Understanding and Expanding Zinc Cation/Amine Frustrated Lewis Pair Catalyzed C-H Borylation.

[(NacNac)Zn(DMT)][B(C6F5)4], 1, (NacNac = {(2,6- i Pr2H3C6)N(CH3)C}2CH), DMT = N,N-dimethyl-4-toluidine), was synthesized via two routes starting from either (NacNac)ZnEt or (NacNac)ZnH. Complex 1 is an effective (pre)catalyst for the C-H borylation of (hetero)arenes using catecholborane (CatBH) with H2 the only byproduct. The scope included weakly activated substrates such as 2-bromothiophene and benzothiophene. Computational studies elucidated a plausible reaction mechanism that has an overall free energy span of 22.4 kcal/mol (for N-methylindole borylation), consistent with experimental observations. The calculated mechanism starting from 1 proceeds via the displacement of DMT by CatBH to form [(NacNac)Zn(CatBH)]+, D, in which CatBH binds via an oxygen to zinc which makes the boron center much more electrophilic based on the energy of the CatB-based LUMO. Combinations of D and DMT act as a frustrated Lewis pair (FLP) to effect C-H borylation in a stepwise process via an arenium cation that is deprotonated by DMT. Subsequent B-H/[H-DMT]+ dehydrocoupling and displacement from the coordination sphere of zinc of CatBAr by CatBH closes the cycle. The calculations also revealed a possible catalyst decomposition pathway involving hydride transfer from boron to zinc to form (NacNac)ZnH which reacts with CatBH to ultimately form Zn(0). In addition, the key rate-limiting transition states all involve the base, thus fine-tuning of the steric and electronic parameters of the base enabled a further minor enhancement in the C-H borylation activity of the system. Outlining the mechanism for all steps of this FLP-mediated process will facilitate the development of other main group FLP catalysts for C-H borylation and other transformations.

A zwitterionic disilanylium from an unsymmetrical disilene.

The reaction of PhC(NtBu)2SiSi(SiMe3)3 (1) with Me3SiCH2Cl afforded an unsymmetrical sp2-sp3 disilene, 2, with concomitant elimination of Me3SiCl. The analogous reaction with PhC(NtBu)2SiCl resulted in the oxidative addition of the C-Cl bond at the Si(II) atom (3). The reactions of 2 with sulfur and selenium led to compounds with SiE (ES (4) and Se (5)) double bonds. Tellurium reacted differently with 2 and furnished a zwitterionic compound, 6.

Deoxygenative hydroboration of primary and secondary amides: a catalyst-free and solvent-free approach.

In contrast to the recent reports on catalytic hydroboration of amides to amines with pinacolborane (HBpin), a simple catalyst-free and solvent-free method for the hydroboration of a variety of amides has been realized. To get the mechanistic insights, DFT calculations have been performed.

Unsymmetrical sp2 -sp3 Disilenes.

Disilenes with differently coordinated silicon atoms are not known. Here, we have shown the high yield synthesis of a range of disilenes (2-4 and 6) upon reaction of a hypersilyl silylene PhC(NtBu)2 SiSi(SiMe3 )3 (1) with aliphatic chlorophosphines. The most striking characteristic of these disilenes is the presence of two differently coordinated Si atoms (one is three-coordinated, the other four-coordinated). The analogous reaction with Ph2 PCl did not afford the desired disilene, but, surprisingly, led to the first tetraphosphinosilane (8). DFT calculations were performed to understand the bonding in disilenes and differences in reactivity of the complexes.

Diverse reactivity of carbenes and silylenes towards fluoropyridines.

The reaction of IDipp with C5F5N led to functionalization of all three carbon atoms of the imidazole ring with HF2- as the counter-anion (1). Reactivity with 2,3,5,6-tetrafluoropyridine gives only C-F bond activation leaving C-H bonds intact (5b). The reaction of SIDipp with C5F5N in the presence of BF3 afforded the ring cleavage product (3). Analogous reactions with silylene led to oxidative addition at the Si(ii) center.

Lithium compound catalyzed deoxygenative hydroboration of primary, secondary and tertiary amides.

A selective and efficient route for the deoxygenative reduction of primary to tertiary amides to corresponding amines has been achieved with pinacolborane (HBpin) using simple and readily accessible 2,6-di-tert-butyl phenolate lithium·THF (1a) as a catalyst. Both experimental and DFT studies provide mechanistic insight.

Diverse Reactivity of Hypersilylsilylene with Boranes and Three-Component Reactions with Aldehyde and HBpin.

The recently reported hypersilylsilylene PhC(NtBu)2SiSi(SiMe3)3 (1) reacts with BH3, 9-BBN, and PhBCl2 to yield the respective Lewis acid base adducts 2-4, respectively. Compound 4 undergoes isomerization to form a ring expansion product 5. The same silylene was found to initially form an adduct with HBpin (6) and subsequently isomerized to 7 via the rupture of the B-H bond of HBpin (7), where the hydride was bound to the carbon atom of the amidinate ligand and the Bpin unit was attached to the silicon center. Surprisingly, the reaction of 1 with HBcat results in PhC(NtBu)2Bcat (8). Subsequently, we have shown that HBcat forms the same product when it reacts with related silylene PhC(NtBu)2SiN(SiMe3)3 (1'). With all of these reactions in hand, we ponder if silylene can activate two small molecules at one time. In this work, we delineate the three-component reactions of silylenes 1 and 1' with 4-fluorobenzaldehyde and HBpin, which afforded unusual coupling products, 9 and 10, respectively. Note that 9 and 10 were prepared from the cleavage of the B-H and C═O bonds by silylene in a single reaction and are the first structurally attested Si-C-O-B coupled products.

Lithium compounds as single site catalysts for hydroboration of alkenes and alkynes.

The hydroboration of alkenes and alkynes using easily accessible lithium compounds [2,6-di-tert-butyl phenolatelithium (1a) and 1,1' dilithioferrocene (1b)] has been achieved with good yields, high functional group tolerance and excellent chemoselectivity. Deuterium-labeling experiments confirm the cis-addition of pinacolborane. The methodology has been further extended to myrcene, which undergoes selective 4,3-hydroboration. DFT calculations provide insights into the mechanism.

Synthesis and Reactivity of a Hypersilylsilylene.

Stabilization of an amidinatosilylene with a bulky tris(trimethylsilyl)silyl substituent was realized with the preparation of PhC(NtBu)2Si{Si(SiMe3)3} (1) from PhC(NtBu)2SiHCl2 with K{Si(SiMe3)3} in more than 90% yield. The highly deshielded 29Si NMR resonance (δ = 76.91 ppm) can be attributed to the absence of a π-donating substituent. The molecular structure of 1 shows a trigonal-planar geometry around the SiII center with a SiII-SiIV bond length of 2.4339(13) Å. A series of reactions of 1 with Me3NO, S, Se, and Te were performed. While siloxane derivatives (2 and 3) are obtained from reactions with Me3NO, silachalcogenones (4-6) are formed with other chalcogens. The presence of Si═E (E = S, Se, and Te) bonds in 4-6 have been confirmed by single-crystal X-ray studies. Silaoxirane (7) formation was observed when 1 was treated with acetone, demonstrating the importance of the tris(trimethylsilyl)silyl group to kinetically and thermodynamically protect the silaoxirane derivative with less bulky substituents on the C atom.

Easily accessible lithium compound catalyzed mild and facile hydroboration and cyanosilylation of aldehydes and ketones.

Simple and readily accessible lithium compounds such as 2,6-di-tert-butyl phenolate lithium (1a), 1,1' dilithioferrocene (1b) and nacnac lithium (1c) are found to be efficient single site catalysts for hydroboration of a range of aldehydes and ketones with HBpin at room temperature. The efficacy of 1a-1c as catalysts is extended to the cyanosilylation of aldehydes and ketones with Me3SiCN.

Metal free mild and selective aldehyde cyanosilylation by a neutral penta-coordinate silicon compound.

This study demonstrates the preparation and structural characterization of a Si(iv) hydride (PhC(NtBu)2SiH(CH3)Cl) (1) and its use as a catalyst for the cyanosilylation of a variety of aldehydes. Compound 1 represents the first neutral penta-coordinate silicon(iv) species that catalyzes cyanosilylation of aldehydes under mild conditions.

Transition metal free catalytic hydroboration of aldehydes and aldimines by amidinato silane.

The transition metal free catalytic hydroboration of aldehydes and ketones is very limited and has not been reported with a well-defined silicon(iv) compound. Therefore, we chose to evaluate the previously reported silicon(iv) hydride [PhC(NtBu)2SiHCl2], (1) as a single component catalyst and found that it catalyzes the reductive hydroboration of a range of aldehydes with pinacolborane (HBpin) under ambient conditions. In addition, compound 1 can catalyze imine hydroboration. DFT calculation was carried out to understand the mechanism.