Organozinc compound

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Organozinc chemistry
Organozinc compounds in organic chemistry contain carbon to zinc chemical bonds. Organozinc chemistry is the science of organozinc compounds describing their physical properties, synthesis and reactions.[1][2][3][4]

Organozinc compounds were among the first organometallic compounds made. They are less reactive than many other analogous organometallic reagents, such as Grignard and organolithium reagents. In 1848 Edward Frankland prepared the first organozinc compound, diethylzinc, by heating ethyl iodide in the presence of zinc metal.[5] This reaction produced a volatile colorless liquid that spontaneous combusted upon contact with air. Due to their pyrophoric nature, organozinc compounds in general are prepared in air using air free techniques. They are unstable toward protic solvents. For many purposes they are prepared in situ, not isolated, but many have been isolated as pure substances and thoroughly characterized.[6]

Organozinc can be categorized according to the number of carbon substituents that are bound to the metal.[2][3]

  1. Diorganozinc (R2Zn): A class of organozinc compounds in which two alkyl ligands. These may be further divided into subclasses depending on the other ligands attached
  2. Heteroleptic (RZnX): Compounds which an electronegative or monoanionic ligand (X), such as a halide, is attached to the zinc center with another alkyl or aryl substituent (R).
  3. Ionic organozinc compounds: This class is divided in to organozincates (RnZn-) and organozinc cations (RZnLn+).

Bonding

In its coordination complexes zinc(II) adopts several coordination geometries, commonly octahedral, tetrahedral, and various pentacoordinate geometries. These structural flexibility can be attributed to zinc’s electronic configuration [Ar]3d104s2. The 3d orbital is filled. therefore, ligand field effects are nonexistent.Coordination geometry is thus determined largely by electrostatic and steric interactions.[2] Organozinc compounds usually are two- or three-coordinate, reflecting the strongly donating property of the carbanionic ligands.

Typical diorganozinc complexes have the formula R2Zn. A polar covalent bond exists between carbon and zinc, being polarized toward carbon due to the differences in electronegativity values (carbon: 2.5 & zinc: 1.65). The dipole moment of symmetric diorganozinc reagents can be seen as zero in these linear complexes, which explains their solubility in nonpolar solvents like cyclohexane. Unlike other binary metal alkyls, the diorganozinc species show a low affinity for complexation with ethereal solvent. Bonding in R2Zn is described as employing sp-hybridized orbitals on Zn.[2]

These structures cause zinc to have two bonding d-orbitals and four low-lying non-bonding d-orbitals (see non-bonding orbital), which are available for binding. When zinc lacks electron donating ligands it is unable to obtain coordination saturation, which is a consequence of the large atomic radius and low electron deficiency of zinc. Therefore it is rare for bridging alkyl or aryl groups to occur due to the weak electron deficiency of the zinc atom. Although, it does occur in some cases such as Ph2Zn (Shown below) and which halogens are the organozinc can form metal clusters (see cluster chemistry). When a halogen ligand is added to the zinc atom both the acceptor and donor character of zinc is enhanced allowing for aggregation.[2]

Saturated diorganozinc reagents with bridging aryl groups

Synthesis

Several methods exist for the generation of organozinc compounds.

Oxidative Addition

Frankland’s original synthesis of diethylzinc involves oxidative addition of ethyl iodide to zinc metal. The zinc must be activated in some way to facilitate this redox reaction. One of such activated form of zinc employed by Frankland is Zinc-copper couple.[5]

Riecke zinc, produced by in situ reduction of ZnCl2 with potassium is another activated form of zinc. This form has proven useful for reactions such as Negishi coupling and Fukuyama coupling. The reaction is facilitated by electron-withdrawing substitutents on the alkyl or aryl halide, e.g., nitriles and esters.[7][8]

Functional group exchange

The two most common zinc functional group interconversion reactions are with halides and boron, which is catalyzed by copper iodide (CuI) or base. The boron intermediate is synthesized by an initial hydroboration reaction followed by treatment with diethyl zinc. This synthesis shows the utility of organozinc reagents by displaying high selectivity for the most reactive site in the molecule, as well as creating useful coupling partners.[9]

This group transfer reaction can be used in allylation, or other coupling reactions (such as Negishi coupling).[10]

β-Silyl diorganozinc compounds

One of the major drawbacks of diorganozinc alkylations is that only one of the two alkyl substituents is transferred. This problem can be solved by using Me2SiCH2 (TMSM), which is a non-transferable group.[11]

Transmetallation

Transmetallation is similar to the interconversions displayed above zinc can exchange with other metals such as mercury, lithium, copper, etc. One example of this is diphenylmercury reacting with zinc metal to form diphenylzinc and metallic mercury:

HgPh2 + Zn → ZnPh2 + Hg

The benefit of transmetalling to zinc it is often more tolerant of other functional groups in the molecule due to the low reactivity which increases selectivty.[12]

  • In the synthesis of Maoecrystal V, an directed ortho metalation gives the initial aryl-lithium species, which is transmetallated to the desired arylzinc compound. The arylzinc compound is significantly less reactive than the aryl-lithium species and thus better tolerates the functionality in the subsequent coupling with methyl chlorooxaloacetate. Esters are significantly stable against organozinc reagents.[13]

Organozinc can be obtained directly from zinc metal:[14][15]

In this method zinc is activated by 1,2-dibromoethane and trimethylsilyl chloride. A key ingredient is lithium chloride which quickly forms a soluble adduct with the organozinc compound thus removing it from the metal surface.

Reactions

In many of their reactions organozincs appear as intermediates.

  • In the Frankland–Duppa reaction (1863) an oxalate ester (ROCOCOOR) reacts with an alkyl halide R'X, zinc and hydrochloric acid to the α-hydroxycarboxylic esters RR'COHCOOR[16]

Reformatsky reaction

This organic reaction can be employed to convert α-haloester and ketone or aldehyde to a β-hydroxyester. Acid is needed to protonate the resulting alkoxide during work up. The initial step is an oxidative addition of zinc metal into the carbon-halogen bond, thus forming a carbon-zinc enolate. This C-Zn enolate can then rearrange to the Oxygen-Zinc enolate via coordination. Once this is formed the other carbonyl containing starting material will coordinate in the manner shown below and give the product after protonation.[17] The benefits of the Reformatsky reaction over the conventional aldol reaction protocols is the following:

  1. Allows for exceedingly derivatized ketone substrates
  2. The ester enolate intermediate can be formed in the presence of enolizable moieties
  3. Well suited for intramolecular reactions

Below shows the six-membered transition state of the Zimmerman–Traxler model (Chelation Control, see Aldol reaction), in which R3 is smaller than R4.[18]

The Reformatsky reaction has been employed in numerous total syntheses such as the synthesis of C(16),C(18)-bis-epi-cytochalasin D:[19]

The Reformatsky reaction even allows for with zinc homo-enolates.[20] A modification of the Reformatsky reaction is the Blaise reaction.[18]

Simmons-Smith reaction

The Simmons-Smith reagent is used to prepare cylclopropones from olefin using methylene iodide as the methylene source. The reaction is effected with zinc. The key zinc-intermediate formed is a carbenoid (iodomethyl)zinc iodide which reacts with alkenes to afford the cyclopropanated product. The rate of forming the active zinc species is increased via ultrasonication since the initial reaction occurs at the surface of the metal.

Although the mechanism has not been fully elaborated it is hypothesized that the organozinc intermediate is a metal-carbenoid. The intermediate is believed to be a three-centered "butterfly-type" intermediate This intermediate can be directed by substituents, such as alcohols, to deliver the cyclopropane on the same side of the molecule. Zinc-copper couple is commonly used to activate zinc.[18]

Negishi Coupling

This is a powerful carbon-carbon bond forming cross-coupling reaction using an organic halide, an organozinc halide reagent, and a nickel or palladium catalyst. The organic halide reactant must either be alkenyl, aryl, allyl, or propargyl. A key step in the catalytic cycle is a transmetalation in which a zinc halide exchanges its organic substituent for another halogen with the metal center. Although in comparison to other cross-coupling reactions such as Suzuki, Heck or Stille coupling, the Negishi reaction has been underdeveloped. Either diorganic species or organozinc halides can be used as coupling partners during the transmetallation step in this reaction. Despite the low reactivity of organozinc reagents on organic electrophiles, these reagents are among the most powerful metal nucleophiles toward palladium.[21]

An elegant example of Negishi coupling is shown below with Furstner and co-workers’ synthesis of amphidinolide T1.[22]

Fukuyama Coupling

Fukuyama coupling is a palladium involving the coupling of an aryl, alkyl, allyl, or α,β- unsaturated thioester compound. This thioester compound can be coupled to a wide range of organozinc reagents in order to reveal the corresponding ketone product. This protocol is useful due to its sensitivity to functional groups such as ketone, acetate, aromatic halides, and even aldehydes. The chemoselectivity observed indicates ketone formation is more facile than oxidative addition of palladium into these other moieties.[23]

An interesting example of this coupling method is the synthesis of (+)-biotin. In this case, the Fukuyama coupling takes place with the thiolactone.[24]

Barbier Reaction

The Barbier reaction is a nucleophilic addition reaction into a carbonyl, similar to the Grignard reaction. The organozinc reagent is made via an oxidative addition into the alkyl halide. The reaction produces a primary, secondary, or tertiary alcohol via a 1,2-addition. The benefit of using the Barbier reaction is that it allows convenient one-pot synthesis of the organozinc reagent in the presence of the carbonyl compound. Organozinc reagents are also less water sensitive and thus running the reactions in water are permitted. Similar to the Grignard reaction, a schlenk equilibrium is displayed, in which the more reactive dialkylzinc can be formed.[18]

The mechanism resembles the Grignard reaction, in which the metal alkoxide can be generated by a radical stepwise pathway, through single electron transfer, or concerted reaction pathway via a cyclic transition state. An example of this reaction is in Samuel Danishefsky’s total synthesis of Cycloproparadicicol. By using the organozinc addition reaction conditions the other functionality of the dienone and the alkyne are tolerated.[25]

Asymmetric Variants

Among the Group 12 elements zinc is the most reactive. Commercially available diorganozinc compounds are dimethylzinc, diethylzinc and diphenylzinc. These reagents are expensive and difficult to handle. In one study[26][27] the active organozinc compound is obtained from much cheaper organobromine precursors:

The synthesis of (+)-aspicillin, starts first with a hydroboration, then transmetallation to zinc which can then do an addition into the aldehyde substituent.[28]

Zinc acetylides

The formation of the zinc acetylide goes via the intermediacy of a dialknyl zinc (functional group exchange). Catalytic processes have been developed such as Merck’s ephedrinee process.[29] Propargylic alcohols can be synthesized via this zinc acetylide route. These versatile intermediates can then be used for a wide range of chemical transformations such as cross-coupling reactions, hydrogenation, and pericyclic reactions.[30]

When no ligand is present the reaction goes extremely slow with low yields (30%). Addition of a chiral ligand gives high conversion with low reaction times (ligand acceleration). Ryoji Noyori determined the zinc-ligand monomer is the active species in the reaction.[31]

Diastereoselectivity for addition of organozinc reagents into aldehydes can be predicted by the following model by Noyori and David A. Evans:[32]

Zinc-acetylides are used in the HIV-1 reverse transcriptase inhibitor Efavirenz as well as in Merck’s ephedrine derivatives .[33]

Organozincates

The first organozinc ate complex (organozincate) was reported in 1858 by James Alfred Wanklyn,[34] an assistant to Frankland and concerned the reaction of elemental sodium with diethylzinc:

2 Na + 3 ZnEt2 → 2 NaZnEt3 + Zn

Organozinc compounds that are strongly Lewis acidic are vulnerable to nucleophilic attack by alkali metals, such as sodium, and thus form these ‘ate compounds’. Two types of organozincates are recognized: tetraorganozincates ([R4Zn]M2), dianionic and triorganozincates ([R3Zn]M), monoanionic. Their structures, which are determined by the ligands, have been extensively characterized.[3]

Synthesis

Tetraorganozincates such as [Me4Zn]Li2 can be formed by mixing Me2Zn and MeLi in a 1:2 molar ratio of the rectants. Another example synthetic route to forming spriocyclic organozincates is shown below:[3]

Triorganozincates compounds are formed by reacting a diorganozinc such as (Me3SiCH2)2Zn with an alkali metal (K), or an alkali earth metal (Ba, Sr, or Ca) to afford the triorganozincate, [(Me3SiCH2)3Zn]K. Triethylzincate degrades thermally to sodium hydridoethylzincate(II) as a result of beta-hydride elimination:[35]

2 NaZnEt3 → Na2ZnH2Et4 + 2 C2H4

The product is an edge-shared bitetrahedral structure, with bridging hydride ligands.

Reactions

Although less commonly studied, organozincates often have increased reactivity and selectivity compared to the neutral diorganozinc compounds. They have been useful in stereoselective alkylations of ketones and related carbonyls, ring opening reactions. Aryltrimethylzincates participate in vanadium mediated C-C forming reactions.[3]

Organozinc(I) compounds

Low valent organozinc compounds having a Zn–Zn bond are also known. The first such compound, decamethyldizincocene, was reported in 2004[36]

See also

CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
↓
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown

References

  1. ↑ Knochel, P.; Millot, N.; Rodriguez, A.; Tucker, C. E. Org. React. 2001, 58, 417. doi:10.1002/0471264180.or066.01
  2. ↑ 2.0 2.1 2.2 2.3 2.4 The Chemistry of Organozinc Compounds (Patai Series: The Chemistry of Functional Groups), (Eds. Z. Rappoport and I. Marek), John Wiley & Sons: Chichester, UK, 2006, ISBN 0-470-09337-4.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Organozinc reagents – A Practical Approach, (Eds. P. Knochel and P. Jones), Oxford Medical Publications, Oxford, 1999, ISBN 0-19-850121-8.
  4. ↑ Synthetic Methods of Organometallic and Inorganic Chemistry Vol 5, Copper, Silver, Gold, Zinc, Cadmium, and Mercury, W.A. Herrmann Ed., ISBN 3-13-103061-5
  5. ↑ 5.0 5.1 E. Frankland, Liebigs Ann. Chem.,1849, 71, 171
  6. ↑ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
  7. ↑ Rieke, R. D. (1989). "Preparation of Organometallic Compounds from Highly Reactive Metal Powders". Science 246 (4935): 1260–1264. Bibcode:1989Sci...246.1260R. doi:10.1126/science.246.4935.1260. PMID 17832221. 
  8. ↑ Negishi,E. J. Organomet. Chem. 2002, 653, 34-40 (doi:10.1016/S0022-328X(02)01273-1)
  9. ↑ Knochel, P. et al. J. Org. Chem. 1996, vol. 61, 8229-8243.
  10. ↑ Naka,H; et. al.New J. Chem., 2010, 34, 1700–1706
  11. ↑ Knochel,P.; et. al. Angel. Chem. Int. Ed. Engl. 1997, volume 36, 1496-1498
  12. ↑ Markies, P; Schat, Gerrit; Akkerman, Otto S.; Bickelhaupt, F.; Spek, Anthony L. (1992). "Complexation of diphenylzinc with simple ethers. Crystal structures of the complexes Ph2Zn·glyme and Ph2Zn·diglyme". J. Organomet. Chem. 430: 1. doi:10.1016/0022-328X(92)80090-K. 
  13. ↑ J. Am. Chem. Soc., 2013, 135 (39),4552–14555 (doi:10.1021/ja408231t)
  14. ↑ Efficient Synthesis of Functionalized Organozinc Compounds by the Direct Insertion of Zinc into Organic Iodides and Bromides Arkady Krasovskiy, Vladimir Malakhov, Andrei Gavryushin, Paul Knochel, Angewandte Chemie International Edition, Volume 45, Issue 36 , Pages 6040–6044 2006 doi:10.1002/anie.200601450
  15. ↑ In this example the arylzinc iodide continues to react with allyl bromide in a nucleophilic displacement
  16. ↑ Frankland–Duppa reaction
  17. ↑ Fürstner, A. Synthesis 1989(8),571-590(doi:10.1055/s-1989-27326)
  18. ↑ 18.0 18.1 18.2 18.3 Kurti, L.; Czako, B. ‘‘Strategic Applications of Named Reactions in Organic Synthesis’’; Elsevier: Burlington, 2005.
  19. ↑ Vedejs,E. J. Org. Chem., 2000, vol. 65 (19), 6073–6081 (doi:10.1021/jo000533q)
  20. ↑ Kumwaijima,I.; et. al. J. Am. Chem. 1987, 109, 8056
  21. ↑ Nicolaou,K.C.; et. al.Angew. Chem. Int. Ed. 2005, 44,4442 – 4489(doi:10.1002/anie.200500368)
  22. ↑ Alssa,C.; Riveiros,R.;Ragot,R.; Frustner,A. J. Am. Chem. Soc. 2003, 125, 15512 – 15520 (doi:10.1021/ja038216z)
  23. ↑ Fukuyama,T. et. al. Tetrahedron Letters. 1998, 39, 3189-3192 (doi:10.1016/S0040-4039(98)00456-0)
  24. ↑ Seki,M.; et. al.Tetrahedron Letters. 2000, 41 5099-5101 (doi:10.1016/S0040-4039(00)00781-4)
  25. ↑ Danishefsky,S.; et. al. J. Am. Chem. Soc., 2004, 126 (25), 7881–7889 (doi:10.1021/ja0484348)
  26. ↑ From Aryl Bromides to Enantioenriched Benzylic Alcohols in a Single Flask: Catalytic Asymmetric Arylation of Aldehydes , J.G. Kim and P.J. Walsh, Angewandte Chemie International Edition, Volume 45, Issue 25 , Pages 4175–4178, 2006, doi:10.1002/anie.200600741
  27. ↑ In this one-pot reaction bromobenzene is converted to phenyllithium by reaction with 4 equivalents of n-butyllithium, then transmetalation with zinc chloride forms diphenylzinc which continues to react in an asymmetric reaction first with the MIB ligand and then with 2-naphthylaldehyde to the alcohol. In this reaction formation of diphenylzinc is accompanied by that of lithium chloride, which unchecked, catalyses the reaction without MIB involvement to the racemic alcohol. The salt is effectively removed by chelation with tetraethylethylene diamine (TEEDA) resulting in an enantiomeric excess of 92%.
  28. ↑ De Brabander,J;et. al. Tetrahedron Letters, 1995, Vol. 36, No. 15, pp. 2607-2610
  29. ↑ Li, Z.; Upadhyay, V.; DeCamp, A. E.; DiMichele, L.; Reider, P. J. Synthesis 1999, 1453-1458.
  30. ↑ Niwa,S.; et. al. Chem. Rev., 1992, 92 (5),833–856. (doi:10.1021/cr00013a004)
  31. ↑ Noyori,R.;et. al.Angew. Chem. Ini. Ed. Engl. 1991 30 49-69. (doi:10.1002/anie.199100491)
  32. ↑ Evans, D. A. Science. 1988, 240, 420-426 (doi:10.1126/science.3358127)
  33. ↑ Thompson, A. S.; Corley, E. G.; Huntington, M. F.; Grabowski, E. J. J. Tetrahedron Lett. 1995, 36, 8937-8940
  34. ↑ J. A. Wanklyn (1858). "Ueber einige neue Aethylverbindungen, welche Alkalimetalle enthalten". Liebigs Annalen 108 (67): 67–79. doi:10.1002/jlac.18581080116. 
  35. ↑ Facile Synthesis of Well-Defined Sodium Hydridoalkylzincates(II) Anders Lennartson, Mikael Hakansson, and Susan Jagner Angew. Chem. Int. Ed. 2007, 46, 6678–80. doi:10.1002/anie.200701477
  36. ↑ Schulz, Stephan (2010). "Low-Valent Organometallics-Synthesis, Reactivity, and Potential Applications". Chemistry – A European Journal 16 (22): 6416–28. doi:10.1002/chem.201000580. PMID 20486240. 

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