Carbenium Tetrafluoroborate

The thermal reaction of 1,3-dienes with Co2(CO)8 gives the corresponding bimetallic dimers [(diene)Co(CO)2]2 132 as orange red solids in good yields (Scheme 27)28,133. Oxidation of the dimeric complexes 132 with ferricinium tetrafluoroborate or triphenyl-carbenium tetrafluoroborate gives the monomeric (diene)Co(CO)3+ cations 133 in modest... 

Reduction of tri-(2-thienyl)methyl carbenium tetrafluoroborate by zinc in DME at 65°C produces, via the radical (341), two dimers (342) and (343). The former is the kinetically favored product, which changes to the thermodynamically preferred (343) on thermal equilibration. This obviously takes place by dissociation into the radical (341) (90TL2627). 

When l,l-dimethoxy-4-propyl-2,6-diphenyl-A5-phosphorin (53) is transformed into its 4-carbenium tetrafluoroborate (54) and treated with a base the 4-(l-propenyl) derivative (55) is formed. This, by a two phase oxidation with KMn04, gives the aldehyde (56) which can be further transformed, e.g. into the nitrile (57 equation 34) (75AG(E)lll). An Arbusov type dealkylation of the cation salts (49) (or 54) with LiBr gives the phosphinic ester (58 equation (35)). 

Shortly afterwards, in 1960, Fischer discovered that addition of triphenyl-carbenium tetrafluoroborate to T -cyclohexa- 1,3-diene iron tricarbonyl 4 produced the novel in -cyclohexadienyl iron tricarbonyl cation 5 as a stable salt, and the reactivity of such compounds towards C-C bond formation was soon being explored. The application of organometallic complexes to organic synthesis had begun. 

Related Reagents. Diethyl Phenyl Orthoformate Dimethoxy-carbenium Tetrafluoroborate Dimethylchloromethyleneammo-nium Chloride Ai,Ai-Dimethylformamide A, A -Dimethyl-formamide Diethyl Acetal Methyl Formate. 

To further demonstrate the stability and unreactivity of the isopropenylferrocene carbenium ion, a stable derivative of this ion was prepared and evaluated as an initiator for the cationic polymerization of styrene. The derivative prepared for this purpose was the tetrafluoroborate salt of the isopropenylferrocene carbenium ion. Even at 20°C a 26% yield (Mjj = 11,315, = 18,815) of... 

FIGURE 2.19 Pyrrolidino instead of dimethylamino substitutents for the environmental acceptability of phosphonium and carbenium salt-based reagents.56 Tetramethylurea from O-benzotriazol- l -yl/V./V./V./V -tetramethyluronium hexafluorophosphate and tetrafluoroborate is more volatile and is cytotoxic. The product released from PyBOP is not environmentally objectionable. PyBOP = benzotriazol-l-yl-oxytripyrrolidinophosphonium hexafluorophosphate. 

In trifluoroacetic acid [0.4 M TBABF4 (tetrabutyl ammonium tetrafluoroborate)] unbranched alkanes are oxidized in fair to good yields to the corresponding triflu-oroacetates (Table 2) [16]. As mechanism, a 2e-oxidation and deprotonation to an intermediate carbenium ion, that undergoes solvolysis is proposed. The isomer distribution points to a fairly unselective CH oxidation at the methylene groups. Branched hydrocarbons are preferentially oxidized at the tertiary CH bond [17]. 

However, with triphenylcarbenium-tetrafluoroborate, l.l-dimethoxy-2.6-di-phenyl-4-methyl-X -phosphorin 220 easily splits off a hydride ion to form a resonance-stabilized carbenium-phosphonium-oxonium-ion 221, as Schafer has found The high electron density of the X -phosphorin system, which is in exellent agreement with the theoretical model of Schweig and coworkers (see p. 115), facilitates this interesting reaction. 

Another and still more stabilized carbenium-phosphonium-oxonium-tetra-fluoroborate 225 can be prepared when bis-4.4- (l, l-dimethoxy-2.6-diphenyl-X -phosphorin)-methane 224 is treated with triphenylcarbenium-tetrafluoroborate. 

When nitronium tetrafluoroborate was attempted to react with the trityl cation in CH2CI2 or sulfolane, no nitration occurred due to the deactivating effects of the carbenium ion center in 215. Nitration of deactivated substrates is also readily accomplished by reaction with NO2CI with three mole excess AICI3 suggesting Lewis acidic electrophilic solvation of the nitronium cation (217, eq 62).105... 

The positive chain end is assumed to be a carbenium ion. This mechanism is not applicable to polymerizations initiated with strong alkylating agents such as triethyl-oxonium tetrafluoroborate because with these initiators no zwitter ions can be produced. Since no difference was observed between the products resulting from BF3 or from oxonium initiators (6), it is unlikely that tetramer would be formed by two different mechanisms in these two cases and consequently the proposed mechanism is dubious. 

In 1960, Olah and coworkers reported a study on the polymerisation of olefins by various carbenium salts prepared at low temperature just before the addition of the monomer. AUcyl tetrafluoroborates in particular were drown to be fairly active initiators, but the molecular weight of the products were low. The above salts are not stable under normal conditions, but can be hailed in an inert atomsphere at low temperature and their reactivity is such that they can attack alij atic olefins such as butene and isdrutene. However, the very instability of these initiators is an obvious stumbling block inst fundamental studies. 

An alternative and excellent route to oxaspiropentanes 83 and thus of cyclobutanones 84 proceeds through the condensation of cyclopropyldiphenylsulfonium tetrafluoroborate (82) with aldehydes and ketones. Table 2 presents some illustrative examples. With cyclopropyl methyl ketone and benzophenone as substrates, the oxaspiropentanes cannot be isolated but rapidly rearrange to the corresponding cyclobutanones." Here ring expansion is facilitated by the presence of excellent carbenium ion stabilizing groups. 

Another highly interesting reaction has been recorded for (45) involving a net transfer of hydride leading to the formation of (96) when a solution of (45) in dry CHCI3 is treated with triphenyl-carbenium ion tetrafluoroborate (Equation (2)) <80JA6365>. 

Organocatalytic S l-type reactions with carbenium ions have not been employed in natural product synthesis, as they are of limited utility. A more effective and useful reaction for use in natural product synthesis has been described by the Cozzi group, which explores the chemistry of the readily available benzodithiolylium tetrafluoroborate 1 in organocatal3d ic enantio-selective reactions (Scheme 18.1). ... 

The addition of strong acids to a-ferrocenyl alcohols 12 results in rapid formation of a-ferrocenylcarbenium ions 13, via protonation of the alcohol oxygen and loss of water. Unlike the vast majority of purely organic carbenium ions, these species are easily isolated as stable tetrafluoroborate or hexafluoro-phosphate salts. Their stability stems from the delocalisation of positive charge over the ferrocenyl group, as a consequence of the overlap between the empty p-orbital of the a-sp hybridised carbon and filled metal orbitals. This delocalisation is represented by the two canonical forms 13x and 13y, with the actual structure of a-ferrocenylcarbenium ions lying somewhere between the two. 

Sn2 substitutions of allylic electrophiles, additions to oxo-carbenium ions, and many others. Suitable leaving groups in these substitution reactions include alkyl chlorides, bromides, iodides, mesylates, tosylates, and acetates. Often these processes are conducted under phase-transfer conditions, or require the use of 18-crown-6 to solubilize the salt and enhance the nucleophilicity of the acetate anion. The use of ionic solvents, such as butyl methylimidazolium tetrafluoroborate (Btnim BF4), has also proven useful. The following examples are representative (eqs 2-9). 

Very recently, Cozzi et al. successfully demonstrated the application of this methodology for the a-alkylation of the formyl group to enohzable aldehydes. This was achieved by a reaction of benzodithiolylium tetrafluoroborate (a stabilized carbenium ion) in the presence of several different substituted imidazolidin-4-ones [48]. After reductive removal of the benzodithiol group access to chiral a-branched primary alcohol is given. Using this method optically pure arundic acid is obtained by a three-step synthesis. 

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