Alkenyl copper

The dienone 858 is synthesized by coupling of the alkenyl copper reagent 856 with crotyl chloride (857) in the presence or absence of ZnCl2[731], Tetrabutyllead (859) reacts with benzoyl chloride to afford butyl phenyl ketone[732]. 

The mechanism of carbometallation has been explored computationally.77 The reaction consists of an oxidative addition to the triple bond forming a cyclic Cu(m) intermediate. The rate-determining step is reductive elimination to form a vinyl magnesium (or zinc) reagent, which then undergoes transmetallation to the alkenyl-copper product. 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

The remaining alkenyl copper moiety in 50 can react intramolecularly with the carbon—carbon triple bond to give metallofulvene derivatives 53. However, the coupling of the intermediate with the second molecule of alkynyl halide would seem to be faster than the cyclization reaction. Therefore, the dienediyne is obtained as the major product. 

The coupling with alkynyl iodides in the presence of CuCl and DMPU proceeds quite differently from that with alkynyl bromides. Although the first step of the coupling is the same, the subsequent Cu/I exchange reaction of the intermediate is different. As the final product, iododienyne 54 is obtained in high yields, as shown in Eq. 2.37 [35]. In the case of alkynyl bromides, Cu/Br exchange does not proceed. Therefore, the alkenyl copper moiety couples with the second alkynyl bromide molecule. 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

With glyceraldehyde-derived enones and enoates, it has been found that addition of aryl or alkenyl copper reagents is almost independent of the enone geometry [24, 25]. In agreement with the modified Felkin-Anh model, Z enoates usually provide high levels of anti selectivity (Scheme 6.11). Hence, the Z derivative 64 reacted with complete stereochemical control, whereas the -enoate 64 gave a lower selectivity of 4 1 in favor of the anti-conjugate adduct [25]. 

In addition to alkenylboron compounds, alkenylalane, alkenylzirconium, alkenyltin, alkenyl copper, and alkenylmagneslura reagents are reported to undergo a related alkenyl-alkenyl coupling reaction to give l,3-a1kad1enes. 

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. 

Attempted preparation of fluorinated alkenyl copper reagents from the corresponding alkenyl iodides with copper powder was not successful. The symmetric dimer was obtained in good yield [172-179] (Scheme 62). 

A practical method for preparation of fluorinated alkenyl copper reagents has been recently developed from cuprous halides metathesis of the corresponding zinc or cadmium reagents [180]. These copper reagents exhibit excellent stability at room temperature and undergo a variety of coupling reactions with methyl, allyl, vinyl, aryl and acid halides [180] (Scheme 63). More recently, preparation of cyclic perfluoroalkenyl copper reagents has been reported by the same route [156-158]. 

A reaction mechanism was tentatively proposed by Takaya et al. which involved, in a first step, the formation of aryl- or alkenyl-copper(I) species formed by... 

Alkenyl Iodides from alkenyl-copper derivatives, 62, 1... 

All the starting materials were very easily prepared in a single-pot operation by treatment of the alkoxy-allene 96 [70] with lithium organocuprate either in Et20 (for the formation of the Z- vinyl copper intermediate Z-97) or in THF (for the formation of the -vinyl copper intermediate -97) and trapping the resulting alkenyl copper E- and Z-97 with different unsaturated alkyl halides to give 98a-j (Scheme 35) [71]. 

Complex alkenyl-copper and -cuprate derivatives, and also alanates, have been generated with high stereochemical purity, and upon treatment with iV-chloromethyl-iV-methylformamide or iV-chloromethyl-phthalimide provide access to highly functionalized allylic amides (entries 1 and 2, Table 7). Addition to acyliminomalonic esters or acyliminophosphonates provides acylaminomalonates and acylamino-alkylphosphonates in modest yield (entries 3 and 4, Table 7). 

Alkenyl copper reagents, such as 87-%, also prepared via vinyl iodides such as 85 or %, have proven to be reliable reagents for the introduction of the standard as well as many modified p-chains (22,23.26.29.31.32.34-37,72). The alkenyl-copper reagents are of three types lithio divinylcuprates (87), lithio mono vinylcuprates (%, ) containing an inert ligand, and vinylcopper reagents (90) which have received little synthetic attention (Scheme 16). scheme is... 

When an alkenyl copper species is used in the displacement reaction, then the alkenyl species reacts with retention of configuration of the double bond (1.172). Alkenyl radicals can interconvert readily and these results suggest, therefore, that alkenyl radicals are not involved in these reactions. 

An alkenyl copper species containing a sulfonylalkenyl group 92 with a-iodoalkylzinc reagent produces 93 (Ohno et al. 2000 Varghese et al. 2000, respectively) (Scheme 7.35). 

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