Boranes in Organic Chemistry 1. α-Carbonylalkyl- and β-Oxyalkylboranes in Organic Synthesis

This review is devoted to the synthesis of a-carbonylalkyl- and β-hydroxy-alkyl boranes and their use in organic synthesis. a-Carbonyl-alkylboranes include several heteroatomic compounds, in particular, [1.2.3]-diazaborinines, uracyl boronic acids, and [1.2.3.4]-diaza-diboretes. The latter type has been obtained by the ketene aminoborations. The reactions of halogenboranes with diazoesters and sulfur ylides resulting in formation of a-carbonyl alkylborates containing diazofunction or ylide structural fragment are described. Amino and halogen boration of acetylenic acid esters was also used for the synthesis of a-carbonyl alkyl boranes. Reactions involving Cr-carbene complexes and acetylenic borone esters were presented for the synthesis of naphthoquinone boronic acids. The formation of amidoboranes by boration of dichloroacetanilides was remined. Boration of 4,8-dimethoxy-2-quinolone with trimethylborates leading to 2-quinolone-3-boronic acid was described. The common synthetic method to a-carbonyl alkyl boranes based on the hydroboration of acrylic acid derivatives was discussed. The results of enhydrazones hydroboration, leading to stable cyclic complexes have been mentioned. The interaction of a-bromoketones with trialkyl or dialkylboranes represents as a general synthetic method to a-carbonyl alkyl boranes. Synthetic approaches to â-hydroxy alkyl boranes are performed. The wide spread hydroboration of vinyl and allyl esters received a well-described attention. The hydroboration of cyclanone enol acetates, 3-keto- and 17-keto-steroids and cyclic allyl alcohol acetates was discussed. The results of aliphatic and alicyclic vinyl esters (including dihydrofuran derivatives) boralylation leading to β-hydroxy alkyl boranes have been envisaged. The synthesis of optically active β-hydroxy alkyl boranes using chiral borane hydrides was discussed. The heterocyclic boran dihydrides are obtained by the hydroboration of dihydropyranes, chromenes and flavenes. Borosilylation of allyl allenylic esters was also been envisaged. The synthetic scheme to optically active boranes and further optically active alcohols were presented. The problems of selectivity regularities in hydroboration reaction by intermolecular complex formations have been discussed.


Introduction
There are a lot of examples of the application of organoboron compounds as reactive intermediates and their role in modern organic synthesis has been reviewed [1][2][3][4][5]. Boron appears not only as an essential element in living organisms but also as a constituent of some antibiotics such as asplamomycin, boromycin, and borophycin [6]. For the last fifty years there have been many incentives to incorporate boron into different biologically active molecules [4], particularly for medicinal application as boron neutron capture therapy of brain tumors [6]. Other methods of synthesis and applications of boron-containing analogues of biomolecules or boron compounds having biological interest have been observed in some reviews [1,2,4,6].

Reactions of trifluorovinyl-trifluoromethylboron derivatives
Pawelke et al. [18,19] have shown that dimethylamino-bis(trifluoromethyl)-borane enters into numerous and novel reactions in which the boron atom increases its coordination number from three to four. Thus, ozonolysis of bis-trifluoromethyl-trifluorovinylborane gave (bis-trifluoromethylboranyl)-oxo-acetic acid 18. If the reaction was carried out in CHCl 3 which has not been carefully dried, the carboxyborane 19 precipitated from the solution. The initially colourless mother liquid, which contained the trifluorooxiranylborane 17, slowly turned yellow. This colour change resulted from the hydrolysis of 17 to form the yellow oxocarboxyborane 18 according to [19] (Scheme 10).
In general, direct oxidation of the crude reaction mixture after benzannulation provided a simple and routine method for the isolation of quinone boronic acid ester compounds 20 and 21 [21] (Scheme 12 and Table 1).

Scheme 12
organylboranes quantitatively to form the corresponding amidoboranes. In certain cases these were in equilibrium with the dimeric forms [22]. Among these reactions in one case the α-carbonylalkylboranes 23 was formed (Scheme 13). Thus, N-phenyl-N-trimethylsilyl-dichloroacetamide reacted with bromodimethylborane to form 23.

Hydroboration of functional derivatives of alkenes
Hydroboration of enamines with five-membered rings gives a stable α-carbonylalkylboranes 24 [23] (Scheme 14). Oxidation of these compounds with hydrogen peroxide in alcohol formed the corresponding carboxylic acids.

Hydroboration of unsaturated esters
Hydroboration of unsaturated esters was observed by Brown and Keblys [28]. The authors found that the unusual reactivity of ethyl acrylate, suggesting that the hydroboration-reduction of this ester must proceed at very different rates. The first step might involve 1,2-addition with formation of the unstable α−carbonylalkylboranes 32 which followed by the rapid transfer of boron from carbon to the neighboring oxygen (Scheme 17).

Hydroboration of vinyl and allyl ethers
According to Mikhailov et al. [31,32] the reaction of diborane and vinyl ethyl or vinyl butyl ether in ethereal solutions at -70 o C followed by slow heating to room temperature, leads to thermally unstable boranes 47 [33] (Scheme 20).
The hydroboration of β-ethoxystyrene with diborane in tetrahydrofuran produced β-oxyalkylboranes 48 and 49 [34] and formation of the latter two alcohols was explained by the hydroboration-oxidation of styrene. Interestingly, in a study of the deuteroboration of cis-β-ethoxystyrene by Pasto and Snyder [35] cis-β-ethoxystyrene spontaneously underwent cis-elimination to form trans-β-deuterostyrene via β-borylethers. In the presence of a basic (C 4 H 9 Li) or acid catalyst (BF 3 ), a trans-elimination with the formation of cis-β-deuterostyrene was observed (Scheme 21).
Hydroboration-oxidation of 1-ethoxycyclohexene in tetrahydrofuran to form trans-2-ethoxycyclohexanol, indicated that the addition of boron occurred at the β-position according to the relative thermal stability of the β-oxyalkylborane 50 than α-oxyalkyl-borane 51 [34] (Scheme 22). The addition of boron trifluoride to the hydroboration products caused decomposition of the β-boryl alkyl ethers.
The hydroboration of cis-verbenyl acetate proceeded from the side opposite to the gem-dimethyl group at β-position 61 with respect to the acetoxy group [43]. Oxidation resulted in a mixture of four compounds (Scheme 29).

Synthesis of β β β β β -oxyalkylboranes from butenyl derivatives
Isobutenyl ethyl ether reacted with borane to give β-oxyalkylborane 75 as the final compound [50] (Scheme 35). The ethoxy group caused the olefin to be highly reactive and, further, reversed the addition pattern of the isobutylene system. A trace amount of iso-butyraldehyde was found among products in this reaction.

Transfer reactions
The kinetics of the Lewis acid-catalyzed dealkoxyboronation of esters of trans-2-ethoxycyclohexaneboronic acid in a variety of donor solvents and with a variety of Lewis acids have been studied by Pasto and Timony [59]. β-Oxyalkylboranes 81-85 were obtained. Preparation of dimethyl 2-ethoxy-1-phenyl-1-ethaneboronate 81 by hydroboration of β-ethoxystyrene in tetrahydrofuran followed by methanolysis was reported. The borinate 83 was prepared by reac-tion of 82 with methyl-magnesium iodide in ether at -
High diastereoselectivity was found for allylic tin compound 120 that was converted to diol 121 via β-

Hydroboration of the functional derivatives of alkenes
An hydroxy group in some cases may direct attack from the same side via intermediate βoxyalkylborane 127 (Scheme 56) [78].