Interligand Interactions Dictate the Regioselectivity of trans-Hydrometalations and Related Reactions Catalyzed by [Cp*RuCl]. Hydrogen Bonding to a Chloride Ligand as a Steering Principle in Catalysis

Abstract

Reactions of internal alkynes with R3M-H (M = Si, Ge, Sn) follow an unconventional trans-addition mode in the presence of [Cp*Ru(MeCN)3]PF6 (1) as the catalyst; however, the regioselectivity is often poor with unsymmetrical substrates. This problem can be solved upon switching to a catalyst comprising a [Ru-Cl] bond, provided that the acetylene derivative carries a protic functional group. The R3M unit is then delivered with high selectivity to the alkyne-C atom proximal to this steering substituent. This directing effect originates from the ability of the polarized [Ru-Cl] bond to engage in hydrogen bonding with the protic substituent, which helps upload, activate, and lock the alkyne within the coordination sphere. An additional interligand contact of the chloride with the -MR3 center positions the incoming reagent in a matching orientation that translates into high regioselectivity. The proposed secondary interactions within the loaded catalyst are in line with a host of preparative and spectral data and with the structures of the novel ruthenium π-complexes 10 and 11 in the solid state. Moreover, the first X-ray structure of a [Ru(σ-stannane)] complex (12a) is presented, which indeed features peripheral Ru-Cl···MR3 contacts; this adduct also corroborates that alkyne trans-addition chemistry likely involves σ-complexes as reactive intermediates. Finally, it is discussed that interligand cooperativity might constitute a more general principle that extends to mechanistically distinct transformations. The presented data therefore make an interesting case for organometallic chemistry that provides inherently better results when applied to substrates containing unprotected rather than protected -OH, -NHR, or -COOH groups.

Stereo‐ and Regioselective Alkyne Hydrometallation with Gold(III) Hydrides

“…With silyl acetylenes, the gold is distal to the SiR 3 group. This regioselectivity is in contrast to that obtained in a number of alkyne hydrometallation products, for example hydrostannations,18, 19 hydrosilylations,20 hydroaluminations,21 and hydropalladations,22 where the metal is predominantly attached α to the phenyl substituent.…”

Two Enabling Strategies for the Stereoselective Conversion of Internal Alkynes into Trisubstituted Alkenes

“…This is particularly true for products derived from propargyl alcohold erivatives, which react in ah ydroxy-directed manner with notably high levels of regio-and stereoselectivity. [12,13] The resulting prodScheme3.Hydroxy-directed hydrosilylation of propargyl alcohols followed by C-alkylation via Brook rearrangement. ucts of type B provide ample opportunity for downstream functionalization: [42] The copper-mediated C-alkylation reactions described above further complement the portfolioi n that they open ar eliable, functional group toleranta nd hence potentially broadly applicable entry into stereochemically welldefined trisubstituted alkenes;m ost notably,t he (E)-2-methylbut-2-en-1-ol motif is well within reach,w hich is prevalent in innumerousb ioactive natural products.…”

“…[34,35] We conjectured that the mandatory use of triphenylsilanes likely reflects the need to lower the energy of the C-Si antibondingo rbital as ap reludef or successfula ttack of the adjacent alkoxide onto the silicon atom that triggers the critical Brook rearrangement; [36,37] this step is arguably challenging as it passes throughaf our-membered intermediate O.U nder the premise that orbitale nergy lowering is the key to success, however,s ilanes of type B carrying electronegative heteroatoms at silicon should also qualify as substrates. [38] Alkoxysilanes B (X = OR), which are easy to make via the ruthenium catalyzed trans-hydrosilylation referred to above, [12,13] fall into this category and were therefore deemed ap otentially very attractive and versatile class of startingm aterials.…”

“…Moreover,r ecent progress in metal-catalyzed trans-hydrometalation of alkynesm akes the required organotin precursors of type B (E = Sn) readily available in isomerically pure form; [12][13][14] the analogousa lkenylsilanes B (E = Si)a re equallyw ell accessible and hence also deserve consideration as possible substrates. [12,15,16] Outlined below is as ummary of our investigations in this field, whichn ow allow us to engage either type of precursor into productive and selectivem ethylation reactions. Interestingly though, we learned that what seemed to be a regular Stille/Migita crosscoupling at the outset of our study is most likely ap urely copper-mediated process, in which the added palladium catalyst serves as an adjuvantr ather than an indispensable catalysti nm any cases.…”

Progress in the trans-Reduction and trans-Hydrometalation of Internal Alkynes. Applications to Natural Product Synthesis

“…36 The Fürstner group discovered a very profound impact of the catalyst structure on the regioselective course of the transhydrosilylation in the case of non-symmetrical alkyne substrates bearing protic functionality. 37 The example of the propargyl alcohol 1 depicted in Scheme 2 is representative: Whereas the conditions originally developed by Trost and coworkers using [Cp*Ru(MeCN) 3 ]PF 6 in CH 2 Cl 2 favor isomer 2 with the silyl group at the position distal to the OH group, the neutral chloride-containing complex [Cp*RuCl] 4 affords the "proximal" isomer 3 with similar (in many cases actually higher) selectivity and excellent yield. 37 Mechanistic investigations suggest that hydrogen bonding between the polarized [RuCl] unit and the OH group lock the substrate in a particular orientation within the coordination sphere of the Scheme 1.…”