Use of Piers-Rubinsztajn Chemistry to Access Unique and Challenging Silicon Phthalocyanines

Tris(pentafluorophenyl)borane-catalyzed Hydride Transfer Reactions in Polysiloxane Chemistry-Piers-Rubinsztajn Reaction and Related Processes

Tris(pentafluorophenyl)borane (TPFPB) is a unique Lewis acid that catalyzes the condensation between hydrosilanes (Si-H) and alkoxysilanes (Si-OR), leading to the formation of siloxane bonds (Si-OSi) with the release of hydrocarbon (R-H) as a byproduct-the so-called Piers-Rubinsztajn reaction. The analogous reactions of hydrosilanes with silanols (Si-OH), alcohols (R-OH), ethers (R-OR') or water in the presence of TPFPB leads to the formation of a siloxane bond, alkoxysilane (Si-OR or Si-OR') or silanol (Si-OH), respectively. The above processes, often referred to as Piers-Rubinsztajn reactions, provide new synthetic tools for the controlled synthesis of siloxane materials under mild conditions with high yields. The common feature of these reactions is the TPFPB-mediated hydride transfer from silicon to carbon or hydrogen. This review presents a summary of 20 years of research efforts related to this field, with a focus on new synthetic methodologies leading to numerous previously difficult to synthesize well-defined siloxane oligomers, polymers and copolymers of a complex structure and potential applications of these new materials. In addition, the mechanistic aspects of the recently discovered reactions involving hydride transfer from silicon to silicon are discussed in more detail.

Silicon(IV) complexes of octaaryl-substituted porphyrazines and corrolazines: influence of macrocycle contraction on spectral-luminescence, acid-base, and redox properties

The template cyclomerization of the iminoimide derivatives (obtained by the treatment of diarylfumarodinitriles with NH₃ in methanol containing catalytic amounts of dissolved Na) in the presence of SiCl₄ in pyridine leads to silicon(IV) octaarylporphyrazine complexes ((HO)₂SiPzAr₈, Ar = Ph, ^tBuPh) as a main reaction product. In the case of phenyl-substituted derivative, the formation of a distinctive Si(IV) complex as a byproduct was observed, which according to mass-spectroscopy measurement contains the macrocycle with five diphenylpyrrolic units. The treatment of bishydroxy complexes with tripropylchlorosilane in the presence of magnesium in pyridine leads to the formation of axially siloxylated porphyrazines (Pr₃SiO)₂SiPzAr₈, followed by the reductive contraction of the macrocycle and formation of the corresponding corrolazine complexes (Pr₃SiO)SiCzAr₈. It is shown that the addition of acid (trifluoroacetic acid, TFA) facilitates the detachment of one siloxy group in (Pr₃SiO)₂SiPzAr₈ and is essential for its Pz → Cz transformation. In the presence of TFA, only one meso-nitrogen is protonated in the porphyrazine complexes (Pr₃SiO)₂SiPzAr₈ (stability constant of the protonated form pK_s1 = -0.45 for Ar = Ph; pK_s1 = 0.68 for Ar = ^tBuPh), while two consequent protonation stages are observed for the more basic corrolazine complex (Pr₃SiO)SiCzPh₈ (pK_s1 = 0.93, pK_s2 = 0.45). Both types of Si(IV) complexes are poorly fluorescent (Φ_F < 0.07). The porphyrazine complexes have low ability to generate singlet oxygen (Φ_Δ < 0.15), whereas the corrolazine derivative (Pr₃SiO)SiCzPh₈ is a very efficient photosensitizer (Φ_Δ = 0.76).

The ultimate Lewis acid catalyst: using tris(pentafluorophenyl) borane to create bespoke siloxane architectures

The breadth of utility of a commercially available and stable strong Lewis acid catalyst, tris(pentafluorophenyl)borane, has been explored, highlighting its use towards a wide range of unique siloxane products and their corresponding applications. This article focuses on the variety of different outcomes that this impressive borane offers in controlled and selective manners by the variation of reaction conditions, precursor functionalities, reagent or catalyst loading, and the mechanistic considerations that contribute. With a predominant focus on the Piers-Rubinsztajn reaction and its modifications, tris(pentaflurophenyl)borane's utility is highlighted in the synthesis of linear, cyclic and macrocyclic siloxanes, aryl-/alkoxysiloxanes, and other bespoke products. The significance of the catalytic transformation within the field of siloxane chemistry is discussed alongside some of the challenges that arise from using the borane catalyst.