Synthetic Transformations via Vanadium-Induced Redox Reactions

Chapter 1

Synthetic Transformations via Vanadium-Induced Redox Reactions Toshikazu Hirao Downloaded by UNIV OF MEMPHIS on June 7, 2012 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch001

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan

Low-valent vanadium-catalyzed reduction reactions including dehalogenation, pinacol coupling, and the related radical reaction have been developed by constructing a multi— component redox system in combination with a co-reductant and an additive promoter. High stereoselectivity is attained in these catalytic transformations. Oxovanadium(V) compounds, which are evaluated as Lewis acids with oxidation capability, induce one-electron oxidative desilylation of organosilicon compounds. Oxidative ligand coupling of organoaluminums, organoborons and organozincs, and the ate complexes is achieved by treatment with oxovanadium(V) compounds. A catalytic reaction of organoborates proceeds under molecular oxygen.

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© 2007 American Chemical Society

In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

Downloaded by UNIV OF MEMPHIS on June 7, 2012 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch001

3 Vanadium is a biologically essential element. Its inclusion in enzymes such as haloperoxidase and nitrogenase reveals the importance of its redox chemistry. Vanadium complexes, including organovanadium compounds, exist in a variety of configurations depending on their oxidation states and coordination numbers. Vanadium exists in oxidation states ranging from -3 to +5 and generally converts between states via a one-electron redox process. The properties permit the development of a wide range of organic reactions by controlling the redox processes of vanadium compounds. Radical species are useful intermediates in organic synthesis. A variety of methods have been developed for the selective generation of radical species. One-electron reduction or oxidation of organic compounds using the redox process of metals provides a useful and practical route to generate anion radicals or cation radicals, respectively. Particularly, the redox properties of early transition metals including vanadium, titanium, and manganese, have been employed from this point of view, as exemplified by Scheme 1 (1,2). The redox function is tuned by a ligand or a solvent, allowing a more efficient interaction through the orbitals of metals and substrates for facile electron transfer.

Scheme 1 V(III)

V(II) V ( V )

Y

V(IV)

Y

The redox process of V(II) to V(III) is known to induce one-electron reduction reactions. Vanadium(I) species also can serve as similar reductants. Pentavalent vanadium compounds, which exist in one of five possible configurations (e.g., V O C l , tetrahedral; V F , octahedral), are generally considered to be one-electron oxidants which utilize the V(V)-V(IV) couple (Scheme 1). The redox potential of this couple increases with acidity, so the reactions are usually carried out in acidic aqueous media. One-electron oxidation is also possible with the V(IV)-V(III) couple (E , 0.38 V ) , but the V(V)-V(III) couple (E , 0.68 V ) is less useful for organic oxidation. 3

5

0

0



One of the synthetic limitations exists in the use of stoichiometric or excess amounts of metallic reductants or oxidants to complete the reaction. A catalytic system should be constructed to avoid the use of stoichiometric, expensive and/or toxic metallic reagents. Selection of stoichiometric co-reductants or cooxidants is essential for the reversible cycle of a catalyst (Scheme 2). A n alternative method for recycling a catalyst is achieved by electrolysis (3). A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid to facilitate the reduction reaction or, on the contrary, impede the reaction. Taking these interactions into account, the requisite catalytic system is envisioned to be formed by multicomponent interaction. Steric control by means of coordination may permit the stereoselective and/or stereospecific transformations. Scheme 2 Oxized form of Coreductant

Coreductant

Additive Effect on Reduction Susceptibility Substrate

9 8 % 27 The ate complexes 29 of the organoborons undergo more facile oxidation with VO(OEt)Cl , as observed in the oxidation of the aluminum ate complexes. The organic groups are effectively differentiated in the coupling reaction. Although small amount of the Z isomer is obtained in the case of the BuLi2


17 Scheme 12 R

R'

/

BuLi orCsF

B—R'

\

R' VO(OEt)Cl B / \ X R'

CH2CI2

X = Bu, F 29


2

R'

R=Ph, R'=e-hexyl BuLi: E : Z = 9 : 1 C s F : £ only

derived ate complex, use of the organoborate derived from CsF (45) improves the stereoselectivity, giving the E isomer exclusively (Scheme 12). Alkenyltrialkylborates have been reported to be oxidized to alkylated alkenes with I or B r C N (46). Biaryl formation also occurs by electrochemical, photochemical, and chemical oxidation, for example with Ir(IV), of tetraarylborates (47). The oxovanadium(V)-induced ligand coupling provides another promising method for the carbon-carbon bond formation on boron. Catalytic oxidative ligand coupling of the organoborates 30 is promoted by VO(OEt)Cl under oxygen atmosphere, leading to a versatile method for the selective synthesis of symmetrical or unsymmetrical biaryls (Scheme 13). 2

2

Scheme 13 R

Li Ph B 3

R=electron-donating group

Oxidative transformation of organozinc compounds Organozinc compounds can tolerate a broad range of functional groups. Cross-coupling reactions between organozinc reagents and electrophiles such as organic halides are catalyzed by transition metal complexes (48). However, examples for the selective cross-coupling of two ligands of organozinc


18 compounds are limited to a few cases, which include 1,2-migration of zincate carbenoids and intramolecular coupling of organozinc compounds by organocopper reagents (49). The organozinc compound 31a, prepared in situ by transmetallation of the methylzinc chloride with the aryllithium, is oxidized with Cp FePF , to give the homo-coupling compound 33a selectively (Scheme 14). A g B F serves as a useful oxidant to give the desired cross-coupling compound 32a, probably via a one-electron oxidation process. Using VO(OEt)Cl instead of A g B F , the crosscoupling reaction proceeds in preference to the homo-coupling reaction (50). Higher selectivity for the cross-coupling is observed with VO(OEt)Cl than with VO(OPr-/)Cl or VO(OPr-/) Cl. 2

6

4

2

4

2


2

2

Scheme 14 Me

32a

33a

The coupling reaction of organozinc compounds 31 bearing an o-methoxy, o-phenyl, or o-methylthio group on the arene ring proceeds smoothly, but the ocyano-substituted alkylarylzinc exhibits a lower reactivity, although organoaluminum compounds bearing an electron-withdrawing substituent do not undergo oxidative coupling under the similar conditions. Alkyl and 1-alkynyl groups can couple with the aryl group successfully (Scheme 15). Triorganozincates 34 are readily oxidized with VO(OEt)Cl smoothly to give the cross-coupling compounds 32 (Scheme 16) (50). If the conversion to 34 is not complete, the homo-coupling product derived from the oxidation of aryllithium compound with oxovanadium(V) compound is accompanied (42). Such a step is able to be avoided by the preparation of the ate complex by iodine-zinc exchange. Arylzincate 34, prepared from R Z n L i (57) and aryl iodide, is oxidized to give the cross-coupling product 32 exclusively (Scheme 17) (50). 2

3


19


Scheme 15

Scheme 17

Ar—I

R I Ar—Zn—R

R.ZnLi

VO(OEt)C!

2

Ar—R

THF

32 34

The organozincates 35, obtained from M e Z n L i and various bromoarenes (57), are similarly oxidized with VO(OEt)Cl to give the methylarene 32a (Scheme 18) (52). Thus, the coupling between s/? -carbon (aryl group) and sp carbon (methyl group) of aryltrimethylzincates is achieved chemoselectively. 4

2

2

2

3

Scheme 18 Me ZnLi 4

Ar—Br

2

Me I Ar—Zn—Me I Me

2VO(OEt)Cl

2

Ar—Me THF 32a

35


20 The above-mentioned method is applied to the selective carbon-carbon bond formation between s/? -carbons (52). A bromine-zinc exchange reaction of 36 selectively occurs at the position cis to the phenyl group by treatment with M e Z n L i (53). The oxidation of the thus-obtained zincate 37 with VO(OEt)Cl leads to the stereoselective formation of l-bromo-l-methyl-2-phenylcyclopropane, 38. On the other hand, when the reaction mixture is warmed up to 0 °C, followed by treatment of VO(OEt)Cl , dimethylation takes place to give the dimethylcyclopropane 40 via the organozinc 39 (Scheme 19). 3

4

2

2

2


Scheme 19

*\

f

M ZnLi,

"!

e

Synthetic Transformations via Vanadium-Induced Redox Reactions

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