Boranes alkynyl

Vinyl boranes add to conjugated ketones in the presence of a rhodium catalyst (with high asymmetric induction in the presence of BINAP) 7 Alkynyl-boranes also add to conjugated ketones, in the presence of... 

The doso-carboranes with a C2 B3 skeleton are of particular interest because their structure and bonding have been described as classical and non-classical. Koster et al. [49] reported the preparation of pentaalkyl-1,5-dicarba-doso-pentaboranes by hydroboration of dialkyl(l-alkynyl)boranes with tetraalkyl-diboranes(6) (R2BH)2. By using a large excess of (Et2BH)2 as a hydride bath the route to pentaethyl-1,5 -d ic ar ba- doso-pe n tabo ra n e(5) was found to proceed via the 1-carba-aradwo-pentaborane(lO) [50] (see also Section 3.2.2.4 [33]). 

For instance, attempts at isolating donor-free alkynylboranes have been hampered by spontaneous polymerization. The first donor-free tris(alkynyl)borane, tris(3,3-dimethyl-l-butynyl)borane (30), has only recently been obtained by reaction of deprotonated 3,3-dimethyl-l-butyne with boron trichloride at —78°C in pentane (equation 32). The alkynylborane (30) was fully characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. The NMR signal of (30) shows a remarkable shift of 5 = 48, which is upfield from typical shifts of alkyl-and alkenylboranes. The crystal structure of (30) shows similar C-C bond lengths, but significantly shorter B-C bonds in comparison to those observed for the donor-stabilized complexes (30) D indicative of a small degree of p - p interactions between the sp -boron atom and the sp-hybridized carbon. Weak r-overlap was further confirmed by ab initio calculations. It is this r-interaction that has been exploited for the development of nonlinear optical (NLO) materials based on alkynylboranes as described in Section 7.1. ... 

A ntimber of penta-alkyl-l,5-dicarba-c/oso-pentaboranes(5) (9 R = Me, R = Me, Et, or Pr) have been produced from reactions between the dialkyl(l-alkynyl)boranes RjBC R and the tetra-alkyldiboranes (R2BH)2. Tris(diethyl-boryl)methane, (Et2B)3CH, has also been reported, and this gives, on treatment with triethylaluminium, both 2,3,4-triethyl-l,5-dicarba-c/oso-pentaborane(5) (10) and the adamantane derivative (11). ... 

Good yields of dialkylalkynylboranes, RaBC=CR, are obtained by reaction of RaBOMe with R C=CLi followed by trifluoroborane etherate. w-Alkynyl-boranes can be obtained from its isomers in which the triple bond is in a different position by isomerization with potassium 3-aminopropylamide (KAPA). 1-Bora-adamantane has been synthesized by sequences involving allylboronation and hydroboration reactions, and has been converted into adamantan-l-ol by cyanida-tion or carbonylation. Scheme 12 shows the application of anions derived from... 

In the hydroboration of terminal alkenes, carrying a ketone or aldehyde group, with a variety of borane reagents, dicyclohexylborane has been identified as the most efficient reagent. Analogous hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation. ... 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

The synthetic utility of the alkyne/B-X-9-BBN adducts has also been explored.152 (Z)-l-Alkynyl-2-halo-1-alkenes are the result of lithium alkynide action on these latter boranes (Scheme 76).157 The adducts resulting from the treatment of alkynes with B-X-9-BBN have been shown to be labile under protic conditions. However the decomposition has been harnessed and is an efficient preparation of 2-halo-1 -alkenes.158... 

In 2008, Erker extended this methodology to the hydroboration of internal alkynyl-phosphines. Using Piers borane HBfCf Fs) , the PBs 39b-d featuring E-CH=C(R) linkers were isolated in excellent yields (Scheme 27).51 NMR (XH, 13C... 

Alkynylboration has been achieved in the reaction of alkynyl(pinacol)boranes with alkynes in the presence of nickel catalysts, giving d.v-1 -borylbul-1 -en-3-yne derivatives. 1-Arylalk-l-ynes underwent the alkynylboration regioselectively with the selective introduction of the alkynyl groups at their 1-positions, where the aryl groups were attached.45... 

The CBS reduction has also proven to be an efficient method for asymmetric reduction of a,ft-unsaturated enones14 and ynones15 (Scheme 4.31). The asymmetric reduction of alkynyl ketones affords propargylic alcohols 30 with high levels of enantioselectivity and in moderate to good yields. Optimized reaction conditions for the reduction are the use of THF at — 30° C, 2 equivalents of chiral oxazaborolidine 28b, and 5 equivalents of borane methyl sulfide complex. 

Midland and others reported that B-isopinocampheyl-9-borabicyclo[3.3.l]no-nane [Alpine-Borane (7 )-79] is an effective reagent for the highly asymmetric reduction of alkynyl ketones to afford the propargylic alcohol 8030 (Scheme 4.3z). The reagent (R)-19 is prepared from (+)-a-pinene and 9-borabicyclo[3.3.1]no-nane (9-BBN) and often represented as 19banana. The levels of asymmetric... 

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