Pyridine, complexes with boron 2-

The derivatives of cyclic boryloxyalkyl phosphines readily undergo complexation with tertiary amines and especially pyridine with the participation of the boron atom [Eq. (113)] (83IZV2545 86IZV643). 

The synthesis of the representative compound of this series, 1,4-dihydro-l-ethyl-6-fluoro (or 6-H)-4-oxo-7-(piperazin-l-yl)thieno[2/,3/ 4,5]thieno[3,2-b]pyridine-3-carboxylic acid (81), follows the same procedure as that utilized for compound 76. Namely, the 3-thienylacrylic acid (77) reacts with thionyl chloride to form the thieno Sjthiophene -carboxyl chloride (78). Reaction of this compound with monomethyl malonate and n-butyllithium gives rise to the acetoacetate derivative (79). Transformation of compound 79 to the thieno[2 3f 4,5]thieno[3,2-b]pyhdone-3-carboxy ic acid derivative (80) proceeds in three steps in the same manner as that shown for compound 75 in Scheme 15. Complexation of compound 75 with boron trifluoride etherate, followed by reaction with piperazine and decomplexation, results in the formation of the target compound (81), as shown in Scheme 16. The 6-desfluoro derivative of 81 does not show antibacterial activity in vitro. 

Alkyl-substituted pyridines have been complexed with a wide variety of other boron Lewis acids, BH3, B(OH)3, and BX3, where X is a selection of alkyl, aryl, hydroxy, alkoxy, and aryloxy groups <87JCS(P2)77l). 

The complexes formed with boron trihalides are decomposed to pyridine by boiling water. Complexes with other Lewis acids behave similarly. 

If cyclopentadienyliron dicarbonyl halides are allowed to react with themselves in the presence of Lewis acids, cations are formed in which the new substituent is the cyclopentadienyliron dicarbonyl halide itself, for example, [CjH6Fe(CO)i-X-(CO)2-FeC8H8]+ (X = Cl, Br, I). All three cations can be prepared best by treatment of the corresponding halides with boron trifluoride diethyl etherate all are isolated as tetrafluoro-borates.18 The bromine complex can also be obtained by a more complicated procedure by the reaction between C8H8Fe-(CO)2Br and AlBr3 in liquid sulfur dioxide 16 the iodine cation can be isolated from a melt of cyclopentadienyliron dicarbonyl iodide and aluminum chloride.17 In the latter two cases the hexafluorophosphate salts can be obtained. These binuclear cations are of special interest, because they are cleaved by electron donors,16-17 e.g., aniline, pyridine, benzonitrile, acetonitrile, acrylonitrile, with the formation of the corresponding [C8H8Fe(CO)2L]+ cations and the parent halide. Equations for preparation of the tetrafluoroborate are ... 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Boron trifluoride-diethyl ether complex is a very versatile and useful Lewis acid in several organic reactions. The polymeric ether-BFs complex poly(p-methoxystyrene)-BF3 (14) has been prepared and is more stable and has higher activity in several organic reactions such as isomerization and epoxide rearrangement [26]. The polymeric version of pyridine-BF3 complex 15 has also been prepared from poly(vinylpyridine) and BF3 [27]. By analogy with the polystyrene-AlCls complex, simple crosslinked polystyrene also forms a stable complex in chloroform with boron trifluoride 16 [27]. 

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