Dimetallic species

Allylzinc bromide is able to add to a variety of alkenyl Grignard reagents in THF at 35 °C123. The structure of the hydrocarbons 184, obtained in moderate yields after hydrolysis (due to their volatility), suggested that 1,1-dimetallic species tentatively formulated as 185 were regioselectively formed (equation 89). Additional examples were subsequently reported but the scope of this intriguing reaction remained rather unexplored and the reactivity of 185 towards various electrophiles was not investigated124,125. 

Another option for the preparation of allenyllithium reagents involves the carbolithia-tion of activated conjugated enynes15. Thus, addition of BuLi to the enyne 281 led to an allenyllithium compound 282 (in metallotropic equilibrium with its propargylic counterpart). Subsequent addition of allylzinc bromide generated the allylic 1,1-dimetallic species... 

The formation of 286 was rationalized by considering that the initially generated ally lie 1,1-dimetallic species 287, which was presumably less stabilized in the absence of appropriately located heteroatoms and/or silyl groups, reacted with another molecule of allenymetal to produce a new ally lie dimetallic species 288. This process could in principle happen again and induce polymerization but instead, compound 288 underwent a type ... 

When 303 was directly treated with Me2Cu(CN)Li2, the transmetallation failed to discriminate between the two carbon-metal bonds. By contrast, the allylzincation of the alkynyllithium derived from the propargylic alcohol 309 produced the alkenyl 1,1-dimetallic species 310, in which the two carbon-metal bonds exhibit different reactivities due to the presence of a metal-alkoxide. Indeed, transmetallation with Me2Cu(CN)Li2 led to the alkenyl copper-zinc species 311, which was relatively poorly reactive towards electrophiles but underwent successful 1,4-addition to ethyl propiolate leading to 312 in satisfactory overall yield (equation 145)180. 

Z-Alkenyllithium reacts with crotylmagnesium bromide and zinc dibromide in ether at — 50 °C to give the stable 1,1-dimetallic species via the postulated transition state involving a zinca-ene reaction. After acidic hydrolysis, anti -(35, 4/ )-dimethyl-l-nonene was obtained in 87% yield with a diastereoselection of 92 8 [101, 105] (Scheme 7-89). 

Despite the formal similarity of the reaction, the mechanism of Cp2ZrCl2-catalyzed ethylalumination [64] with AlEt3 is different from that of either methylalumination with AlMe3 or ethylalumination with Et2AlCl [62]. The involvement of dimetallic species was confirmed by NMR spectroscopy as well as deuterolysis (Scheme 8.31). The proposed mechanism features an interesting zwitterionic bimetallic species, in which the zirconium center is cationic. A highly instructive treatise on the mechanistic pathways of carbometalation is presented in [65],... 

An alkoxy group at the -position induces a cyclopropanation reaction of the dimetallic species as shown in equation 46. The formation of the gem-dimetal species proceeded diastereoselectively as described above. The ring closure proceeds with inversion of configuration to form stereospecifically the cyclopropane ring. An alkoxy substituent at the -position of the allylzinc reagent also induces the cyclopropanation reaction (equation 47)61a 72. 

Transmetallation of the dimetallic species 186a or 186b with CuCN led to a new organometallic species formulated as 191 which displayed remarkable thermal stability. Addition of reactive alkylating agents such as allyl bromide to 191 directly produced compound 192. Indeed, once the first alkylation occurred, the copper salts present in... 

Combining substrate-induced diastereoselection and mutual diastereoselectivity, as illustrated for the crotylzincation of the alkenyllithium derived from 209, led to excellent results as the gewt-dimetallic species 217 was obtained in a highly stereoselective fashion. The stereochemical outcome was explained by the addition of the kinetically reactive cisoid metallotropic form of the crotylzinc reagent anti to the propyl group in the chelated allyl alkenylzinc intermediate. After hydrolysis, compound 218 was obtained as a single diastereomer (equation 106)148,149. 

If two different external electrophiles are to be added to the dimetallic species, diastere-oselectivity can only be achieved if the two carbon-metal bonds are properly discriminated against the reaction with a first electrophile. Moreover, the resulting monometallic species has to exhibit significant configurational stability. Coordination by a heteroatom, which turned out to be essential for achieving substrate-induced diastereoselection, also nicely served these purposes. 

When the dimetallic species 210 was first deuteriated with MeOD and then subjected to iodinolysis, a 60/40 mixture of the two diastereomeric a-deuteriated iodides 267a and 267b was obtained. Although the use of a first small electrophile such as a MeOD did not enable efficient differentiation of the two carbon-metal bonds, a reversal of diastereose-lectivity was observed when the a-deuteriated dimetallic species 268 was first protonated with MeOH and then reacted with iodine. This result points towards the configurational stability of the resulting monoorganozinc species generated after reaction with a first electrophile, presumably due to the coordination by fert-butyl ether moiety (equation 127)162. 

Thus, the alkynyllithium derived from the propargylic ether 302 underwent allylzincation under the above-mentioned conditions and led to the dimetallic species 303. Whereas treatment with NBS resulted in the formation of the dibromoolefin 304, reaction of 303 with the less reactive benzenesulfonyl chloride produced an ce-chlorozinc sulfinate 305. The latter could in turn react with different electrophiles and afforded the corresponding tri- or tetra-substituted olefins of type 306 as single geometric isomers (equation 144)179. 

In view of the extensive documentation outlined above, the usefulness of the polarity alternation concept as a primary guide for evaluation of substituent effects can hardly be denied. The influence of a substituent on the ipso site has not been discussed in this article but an even more direct and important effect is implicit. Among the innumerable examples one may cite the preferential formation of geminal dimetallic species [5] in hydrometalation and carbometalation of vinylmetals and acetylenes. On the other hand, chemical systems are usually very complex, inter- and intramolecular forces including steric and stereoelectronic factors may dominate over polarity alternation. Thus, chelation by a proximal donor often directs metalation and stabilizes certain organometallic entities. In these instances the stability gaining from polarity alternation is overwhelmed. 

The cleavage of the Si—Si bond in the 9,10-disilanthracene 8 dimer by lithium metal in THF allows the synthesis of a dimetalated species 9 (Scheme 4)42. The reaction can be quenched at this point with electrophiles such as methyl iodide. Extended reaction times in the presence of excess metal produced 9,10-dimetalla-9,10-disilaanthracene 10 (M = Li). The corresponding potassium derivative 10 (M = K) was obtained directly from 8 upon treatment with excess metal. These 1,4-dimetalla species 10 have been used to synthesize a variety of civ-substituted disilaanthracene derivatives42. 

Peris and coworkers have also disclosed Ir and Rh complexes 33 and 34 which can catalyze the hydrosilylation of alkynes [77]. Again, poor selectivity was observed as mixtures of the -trans, [ -cis, and a addition products were obtained. Generally speaking, it was found that Rh catalysts were more reactive than the Ir catalyst and the dimetallic complexes were much more active than their monometallic counterparts. It is believed that the difference in reactivity between the dimetallic and monometallic complexes arises from the dimetallic species ability to oxidize to the corresponding M(III) species, thus preventing oxidative addition of the silane. 

Mechanism Two pathways are suggested for this reaction (Scheme 4.49). The titanium-carbene complex A is formed as a key intermediate, which reacts with carbonyl compound to form an alkene via the oxatitanacyclobutane B (Path A). Alternatively, the addition of gem-dimetallic species C to a carbonyl compound gives the adduct D, which eliminates (TiCp2 RS)20 to give an alkene (Path B). 

The related addition reaction of allylzuic bromide to alkynil zinc reagents in refluxing THF leads to a mixture of vinyhc 1,1-organo-ggw-dimetallic species and gem-trimetalhc species, which respectively result from single and double additions (equation 51). The presence of a Lewis basic (see Lewis Acids Bases) group suitably placed for intramolecular (see Intramolecular) chelation, and bearing a secondary substituent, avoids the formation of double addition products and allows the reaction to take place in mild conditions (equation 52). 

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