Alkenylboronic acid derivatives

A Prelude Preparation of Haloalkenes and Alkenylboronic Acid Derivatives, Important Building Blocks for Palladium-Mediated C,C Couplings Carbocupration of Alkynes... 

In 2006, Ukai et al. proposed an interesting alternative with a rhodium(I)-catalyzed carboxylation of aryl- and alkenylboronic esters proceeding under mild conditions, and leaving ancillary reactive functional groups such as carbonyl- and cyano unreacted [51] (Scheme 5.13). Considering that organoboronic esters are easily available, and that various functional groups are tolerated, this reaction appeared to be particularly useful for the preparation of functionalized arylcarboxylic acids, such as benzoic and cinnamic acid derivatives. 

To avoid the formation of small amounts of the dihydroborated derivatives, it is desirable to use 10% excess of alkyne. Hydrolysis or alcoholysis of the resulting alkenyldichloroborane provides a simple preparation of the desired alkenylboronic acid or esters via hydroboration. 

The vicinal diol cyclic sulfate from dimethyl tartrate undergoes nucleophilic opening to give substituted malate esters., However, for this application diethyl and diisopropyl L-tartrates give superior yields and selectivities. The asymmetric cyclopropana-tion of the 1 -alkenylboronic ester derived from dimethyl L-tartrate (eq 5) is another example where other tartaric acid derivatives surpass the performance of dimethyl tartrate. ... 

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. 

The carbenoid generated from diiodomethane/diethylzinc is often found to be more reactive than the conventional Simmons-Smith reagent. Thus, in the case of 1-alkenylboronic acid esters, where diazomethane failed to cyclopropanate trisubstituted derivatives (Section 1.2.1.1.1.) diiodomethane/diethylzinc gave good yields of the required products 16. ... 

Alkenylboronic acids 58 were esterified with chiral diols [e.g. ( + )-diisopropyl tartrate] and the ester 59 subjected to the original Simmons-Smith procedure to give the cyclopropyl derivatives 60 as the intermediate product. Cyclopropyl derivative 60 was directly oxidized with alkaline 30% hydrogen peroxide to /ranj-cyclopropanols 61 in 73-86% ee. This method gives access to secondary cyclopropanols bearing a vicinal substituent. 

Alkene derivatives such as alkenylboronic acids and alkenylzir-conanes reacht with PhI(OAc)2 to furnish alkenyliodonium salts (eq 46). These transformations proceed with retention of olefin configuration. Similarly, alkenylboron species add to PhI(OAc)2 in the presence of Nal to give vinyl acetate products (eq 47). In these examples, ( )-alkenylboronates give stereochemically pure (Z)-configured enol acetates. ... 

Alkenylboron derivatives are readily available by stereo-defined hydroboration of alkynes and used for the preparation of styrene derivatives by alkenyl-aryl coupling. Vinylboronic acid (81), the most simple alkenylboronic acid, is difficult to handle, because it undergoes uncontrollable polymerization. Its anhydride, 2,4,6-trivinylcyclotriboroxane (82)-pyridine complex, is stable, and can be used conveniently for the coupling. Vinylation of 2-bromoanisole with 82 proceeds in refluxing DME using K2CO3 as a base fo provide 2-vinylanisole (83) [80]. 

R. A. Batey in Boronic acids preparation and applications in organic synthesis and medicine (D. Hall, Ed.). Wiley-VCH Verlag GmhH, Weinheim, Germany, 2005, pp. 279-304. NucleophUic addition reactions of aryl and alkenylboronic acids and their derivatives to imines and iminium ions. 

Compared to aryl- and alkenylboronic adds, alkylboronic acids and esters have found limited use as synthetic intermediates aside for their oxidation into alcohols (Section 1.5.2.1). This is due in part to their inferior shelf-stability. In addition, their trans-metallation with transition metal catalysts such as palladium is presumed to be more difficult than that of the unsaturated and aromatic boronic acid derivatives [296]. For example, alkylboronic adds have long been known to be reluctant substrates in the Suzuki-cross-couphng reaction, and they have become eflfident in this apphcation only very recently with the use of special bases and the advent of new and highly active catalyst systems (Section 1.5.3.1). Perhaps the most synthetically useful class of alkylboronic adds are the a-haloalkyl derivatives popularized by Matteson (Section 1.3.8.4), and their elegant chemistry is described in Chapter 8. 

Nucleophilic Addition Reactions of Aryl and Alkenylboronic Acids and Their Derivatives to Imines and Iminium Ions... 

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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