Carbanions vinyl halides

Among special chemical methods that facilitate the Diels-Alder reaction can be included the temporary metal connection strategy [101] that is illustrated in Table 4.27. Si, Mg and A1 are used as temporary connectors of diene and dienophile moieties. The cycloaddition occurs easily due to its intramolecular nature and because the dienophilic component of reagent is now formally a vinyl carbon ion (i.e. a vinyl carbanion in 154 with M = AlEt ). Thus the metal-tethered 154, prepared from lithium alkoxide of 153 with the suitable metal vinyl halide, gives, by heating, the cycloadducts 156 and 157, through the... 

The partial loss of configuration often observed during reduction of cyclopropyl halides may actually occur via the corresponding cyclopropyl radicals, which lose configuration rapidly °>. In that event, their behavior would resemble that of vinyl halides, as exemplified by the 3-iodo-3-hexenes 16>. Occasional cases of partial inversion could be associated with shielding of the cyclopropyl carbanion by the electrode surface, with concomitant protonation on the other face of the carbanion S8>. 

The consecutive reaction of vinyl halides and alkenes with activated methylene systems [42] in the presence of a palladium catalyst and phase-transfer catalyst results from the addition of the methylene carbanion with the initially formed Heck product (Scheme 6.31) an intramolecular version of the reaction leads to the formation of bicycloalk-l-enes (Scheme 6.31) [42], The analogous combined coupling reaction of iodoarenes and activated methylene compounds with non-conjugated dienes under similar conditions forms the monoalkene (Scheme 6.31) [43]. 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

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 production of the (Z)-haloalkenes is thought to proceed via initial exchange of the tetrafluoroborate and halide ions and collapse of the resulting vinyliodonium halides by the addition-elimination (Ad-E) mechanism (equations 203 and 204)84. As with Ad-E reactions of moderately activated vinyl halides (X = Cl, Br), which typically occur with configurational retention (> 95%)143 145, the intermediate carbanions apparently prefer a least motion rotation of 60° prior to the expulsion of iodobenzene. It has been demonstrated by an NMR study that anion exchange between (Z)-(2)-phenylsulfonyl-l-decenyl)-phenyliodonium tetrafluoroborate and tetrabutylammonium chloride occurs instantaneously in deuteriochloroform84. Furthermore, when authentic halide salts of the... 

The coupling takes place as if a carbanion (R ) were present and the carbanion attacked the alkyl halide to displace the halide ion. This is probably not the actual mechanism, however, because dialkylcuprates also couple with vinyl halides and aryl halides, which are incapable of undergoing SN2 substitutions. 

A useful, although somewhat limited, method of preparing vinyl azides involves the addition of azide ion to vinyl halides with subsequent elimination of halide ion. This reaction is only possible when the halogen is located to a group which can effectively stabilize the intermediate carbanion. A number of alkyl and aryl substituted )3-chlorovinyl ketones (23) have been found to react in this way ". In many cases, however, the resulting /3-azidovinyl ketones (26) cannot... 

Tandem cyclization was not obtained for 88. However, this could be achieved for 89 in which the vinyl halide was further activated with an acetyl group [Eq. (54)] and the catalyst was used in a larger amount (0.3-0.5 eq.) [238]. In explanation of the difference, it was proposed that the subsequent formation of the cyclopropyl ring by 3-. vu-cyclization is reversible. The electron withdrawing group at the final radical center promotes its rapid reduction [by the excess Ni(I) complex] to a carbanion that is quickly protonated [238]. 

Simple sulfonyl carbanions which do not contain additional carbanion-stabilising groups, e.g. carbonyl groups or heteroatoms, can be readily alkylated in high yield by modern techniques with the use of alkyllithiums and lithium amide bases. A number of allylic halides have been successfully used. In allylic halides, the halogen directly attached to the double-bonded carbon is relatively inert towards nucleophilic attack (Scheme 41). In this way, sulfones (96) can be transformed via desulfonation into vinyl halides (97) or into ketones (98) by hydrolysis (Scheme 41). In contrast to ordinary alkyl sulfones, triflones (99) can be alkylated under mildly basic conditions (potassium carbonate in boiling acetonitrile) (Scheme 42). The ease of carbanion formation from triflones (99) arises from the additional electron-withdrawing (-1) effect of the trifluoromethyl moiety. 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

Dicarborative 1,2-additions of aryl halides, vinyl halides or enol triflates and stabilized carbanions to allenes in the presence of similar palladium catalyst systems reportedly occur with high EfZ stereoselectivity70. They have been applied in an intramolecular version36 and in a two-step synthesis of a steroidal skeleton 1. 

Diketones. This alkylation of ketimines has been extended to the synthesis of 1,4-diketones by use of a 2,3-dihalopropene as the alkylating reagent. The reaction is carried out with the carbanion, prepared as described above and then diluted with IHF at -60°. The resultant vinylic halide is then hydrolyzed with sulfuric acid at -20 to 0°. 

Vinyl halides and their carbanions (transition from -interactions)... 

A third method for preparation of carbanions is metal halogen exchange. An aryl or vinyl halide may be treated with an alkyllithium that gives the aryl or vinyllithium and the alkyl halide (Eq. 7.4) [12]. Vinyl bromide can be converted to vinyllithium by treatment with fert-butyllithium in ether at -78°C [13]. 

Vinyl halides undergo elimination by an ElcB mechanism more readily than alkyl halides because the carbanion intermediate formed from a vinyl halide has an sp hybridized C, while the carbanion derived from CH3CH2CI is sp hybridized. The higher percent -character of the sp hybridized anion makes it more stable, and therefore, it is formed more readily. 

Dialkyl cuprates may also be added to aryl vinyl sulphoxides and the resulting a-sulphinyl carbanions can be treated with various electrophiles such as aldehydes, ketones and alkyl halides (equation 350)643. 

The kRrlka ratios for the elimination-rearrangement process at 120° are 20,46 and 41 for R = H, Me and MeO, respectively. Bond-breaking is therefore important, and slow elimination of halide ion from vinyl carbanions, which are formed in pre-equilibrium, seems plausible (Jones and Damico, 1963). 

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