Alkynes boration

Abstract Transition metal catalyzed alkene and alkyne boration reactions are attractive... 

The ruthenium-catalyzed direct addition of saturated aliphatic alcohols to non-activated alkynes remains a challenge. Only ally alcohol has been successfully involved in the intermolecular addition to phenylacetylene to produce an ether and the enal resulting from Claisen rearrangement (Equation 10.7) [24]. Thus, in refluxing toluene, in the presence of a catalytic amount of RuCl(tris(pyrazolyl) borate) (pyridine)2, a 1 1 mixture of ally P-styryl ether and 2-phenylpent-4-enal was obtained in 72% overall yield. 

The formation of a ruthenium vinylidene is proposed as the key intermediate in the regioselective addition of hydrazine to terminal alkynes [55]. This reaction, which proceeds via addition of the primary amino group of a 1,1-disubstituted hydrazine followed by deamination, provides an unprecedented access to a variety of aromatic and aliphatic nitriles. The tris(pyrazolyl)borate complex RuCl(Tp)(PPh3)2 gave the best catalytic activity in the absence of any chloride abstractor (Scheme 10.17). 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

Bis(phosphoranimine) ligands, chromium complexes, 5, 359 Bis(pinacolato)diboranes activated alkene additions, 10, 731—732 for alkyl group functionalization, 10, 110 alkyne additions, 10, 728 allene additions, 10, 730 carbenoid additions, 10, 733 diazoalkane additions, 10, 733 imine additions, 10, 733 methylenecyclopropane additions, 10, 733 Bisporphyrins, in organometallic synthesis, 1, 71 Bis(pyrazol-l-yl)borane acetyl complexes, with iron, 6, 88 Bis(pyrazolyl)borates, in platinum(II) complexes, 8, 503 Bispyrazolyl-methane rhodium complex, preparation, 7, 185 Bis(pyrazolyl)methanes, in platinum(II) complexes, 8, 503 Bis(3-pyrazolyl)nickel complexes, preparation, 8, 80-81 Bis(2-pyridyl)amines... 

The cA-bis(boryl)alkenes 482 are obtained by bis-boration of terminal alkynes catalysed by a Pt complex [185]. Pd and Rh complexes are inactive. Then the (Z)-1,2-diphenylalkene 483 is prepared by Pd-catalysed Suzuki-Miyaura coupling of 482 with iodobenzene. 

Pinacolborane (PBH, 537) sluggishly hydroborates alkynes and alkenes. Hydro-boration of alkyne with PBH is catalyzed by hydrozirconocene chloride (HCp2ZrCl) [206], CpNi(Ph3P)Cl and Rh(CO)(Ph3P)2Cl [207] at room temperature. Hydrobora-tion of 4-octene with PBH at room temperature gives either terminal or internal boranes 538 or 539 regioselectively, depending on the catalyst used [207]. PBH is more stable than CBH, and easier to handle. 

This section does not review comprehensively all the tetrahydro-borate derivatives, since these have been adequately covered elsewhere (50). The aluminohydride compound Cp2Zr(H)AlH4 has been mentioned, but several similar hydrides have been identified. Such compounds are relevant in particular to known reactions where titanium(IV) or zirconium(IV) compounds catalyze the addition of Al-H bonds to alkenes and alkynes (114, 115). 

Once an alkyne moiety is introduced into a peptide, e.g., as propargyl glycine (Pgl) in place of a simple glycine in Enk, it can of course be used directly for complexation to a metal complex. Along those lines, we have reacted Co2(CO)g with the peptide Ac-Tyr-Gly-Pgl-Phe-Leu-NH2, to yield the Co2(CO)6 complexed to the Pgl alkyne side chain [32]. In a real SPPS scheme with a pre-formed metal-alkyne complex, the amino acid building block [Tp WI(CO)(ri2-Fmoc-Pgl-OH)] was incorporated to yield the side-chain substituted Enk derivative H-Tyr-Gly-[Tp W(I)(CO)(r 2-Pgl)]-Phe-Leu-OH (Tp tris(3,5-dimethylpyrazolyl)-borate) [44]. 

Two methods for the direct conversion of terminal alkynes to alkynyl(phenyl)iodo-nium tetrafluoroborates have been reported, but their generality has not been documented in the literature. The ter -butylethynyl salt has been prepared by the generation of lithium tert-butylethynyl(trifluoro)borate and its coupling with iodosylbenzene by the standard method (equation 29)80. The treatment of 1-pentyne and phenylacetylene with diphenyl-ju-oxodiiodine(III) bistetrafluoroborate in dichloromethane likewise affords the corresponding alkynyliodonium tetrafluoroborates (equations 30)29. 

Selected 1-alkenylboronates synthesized by Equation (56) are shown in Scheme 17. The reaction afforded cyclic and acyclic vinylboronic esters possessing various functional groups, which are not available by conventional hydro-boration of alkynes or by a transmetallation method, for example 160-167. 

Iron(n) salts mediated the coupling of trialkylboranes with KSGN to give RSCN via a radical process.553,554 Stereochemically pure ( )-l-thiocyanato-alkenes or ( )-l-azide-alkenes were obtained from alkynes when hydro-boration with disiamylborane was followed by reaction with potassium thiocyanate or sodium azide in the presence of copper(n) nitrate, copper(n) acetate, and small amount of water in polar aprotic solvent (Equation (116)).555... 

As illustrated in Equation (149), 2-alkyne-substituted THFs could be ring-opened via intramolecular transfer hydrogenation using TpRuPPh3(MeCN)2PE5 (Tp = tris(l-pyrazolyl)borate) as a catalyst to provide dienyl ketones <2004JOC4692>. 

---