Alkenyl from alkenylboronic acids

These boronic esters are easily hydrolyzed to give /ra .v-alkenylboronic acids with complete retention of their stereochemistry (C in Figure 16.15). Alkenylboronic esters and alkenylboronic acids are organometallic compounds that can be alkenylated and arylated in Pd-catalyzed reactions (Section 16.4.2). Aside from this, the iraws-alkenylboronic acid esters as well as the /rau.v-alkenylboronic acids are valuable precursors of haloalkenes (Figure 16.15). 

Alkenyl, aryl, and heteroaryl boronic acids reacted with 1,2-diamines and glyoxylic acid giving directly piperazi-nones (Equation (123)).571 2-Hydroxymorpholines were synthesized from glyoxal, 2-aminoethanols, and aryl- or 1-alkenylboronic acids (Equation (124)).572... 

Alkenyl boronic acids synthesized from internal alkynes fail to undergo the reaction with iodine and base. However, alkenylboronic acids and esters obtained from internal alkynes react with bromine in the presence of base to afford in excellent yields the corresponding alkenyl bromides (Eq. 112)174),... 

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

Alkenylboronic acids can be synthesized from reactive alkenylmetal species in a way similar to that described above for arylboronic acids (Section 1.3.1.1.1) [224]. Typically, alkenyl bromides or iodides are treated sequentially with n-Buli and a trialkylbo-rate (entry 1, Table 1.4). A nonpolar trienylboronic acid was synthesized using this approach [226]. As described in Section 1.2.2.2, small boronic adds tend to be highly soluble in water and may be difficult to isolate when made using the traditional approach involving an aqueous workup. In these cases, exemplified with the polymerization-prone ethyleneboronic acid synthesized from vinyknagnesium bromide, it has proved more convenient to isolate the product as a dibutyl ester by extraction of the addic aqueous phase with butanol [227]. Recently, aUcoxy-functionaHzed butadi-enyl- and styrenyl boronic esters were synthesized from a,P-unsaturated acetals by treatment with Schlosser s base and subsequent trapping with triisopropylborate (entry 2) [228]. 

The first example of an addition reaction of an C(sp -B based organoboronic add to an iminium ion was reported by Petasis and Akritopoulou in 1993 [24]. They demonstrated the addition of ( )-alkenylboronic acids to preformed iminium ions derived from secondary amines and formaldehyde, to generate allylic amines 1 (Scheme 7.1). Typically, a two-stage process was employed for the formation of 1. Initially, a secondary amine (including, dialkyl, acyclic and cydic examples) was heated with paraformaldehyde in dioxane or toluene solvent at 90 °C for 10 min. An ( )-alkenyl-boronic acid was then added, and the solution either stirred at 90 °C for 10 min, or at room temperature for 3 h. The allylic amine product 1 was then isolated through a standard aqueous work-up (sequential treatment with aqueous HCl and NaOH). The synthetic utility of this chemistry was demonstrated in this first report by a synthesis of the oral antifungal agent naftifine (2). 

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