Platinum(II)‐Catalyzed Intramolecular Cyclization of o‐Substituted Aryl Alkynes through sp³ CH Activation

Abstract

Ring leader: PtCl(2) catalyzes intramolecular cyclization of o-isopropyl or o-benzyl aryl alkynes to give substituted indene derivatives with good yields and high selectivity. This reaction appears to proceed through an sp(3) C-H activation and 1,4-hydrogen migration pathway (see scheme).

Direct coupling of unactivated alkynes and C(sp3)–H bonds catalyzed by a Pt(ii, iv)-centered catalyst: a computational study

“…Although C-H bond functionalization with transition-metal catalysis provides strategically new opportunities in carboncarbon bond formation, 1-7 activation of C(sp 3 )-H bonds is still considered a difficult challenge because of their high dissociation energy. [10][11][12][13][14][15][16][17] Since the platinum(II)-catalyzed oxidation of methane to methanol, 18 catalysis with Pt-salts is a rapidly evolving area of research for development of new reactions triggered by p-activation of alkynes and which nowadays has initiated extensive investigation of C(sp 3 )-H bond activation. [10][11][12][13][14][15][16][17] Since the platinum(II)-catalyzed oxidation of methane to methanol, 18 catalysis with Pt-salts is a rapidly evolving area of research for development of new reactions triggered by p-activation of alkynes and which nowadays has initiated extensive investigation of C(sp 3 )-H bond activation.…”

“…[10][11][12][13][19][20][21][22][23] Recently, Pt-centered catalysts have shown their efficiency in a series of transformations involving the transfer of a nucleophilic group onto an alkyne, followed by ring closure on the resulting carbocationic intermediate. [10][11][12][13] Sames and co-workers reported a distinct reaction, in which the unactivated terminal alkynes serve as hydride acceptors in the through-space hydride transfer and undergo catalytic intramolecular hydroalkylation at the a-position of saturated heterocyclic ethers, providing rapid access to bicyclic products. [10][11][12][13] Sames and co-workers reported a distinct reaction, in which the unactivated terminal alkynes serve as hydride acceptors in the through-space hydride transfer and undergo catalytic intramolecular hydroalkylation at the a-position of saturated heterocyclic ethers, providing rapid access to bicyclic products.…”

“…[10][11][12][13] Sames and co-workers reported a distinct reaction, in which the unactivated terminal alkynes serve as hydride acceptors in the through-space hydride transfer and undergo catalytic intramolecular hydroalkylation at the a-position of saturated heterocyclic ethers, providing rapid access to bicyclic products. [10][11][12]31 Three types of mechanistic pathways of these formal cycloisomerizations were proposed: (1) the cycloisomerizations were proposed to proceed via the initial formation of a M-vinylidene species, [10][11][12] (2) a 1,4-hydrogen shi along the p-system 13,14 and (3) a hydrogen shi with M n+ aided process. [10][11][12]31 Three types of mechanistic pathways of these formal cycloisomerizations were proposed: (1) the cycloisomerizations were proposed to proceed via the initial formation of a M-vinylidene species, [10][11][12] (2) a 1,4-hydrogen shi along the p-system 13,14 and (3) a hydrogen shi with M n+ aided process.…”

“…30 The hydroalkylation of terminal alkynes has previously been limited to aromatic substrates and to give substituted indenes via an overall cyclization (Scheme 1). 13,14,32 The above three mechanisms were hypothesized based on the unsaturated aryl alkynes aromatic substrates. 13,14,32 The above three mechanisms were hypothesized based on the unsaturated aryl alkynes aromatic substrates.…”

“…13,14,32 The above three mechanisms were hypothesized based on the unsaturated aryl alkynes aromatic substrates. [10][11][12][13]33 Although a few groups have discussed the mechanism of this type of reaction [10][11][12][13]23,30 and previously, Zhao and co-workers 34 discussed several possible pathways of the bicyclic products formation from substrate 1 (Scheme 2) with PtI 4 as the catalyst, there are few detailed theoretical studies available in the literature where mechanistic possibilities are compared between the Pt(II)-and Pt(IV)-catalyzed transformation and why the different substrates 2 and 3 (Scheme 2) possessing the different yield (62% and 33% respectively) in experiment comparing to the substrate 1 (86%) is unclear. 30 The remarkable progress in the hydroalkylation of terminal alkynes has mostly been focused on synthesis and characterization of new complexes, without utilizing computational chemistry to validate mechanistic speculation.…”