P. Maher, 89,
Chapter 6 Group IV: The Silicon Group D. A. Armitage, 137,
Chapter 7 Group V: Arsenic, Antimony, and Bismuth By J. L. Wardell, 215,
Chapter 8 Metal Carbonyls By E. W. Abel and F. G. A. Stone, 224,
Chapter 9 Organometallic Compounds containing Metal–Metal Bonds By N. G. Connelly, 234,
Chapter 10 Substitution Reactions of Metal and Organometal Carbonyls with Group V and VI Donor Ligands By R. J. Mawby, 273,
Chapter 11 Carbene, Nitrene, and Related Complexes By J. A. Connor, 302,
Chapter 12 Complexes containing Metal–Carbon σ-Bonds By M. I. Bruce, 320,
Chapter 13 Hydrocarbon–Metal π-Complexes By M. A. Bennett, 365,
Chapter 14 π-Allyl Complexes By M. Green, 407,
Chapter 15 π-Cyclopentadienyl, Arene, and Related Complexes By R. J. Mawby, 431,
Chapter 16 Substitution Reactions of Hydrocarbon–Metal π-Complexes By S. A. R. Knox, 453,
Chapter 17 Oxidative Addition and Related Reactions By M. Green, 491,
Chapter 18 Homogeneous Catalysis By F. J. McQuillin, 526,
Chapter 19 X-Ray and Electron Diffraction Studies of Organometallic Compounds By R. F. Bryan, 540,
Author Index, 589,

CHAPTER 1

Group I: The Alkali and Coinage Metals

BY J. L. WARDELL

1 Lithium

Structural and Bonding Studies. — From an ab initio SCF–MO calculation, (MeLi)4 was found to be more stable than four monomeric units by 4.8 eV. The extra stability of the tetramer is suggested to be associated with the lithium–lithium bonding in the Li4 tetrahedron and the threefold increase in the number of carbon–lithium bonds, which compensate for the decreased component of the lithium–carbon bond overlap on tetramer formation.

Crystal structures of fluorenyl-lithium bisquinuclidine, triphenylmethyl-lithium (TMED)2, and (TMED-lithium)2 naphthalenide have been obtained. In the first of these complexes there is a large covalent interaction involving the metal and a carbanion three-centre bond, formed from an empty Li orbital (p-type) and two carbon pz orbitals of the highest occupied MO of the carbanion as well as the sigma overlap of a lithium sp2 orbital with the lower-energy MO of the carbanion. These general features were also found in the other complexes.

Lithium 1- and 2-methylnaphthalides exist as contact ion-pairs both in THF and in ether at ambient temperature, but in HMPT these compounds exist as solvent-separated ion-pairs or possibly free ions. The methylene unit in these carbanions is sp2 hybridized and lies in the plane of the aryl ring; in ether solution there is possibly an additional allylic-type coordination, as in (1) and (2). Spectral studies indicate that there is some localization of the carbon–lithium bonds.

In cyclohexylamine–diethylamine, indenyl-lithium exists as an equilibrium mixture of contact (35%) and solvent-separated ion-pairs 65%) at room temperature; in DME, a rise in the solution temperature caused an increase in the number of solvent-separated ion-pairs. Indenyl-lithium in the first excited state was also found to exist as solvent-separated ion pairs.

Rotation of the phenyl groups is hindered in 1,3-diphenylallyl-lithium, which exists as solvent-separated ion pairs in ether; for 2-methyl-1, 3-diphenylallyl-lithium there are additional rotational restrictions about the carbon–carbon bonds of the allyl system. Allylmethyl-lithium compounds were found to rearrange via an ionization–recombination mechanism. Ring-opening of N-lithio-2,3-cis-diphenylaziridine led to 1,3-cis, trans-diphenyl-2-aza-allyl-lithium, which subsequently rearranged to the trans, trans-form. cis- and trans-3-neopentylallyl-lithium, formed from t-butyllithium and buta-1,3-diene in a hydrocarbon solvent, did not equilibriate even at 70 °C. The upfield shift of the γ-protons and the downfield shift of the α-protons in the 1H n.m.r. spectra (as well as u.v. and i.r. data) of the neopentylallyl-lithiums indicated some π-component to the αβ bond, see (3a) and (3b), especially in the aggregates (tetramers, in benzene).

N.m.r. studies of carbon–lithium bonding in organo-lithium compounds have been reviewed. The shift of the CO stretching vibrations from their positions in non-metalloid analogues can be correlated with the ionic character of the carbon–lithium bond in R1R2CLiCO2Et.

In DME, as in THF, the thermodynamic preference is for lithium to be in the equatorial position when substituted into the conformationally fixed 1,3-dithians (4a) and (4b) (K > 100); however, the kinetic preference for formation from the 1,3-dithian (5) is much less pronounced (keq/kax<10).

Monolithium derivatives of acetonitrile and phenylacetonitrile are associated in DMSO via interaction between the nitrile groups and lithium atoms. The species (LiCH2CN)4 and (PhCHLiCN)2 are not electron-deficient compounds; ionization of (LiCH2CV gives Li+, contrary to the behaviour of electron-deficient alkyl-lithium aggregates. p-t-Amylbenzyl-lithium in toluene is dimeric.

The electronic transitions in organo-lithium aggregates have been observed for ethyl- and butyl-lithium reagents and calculated using CNDO/2 for (MeLi)4 at 124 nm and for (MeLi)6 at 171 nm. The lower-energy transition in the tetramer corresponds to the electron going from an MO centred (73 %) on the carbons to one which is 83% on lithium. Based on such information, photochemical reactions of alkyl-lithiums were predicted to involve formation of radicals by the charge-transfer process. Photolysis of phenyl-, p-tolyl-, and 2-naphthyl-lithium compounds in Et2O gave coupled biaryl products but not products of reaction with ether; the use of D2O in the work-up led to no incorporation of deuterium (therefore no biaryl-lithium compounds were present at the onset of the work-up). However, when o-anisyl-lithium was photolysed, the major product was 2-methoxybiphenyl, and deuterium was incorporated during the work-up; the intermediacy of benzyne in the latter procedure was likely. Neophyl-lithium on reaction with oxygen in a hydrocarbon solvent, in which tetrameric neophyl-lithium units are present, at 25 °C gave significant amounts of products derived from the benzyl (dimethyl)-methyl moiety, indicating a free-radical process. However, in solvents containing ethers and amines (in which dimeric units are found), autoxidation proceeds predominantly via non-radical pathways.

Polylithio-compounds. — The first examples of perlithiated compounds, CLi4, C2Li6, and C2Li4, were obtained from the reaction of excess lithium atoms at 800 — 1000 °C with chlorocarbons under high-vacuum conditions. The products, formed in yields greater than 80%, were extremely sensitive to air and moisture. The reaction of n-butyl-lithium with tetrachlorothiophen in Et2O gave 2,5-dilithiodichlorothiophen, whereas in THF the monolithio-derivative was the major product; the reaction of hexachlorobiphenyl with BunLi can also lead to dilithio-derivatives. Metallation of alk-1- and -2-ynes with butyl-lithium in ether produced dilithiated species.

Carbenoids and Related Compounds. — The chemistry of carbenoids and other thermolabile organolithium compounds has been reviewed. The mechanisms of the decomposition of trichloromethyl-lithium and o-halogenophenyl-lithium compounds were studied by d.t.a.; decomposition in the presence of donors occurred via concerted β-elimination of LiX. Tris(phenylthio)-methyl-lithium, (PhS)3CLi, decomposed significantly only above 20 °C to give (PhS)2C=C(SPh)2. As anticipated, addition of LiSPh reduced the rate of the carbenoid’s reactions, and addition of either p-MeC6H4SLi or (p-MeC6 H4S)3CLi resulted in complete scrambling of the arylthio-groups. (PhS)3CLi reacted with electrophiles, e.g. D2O, and nucleophiles, e.g. Ph3P, to give Ph3P:C(SPh)2; (PhSe)3-π. CHπLi (n = 1, 2, or 3) compounds were also produced. Me3SnCXYLi (X, Y = Cl or Br) were stable in THF-rich media only below – 95 °C; their reactions with Me3SiCl and MeI were complex. The preparation and thermal decomposition of other carbenoids, e.g. (6) and (7), were also reported.

Other Reactions. — The rate of reaction of methyl-lithium with 2,4-dimethyl-4′-methylthiobenzophenone [one-fourth-order dependence on MeLi] was reduced on addition of either LiBr or LiI, owing to formation of the less reactive species Li4Me3Br, Li4 Me2Br2, and Li4Me3I. LiX also had an inhibiting effect (X = I>Br>Cl>F) on the exchange between phenyl-lithium and bromobenzene but NH4X, on the other hand, had a catalysing influence (X = Cl>Br>I). The rates of exchange and the equilibrium constants for aryl-lithium reactions with bromobenzene were also reported. An interesting derivative of this reaction, but one, however, with a different mechanism, is the exchange reaction of aryl-lithium compounds with aryl cyanides. In hydrocarbon solvents and under conditions in which C6Cl5Li decomposes to tetrachlorobenzyne, up to 48% yields of the aryl cyanides ArCN can be obtained.

Further studies of reactions of organo-lithium compounds with ethers have been made. THF and its 3- and 4-alkyl derivatives are cleaved to alkenes and stable lithium enolates of aldehydes via α-hydrogen abstractions and [π4s + π2s] cycloreversions (Scheme l). (β-Hydrogen abstraction is only important for the 2-alkyl derivatives.) In a related manner, 5,6-dihydro-2H-pyran on lithiation gave cyclopropyl enolate.

Metallated aryl ethers were shown to be secondary products of cleavage of aryl ethers by lithium arenes, the initial step being formation of radical anions.

Lithiotriphenylphosphinoacetonide, ([MATHEMATICAL EXPRESSION OMITTED]) Li+, an interesting 1,3-dianion obtained from butyl-lithium and [MATHEMATICAL EXPRESSION OMITTED] at –68 °C, gave higher yields of alkylated products [MATHEMATICAL EXPRESSION OMITTED] at – 68 °C, when solvent-separated ion pairs predominate, than at higher temperatures, when contact ion pairs are the more important species.

Deuteriolysis of 9-lithio-10-alkyl-9,10-dihydroanthracene in THF (contact ion pairs) gave equal amounts of the quasi-axial and quasi-equatorial deuterio-isomers, whereas in HMPT (solvent-separated ion pairs) only the quasi-axial deuterio-isomer was obtained. However, 9-lithio-10-methyl-9, 10-dihydro-anthracene in THF on reaction with PriI gave a 59:41 (trans :cis) mixture of the 9,10-dialkyl isomers, and the reaction of MeI with 9-lithio-10-isopropyl-9,10-dihydroanthracene led to a 10:90 (trans:cis) mixture of isomers.

A review on the synthetic uses of organo-lithium compounds has appeared. Contrary to a previous report, several authors have shown that 1,2-additions of organo-lithium reagents to simple quinolines do occur directly rather than by rearrangement of an initially formed 1,4-adduct. The 1,2-adducts were either trapped with ethyl chloroformate or hydrolysed to 2-substituted quinolines. However, the reaction of n-butyl-lithium with 2-ferrocenylquinoline did produce a 1,4-adduct. The reaction products of 3-fluoropyridine with t-butyl-lithium were trapped by pentanone; the maximum yields (50%) of 2-substitution were obtained in THF–TMED and that of the 4-isomer (65%) in Et2O-TMED.

Recent work on the mechanism of formation of phenylcyclopropane from allyl chloride and phenyl-lithium in ether pointed to the intermediacy of cyclopropene and several lithiated phenylcyclopropanes (Scheme 2).

Allyl-lithium compounds, generated in situ from allyl mesitoate esters and lithium, cross-couple with allylic halides; any rearrangement which occurred was only found in the allylic portion from the allyl-lithium reagent. 2-Lithio-2-trimethylsilyl-1,3-dithian reacted with aldehydes and ketones R1R2CO to afford keten thioacetals, [MATHEMATICAL EXPRESSION OMITTED] ; related reactions were those of Me3SiCH(SMe)Li and Me3SiCH(Li)P(S)Ph2 with R1R2CO to give R1R2C=CHSMe and R1R2C=CHP(S)Ph2, respectively.

Reactions of dilithium cyclo-octatetraenide with esters, anhydrides, acyl chlorides, nitriles, and nitroso-compounds have been investigated. Differences in the stereoselectivity of the products from protonation of the sodium and lithium dihydronaphthylide have been observed. The reaction of 2 equivalents of lithium dihydronaphthylide with 1 equivalent of alkyl halide affords alkyl-lithium compounds.

2 Other Alkali Metals

In cyclohexylamine solutions, fluorenylcaesium (between -20 and +90 °C), indenylcaesium, and triphenylmethylcaesium exist as contact ion pairs. The tight ion pairs of fluorenyl salts (fluorenyl- M+, M = K or Na) in ether solvents interacted with macrocyclic polyethers (crown ethersj to form contact ion pairs complexed with the crown ethers as well as crown-separated ion pairs. In THF, the preferred position of the Na+ and K+ cations in indenyl- metal+ ion pairs appears to be over the five-membered ring, unlike that of Li+. The spectra of the contact ion pair, alkali-metal derivatives of 1- and 2-methylnaphthalenes suggested partial localization of the carbon–M bonds, the delocalizations for K, Na, and Li salts being 79, 71, and 58%, respectively.

The dianions ([MATHEMATICAL EXPRESSION OMITTED]) 2M+, formed from benzophenone anil Ph2C = NPh and alkali metals, reacted with isopropyl halides to give PriCPh2NHPh as well as ring-substituted products, the yield of the former increasing in the sequence M = K-alkylated products from the reaction of alkali-metal derivatives of indole with alkyl halides increased in the same order. The major reaction of alkyl halides with ([MATHEMATICAL EXPRESSION OMITTED]) Na+, from Ph2C=CHCH2R and NaNH2, occurred at C-3 for R = H and Me and at C-1 for R = Ph.

The reaction of sodium–potassium alloy with biphenyl in DME must give several polymetallated biphenyl products since hydrolysis produced phenyl-cyclohexane, phenylcyclohexenes, and phenylcyclohexadienes. n-Amyl-sodium and TMED at – 15 °C in hexane is a powerful metallating combination, e.g. 1,3-dimethylnaphthalene formed the 1,3-disodium derivative in quantitative yield. The use of DABCO in place of TMED did not give such good results. The TMED is thought to disperse the large n-amylsodium aggregates and so enlarge the available surface area of n-amylsodium.

1,4- and 1,5-migrations of phenyl groups were shown to occur in the reactions of Cl(CH2)nCPh3, n = 3 or 4, with caesium but not with the other alkali metals. However, 1,2- and 1,4-migrations of the p-biphenylyl group are somewhat easier, e.g.p-C6H5· C6H4·CH2·CD2Cl on reaction with Cs and K (but not Li) at reflux in THF led to some rearrangement.

Electron-transfer processes involving radical anions and carbanions have been reviewed. The kinetics of the reaction of sodium anthracene with water have been further reported upon. The dihydroanthracenyl anion was an intermediate in this reaction and was even more reactive than the radical anion. An equilibrium between contact and solvent-separated ion pairs was postulated for sodium anthracene in THF but only solvent-separated ion pairs were present in DME. An interesting preparation of the naphthalene radical anion was by the reaction of 4-phenyl-1,3-dioxan with sodium-potassium alloy. Naphthalene radical anion reactions with alkyl halides have been reviewed. Sodium naphthalenide reacted in DME with erythro- and threo-2,3-dibromo-3-methylpentane in a two-electron trans-elimination process with high stereoselectivity, and with polyhalogeno-substrates, such as MeCMe2CCl2Me, to give carbene intermediates.

(Continues…)Excerpted from Organometallic Chemistry Volume 2 by E. W. Abel, F. G. A. Stone. Copyright © 1973 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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