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4.4.7 Silylboron Reagents

DOI: 10.1055/sos-SD-104-00395

Delvos, L. B.; Oestreich, M.Science of Synthesis Knowledge Updates, (2017165.

General Introduction

The research field of silylboron reagents has witnessed immense growth since the initial summary in Science of Synthesis by Hemeon and Singer in 2002.[‌1‌] This progress is reflected by numerous reviews published in recent years, often covering different aspects of this chemistry.[‌2‌‌10‌] The synthesis of more-stable compounds containing a Si—B bond, [di­meth­yl(phenyl)silyl]pinacolborane in particular, made them attractive and routinely used reagents in organic main-group chemistry, and reports on their successful synthetic application have outnumbered publications on their synthesis by far. This section will comprehensively cover all aspects of this progress ranging from synthesis (Section 4.4.7.1) to application (Section 4.4.7.2).

The first section focuses on the synthesis of silylboron reagents, which can be used either directly in a synthetic context or represent key intermediates for the development of synthetically more elaborated derivatives. Generally, the majority of compounds containing a Si—B bond are accessible by direct Si—B bond formation between a silane and a boron reagent through nucleophilic displacement and reductive or transition-metal-catalyzed coupling (Section 4.4.7.1.1). However, the introduction of certain functionalities on the silicon or boron atom requires further manipulation at the thus-obtained silylboron reagents, e.g. by ligand exchange on either main-group element (Section 4.4.7.1.2). Methodologies toward the construction of silylboron reagents that are less important from a synthetic point of view, e.g. silylene insertion reactions[‌11‌‌16‌] and disilene functionalization,[‌17‌] are beyond the scope of this update. Some compounds with multiple Si—B bonds or silylborates are included elsewhere in Science of Synthesis and were not revisited because these compounds rarely find use in organic synthesis.[‌18‌]

The application section is subdivided by functional groups and structural features of the reaction partners. The vast number of subclasses reflects the broad synthetic applicability of silylboron reagents. This is due to versatile Si—B bond activation strategies, which have opened the door to a broad range of distinct chemical transformations. The following paragraph gives a brief outline of these concepts and makes reference to specialized reviews together with mechanistic work, not discussed here in the main text.

Oxidative addition of the Si—B bond to group 10 transition metals is the most frequently used activation method and has led to significant progress in the catalytic (asymmetric) difunctionalization of unsaturated hydrocarbons by silaboration (Scheme 1).[‌6‌,‌7‌,‌9‌,‌10‌] Intermediates formed by Si—B bond activation with nickel, palladium, or platinum catalysts insert into C≡C (Section 4.4.7.2.1), C=C (Section 4.4.7.2.24.4.7.2.4 and 4.4.7.2.8), or activated C—C bonds (Section 4.4.7.2.9) by borametalation with subsequent Si—C bond formation by reductive elimination of the catalyst. Voltammetric studies,[‌19‌] quantum mechanical calculations,[‌20‌,‌21‌] and kinetic analyses[‌22‌] were used for several transformations to investigate the nature of Si—B bond activation, the order of main group—carbon bond-formation events, and the origin of selectivities.

Scheme 1 Si—B Bond Activation by Oxidative Addition to Transition-Metal Complexes

A variation of this concept makes use of amino-functionalized silylboronic esters that undergo β-elimination of the boryl subunit upon oxidative addition to palladium complexes to generate palladium-stabilized silylenes (Scheme 2). These silylene intermediates are known to react in (2 + 2 + 1) or (4 + 1) cycloadditions with alkynes (Section 4.4.7.2.1.4) or conjugated dienes (Section 4.4.7.2.3.4), respectively.

Scheme 2 Si—B Bond Activation by Oxidative Addition with Subsequent β-Elimination of the Boryl Subunit

Another possibility of incorporating both main-group elements into a target molecule is the insertion of carbenoids into the Si—B bond (Scheme 3). Chemoselective addition of a carbenoid to the more Lewis acidic boron atom generates a borate intermediate, which undergoes a 1,2-silyl migration with concomitant substitution of the leaving group to form 1,1-difunctionalized products (Section 4.4.7.2.10).

Scheme 3 Si—B Bond Activation by Carbenoid Insertion

Similarly, the difference in Lewis acidity between silicon and boron is exploited for boron–metal exchange at the silicon atom to generate silicon nucleophiles. Exchange with hard main-group (alkali) metals is achieved by Si—B bond transmetalation with organolithium or organomagnesium reagents (Scheme 4, path a).[‌23‌] In these cases, complete conversion into the corresponding metalated silanes occurs, whereas transformation of silylboronic esters with metalated nitrogen (path b) or oxygen bases (path c) usually leads to partial cleavage of the Si—B bond[‌24‌] or adduct formation[‌25‌,‌26‌] as verified by spectroscopic studies and/or DFT calculations. However, regardless of the precise nature of the silicon nucleophile, it reacts selectively with alkenes (Section 4.4.7.2.2.2) or allylic acceptors (Section 4.4.7.2.7.2).

Scheme 4 Si—B Bond Activation by Transmetalation or Adduct Formation with Metalated Carbon, Nitrogen, or Oxygen Bases

Si—B bond activation by σ-bond metathesis with copper—, rhodium— or, to a lesser extent, nickel—oxygen bonds provides access to transition-metal-stabilized silicon nucleophiles (Scheme 5).[‌2‌‌5‌,‌8‌] In particular, Si—B transmetalation with copper alkoxide catalysts has become a standard technique in organosilicon chemistry. Hence, significant effort was made to fully characterize these copper-based silicon nucleophiles by X-ray crystallography and NMR spectroscopy.[‌27‌,‌28‌] A plethora of fundamental Si—C bond formations were reinvestigated using this approach (often followed by previously unprecedented enantioselective variants), including silacupration of alkynes (Section 4.4.7.2.1.5) or allenes (Section 4.4.7.2.4.2) as well as nucleophilic 1,2-silylation of C=X bonds (Section 4.4.7.2.5), 1,4-silylation of α,β-unsaturated acceptors (Section 4.4.7.2.6), and nucleophilic substitution of allylic or propargylic electrophiles (Section 4.4.7.2.7).

Scheme 5 Si—B Bond Activation by σ-Bond Metathesis with Transition Metal—Oxygen Bonds

An emerging area is the Si—B bond activation by neutral Lewis bases such as N-heterocyclic carbenes or the substrate itself. Lewis acid/base adduct formation increases the nucleophilicity of the silyl group to allow for a selective silicon transfer onto electrophiles such as α,β-unsaturated acceptors (Section 4.4.7.2.6.2 and 4.4.7.2.6.4) or electron-poor hetarenes (Section 4.4.7.2.8.1) (Scheme 6). This activation mechanism was investigated in detail for N-heterocyclic carbenes.[‌25‌,‌29‌]

Scheme 6 Metal-Free Si—B Bond Activation by Neutral Lewis Bases

Photochemical Si—B cleavage completes the studied activation mechanisms (Scheme 7). The released silyl radicals were used in formal hydrosilylations and silylative cyclization reactions (Section 4.4.7.2.2.3).

Scheme 7 Photochemical Cleavage of the Si—B Bond

References