Alkynyl hydroboration

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]). 

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. ... 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

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... 

The hydroboration of allylic silanes proceeds with high diastereoselectivity as demonstrated by Fleming and Lawrence.87 It is difficult to use the newly formed carbon-boron bond for making new carbon-carbon bonds due to its moderate reactivity. However, the B/Zn exchange converts the unreactive carbon-boron bond to a reactive carbon-zinc bond, as in compound 24. A further transmetallation with the THF soluble salt CuCN-2LiCl provides copper reagents, which can be allylated, alkynylated, or acylated (Scheme 6). 

Instead of adding two hydrogen atoms to an alkynyl silane we could add H and SiMe3 to a simple alkyne by hydrosilylation (addition of hydrogen and silicon). This is a cis addition process catalysed by transition metals and leads to a tram (E-) vinyl silane. One of the best catalysts is chloroplatinic acid (H2PtCl6) as in this formation of the E-vinyl silane from phenylacetylene. In this case photochemical isomerization to the Z-isomer makes both available. Other than the need for catalysis, this reaction should remind you of the hydroboration reactions earlier in the chapter. The silicon atom is the electrophilic end of the Si-H bond and is transferred to the less substituted end of the alkyne. 

A similar example is seen in the [Pd2(dba)3]-catalyzed hydroboration of 2-methyl-l-buten-3-ynes [274]. While PPhj and PPh2(CgF5) favor the 1,4-addition product allenylborane 100 all diphosphines yield the 1,2-addition product ( )-dienylborane 102 exclusively (Table 1-13). This remarkable difference in selectivity is explained based on an 1,3-enyne monophosphine complex 103 and an alkynyl diphosphine complex 104 as intermediates. Dppf exhibits the best product yield among the phosphines tested. Similar observation was noted in the asymmetric hydroboration (Scheme 1-44) [275]. The action of catecholborane on 1-phenyl-1,3-butadiene also proceeds regioselectively to give, after oxidation, anti-l-phenyl-l,3-butanediol... 

Cross-coupling reactions between 1-alkynyl halides and 1-alkenylboranes, which are readily available via hydroboration of alkynes, can also be catalyzed by Pd-phosphine complexes. Stereoselective synthesis of conjugated ( )-enynes 143 was achieved by the coupling of the alkynyl bromide with an alkenylborane [Eq. (48)] [69]. 

While the highly versatile rare-earth metal-catalyzed hydrogenation, hydroboration, hydrosilylation, hydroamination, and hydrophosphination has been studied over the last two decades, the rare-earth metal-mediated activation of the hydroxyl 0-H bond for an analogous hydroalkoxylation process [87,190] has been reported only recently. Homoleptic trisamides were reported to catalyze the smooth cyclization of allenyl alcohols [191] and alkynyl alcohols [191,192] to form the corresponding unsaturated ethers. 

If redistribution proceeds through the formation of three-center, two-electron bonds (Scheme 1), alkenyl, aryl, alkynyl, allyl, benzyl and propargyl groups should participate more readily than the alkyl group, but this point is not established. Catalysis by boron hydrides is not applicable where alkenyl or alkynyl groups are present, because the boron hydrides are consumed through hydroboration. 

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