Reactions of Alkynes and Allenes

Trost and coworkers [131] synthesized oxaheterocycles by a Pd-catalyzed addition of terminal alkynes onto hydroxyalkynoates, followed by an intramolecular addition of the hydroxyl functionality on the triple bond. Simple lactonization may take place as a side reaction. 

Some single examples of this type of reaction were discussed in earlier sections, but due to the importance of these transformations, an additional overview will be provided here. One of the first transformations based on this strategy was published by Inoue and coworkers [134] using propargylic alkoxide, an arylhalide or vinyl bromide and C02 to give cyclic carbonates. The Balme group used this ap- 

Allylic chlorides can also be used as coupling partners, as shown by Lu and Liu [137]. In addition, the domino process could be used for the synthesis of benzylidene pyrrolidines, as shown for the reaction of 6/1-285 with benzylidene malonate 

Furthermore, the Wu group [141] has prepared isoquinolines and iso-indoles starting from 2-(2-phenylethynyl)-benzonitrile, while Yamamoto and coworkers [142] prepared benzopyrans from alkynylbenzaldehydes, the Cacchi group [143] synthesized benzofurans, and Balme and coworkers [144] furo[2,3-fo]pyridones. 

Mouriho and coworkers have shown, in a latest publication, an impressive combination of a palladium-catalyzed 6-exo-cyclocarbopalladation and a Negishi cou- 

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The cationic gold complex with CAAC ligand 29 can also catalyze the unprecedented hydroamination reaction of alkynes and allenes using ammonia [59, 60]. It was also demonstrated that it can catalyze the simple hydroamination reaction. It... 

Similar to the alkenes, the carboamination reactions of alkynes" " and allenes" are also weU known in the literature. However, in aU cases, nonstereoselective products were obtained as a sp carbon of the alkyne was converted into a sp carbon after the carboamination step hence, a new chiral center could not be generated. 

When the transition-metal-catalyzed hydrothiolation of unsaturated compounds is performed in the presence of carbon monoxide, carbonylation reactions may proceed with the introduction of sulfanyl groups. In fact, a series of carbonylative thiolation reactions of alkynes and allenes are reported. These reactions provide useful tools to synthetically important organosulfur compounds [104, 105]. For example, the rhodium-catalyzed reaction of alkynes with thiols and CO provides the corresponding thioformylation products regioselectively [106, 107]. Switching the catalyst from rhodium complex to platinum complex leads to a sharp reversal of regioselectivity of CO introduction [108, 109] (Scheme 32). 

Group 4 metal based catalysts have been studied intensively in hydroamination reactions involving alkynes and allenes [77 81], but (achiral) hydroamination reac tions involving aminoalkenes were only recently reported [82 84]. The reactivity of these catalysts is significantly lower than that of rare earth, alkali, and alkaline earth metal based catalysts. In most instances, gem dialkyl activation [37] of the aminoalk ene substrate is required for catalytic turnover. 

The metallocene complex 27 containing a M=X double bond undergoes overall [2 + 2] cycloaddition with an internal alkynes to give heterometallacyclobutenes (28) [77], A formal [2 + 2] cycloaddition of CpjZr (=N Bu)(thf) with imine affords a 2,4-diazametallacyclobutane, whose further reaction with imines results in an imine metathesis reaction [78] azametallacyclobutene is an intermediate in the Cp2Zr(NHR)2-assisted hydroamination of alkynes and allene [79],... 

The first systematic B3LYP DPT investigation into the hydroformylation of alkynes and allene was carried out by Huo et al. [92,93]. It was fotmd that acetylene hydroformylation has very similar catalytic mechanism as that of propene, therefore only the differences are emphasized, and no general comparison is made. Scheme 17 shows the activation and reaction free energies. 

The mechanism of the hydroaminations of alkynes and allenes catalyzed by neutral group 4 metal complexes has been studied in detail, and this reaction has been shown to occur through metal-imido complexes. The basic steps of this mechanism are shown in Scheme 16.20. These reactions are initiated with either a bis(amido) complex, as... 

Several examples of Pd°-catalyzed carboamination reactions between allenes and aryl or alkenyl halides have been reported [50[. For example, treatment of allene 52 with iodobenzene in the presence of K2CO3 and 2mol% Pd(PPh3)4 afforded pyrrolidine 53 in 78% yield (Eq. (1.25)) [50a]. Mechanisms involving aikene amino-palladation (similar to the reactions of alkynes and alkenes noted above) have occasionally been invoked to explain these reactions. However, in many instances these transformations may involve intermediate Ji-allylpalladium complexes. Due to this mechanistic ambiguity, these transformations have been included in this section for comparison with the related reactions of alkenes and alkynes. Similar reactions involving allylic halides have also been described (Eq. (1-26)) [51]. 

Frequently seen are Parham cyclizations onto alkene side chains. For example, bromoalkene 39 undergoes cyclization and electrophilic trapping to afford 40. A series of allyl 2-lithioaryl ethers undergo a tandem Parham cyclization-y-elimination to afford 2-cyclopropylphenols, e.g., 41- 42. Intramolecular carbolithiation reactions of alkenes have led to 2-azabenzonorbomanes and tetrahydroisoquinolines. Similarly, carbolithia-tions of alkyne and allene side chains have been reported. Thus, both 43 and 45 undergo iodine-lithium exchange and cyclization to provide benzofuran 44 and fiiropyridine 46, respectively. 

Several examples of the transition-metal-catalyzed carbonylative thiolation of alkynes and allenes with carbon monoxide were reported recently. While thioformylation products (R(PhS)C=CCHO) are obtained regioselectively by the RhH(CO)(PPh3)3-catalyzed reaction of alkynes (RC=CH) with PhSH and the use of Pt(PPh3)4 as the catalyst... 

Ni(COD)2-catalyzed tandem carbostannylation of alkynes and allenes with alky-nylstannes in the presence of a phosphine ligand results in highly chemoselective synthesis of dienyne stannanes. The stereoselectivity of the reaction depends strongly on the chelation nature of the ligands [237]. For example, when tris(4-tri-fluoromethylphenyl)phosphine is used, the D/E ratio is 98 2. On the other hand, when a bidentate ligand (e.g. [o-(dimethylamino)phenyl)diphenylphosphine] is employed, the D/E ratio becomes 6 94. 

The intermolecular hydroamination reactions of alkynes and alkenes occur with Markovnikov or anti-Markovnikov selectivity. The nucleophilic addition to aUenes occurs at terminal carbon of allenes not at central one. 

Abstract This chapter deals with the transition-metal-catalyzed hydrothiolation and hydroselenation of alkynes and allenes and related imsaturated compounds with thiols and selenols. In these reactions, the regio- and/or stereoselectivities of the addition products can be controlled by switching the transition metal catalysts. Metal sulfides and selenides (RE-ML , E = S, Se, M = Ni, Pd, Rh, Zr, Sm, etc.) play an important role as key catalyst species in these hydrothiolation and hydroselenation. The introduction of carbon monoxide into these hydrothiolation and hydroselenation systems leads to novel carbonylation with simultaneous addition of thio and seleno groups to unsaturated bonds. 

Benzeneselenol as a representative selenol is a colorless liquid of greater acidity than benzenethiol (p a = 5.9 (PhSeH) 6.5 (PhSH)). The bond energy of Se-H is 73 kcal/mol, is smaller than S-H (87 kcal/mol) [82]. These properties may contribute to the efficiency in the oxidative addition of selenols to low-valent transition metals, ligand-exchange reaction between high-valent transition metal complexes and selenols, and protonation process of carbon-metal bonds. Indeed, several transition metal complexes catalyze the highly selective hydrothiolation of alkynes and allenes. 

Nonstabilized carbonyl ylides (41) prepared by reaction of a-iodosilyl ethers with Smlj, can be trapped with various alkenes, alkynes and allenes to form furans of type 42, 43, and 44... 

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

As seen in Scheme 55, with the simple addition of a balloon of CO to the [5 + 2]-reaction of VCPs and alkynes or allenes, the [5 + 2]-reaction can be diverted via a nine-membered metallacycle to produce initially an eight-membered ring product. This Rh(l)-catalyzed three-component [5 + 2 + l]-reaction is mechanistically similar to the previously mentioned [6 + 2]-reaction (Section 10.13.2.5.2) differing only in regard to the insertion of CO in the pathway versus its incorporation in starting material (Scheme 56). 

In addition to alkenes and alkynes, allenes have attracted considerable interest due to their unique reactivity and multireaction sites. Therefore, transition-metal-catalyzed nucleophilic addition reaction of amines and imines to allenes has been extensively studied to prepare biologically important amines and nitrogen-heterocycles.31,31d... 

The isomerization reactions of alkynes via [2,3]- or [3,3]-sigmatropic rearrangements are probably among the most important methods available for the introduction of the allene group into an organic molecule. The usefulness of this synthetic method [61, 62] and the access to new functionalized allenes [63] have been reviewed recently. 

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