Alkenyl-metal compounds

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. 

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). 

Acetylenes react with the reagents obtained by mixing alanes with [Cp2ZrCl2] to produce alkenyl-metal compounds in high yields, the reaction involves a clean cw-addition, reaction (13). The exact nature of the metal species MLn... 

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]. 

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. 

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

Transition metal-catalyzed cross-coupling reactions between vinyl organometallic compounds and unactivated alkyl halides that can be usually performed with palladium, nickel and cobalt are of particular synthetic interest [37-39]. Recently, the groups of Cahiez [48] and Cossy [49] concurrently reported the first iron-catalyzed reaction of alkenyl Grignard compounds with primary and secondary alkyl halides (X=Br, I) (Scheme 5.15). The two protocols basically differ in the iron source... 

The selectivity of the alkenylation reactions and the yields of products can be dramatically improved by carrying out the reaction of alkenyliodonium salts with carbon nucleophiles in the presence of transition metal compounds in stoichiometric or catalytic amounts. Thus, the reactions of bicycloalkenyldiiodo-nium salts 62 with cyanide anion or with alkynyllithium in the absence of transition metals are non selective and lead to a wide spectrum of products, while the same reactions in the presence of the equimolecular amount of copper(I) cyanide afford the respective products of vinylic nucleophilic substitution in good yields (Scheme 29) [52,53]. 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

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). 

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. 

With the notable exceptions of aUyl-allyl coupling (Sect, in.2.10) and enolate allylation (Sect. V.2.1.4), Pd-catalyzed allylation has been achieved mostly with alkenyl-metals (alkenyl-allyl couphng) and arylmetals (aryl-aUyl couphng). The use of alkylmetals and alkynyhnetals has been rare perhaps because their allylation can be performed satisfactorily with appropriate Grignard reagents, whose reactivity can be modified favorably with either stoichiometric or catalytic amounts of Cu compounds, as needed. 

General Procedures. - Normant and Alexakis have reviewed the use of alkyne metal compounds in the stereospecific synthesis of alkenyl compounds. The isomerisation of internal alkynes to ethynyl compounds effected by the catalyst derived from sodium or potassium hydride in 1,3-diaminopropane is illustrated in the conversion of nonadec-2-yn-l-ol (23) to nonadec-18-ynol (24) in 85% yield ... 

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