Alkenyl-metals

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

This process seems to be contra-intuitive, since two nucleophiles lead to a third nucleophile, but is much easier than the allylzincation of simple alkenes. The reaction is profiled by ab initio calculations, and the energy diagram in Scheme 4 explains the rapid reaction of alkenyl metal with allylzinc halide12. A detailed and advanced computational study for this process by Nakamura and coworkers shows the existence of two possible pathways. One is the metalo-ene pathway whereas the second one corresponds to a metala-Claisen reaction (Scheme 5)13. In both cases, allylzinc and vinyl metal react via a six-membered transition state. 

As shown in Scheme 6, the addition forms two stereogenic centers via the favorable chair-like transition state. The diastereoselective construction of stereogenic centers has been studied extensively by Marek and Normantla. For the control of stereochemistry, one should think about the configuration of allylzinc compounds and the alkenyl metal. Interestingly, a comparison of four possible transition states (Scheme 7) by calculation concludes that Z-crotylzinc bromide is the most favorable transition state. This means that it is not necessary to think about the stereochemistry of crotylzinc bromide as its configuration changes via 1,3-transposition of the zinc atom (Scheme 6)12. 

As described in Section II, the preparation of 1,1-dizincioalkane can be achieved not only by reduction of 1,1-dihaloalkane, but also by allylzincation of alkenyl metal compounds. The reaction of allylzinc with alkenyl Grignard reagent, which is followed by the addition of aldehydes, gives dienes as shown in Scheme 1511,41. 

Allenylzinc reagent also adds to alkenyl metal to afford. s/r -gcminatcd organodimetal compounds. In equation 40, the alkynyl group was introduced diastereoselectively61... 

These stable and easily handled alkenylmercurial compounds are important synthetic intermediates and can be used directly for the preparation of other alkenyl metallics. One such application is illustrated in Scheme 5 for the synthesis of a prostaglandin analogue 197). 

Relatively few examples of metal trifluorophosphine complexes of formula [MRx(PF3)y are known (see Table XVI). 

Among all the alkenyl metals selected, Zn, Zr and A1 give best results for the Pd-catalyzed cross-coupling reactions of alkenyl metals. 

The method was made considerably more general by inclusion of a catalytic amount of a palladium(O) complex during addition of the organometallic. Alkenylcopper reagents actually react relatively slowly with acid halides but, in a fashion analogous to other alkenyl metal species (see Section 1.13.4), they may be readily transmetallated to form an acylpalladium(II) complex which then undergoes reductive elimination to the product (Scheme 29)." A further discussion of acylation mediated by palladium complexes is included in Section 1.13.4. Interestingly, a,P-unsaturated acid chlorides react under these conditions to form divinyl ketones. 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

C—SeMe and the C—Cl bonds and often faster than that of the C— Br bond The reduction is highly chemoselective and leads to alcohols usually in almost quantitative yield (Scheme 161, a Scheme 164, a Scheme 168, a and b). In rare cases, however, such as when a ca n-carbon double or triple bond is present in a suitable position, the formation of a five- or six-membered ring takes place by trapping of the radical intermediate (Scheme 118). ° Tin hydride reduction has been advantageously extend (g P-hydroxy-y-alkenyl and -hydroxy-a-alkenyl selenides displayed in Scheme IM (a) and Scheme 168 (a and b) and derived from a-selenoalkyllithiums and enenones, and from 1-seleno-l-alkenyl metals and carbonyl compounds, respectively. 

Like hydroalumination and hydrozirconation, hydroboration of alkynes also provides a convenient and Stereospecific route to alkenyl metal reagents. However, initial attempts to achieve palladium-catalyzed cross-coupling of alkenylboranes with alkenyl halides were unsuccessful, due to the poor carbanionic character of these reagents. Later, Suzuki discovered that the desired transformation could be effected in the presence of an alkoxide or hydroxide base weaker bases, such as sodium acetate or triethylamine, were not generally effective. The reaction is suitable for the preparation of ( , )-, ( ,Z)- and (Z,Z)-dienes. Since reactions of alkenylboronates are higher yielding than those of alkenylboranes, the recent availability of (Z)-l-alkenylboronates " substantially improves the Suzuki method for the preparation of (Z)-alkenes. An extension of the methodology to the synthesis of trisubstituted alkenes has also been reported. " ... 

Reactions of 1-Alkenyl Metals with 1-Alkynyl Halides Reactions of Terminal Alkynes with -Carbon Halides General aspects Synthesis of terminal alkynes Stereospecific synthesis Phase-tranter catalysis... 

Reactions of 1-Alkenyl Metals with 1-Alkynyl Halides... 

The most common routes to alkenyltin compounds involve reaction between an alkenyl-metallic compound and a tin halide, or the hydrostannation of an alkyne (see Section 4.4). In recent years, the stannylmetallation of alkynes has become increasingly important. Direct hydrogenation of an alkynylstannane to a vinylstannane does not appear to be practicable, presumably because the catalyst is poisoned by the tin compound, but cis addition of dihydrogen can be achieved by hydrozirconation followed by hydrolysis,1 or in some cases, by cyclotitanation followed by hydrolysis2 (Section 8.2.2 below). 

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