N‐Methyl‐Benzothiazolium Salts as Carbon Lewis Acids for Si−H σ‐Bond Activation and Catalytic (De)hydrosilylation

Abstract N−Me‐Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si−H σ bonds. These carbon‐centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si−H σ‐bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3PO and H2O in contrast to triarylboranes. The N−Me‐benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si−H bond activation and as catalysts for the hydrosilylation of imines.


Introduction
The use of hydridophilic main-group Lewis acids,p articularly B(C 6 F 5 ) 3 ,i nS i ÀHb ond activation, [1] and more broadly in "frustrated Lewis pair" (FLP) chemistry, [2] has generated significant recent breakthroughs. [3] This includes their use as versatile catalysts for dehydrosilylation and hydrosilylation reactions. [4] Generally,t he Lewis acids employed are tri(fluoroaryl)boranes which have sufficient Lewis acidity towards hydride to heterolyticallyc leave SiÀHb onds (in combination with an appropriate Lewisb ase) and generate borohydrides that are able subsequently to reduce electrophilics ubstrates. Mechanistic studies revealed that SiÀHh eterolysis is an S N 2t ype process proceeding via a" partially" activated SiÀHb ond, which can be viewed as a" Si-H-B" 3c-2e interaction, [5,6] with one example recently crystallographicallyc haracterised. [7] In the absence of an appropriate nucleophile no silylium ions are formed from combining B(C 6 F 5 ) 3 andR 3 SiH, although silane H/D scramblings till proceeds via af our-membered transition state (inset Scheme 1). [7] In more recent studies, weaker boronL ewis acids,s uch as BPh 3 ,a lso have been shown also to be effective in SiÀHb ond activation. [8] Nevertheless, the high oxophilicity of boron Lewis acids requires rigorously dried conditions (or an excesso fh y-dride) [9] and leads to substrate scope limitations. [10] Consequently,the development of new Lewis acids that have low oxophilicity but retain sufficient Lewis acidity towards hydride to activate EÀH( E= Ho rR 3 Si)b onds is desirable. [11] Lewis acids in which carbon is the locus of electrophilic character have significant potential in this area as the highere lectronegativity of carbon (relativet ob oron)r esults in ar eduction in "hard" Lewis acidity. [12] Trityl salts are amongst the most widely utilised carbon Lewis acids including in catalytic applications.H owever, these catalytic transformationsg enerally proceed by activation of the substrate by coordinationt ot he electrophilicc arbon centrei nt rityl and not by an FLP-type mechanism. [12,13] The use of trityl salts and other carbon Lewis acids in FLP chemistry,i ncluding the activation of H 2 ,h as significantly less precedent, although al imited number of examples have been recently reported. [14] Carbocationsincluding trityl, are well documented to irreversibly cleave SiÀHb onds to form silylium cations and Ph 3 CH. [15] In contrast, carbon Lewis acids that "partially" activate SiÀH bonds( e.g.,e xhibit analogousr eactivity to B(C 6 F 5 ) 3 )a re extremely rare to the best of our knowledge. One recent example from our group are N-methyl-acridinium salts which activate SiÀHb onds (Scheme 2) as indicated by SiÀH/SiÀDs crambling experiments but no silylium cations are observed in solution. [16] However,t he Lewis acidity of the NÀMe-acridinium cation (1 + )t owardsh ydride is greater than that of B(C 6 F 5 ) 3. [17] This makes the conjugate organic hydride, N-Me-acridane (1- H), formed for example on Si-H heterolysis by 1 + and an appropriate nucleophile, ap oor reductant. This fact combined with the propensity of NÀMe-acridinium salts to initiate photoactivated radical reactivity [18] led us to search for other carbon Lewis acids able to "partially"a ctivate SiÀHb onds but that are weaker Lewis acids towards hydride than 1 + .
Previous work has shown that C2 substituted benzothiazolines are highly effective organic hydrides for the reduction of imines catalyzed by phosphoric acids (Scheme 3, top). [19] Based on this precedence, we targeted the oxidised form, the benzothiazole, as ap otentialc arbonL ewis acid after methylation at nitrogen to increaset he electrophilicity at the C2 position. N-Me-2-R-benzothiazolium cations are attractive Lewis acids as they are simple to make using established routes and can be readily fine-tuned e.g.,b ya lteringt he C2 substituent. [19] Furthermore, they represent ah itherto underexplored class of Lewis acid in FLP-typec atalysis, namely aL ewis acid based on an iminium cation, am oiety whichi sg enerally considered as as ubstrate for reduction and not as ac atalyst. Herein we demonstrate that N-Me-2-aryl-benzothiazolium salts are able to activate SiÀHb onds and are effective as catalysts for dehydrosilylation and hydrosilylation reactions.

Results and Discussion
Initially,t he hydride ion affinity (HIA) 20 of the N-methyl-2-phenylbenzothiazolium cation (2 + )r elative to BEt 3 was computationally determined and found to be À45 kcal mol À1 (Scheme 4). This is comparable to that calculated previously for B(C 6 F 5 ) 3 (À41, kcal mol À1 M06-2X/6-311G(d,p) with dichloromethane (DCM)s olvation( Polarizable Continuum model,P CM). Furthermore, this HIAv alue is less than that found for 1 + (À53 kcal mol À1 ) [15] suggesting [2] + is am ore appropriate Lewis acid for use in catalytic imine reductions as its conjugate organic hydride will be more reducing than 1-H.T he C2-penta-fluorophenyl analogue, [3] + , also was calculated and found to have an HIA of À51 kcal mol À1 indicating it is less suitable for use in catalytic reductions( as itsc onjugate hydride, 3-H,w ill be apoorerreductant).

Scheme2.EÀHb ond activationusing [N-Me-acridinium] + .
Scheme3.To p, Brønsteda cid catalyzedt ransfer hydrogenation of imines using stoichiometric benzothiazoline. Bottom,b enzothiazolium cation catalyzed hydrosilylation and dehydrosilylation using stoichiometric silane.  Recent studies have highlighted the importance of correctly balancing the electrophilicity of the substrate (e.g.,a ni minium cation) and the reducing power of the organic hydride to achieve successful transfer hydrogenation. [25] Thus, we investigated the diphenylphosphoric acid initiated reduction of N-benzylidene-aniline, employing [2-H] as the hydride source (Scheme 5). Using equimolar ratios this produced N-benzylaniline in around 50 %c onversion, with conversion limited due to the amine product deprotonating the iminium cation (full imine reduction can be achieved by using > 2equivalent of the phosphoric acid). These reactions confirm that 2-H is am ore accessible source of hydride for reductionsthan 1-H.
An umber of controlr eactions were performed to ensure that protonolysiso ft he borate anion is not leading to an active borane catalyst under these conditions. Anion decomposition has been previously reported on heating NaBPh 4 in wet solvents which led to the formation BPh 3 . [27,31] 4 ]i nM eCN indicates that B(Aryl) 3 (or any other borane able to initiate dehydrosilylation) is not being formed by anion decomposition. These results combined confirm that under these conditions catalytic BnOH dehydrosilylation is initiatedb yt he benzothiazolium salts and not by Lewis acidic boranes derived from anion decomposition. [32] [5][BArCl] also catalyzest he dehydrosilylation of BnOH in DCM with Ph 3 SiH with 57 %c onversion at6 08Ca fter 24 h. This is slower than with B(C 6 F 5 ) 3 [30] using the same silane/alcohol which proceeds at room temperature. This indicates as ignificantly greater kinetic barrier using the benzothiazolium salt as catalyst. Phenol also underwent dehydrosilylation catalyzed by [4][BArCl] (76 %c onversion after 2hat 20 8Ci nD CM) precluding an alcohol dehydrogenation/c arbonylh ydrosilylation mechanism. Benzothiazolium salts also dehydrosilylate water to form the respectives iloxane, thusa lcohol dehydrosilylation proceeds using non-purified solvents (using excess silane) and also in more environmentally friendly solvents such as methyl tert-butyl ether (MTBE).
Another established B(C 6 F 5 ) 3 catalyzed reaction is the hydrosilylationo fc arbonyls reported by Piers and co-workers. [5] Using the three benzothiazolium[BArCl] salts for carbonyl hydrosilylation led to differences in the rate of benzaldehyde (Table 2e ntries 1-3) and acetophenone (Table 2e ntries [4][5][6] hydrosilylation. With both substrates the C2-(1-naphthyl)s ubstitutedc atalyst [5][BArCl] resultsi nt he slowest rate of hydrosilylations uggesting that either greater steric hindrance aroundt he C2 centre or stronger cation-anion interactions are retarding the rate of hydrosilylation using this catalyst. HIA calculationso n[5] + revealed it has an effectively identical Lewis acidity towards hydride as found for [2] + (À45.5 and À45.4 kcal mol À1 ,r espectively)p recluding the reactivity disparity originating from different degrees of Lewis acidity to hydride (which in turn would lead to differing degrees of silane activation and differing reducing powers of 5-H and 2-H).
Other features of carbonyl hydrosilylation using benzothiazolium salts are comparable to that reported for B(C 6 F 5 ) 3 .F or example, an equimolar mixture of acetophenone/benzaldehyde/PhMe 2 SiH led to hydrosilylation of the more nucleophilic substrate, benzaldehyde, preferentially in the presence of 5mol % [4][BArCl].M oreover,a ttempts to selectively hydrosilylate ethyl benzoate using 1.2 equivalents of PhMe 2 SiH in the presence of [4][BArCl] led to mixtures of products consistent with the silyl acetal product undergoing further reduction competitively to ester hydrosilylation, again analogous to that observed using B(C 6 F 5 ) 3 . [5] Finally,acontrol reactionu sing 5mol %N aBArCl in the hydrosilylation of acetophenone resulted in no reaction (after 60 minutes at 20 8Ci nD CM) indicating that catalytic activity is again due to the benzothiazolium salt.

Conclusion
N-Me-C2-Aryl-benzothiazolium cations represent an ew family of readily tuned Lewis acids that show activity in frustrated Lewis pair (FLP) chemistry.T hey are based on cationic iminium moietiesc ontaining an electrophilic carbon centre that has aL ewis acidity towards hydride comparable to the triarylboranes widely used in FLP reactivity.H owever,i nc ontrast to the triarylboranes thesec ations show little propensity to bind hard Lewis bases such as, H 2 O, Et 3 PO and 4-DMAP.Ar ange of benzothiazolium salts "partially" activate the SiÀHb ond of silanes, as indicatedbyH/D scrambling. Furthermore, they are effective catalysts [35] in ar ange of established FLP-type (de)hydrosilylation reactions with rational tuning of the C2-aryl substituent enhancing catalytic activity.T he ability of ac ationic iminium moietyt oi nitiate catalytic (de)hydrosilylation reactions by activation of silane s-bonds is notable as these moieties are viewedg enerally as substrates for reductioni nF LP chemistry and not as catalysts themselves.

Experimental Section
Supporting Information for this article includes experimental details, spectra, computational and crystallographic data. CCDC 1501133 contains the supplementary crystallographic data for this paper.T hese data are provided free of charge by The Cambridge Crystallographic Data Centre. supporting this publication are available as supplementary information accompanying this publication.