Angewandte
Communications
Chemie
Scheme 2. a) 10 (2.5 mol%), CH2Cl2, 08C, 86%, d.r.=11:1. b) 11
(10 mol%), CH2Cl2, 08C, quant. (NMR), d.r.=2:1. c) 10 (0.27 equiv),
CH2Cl2, 08C, 18 (35%, see text). d) 10 (12.5 mol%), CD2Cl2, 08C, 22
(quant., NMR). e) 10 (0.125 equiv), [D8]THF, 08C, 20 (quant., NMR).
THF=tetrahydrofuran, Ts=p-toluenesulfonyl.
Figure 2. Structure of the complex 20 in the solid state.[33] Thermal
ellipsoids shown at 50% probability.
reluctant to ligate the [Ru]+ center in 11, probably for simple
geometric reasons.[24] That is why we propose the alignment
shown in E to explain the intermolecular cases catalyzed by 11
(see Scheme 1), wherein the crotyl alcohol makes the primary
contact to [Ru]+.[25] This coordination, in turn, acidifies the
OH proton and leads to peripheral hydrogen bonding with
the incoming propargyl alcohol. The inverse scenario with the
propargylic OH being docked onto the metal center is much
less likely.
of the acetylene away from linearity [C2-C3-C4 154.9(1)8]. As
expected, coordination of the triple bond is supported by tight
hydrogen bonding between the OH group and the chloride
(2.342 ꢀ).[28,29] Since the resulting chelate structure locks the
adjacent methyl groups, C1 and C9, in different chemical
environments, it is predisposed for relaying stereochemical
information during cyclization from a propargylic center to
À
the newly developing C C bond. Note that the tetrahedral
In an attempt to study the selectivity-determining periph-
eral interactions in more detail, we reacted compound 15,
comprising a terminal rather than an internal alkene, with
stoichiometric [Cp*RuCl] (Scheme 2). Since the resulting
metallacycle 16 lacks exocyclic H atoms for b-hydride elim-
ination, this intermediate was deemed amenable to spectro-
scopic and/or crystallographic characterization. However, the
13C NMR spectrum suggested formation of four different
carbonyl-containing products. The major one (18) was
isolated as orange-red crystals and characterized by X-ray
diffraction (see the solid-state structure in the Supporting
Information). While the formation of a ruthenium diene
complex per se is not surprising, the ligand constitution in 18
is unusual in that it mandates formal loss of H2 either before
or after cyclization; Scheme 2 shows a plausible scenario.
To block this unexpected escape route, the relevant
protons in 15 were formally replaced by methyl groups
(Scheme 2). As expected, exposure of enyne 19 to 10 in
CD2Cl2 afforded a single ruthenium complex, which proved
metastable and evolved into the cyclobutene 22, presumably
by reductive elimination of the metallacycle 21 and subse-
quent isomerization of the bridgehead olefin primarily
formed.[26] Gratifyingly though, an intermediate derived
from 19 was sufficiently long-lived in [D8]THF to allow
isolation in the crystalline form. To the best of our knowledge,
20 (Figure 2) is the first ruthenium complex comprising an
intact enyne ligand.
coordination sphere renders the Ru center in the piano-stool
complex 20 chiral.[19] Binding of the alkene must also be
appreciable as judged from the extended C7-C8 distance
[1.406(2) ꢀ].[27] Despite these clear signs of activation of
either p system, the enyne has not yet succumbed to oxidative
cyclization with formation of a planar ruthenacycle as
evidenced from the significant tilting of the alkene against
the alkyne unit (C3-C4····C7-C8 69.88) and the nonbonding
C4···C7 distance (2.637(3) ꢀ). A clash between the pseudoax-
ial but slightly inwardly-bent methyl groups, C9 and C10, on
the rim seems to prevent further contraction and hence
spontaneous oxidative cyclization from occurring. NMR
spectra recorded at À508C show that these structural features
are largely maintained in solution. Warming of the sample to
ambient temperature, however, entails line broadening and is
suggestive of rapid exchange between at least two species. We
suppose that de-coordination/re-coordination of the alkene
accounts for this dynamic phenomenon.[30] It is therefore
readily understood why more highly substituted alkenes are
handicapped substrates, particularly in intermolecular set-
tings. They qualify, however, if a proximal OH group assists in
binding (Table 2).
To understand the origin of the experimentally observed
regio- and diastereoselectivity of the ruthenium-catalyzed
coupling reaction with 7, we performed density-functional
theory (DFT) calculations at the SMD-M06L/def2-TZVP
level of theory.[31] Achiral but-2-yn-1-ol and chiral pent-3-yn-
2-ol were chosen as reaction partners. In the Supporting
Information, we describe the computational methods, the
The firm binding of the alkyne unit in 20 is manifested in
the elongated C3–C4 distance [1.250(2) ꢀ][27] and the bending
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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