Alkynes protonolysis

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne. This avoids the complications which occur with borane that lead to polymeric structures. Catecholborane is a particularly usefiil reagent for hydroboration of alkynes. Protonolysis of the adduct with acetic acid... 

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). 

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. 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

Derivatization of some mono(cyclopentadienyl) complexes to yield new monosubstituted species can often be accomplished by metathetical exchange (Equation (26)) or protonation reaction.295 Protonolysis of (CsPr 4H)Ca[N(SiMe3)2](THF) with several terminal alkynes HC CR in either toluene or hexanes produces the... 

Like alkynes, a variety of mechanistic motifs are available for the transition metal-mediated etherification of alkenes. These reactions are typically initiated by the attack of an oxygen nucleophile onto an 72-metalloalkene that leads to the formation of a metal species. As described in the preceding section, the G-O bond formation event can be accompanied by a wide range of termination processes, such as fl-H elimination, carbonylation, insertion into another 7r-bond, protonolysis, or reductive elimination, thus giving rise to various ether linkages. 

Regio- and stereoselective addition of 9-(phenylthio)-9-BBN to terminal alkynes is catalyzed by Pd(PPh3)4 to produce 9-[(Z)-/ -(phenylthio)alkyenyl]-9-BBN (Scheme 73) 283 Addition of styrene avoids catalyst deactivation by trapping free thiophenol generated in the reaction mixture. The produced alkenylboranes exhibit high reactivities for protonolysis with MeOH to produce 2-phenylthio-l-alkenes. 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

Isonitrile insertion into zirconacycles to afford iminoacyl complexes 28 is fast, but rearrangement to q2-imine complexes 30 is slow. In the case of tBuNC, the rearrangement does not occur. Amines 32 are formed on protonolysis of the q2-imine complex. The q2-imine complexes 30 readily undergo insertion of Ti-components (alkenes, alkynes, ketones, aldehydes, imines, isocyanates) to provide a wide variety of products 37 via zirconacycles 36. The overall sequence gives a nice demonstration of how a number of compo-... 

The reaction of nBu2ZrCp2 with 2 equivalents of PhC CPh provides the novel bicyclic gem-dizirconium complex 140 [236] (Scheme 7.42). Protonolysis of complex 140 with 3 n HC1 gives bibenzyl in 88% yield, while its deuterolysis with D20 provides tetradeuterio-bibenzyl 141 with 92 % deuterium incorporation. The dual path nature (142 versus 140) of the reaction of Cp2Zr with alkynes is an important factor in designing Zr-promoted cyclizations of alkynes, enynes, and diynes. 

Trisubstituted alkenes. The (Z)-2-bromo-l-alkenylboranes (1), obtained by bromoboration of 1-alkynes with BBr, (13, 43), undergo coupling with organozinc chlorides in the presence of Cl2Pd[P(C6H5)3]2 to provide, after protonolysis, disub-stituted alkenes (3). However, the intermediate alkenylborane (2) can undergo a... 

Protonolysis of these allenylboranes can be effected by addition of water or acetic acid. In the former case the reaction occurs by an Se2 pathway affording alkynes in high yield (Table 9.9). In contrast, acetic acid effects protonolysis without isomerization to yield the corresponding allenes, also in high yield (Table 9.10). 

These trends were further confirmed through reactions of the foregoing chiral carbonate and phosphate derivatives with other electrophiles (Eqs. 9.32-9.34) [39]. For example, on protonolysis or deuterolysis, the allenyltitanium intermediate derived from the tertiary carbonate of Eq. 9.32 afforded an alkyne of 90% enantiopurity. Based on the configuration of this product and the assumption of a syn elimination to form the allenyltitanium, the protonolysis was suggested to take place by a syn SE2 pathway. In contrast, chlorination of this allenyltitanium intermediate follows an anti pathway (Eq. 9.33). 

Terminal alkynes also undergo HI addition in good yields upon reaction with h and dehydrated alumina (equation 149),189 or by iodoboron addition followed by protonolysis (equation 150).174 These methods appear quite useful. 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

Steps a-c (hydroboration-protonolysis-oxidation) represent a method for the selective reduction of a terminal alkyne in the presence of an alkene. 

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

Yamamoto et al. have reported that reaction of internal alkynes with o-bromo-benzaldehydes [5] or analogous ketones [6] under different reaction conditions affords indenols (Scheme 4). It is believed the mechanism is similar to that of Scheme 3, but using path A, except that the protonolysis of intermediate 9 in Scheme 3 occurs, rather than /3-hydride elimination, to form indenols. Therefore, no C-H bond transformation is involved in this indenol process. 

Protonolysis of alkenylboranes by carboxylic acids occurs readily. The stereochemistry of the alkenyl group is retained during the reaction and so hydroboration/protolytic cleavage of alkynes leads to cis alkenes. Deuterated... 

Hydroalumination of 1-alkynes generally proceeds without the aid of transition metal catalysts. However, the reaction sometimes suffers from undesirable side-processes, including the formation of alk-l-ynylalanes or protonolysis of the intermediate alk-l-enylalanes. In particular, these side-products increase significantly in the reaction of... 

Complex 3c, a catalytic precursor for addition reactions to alkynes (65), reacts at room temperature with a variety of terminal alkynes in alcohols to produce stable alkoxyl alkyl carbene ruthenium(II) derivatives 109 in good yields (Scheme 7). Reaction of 3c (L = PMe3), with trimethylsilyacetylene in methanol gives the carbene ruthenium complex 110, by protonolysis of the C—Si bond, whereas with 4-hydroxy-l-butyne in methanol the cyclic carbene complex 111 is obtained (65,66). 

In the previous section we have discussed that 1-halo-1-alkenes can be conveniently synthesized. However, with these methods it is not possible to synthesize 2-halo-l-alkenes. Although the halometallation reaction would be a powerful tool for the preparation of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose 183). Recently, it has been reported that B-bromo-9-borabicyclo[3.3.1]-nonane (B-Br-9-BBN) and B-iodo-9-borabicvclo[3.3.1] nonane (B-I-9-BBN)1841 react with 1-alkynes, stereo-, regio- and chemoselectively, and after protonolysis, 2-halo-1-alkenes are obtained in excellent yields (Eq. 118)1851. 

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