Protonolysis

In a typical reaction, two of the alkyl groups of the trialkylborane are removed at ambient temper-ature on treatment with propionic acid. Protonolysis of the third alkyl group requires refluxing in diglyme, although the yields of hydrocarbon derived from the alkyl portion of the horane are excellent. Primary alkyl groups 

Protonolysis of alkenylboranes generates an alkene and this reaction is faster than the analogous protonolysis of alkylboranes.57 Once again, protonolysis proceeds stereospecifically with retention of configuration. In Negishi s synthesis of the sex pheromone of Lobesia botrana (65), hydroboration of 64 was followed by protonolysis with acetic acid at 0°C to give 65, for a net conversion of an alkyne to an alkene.58 As with alkylboranes, protonolysis of vinylboranes with deuteroacetic acid allows the synthesis of deuterated alkenes. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a 

---

The 2-alkylideneindanone 282 is formed by carbopalladation via ring expansion of the alkynylcyclobutenol 280 with palladium trifluoroacetate to yield an intermediate 281 and its protonolysis. 4-Oxygenated 5-alkylidenecyclopente-nones react similarly[139]. 

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

Primary alkyl groups are more reactive than secondary and tertiary. PivaUc acid accelerates the rate of protonolysis of trialkylboranes with water and alcohols (207,208). The reaction can be controlled to give excellent yields of dialkylbotinic acids and esters. 

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

Markovnikov boianes not available by direct hydioboiation can be prepared by protonolysis of alkylethenyl- and alkylalkynylborates (298,299). 

The iacrease ia reactivity of coordinated N2 has been assumed to be associated with iacreased bond length and decreased stretching frequency. A labeling study has shown that this is an oversimplification. In the protonolysis of [Cp 22 (N2)]2 2 hydraziae produced comes equally from terminal and bridging N2. An iatermediate, such as [86165-22-2] was proposed where the bridging and terminal N2 have become equivalent. 

Furthermore, carehil carbonylation of the dimer produces [Cp 22 (CO)]2N which on protonolysis gives no reduced form of N2, iadicatiag that both bridging and terminal N2 are required for reduction (249). 

A similar process has been patented coveriug Cp 2ZrR [86165-24-4] Cp 2ZrR(N2) [86165-25-5] and (Cp 2ZrR)2N2 [86165-23-3] where R = CH[Si(CH3 )3 ]2. Protonolysis of the dinitrogen complexes gives hydraziae and ammonia (250). 

Reactions with zinc or aluminum are typically carried out in hydrocarbon solvents. Many of the methyknetal derivatives undergo protonolysis or oxidation very readily, and must be protected from exposure to air or water. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Simple protonolysis of the vinylalane intermediates produces (Z)-2-alkynylvinylsilanes these species can be readily and cleanly isomerized to the corresponding (E)-isomers. 

Monohydroalumination of terminal propargylsilanes (Chapter 7) with two equivalents of DIBAL proceeds in a stereoselective (9), though not regio-selective, manner to produce an approximately 1 1 mixture of the alkenyl-alanes (1) and (2). Protonolysis of the mixture yields the pure (Z)-allylsilanc... 

Protonolysis of the f/ -styrene zirconium complex Cp Zr(t/ -PhCHCH2) [MeC(NPr )2] with 2 equivalents of Bu NH2 provided a high yield of a novel bis(amido) complex according to Scheme 93. ... 

In another procedure, the addition of a dialkyIborane to a 1-haloalkyne produces an a-halo vinylic borane (82). Treatment of this with NaOMe gives the rearrangement shown, and protonolysis of the product... 

Protonolysis of the allyl group with HCl in ether proceeds smoothly to liberate propene and deliver the PdCl2-carbene dimers 51a-b in quantitative yield and excellent purity. The (R,R) 51a complex was analyzed by X-ray crystallography. 

The initial rate of the model reaction follows a first-order dependence for the activated catalyst, the Michael donor, and the Michael acceptor. The rate determining step is not the C-C bond formation or protonolysis but the decomplexation of the bidentate product. This was evidenced by the relationship between the initial conversion and the reaction time. Extrapolation to fg = 0 h provides a positive intercept. In other words, upon addition of the reagents, the C-C bond formation occurs almost instantaneously. The amount of product at fo correlates within the experimental error to the double precatalyst loading since the dimeric precatalyst forms two active monomeric catalyst species. 

Bourke SC, MacLachlan MJ, Lough AJ, Manners I (2005) Ring-opening protonolysis of Sila... 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

---