Tetrafluoroboric acid, reaction with alkenes

Bis(pyridinium)iodonium tetrafluoroborate [I(Py)2BF4] reacts readily with alkenes to afford 1,2-disubstituted products arising from addition of iodine and pyridine. Synthetically more important is, however, the reaction of unsaturated systems with I(Py)2BF4 in the presence of nucleophiles, which provides a general method for vicinal iodofunction-alization of alkenes. In this regard, the addition of a stoichiometric amount of tetrafluo-roboric acid to the reaction medium is often required to avoid the competitive formation of products resulting from pyridine acting as a nucleophile. 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

When terf-pentyl alcohol was reacted with acetic anhydride and perchloric or tetrafluoroboric acids, the reaction proceeded via the more stable trisubstituted alkene (21) under thermodynamic control and the reaction product was mainly a 2,3,4,6-tetramethylpyrylium salt (25). However, with terf-pentyl chloride, acetyl chloride and aluminum chloride or antimony pentachloride, the diacetylation occurred under kinetic control the alkene equilibration proceeds very fast with these Lewis acids so that the product (26) is derived from the more reaetive but less stable disubstituted alkene (22) 63,64 formulas below, the intermediate monoaeetylation products... 

The aminomercuration-demercuration reaction has provided two examples for primary to secondary amine conversion.In one, Markovnikov addition of the aminomercurial (10) to an alkene, followed by ligand exchange with sodium hydroxide and subsequent reduction with sodium borohydride, yields the secondary amine (11) in a one-pot reaction (Scheme 10). In the other,vicinal diamines (12) are the products from the one-pot reaction of alkenes with tetrafluoroboric acid and mercury(ll) oxide in the presence of excess primary amine (Scheme 11). Both reactions work equally well with secondary amines. 

P-Alkoxyalkyl-Fp complexes are rather labile. Treatment with tetrafluoroboric acid etherate leads to elimination of the alcohol to give cationic alkene-Fp complexes. These can then be readily demetalated either by reaction with sodium iodide in acetone at room temperature or by heating in acetonitrile providing the corresponding olefins (Scheme 4-21). " ... 

Another option to gain access to T -allyliron complexes is the removal of a leaving group from substituted neutral (T] -alkene)iron complexes. This procedure starts from allylic alcohols, allylic ethers, or carboxylates, which are complexed with nonacarbonyldiiron and subsequently treated with tetrafluoroboric acid to aflbrd the cationic Ti -allyl(tetracarbonyI)iron complexes (Scheme 4-78). The reaction is stereoselective with respect to the geometry of the starting allyl alcohols. ( )-AlIyl alcohols give anti products, whereas syn complexes are formed from (Z)-allyl alcohols. The alkene complexes can also be generated in situ to afford the q -allyliron complexes after treatment with acid. ... 

The reaction of A -methylthio(dimethyl)amine with a two-fold excess of an alkene in the presence of trimethyloxonium tetrafluoroborate produced, in moderate yield, the l-dimethylamino-2-methylthio adduct 3, after demethylation of the intermediate ammonium salt by excess trimeth-ylamine120. More conveniently, the same product is directly obtained by using boron trifluo-ride-dimethyl ether complex as Lewis acid. 

Dimethoxy-3,6-dihydropyrazine (109), prepared by methylation of 2,5-piperazinedione with trimethyloxonium tetrafluoroborate, is susceptible to lithiation because the protons at C-3 and C-6 are activated by adjacent imine moieties. The lithium salt of this bislactim ether reacts with the 2-chloro-l-phenylsulfonyl alkene (110) to give the 3-substituted pyrazine (111) (Scheme 25) <89JCS(P1)453>. The bislactim ether from piperazinedione cyclo(L-Val—Gly) is lithiated with butyl-lithium and then treated with ketones, alkyl halides, or others to form, nearly stereospecifically, ran5-3-isopropyl-6-substituted piperazinediones due to the steric influence of the isopropyl group <828866, 838673). Similar stereoselective syntheses have been achieved in reactions starting from cyclo(L-Val—D,L-Ala) <828864, 918939). Acid hydrolysis of these products affords chiral a-amino acids. 

Preparation First representatives of alkenyliodonium salts, dichlorovinyl(phenyl)iodonium species, were reported by Thiele and Haakh in the early 1900s [436]. The first general synthetic approach to alkenyl(phenyl)iodonium salts was developed by Ochiai in the mid-1980s [437,438], This method is based on the reaction of silylated alkenes 299 with iodosylbenzene in the presence of Lewis acids, leading to the stereoselective formation of various alkenyliodonium tetrafluoroborates 300 in good yield (Scheme 2.85). 

The new acidic ionic liquid phenyl(butyl) (ethyl) selenonium tetrafluoroborate, [pbeSe]Bp4, was found to be a new acid co-catalyst and successfully used in the MBH reaction of several electron-deficient alkenes with aromatic and aliphatic aldehydes in moderate to good yields and in relatively short reaction times under mild conditions (Scheme 31.16) [32]. 

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