Quimica De Organocooper

Modern Organocopper Chemistry. Edited by Norbert Krause Copyright > 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-29773-1 (Hardcover); 3-527-60008-6 (Electronic)

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2

Transmetalation Reactions Producing Organocopper Reagents
Paul Knochel and Bodo Betzemeier
2.1

Introduction

Organocopper reagents constitute a key class of organometallic reagents, with numerous applications in organicsynthesis [1]. Their high reactivities and chemoselectivities have made them unique intermediates. Most reports use organocopper reagents of type 1 or 2, which are prepared from organolithiums. This transmetalation procedure confers optimal reactivity, but in many cases it permits only the preparation of relatively unfunctionalized organocopper reagents. More recently, substantial developmentshave been taking place in transmetalations to organocopper reagents starting from organometallic species that tolerate the presence of functional groups [2], while synthetic methods permitting the preparation of functionalized organolithiums and organomagnesium compounds have also been developed. All organometallics in which the metal M is less electronegative than copper, and all organometallicspecies of similar electronegativity but with weaker carbon-metal bonds, are potential candidates for transmetalation reactions [3]. Thus, reaction conditions allowing the transmetalation of organo-boron, -aluminium, -zinc, -tin, -lead, -tellurium, -titanium, -manganese, -zirconium and -samarium compounds have all been found, resulting in a variety of new organocopper reagents of type 3. Theirreactivity is dependent on the nature of the original metal M, which in many cases is still intimately associated with the resulting organocopper reagent (Scheme 2.1) [3–5]. In this chapter, we will emphasize these recent developments, especially those that allow the preparation of organocopper species not accessible through the standard procedures involving organolithiums as precursors and their usein reactions with organic electrophiles.

2.2

Transmetalation of Functionalized Organolithium and Organomagnesium Reagents

Many functional groups are incompatible with organolithium reagents. Execution of transmetalations at very low temperatures, however, enables functionalized

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2 Transmetalation Reactions Producing Organocopper Reagents

Scheme 2.1.

Transmetalationsproducing organocopper reagents.

alkenyllithiums and aryllithiums to be prepared, and subsequent further transmetalation at low temperatures provides the corresponding copper reagents [6]. Thus, treatment of 4-bromobenzonitrile 4 with nBuLi at À100  C in a THF/ether/ pentane mixture provides the corresponding aryllithium within 5 min. (Scheme 2.2), and subsequent treatment with the THF-solublecopper salt CuCNÁ2LiCl [7] then affords the functionalized arylcopper compound 5. Treatment of this with 2-cyclohexenone in the presence of TMSCl [8] furnishes the expected Michael adduct 6 in 93% yield.

Scheme 2.2. Preparation of functionalized arylcopper reagents from functionalized aryllithiums.

In some cases it can be advantageous first to transmetalate the functionalized aryllithium reagentto the corresponding zinc reagent and then to perform a second transmetalation to afford the corresponding organocopper species. Thus, 2-iodo-1-nitrobenzene 7 is converted into the corresponding lithium reagent by treatment with phenyllithium [9]. Subsequent transmetalation, firstly with ZnI2 at

2.2 Transmetalation of Functionalized Organolithium and Organomagnesium Reagents

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À80  Cand then with CuCNÁ2LiCl [7] at À30  C, provides the arylcopper 8. This reacts with 3-iodo-2-cyclohexenone to give the expected addition-elimination product 9 in 70% yield. This method can be extended to the preparation of alkenylcopper compounds. Thus, treatment of the iodoalkenyl azide 10 with nBuLi at À100  C (Scheme 2.3), followed by transmetalation with ZnI2 in THF and then by a second...