Boranes alkenyl halides

On the other hand, it is well known that stereodefined 1-alkyenyldialkylboranes are readily prepared by the monohydroboration of alkynes, i.e. dialkylboranes such as disiamylborane and catecholborane permit the monohydroboration of terminal alkynes, thus making readily available the corresponding (E)-l-alkenyldialkyl-boranes with high stereoselectivity, more than 99% (Eq. 97) . Highly pure (Z)-l-alkenyldialkylboranes (purity, more than 98%) are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamylborane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 98) Consequently, if such 1-alkenyldialkylboranes react with 1-alkenyl halides or 1-alkynyl... 

Trialkylboranes including 9-alkyl-9-BBN and alkyl(disiamyl)borane readily undergo the cross-coupling with 1-alkenyl or aryl halides or triflates [8] (Scheme 20). The reaction is limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN or HB(Sia)2 is the most convenient to furnish the desired boron reagents. Trialkylboranes thus obtained are... 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

Coupling reactions of alkyl boranes, formed by hydroboration of alkenes, with unsaturated halides (or triflates or phosphonates) is possible, and this reaction is finding increasing use in synthesis. For example, coupling of the alkyl borane derived from hydroboration (with 9-borobicyclo[3.3.1]nonane, 9-BBN) of the alkene 200 with the alkenyl iodide 201 gave the substituted cyclopentene 202, used in a synthesis of prostaglandin Ei (1.205). This type of B-alkyl Suzuki coupling reaction is very useful for the synthesis of substituted alkenes. 

Reactions of allenes with halides are terminated by attack of organometallic compounds. The reaction of the 2-(2,3-butadienyl)iodobenzene (457) was terminated with alkenyl borane to give 458 [173]. 3-Methyl-1,2-butadiene (459) reacts with ary iodide, and the reaction is terminated by the attack of arylboronic acid 460 to afford 461 [174]. 

The Suzuki coupling was developed by Professor Akira Suzuki of Hokkaido University. The Suzuki coupling uses a boron compound (R-BYj) and an alkenyl, aryl, or alkynyl halide or triflate (RX) as the carbon sources, with a palladium salt as the catalyst. Bromides and iodides are the most commonly used halides chlorides are less reactive. Alkyl halides can sometimes be used but are subject to elimination. A base is also required. The boron compound can be a borane (R jB), a borate ester (R B(OR)2), or a boric acid (R B(OH)2), where R is alkyl, alkenyl, or aryl. The general reaction is shown in the following scheme, where X is halide or triflate and Y is alkyl, alkoxyl, or OH. A list of the types of components that can be used is given in Table 24.1. This reaction is one of the principal methods now used to prepare biaryls. 

---