Enolates reaction with trialkylboranes

Reaction with trialkylboranes. A trialkylborane (1), readily available by hydro-boration of an alkene, undergoes fast 1,4-addition to methyl vinyl ketone to give an enol borinate ester (2), which on hydrolysis affords a methyl ketone (3).24... 

Among other methods for the preparation of alkylated ketones are (1) Alkylation of silyl enol ethers using various reagents as noted above, (2) the Stork enamine reaction (10-69), (3) the acetoacetic ester synthesis (10-67), (4) alkylation of p-keto sul-fones or sulfoxides (10-67), (5) acylation of CH3SOCH2 followed by reductive cleavage (16-86), (6) treatment of a-halo ketones with lithium dialkylcopper reagents (10-57), and (7) treatment of a-halo ketones with trialkylboranes (10-73). 

In the reaction of trialkylborane with 1-alkyne and lead tetraacetate in hexane, one of the alkyl groups of the trialkylborane migrated to the terminal carbon atom of the triple bond, giving, regiospecifically, an internal enol acetate and an internal alkyne as the main products (Scheme 13.31) [54]. Tlie former compound had the (Z) configuration. 

Reactions of a-diazocarbonyl compounds,halogen-substituted enolates and sulfur ylides with trialkylboranes were subsequently recorded and represent alternative methods of preparation (Scheme 3). 

The initial complex derived from the reaction of a-methoxyvinyl-lithium with trialkylboranes has been found to be stable at — 80 °C and can be iodinated to yield enol ethers. Warming induced alkyl group migration leading to a new alkenyldialkylmethoxyborate salt which, when oxidized, produced methyl ketones. ... 

The utility of this reaction is further extended by the ability to use either the kinetic [KN(SiMe8)a-BEt3] or thermodynamic (KH-BEta) enolate, which in the case of, for example, 2-methylcyclohexanone gives either a,a - or a,a-substitution respectively. Alternatively, in the presence of a large excess of the trialkylborane, the enolates react with aldehydes to give predominantly threo-p-keto-alcohols. The boroxazine (107), prepared from acetonitrile and (108), is hydrolysed to the 1,3-diketone C6HnC(0)CH(Et)C(0)Me in a reaction which is synthetically equivalent to a mixed Claisen acylation. ... 

Enol borinates show promise as enolate equivalents in stereoselective aldol additions. These compounds can be prepared by the reaction of diazoketones with trialkylboranes. Direct reaction gives the E enol borinate, which can be isomerized... 

The E boron enolates of acyclic ketones can be obtained by reaction of a-diazo ketones with trialkylboranes." They can be isomerized to the Z-isomer by a catalytic amount of a weak base such as phenoxide ion. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

Regiospecific synthesis of Mannich bases. Hooz and Bridson2 have described a new regiospecific3 synthesis of certain Mannich bases which involves first reaction of a trialkylborane with an a-diazoketone in THF to form an enol borinate (l)4 after evolution of nitrogen ceases, dimethyl(methylene)ammonium iodide (2) in DMSO is added. The Mannich base (3) is obtained in 85-100% yield after hydrolytic workup. Cosolvent DMSO is crucial for high yields. 

A trialkylborane. for example (3) resulting from reaction of hexene-1 (1) with diborane (2. generated from NaBH4-l- BFa). undergoes remarkably fast 1,4-addition to acrolein to produce the enol borinate (4). which on hydrolysis gives the aldehyde (5).1 The Brown group found it more convenient to add water to a solution of the... 

A study on stereodefined boryl enolates that employed both pure (Z)- and ( )-dialkylboryl enolates has been carried out by Masamune et al Hooz reaction involving a-diazocarbonyi compounds (Scheme 3) and a trialkylborane provides exclusively (0)-alkenyloxyboranes, which can be isomerized cleanly to the corresponding Z(0)-isomers.- The Z((9)-dibutylboryl enolates (17) react with various aldehydes exhibiting uniformly high syn selectivities (syn.anti > 95 5), while the corresponding (0)-isomers (18) are converted into the anti aldol products with somewhat lower selectivities (Scheme 15). 

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