Phosphirenium-borate zwitterion: formation in the 1,1-carboboration reaction of phosphinylalkynes

Reaction of the acetylene Mes(2)P-C≡C-Ar with B(C(6)F(5))(3) at rt gives a zwitterionic phosphirenium product, which reacts further at >100 °C to complete the 1,1-carboboration reaction.     

Rhodium-catalyzed carbothiolation reaction of 1-alkylthio-1-alkynes

A rhodium complex derived from RhH(PPh₃)₄ and Me₂PhP catalyzed the carbothiolation reaction of 1-alkylthio-1-alkynes and 1,4-diaryl-1,3-butadiynes giving (Z)-4-alkylthio-4-aryl-3-arylethynyl-3-buten-1-ynes. Terminal alkynes such as 1-decyne and (t-butylthio)acetylene underwent the carbothiolation reaction using a RhH(PPh₃)₄-dppb catalyst. The reactions proceeded via cis-addition with C-C bond formation at the less hindered acetylene carbon.     

Hydrostannation of 1-stannyl-1-alkynes,a reaction leading to novel vinyl anion precursors

Hydrostannation of 1-stannyl-1-alkynes leads to the formation of 1,1-distannyl- and 1,3-distannyl-1-alkenes while the reactions of 1-silyl-1-alkynes with a tin hydride are less regiospecific. The 1,1-distannyl-1-alkenes give α-stannylvinyl anions on treatment with methyllithium     

Novel cyclization reaction of 1,omega-diiodo-1-alkynes without the loss of iodine atoms

In the presence of 1-hexynyllithium (0.2-0.6 equiv.), 1,omega-diiodo-1-alkynes undergo a new type of cyclization reaction without the loss of two iodine atoms to afford (diiodomethylene)cycloalkanes.     

P-C bond activation chemistry: evidence for 1,1-carboboration reactions proceeding with phosphorus-carbon bond cleavage

A series of diarylphosphinyl-substituted acetylenes of the type (aryl)(2)P-C≡C-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with the strongly Lewis acid reagent B(C(6)F(5))(3) in toluene at elevated temperatures (70-105 °C) to give the 1,1-carboboration products 4. Treatment of bis(diphenylphosphinyl)acetylene with B(C(6)F(5))(3) under analogous conditions proceeded with phosphinyl migration to yield the 1,1-carboboration product 4d, bearing a geminal pair of Ph(2)P substituents at one former acetylene carbon atom and a C(6)F(5) substituent and the remaining -B(C(6)F(5))(2) group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic substitution reaction by means of Ph(2)P attack on the adjacent C(6)F(5) ring to yield the zwitterionic phospha-indene derivative 7. The compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.     

Rhodium-catalyzed intermolecular hydroacylation of 1-alkynes: Effect of phosphines and MK-10 on the reaction selectivity

The use of bulky ligands in the rhodium-catalyzed reaction of aldehydes 7 (R¹ = Ph) and 18 with 1-octyne increased the selectivity for ketones 13 and 20, to the detriment of ketones 12 and 19. Bulky phosphines reduced the hydroacylation reaction rate, leading to competition from the addition of the benzoic acid co-catalyst to the alkynes. This competing reaction can be suppressed by using the clay Montmorillonite K 10 (MK-10) as the co-catalyst instead of benzoic acid.     

How to turn the catalytic asymmetric hydroboration reaction of vinylarenes into a recyclable process

Optically pure rhodium(I) complexes [Rh(cod)(Lbond;L)]X (cod=cyclooctadiene; L-L= (R)-2,2'-bis(diphenylphosphino)1-1'-binaphthyl ((R)-BINAP), (S,S)-2,4-bis(diphenylphosphino)pentane ((S,S)-BDPP), 2-diphenylphosphino-1-(1'-isoquinolyl)naphthalene ((S)-QUINAP); X=BF(4), PF(6), SO(3)CF(3), BPh(4)) were immobilised onto smectite clays such as montmorillonite K-10 (MK-10) and bentonite (Na(+)-M). (19)F, (31)P and (11)B NMR experiments recorded in CDCl(3) during the impregnation process provided evidence that montmorillonite K-10 may immobilise ionic metal complexes throughout the cationic and anionic counterparts. However, when bentonite was used as the solid, only the cationic metal complex was immobilised through cationic exchange while the counteranion remained in solution. When we used these preformed catalytic systems in the hydroboration of prochiral vinylarenes, we obtained high activities and enantiomeric excess with (S)-1-(2-diphenylphosphino-1-naphthyl)isoquinoline-modified rhodium complexes. These activities and selectivities are competitive with the homogeneous counterparts. The significant features of this method are the simple separation and good retention of the active metal in the solid, which allows efficient recycling even on exposure to air.     

Carbocyclization reaction of omega-iodo- and 1,omega-diiodo-1-alkynes without the loss of iodine atoms through a carbenoid-chain process

Atom-economical carbocyclization reactions of omega-iodo-1-alkynes and 1,omega-diiodo-1-alkynes to give products with incorporation of iodine atoms is described. Cycloisomerization of 2-(2-propynyloxy)ethyl iodides is initiated by a catalytic amount of LDA to give 3-(iodomethylene)tetrahydrofurans in high yields. Upon treatment of with a catalytic amount of 1-hexynyllithium, 1,omega-diiodo-1-alkynes efficiently undergo cycloisomerization to give (diiodomethylene)cycloalkanes. The diiodomethylene products are also obtained by iodine atom-transfer-type cyclization of omega-iodo-1-alkynes, using 1-iodo-1-hexyne as an external iodine atom source. Bromine atom-transfer and proton-transfer cyclization proceed as well by employing 1-bromo-1-octyne and 1-octyne, respectively. These reactions are proposed to proceed through a carbenoid-chain process involving exo-cyclization of the lithium acetylide intermediates to give Li,I-alkylidene carbenoids. It is shown that the exo-cyclization proceeded stereospecifically through inversion of the stereochemistry at the electrophilic carbon.     

Indium-catalyzed addition of active methylene compounds to 1-alkynes

An active methylene compound adds to a 1-alkyne in high to quantitative yield upon heating in the presence of 0.05-5 mol % of In(OTf)3 to give an alpha-alkenylated carbonyl compound.     

Alkynylhalocarbenes. 2. Generation of (alkyn-1-yl)-chlorocarbenes by basic solvolysis of 1,1-dichloro-2-alkynes and their reaction with olefins: Synthesis of 1-chloro-1-alkynylcyclopropanes

1,1-Dichloro-2-alkynes R¹CCCHCl₂ (4a–g; R¹=Me, n-Pr, c-Pr, t-Bu, Ad, Nor, Ph) were synthesized with yields of 50–75% by chlorination with PCl₅ of formylacetylenes (3a–g), prepared by oxidation of propargyl alcohols (1a–d) with CrO₃·Py·HCl complex or acidolysis of propargyl acetals (2a–c) in the presence of catalytic quantities of pyridine; the corresponding alkynylchlorocarbenes, R¹CCCCl (5a–g) were generated from them with powdered KOH in a two-phase system or t-BuOK. The latter were trapped by olefins with formation of 1-chloro-1-alkynylcyclopropanes (6a–t) with yields of up to 90%.See [1] for Communication 1.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1128–1135, May, 1992.     

Stereoselective cross-coupling reaction of 1,1-Diboryl-1-alkenes with electrophiles: a highly stereocontrolled approach to 1,1,2-triaryl-1-alkenes

Palladium-catalyzed cross-coupling reaction of 1,1-diboryl-1-alkenes with aryl and alkenyl iodides was found to proceed stereoselectively, giving rise to the corresponding mono-coupled product as a single diastereomer with E-configuration. Second coupling of the initial product with another aryl iodide affords diverse triarylalkenes in their stereochemically pure form. This highly stereoselective approach for triarylalkenes allows one to synthesize both diastereomers in one pot from 1,1-diboryl-1-alkenes.     

Protection of primary and secondary amines in the hydroboration reaction

Trimethylsilyl groups are efficient to protect the NH bonds of olefinic amines in the hydroboration reaction with BMS; deprotection is easily run with methanol and affords aminoorganoboranes which can be used without any oxydation.     

9-Borabicyclo[3.3.2]decanes and the asymmetric hydroboration of 1,1-disubstituted alkenes

The syntheses of the optically pure asymmetric hydroborating agents 1 (a, R = Ph; b, R = TMS) in both enantiomeric forms are reported. These reagents are effective for the hydroboration of cis-, trans- and trisubstituted alkenes. More significantly, they exhibit unprecedented levels of selectivity in the asymmetric hydroboration of 1,1-disubstituted alkenes (28-92% ee), a previously unanswered challenge in the nearly 50 year history of this reagent-controlled process. For example, the hydroboration of alpha-methylstyrene with 1a produces the corresponding alcohol 6f in 78% ee (cf., Ipc2BH, 5% ee). Suzuki coupling of the intermediate adducts 5 produces the nonracemic products 7 very effectively (50-84%) without loss of optical purity.     

Synthesis of 9-alkylidene-9H-fluorenes by a novel, palladium-catalyzed cascade reaction of aryl halides and 1-aryl-1-alkynes.

In the presence of a palladium catalyst and NaOAc, aryl iodides react with 1-aryl-1-alkynes to afford 9-alkylidene-9H-fluorenes in good yields. The products from this reaction are highly dependent on the base employed. This process appears to involve (1) oxidative addition of the aryl iodide to Pd(0), (2) alkyne insertion, (3) rearrangement of the resulting vinylic palladium intermediate to an arylpalladium species, and (4) aryl-aryl coupling with simultaneous regeneration of the Pd(0) catalyst. Consistent with this mechanism is the fact that 9-alkylidene-9H-fluorenes can also be prepared by the Pd-catalyzed rearrangement of 1,1-diaryl-2-iodo-1-alkenes.     

Ruthenium-catalyzed hydration of 1-alkynes to give aldehydes: insight into anti-Markovnikov regiochemistry.

The mechanism of the selective conversion of 1-alkynes to aldehydes by hydration was investigated by isolating organic and organometallic byproducts, deuterium-labeling experiments, and DFT calculations. The D-labeled acetylenic hydrogen of 1-alkyne was found exclusively in the formyl group of the resulting aldehydes. After the reaction, the presence of metal-coordinated CO was confirmed. All of the experimental results strongly suggest the involvement of a metal-acyl intermediate with the original acetylenic hydrogen also bound to the metal center as a hydride, with the next step being release of aldehyde by reductive elimination. Theoretical analyses suggest that the first step of the catalytic cycle is not oxidative addition of acetylene C [bond] H or tautomerization of eta(2)-alkyne to a vinylidene complex, but rather protonation of the coordinated 1-alkyne at the substituted carbon to form a metal-vinyl intermediate. This cationic intermediate then isomerizes to Ru(IV)-hydride-vinylidene via alpha-hydride migration of the vinyl group to the metal center, followed by attack of the vinylidene alpha-carbon by OH(-) to give the metal-hydride-acyl intermediate.