Vinylzinc

Table 4. Preparation of Fluorinated Vinylzinc Reagents from (F)-Vinyl Halides and Zinc Metal [110]...

Because the vinylzinc and vinylcadmium reagents can be prepared directly from the vinyl halides (I, Br) with zinc or cadmium metal, this route avoids cross coupling processes and provides a one-pot in situ preparation of perfluo-rovinylcopper compounds Table 7 shows examples of this method of preparation of vinylcopper reagents from the indicated cadmium or zinc reagent [145]... 

The palladium(0)-catalysed coupling of 2-bromocyclopentene-l-carboxaldehyde NJf-dimethylhydrazone with vinylzinc-, 2-furyl-, or 2-thienylzinc halides provides a route to the corresponding dimethylhydrazones, which cyclize thermally to pyridines, e.g. 9 <95T(51)9119>. 

The aziridine-2-carboxaldehyde 56 can also serve as synthon for the synthesis of sphingosines, which are important biomembrane constituents [64]. One possible route involves the addition of an alanate to the aldehyde. In a later stage of this synthetic plan the aziridine can be opened, either via the intermediacy of an oxazoline or directly with dilute acid. Unfortunately, the reaction of aldehyde 56 with a vinylalanate has a poor diastereoselectivity of 3 2. Therefore, an alternative approach was considered, namely one involving the addition of a vinylzinc reagent to the aldehyde thereby employing our N-tritylaziridinediphenyl-methanol 51 as the chiral catalyst. Gratifyingly, only one diastereomer was obtained. Reductive removal of the trityl function, acetylation of the hydroxy... 

A few vinylzinc reagents have been added to aldehydes in the presence of chiral sulfur-containing ligands. As an example, Seto et al. reported, in 2005, the synthesis of dipeptide A-acylethylenediamine-based ligands by parallel... 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

For enantioselective catalytic addition of vinylzinc reagents to aldehydes, see [52-70]. 

SCHEME 65. Addition of vinylzinc bromide to miinium salts generated from benzotriazole derivatives... 

Recently, Soai and coworkers reported an enantioselective addition of diisopropenylzinc and an -branched vinylzinc reagent to aldehydes (equation 19)61. 

A dimetal species, obtained by treatment of vinylzinc with Schwartz s reagent, leads to a very effective alkylidenation reaction as shown in Scheme 1843. 

Fluorinated vinylzinc reagents can be prepared by three methods (1) capture of the corresponding vinyllithium reagents at low temperatures with a zinc salt (2) capture of the corresponding vinyllithium reagent in situ with a zinc salt at ambient temperatures or (3) direct insertion of zinc into the carbon-halogen bond of a fluorinated vinyl halide. 

The synthetic protocol for many of these reagents is detailed elsewhere9. Note that stereochemistry in EIZ alkenes is preserved in the formation of both the fluorinated vinyl-lithium and the fluorinated vinylzinc reagents, and that hindered bases permit selective metallation vs. metal/halogen exchange. 

Related methodology has been utilized to prepare fluoroenynes either by coupling fluorinated vinyl halides with an acetylenic zinc reagent or via coupling of a fluorinated vinylzinc reagent with l-iodo-l-alkynes2,11, as illustrated in equations 17-21. 

The preparation of fluorinated dienes is particularly advantageous via the palladium catalyzed coupling protocol. By proper selection of the appropriate vinylzinc reagent and vinyl iodide, a wide variety of fluorinated dienes can be stereospecifically prepared, as illustrated in equation 22. 

The in situ formed vinylzinc reagent could subsequently be coupled (with palla-... 

Little mechanistic work has been reported on the direct reaction of F-vinyl halides with Zn°. Jairaj and Burton have studied the mechanistic details of the reaction between Z-l-iodopentafluoropropene with Zn° and have presented mechanistic evidence consistent with the formation of a vinyl carbanion that is captured in situ by zinc halide to form the vinylzinc reagent45. Their mechanism is presented in Scheme 1. 

The solvated vinylzinc reagent does not add to benzaldehyde however, under Barbier conditions, the reaction of Zn°, Z-CF3CF=CFT in DMF stereospecifically gives the L -allyl alcohol consistent with trapping of the carbanion intermediate (Scheme 2). 

The fluorinated vinylzinc reagents formed via direct insertion of zinc into the carbon-halogen bond of a vinyl halide have served as useful synthons for the stereospecific synthesis of a wide variety of fluorinated compounds. Some illustrative examples of some of the synthons developed are summarized in the following sections. 

Nguyen and Burton also reported a practical route to 2,2-difluorostyrenes via the palladium-catalyzed coupling of l-iodo-2,2-difluoroethylene with aryl iodides, as outlined in equation 5239. Surprisingly, F2C=CHBr does not work well in this reaction. The initially formed vinylzinc reagent undergoes a novel acid-base reaction (with unreacted olefin) and both [F2C=CHZnBr] and [F2C=CBrZnBr] are formed, as well as F2C=CH2 (Scheme 3). Thus, F2C=CFII is necessary to achieve selective formation of only the... 

Fluorinated vinylzinc reagents undergo coupling with CuBr2 or FeCl3 to give symmetrical dienes (equations 58 and 59)51,52. 

Fluorinated vinylzinc reagents readily react (particularly with Cu(I)X catalysis) with allyl halides at room temperature (equation 63)57. With a substituted allyl halide, the 2-propenyl reagent gave products of both a- and -attack. The predominant product is derived from attack at the less hindered position (equation 64)5. 

The partially fluorinated vinylzinc reagent, reported by Shi and coworkers, has been utilized to prepare the fluoro analog of Naproxen and for the key intermediate for a novel synthetic pyrethroid (equations 67 and 68)44. 

Similar to the zinc insertion methodology for the preparation of fluorinated vinylzinc reagents (Section II.C), zinc reacts quantitatively with bromo- and iodopentafluorobenzene in coordinating solvents such as THF, Et20, DME, DG (diglyme), DMF, DMAC and DMSO to give bis(pentafluorophenyl)zinc (equation 72)68. These workers also obtained equilibrium constants for the Schlenk equilibrium between the mono/bis species via 19F NMR spectra. 

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