Alkynylboronates, addition

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

In addition to their use as precursors for alkynylboron, tin or zinc compounds, alkynylmagnesium reagents show a moderate reactivity toward the coupling with haloarenes and haloalkenes. They are often commercially available, or easy to prepare. Their main drawback is their low chemoselectivity and high nucleophilicity, which implies incompatibilities with functional groups such as nitro and carbonyl. 

Addition reactions, alkenyl- and alkynylboron compounds, 9, 178 Additives, silicones in, 3, 678... 

The radical addition to 1-alkenyl or 1-alkynylboronic esters or amides took place extremely smoothly because the boron atom stabilizes the resulting ct-radical intermediates. Bu3SnH and PhSH predominated the trans-addition products 287 in the addition to 1-alkynylboronic amides at 90 °G, whereas Bu3SnH and Ph2Ph produced the m-addition products 288 at 0°C (>98% Equation (82)).455 Intramolecular addition to 1-alkenylboronic esters has been demonstrated in boron-tethered radical cyclization that provided 1,3- or 1,4-alkanediols 290 via oxidative workup (Equation (83)).456 Inter- and intramolecular additions of alkyl radical457 and sulfonyl radical458 have also been studied. 

Boron Conjugate addition of alkenylboronic acids PhCH=CHB(OH)2 and alkynylboronic esters to ArCH=CHCOMe (Ar = unprotected indoles), carried... 

The cyclic dialkoxyborane 4,4,6-trimethyl-l,3,2-dioxaborinane (HBhg), the vinylboronic ester 2-ethenyl-4,4,6-trimethyl-l,3,2-dioxaborinane (vinylBhg) and the alkynylboronic ester 2-ethynyl-4,4,6-trimethyl-l,3,2-dioxaborinane were first reported in 1966-67 by Woods and coworkers. The authors stressed the high stability of HBhg towards disproportionation, as compared to the parents 1,3,2-dioxaborinane and 1,3,2-dioxaborolane, and its potential for the hydroboration of alkynes. In addition, the first reported o-metal-boron bond (rhodium) was prepared from HBhg. ... 

A novel class of quinone boronic esters has been prepared via a Dotz annulation of Fischer carbene complexes with alkynylboronates. The origin of the high regioselec-tivity is discussed in terms of steric and electronic effects. Additionally, these compounds undergo Pd-catalyzed coupling reactions to give substituted quinones and hy-droquinones (Scheme 9.49) [102]. 

Addition of diazo compounds to metallic complexes allows the formation of metal carbenoid species which can react with unsaturated molecules to form new carbon-carbon bonds. The Cp RuCl(cod)-catalyzed addition of diazo compoimds to alkynes led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoaUcane and one molecule of alkyne [115,116] [Eqs. (53) and (54)]. The ruthenium carbene, generated from diazo compound, reacts with the C=C bond to produce vinylcarbene intermediate able to add a second molecule of diazo compotmd to generate dienes. The stereoselective formation of these conjugated dienes results from the selective creation of two C=C bonds, probably due to the possibility for (C5Me5)RuCl moiety to accomodate two cis carbene ligands. This reaction occurred with terminal or internal alkynes as well as 1,3-diynes [115] and was applied successfully to alkynylboronates [116]. 

The formal hydrochlorination of alkynylboronates can be achieved through the use of Cp ZrCHICl followed by the addition of NCS (Scheme 6.44) [84], The chemistry was regioselective for the formation of the -isomer, and the yields of the chlorinated vinylboronates were fair to moderate. The incorporation of the chlorine was valuable since it provided a synthetic handle for further functionalization. A variation of this approach was used in the stereoselective hydroboration of terminal alkynes (Scheme 6.45) [85]. 

As described earlier, a variety of transition metal catalysts have been used to promote the addition of B-H bonds across alkynes. The development of metal-free approaches has been challenging to design due to the unique reactivity profile afforded by transition metal catalysts. To this end, a metal-free catalyst system has been devised for the addition of pinacolborane to internal alkynes (Scheme 6.48) [91]. Surprisingly, only a carboxylic acid additive was needed to effect the hydroboration reaction. In addition to common alkynes, an alkynylboronate was successfully borylated using the carboxylic acid catalyst. The functional group tolerance was remarkably broad, and a host of alkynes were successfully functionalized. Given the wide substrate scope, availability of the precursors and catalysts, and the metal-free nature of the chemistry, this is a reasonable approach to selective hydroboration reactions. 

The synthesis of alkynyltrifluoroborate salts begins with the generation of lithium acetylides at low temperature followed the addition of a trialkylborate (Scheme 6.53) [104]. Once the intermediate alkynylboronate was generated, aqueous KHF was added to generate the target compounds. The resulting alkynyltrifluoroborates were isolated in moderate to excellent yields. Both alkyl- and arylalkynes were well tolerated by this chemistry. 

Initial treatment of an alkynyl Grignard reagent with trimethylborate followed by the addition of MIDA generated the MIDA alkynylboronates [106]... 

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