The stannylvinyl cation that never was! New concentration- and temperature-dependent probe studies confirm an entirely free radical mechanism and O–Sn coordinative control of the hydrostannation of propargylically-oxygenated dialkyl acetylenes with stannanes and cat. Et3B

Total synthesis of the HDAC inhibitor (+)-(R)-trichostatin A via O-directed dialkylacetylene free radical hydrostannation with Ph3SnH/Et3B. The unusual inhibitory effect of a proximal α-OPv group on the course of a vinyl iodide Stille cross-coupling

In this paper, a new asymmetric total synthesis of optically pure (+)-trichostatin A (1a) is described via a route that utilises a Marshall chiral allenylzinc addition between 9 and 4-dimethylaminobenzaldehyde (10) and an O-depivaloylation at its early stages. O-Directed free radical hydrostannation of the resulting propargylic alcohol 15 with Ph3SnH/cat. Et3B/O2 in PhMe at rt thereafter provided the (Z)-α-triphenylstannylvinyltin 16 in 80-89% yield, with complete stereocontrol and very high α : β regioselectivity (25 : 1). A stereoretentive I-Sn exchange reaction between 16 and I2 (1.4 equiv.) in CH2Cl2 (-78 °C to rt, 1 h) subsequently secured the vinyl iodide 18 in 84-96% yield. The latter was transformed into the enal 4 by successive TPAP/NMO (Ley-Griffith) oxidation and a high yielding (80%) Stille reaction between the α-iodo enal 20 and Me4Sn, catalysed by Pd(PPh3)4 in DMF at 60 °C, under the Baldwin-Lee conditions, which use CsF and CuI as promoters. A Wittig reaction between 4 and Ph3PCHCO2Et (5), saponification, and DDQ oxidation next afforded (+)-trichostatic acid (22). Helquist's ethyl chloroformate mixed-anhydride/TBSONH2 coupling procedure (ref. 17e) thereafter secured (+)-trichostatin A (1a) in good yield. This new total synthesis of 1a is the first-ever successful application of the O-directed dialkylacetylene free radical hydrostannation with Ph3SnH/cat. Et3B/O2 in a dialkylaniline N-containing disubstituted alkynol system, and it now provides a convenient means of accessing many novel trichostatin analogues for future biological screening.

On the divergent reactivity of allenylstannanes generated from the O-directed free radical hydrostannation reaction of (±)-trans-3-(2-phenylcyclopropyl)prop-2-yn-1-ol. EPR evidence for the reversible addition of Ph3Sn radicals to vinyl triphenyltins

(±)-trans-3-(2-Phenylcyclopropyl)prop-2-yn-1-ol (5) undergoes O-directed rt free radical hydrostannation with 2 equiv. of Bu3SnH or Ph3SnH in PhMe to produce the α-cyclopropyl-β-stannylvinyl radicals 26 and 27, which rapidly ring-open to give the benzylic stannylhomoallenyl radicals 28 and 29. These, in turn, react with the excess stannane that is present to provide 21 and 23 as primary reaction products. The triphenylstannylallene 23 also undergoes a competitive Ph3Sn˙ addition to its central allene carbon. This affords the allylically-stabilised radical 31c, which itself reacts with the stannane to produce (Z)-6-phenyl-2,3-bis(triphenylstannyl)hex-3-en-1-ol (24). EPR studies of the reaction of 5 with Ph3SnH (1 equiv.) and cat. Et3B/O2 in PhMe at 250 K have failed to identify the radicals 27 and 29 in the reaction mixtures. Rather, a sharp dd is always observed whose multiplicity is consistent with it being the tris-Ph3Sn-stabilised free radical 33. The latter is suggested to arise from a reversible O-directed Ph3Sn˙ addition to 24. The radical 33 has 1Hβ values of 1.32 mT (13.2 G) and 0.57 mT (5.7 G) and a g of 2.0020.

Rate constants and Arrhenius parameters for H-atom abstraction from Bu3SnH by the 2,2-dimethylvinyl radical in PhMe. Kinetic evidence for an entirely free radical mechanism for the O-directed hydrostannation of alkynols with stannanes and Et3B/O2

Using the 2,2-dimethylvinyl radical 6 as a horological calibrant for the α-cyclopropyl-β-tributylstannylvinyl radicals 2a and 13 in PhMe, the k values and Arrhenius parameters for their cyclopropane ring-openings have been estimated by competition kinetics over a 293-353 K temperature range. The high log A values (14.95 and 14.55) for these reactions only satisfactorily align with a unimolecular, β-scissive, EH1 radical ring-opening being rate-determining, and the radicals 3a (R = Bu) and 14 undergoing H-atom abstraction from the stannane to give 4a and 15. The log A data for these two reactions only endorse a totally free radical mechanism for the O-directed free radical hydrostannation of dialkyl acetylenes with stannanes and Et3B/O2. An estimated kH-atom abstraction Bu3SnH PhMe 293 K of 1.96 × 108 mol-1 s-1 is proposed for 6 in PhMe, along with an estimated kH-atom abstraction Ph3SnH PhMe 293 K of 1.36 × 109 mol-1 s-1.

anti-Selective Borylstannylation of Alkynes with (o-Phenylenediaminato)borylstannanes by a Radical Mechanism

We have achieved the first anti-borylstannylation of alkynes by using (o-phenylenediaminato)borylstannanes. This reaction afforded 1-boryl-2-stannylalkenes with excellent regio- and stereoselectivity by a radical mechanism. This anti-addition manner is in sharp contrast to the syn-selectivity obtained during transition metal-catalyzed borylstannylation. The mild radical conditions enabled a broad substrate scope, and various types of aromatic and aliphatic alkynes were applicable. The origin of regio- and stereoselectivity was elucidated by DFT calculation of the reaction mechanism. The application of the borylstannylation products to cross- or homocoupling reactions provided ready access to either triarylethenes or bisborylbutadienes.

EPR evidence for α-triphenylstannylvinyl radicals in the O-directed hydrostannation of dialkylacetylenes with Ph3SnH/cat. Et3B/O2

Here we provide definitive EPR evidence for the existence of α-triphenylstannylvinyl radicals in the low temperature O-directed free radical hydrostannation of dialkyl propargylic alcohols with Ph3SnH/cat. Et3B and O2 in PhMe. Isotropic hyperfine splitting patterns and spectral simulations confirm the assignments made. In the case of the α-triphenylstannylvinyl radical (Z)-2, an isotopic 119/117Sn hyperfine coupling constant of 9.5 mT (95 G) was measured along with a 1Hβ hyperfine coupling constant of 1.1 mT.

Palladium-catalyzed dearomative allylation of indoles with cyclopropyl acetylenes: access to indolenine derivatives

A palladium-catalyzed redox-neutral allylic alkylation of indoles with cyclopropyl acetylenes has been disclosed. Various 1,3-diene indolenine framework bearing a quaternary stereocenter at the C3 position were synthesized straightforwardly in good to excellent yields with high regio- and stereoselectivities. The reaction could be further expanded to the dearomatization of naphthols to synthesize functionalized cyclohexadienones with 1,3-diene motifs. The reaction exhibited high atom economy and good functional group tolerance.