Hydrosilylation and Hydrosilylation Catalyst

Introduction

Hydrosilanes are relatively stable compounds, but they can react with unsaturated bonds in the presence of metal catalysts. This method has increasingly developed into a synthetic method of organosilicon compounds. In recent years, it has played an important role in the surface modification of semiconductors and the development of some functional materials.

[CP*RU(MECN)3]PF6：A HIGHLY EFFICIENT HYDROSILYLATION CATALYSTS

Vinylsilanes are versatile organometallic reagents involved in a variety of reactions, such as Tamao–Fleming oxidation, olefin metathesis, palladium-catalyzed cross-coupling, protodesilylation and cycloaddition. Among the available methods for the preparation of vinylsilanes, the hydrosilylation of alkynes is the most straightforward and atomically economical method (Scheme 1). Also, a number of transition metal catalysts have been designed to perform these reactions in a region- and stereocontrolled manner (Figure 1).

Scheme 1.Hydrosilylation of Alkynes

Figure 1.Transition metal catalysts have been devised to execute reactions in a regio- and stereocontrolled fashion.

The hydrosilylation of terminal alkynes was developed recently for the preparation of cis- and trans-β-vinylsilanes. Classical Pt-catalysis (Speier's and Karstedt's catalysts)1-5 as well as Rh-based catalysis ([Rh(cod)2]BF4 and [RhC(nbd)]2)6-9 are still powerful methods for the synthesis of trans-β-vinylsilanes.Wilkinson catalysts have also been verified to yield trans products in polar solvents, while in non-polar media the cis-isomers predominate. Ru-base catalysts such as [Ru(benzene)Cl2 or [Ru(p-cymene)Cl2] give cis-β-vinylsilanes. 10-13 Although the stereoselectivity and regioselectivity of hydrosilylation are highly dependent on the alkyne, the silane and the solvent, Grubbs’ 1st generation catalyst also yielded cis products under certain conditions. Despite the existence of a large number of methods for the preparation of linear β-vinylsilanes, until recently there was no general method for the preparation of 1,1-disubstituted α-vinylsilanes. Furthermore, although selective hydrosilylation within internal alkyne molecules can be achieved, the selectivity of the intermolecular variants is practically unknown. The Trost group at Stanford University has developed a very robust scheme for the hydrosilylation of terminal acetylenes to afford α-vinylsilanes via [CP*RU(MECN)3]PF6.14 This catalyst also provides an efficient method for the regioselective intramolecular and intermolecular hydrosilylation of internal alkynes to produce only Z-trisubstituted alkenes.

Intermolecular Hydrosilylation: Terminal Alkynes

A set of different terminal alkynes underwent rapid and mild hydrosilylation in the presence of [CP*RU(MECN)3]PF6 to afford 1,1-bisubstituted α-vinylsilanes in good to excellent yields and low catalyst loadings (Scheme 2). The reaction is tolerant to a wide range of functional groups including halogens, free alcohols, alkenes, internal alkynes, esters, and amines. In addition, large amounts of silane can be used in reactions with good predictability.

Scheme 2.Intermolecular Hydrosilylation: Terminal Alkynes

Intermolecular Hydrosilylation: Internal Alkynes

As shown in Scheme 1, the non-selective hydrosilylation of internal alkynes will likely yield four isomeric addition products. The Trost group has demonstrated that the hydrosilylation of endoalkynes with [Cp*Ru(MeCN)3]PF6 yields only trisubstituted Z-vinylsilanes as a result of the reverse addition of silanes to alkynes (Scheme 3).15

Scheme 3.Intermolecular Hydrosilylation: Internal Alkynes

Importantly, the hydrosilylation reaction exhibits a high level of regioselectivity. The regioselectivity can be summarized as follows: (i) hydrosilylation of 2-alkynes leads to the formation of Z- alkenes in which silyl groups occupy the less spatially demanding positions (1&2); (ii) for substrates where the alkynes are not in the 2-position, the silyl substituents will occupy the more spatially demanding positions (4); (iii) for propargylic, homopropargylic, and bishomopropargylic alcohol substrates, hydrosilylation occurs such that the silyl groups located distal to the hydroxyl functional group of the Z- alkene (5-9); (iv) in the case of α,β-alkynyl carbonyl groups, the silyl group again selectively occupies the distal position of the Z- alkene (10-13).15,16 For free propargyl, higher propargyl, and higher propargyl substrates, hydrosilylation with a silane bearing a leaving group (e.g., an ethoxy substituent) to form cyclic siloxanes (5&8). Notably, hydrosilylation using [Cp*Ru(MeCN)3]PF6 can be carried out while maintaining the stereochemical integrity of the asymmetric center in the alkyne group (9). Finally, although the nonspatially differentiated alkynes undergo steric but nonregionally selective hydrosilylation (3), the prodesilylation of the product mixture provides a single trans diastereomer. This provides a valuable complement to the cis-selectivity observed under Lindlar reducing conditions.

Even highly reactive silanes can participate in intermolecular hydrosilylation reactions with excellent predictability (Scheme 4). The generated alkenylchlorosilane combines with hexadienol to produce siloxane bonds. Heating the triene leads to an intramolecular Diels-Alder (IMDA) reaction to produce a siloxane with four consecutive stereocenters. The adducts can then be subjected to either original desilylation or Tamao-Fleming conditions to provide primary alcohols or diols, respectively.

Scheme 4. Highly reactive silanes can participate in intermolecular hydrosilylation reaction, with excellent predictability.

It is also feasible to treat the alkene prior to protodesilylation or oxidation. For example, vinylsilanes are readily epoxidized by m-CPBA in a diastereoselective manner (Scheme 5). Subsequent prodesilylation provides the corresponding synthetic epoxy alcohol, while Tamao-Fleming oxidation provides the synthetic diol. Thus, this process can be used as an alternative to hydroxyaldehyde condensation.

Scheme 5.Vinylsilanes are readily epoxidized by m-CPBA in a diastereoselective fashion.

Intramolecular Hydrosilylation

Finally, intramolecular hydrosilylation can be achieved using hydroxyalkynes, as shown in Scheme 6 for the synthesis of 3-hydroxypiperidine alkaloid (+)-spectaline (Scheme 6). 17 Treatment of homopropargyl alcohol with tetramethyldisilazane (TMDS) followed by regioselective (distal) and stereoselective (Z) intramolecular hydrosilylation gave cyclic azidosiloxanes. The product was obtained in appreciable yields by Tamao-Fleming oxidation followed by reduction and concomitant cyclization.

Scheme 6.Concise synthesis of the 3-hydroxypiperidine alkaloid (+)-spectaline

Intramolecular hydrosilylation and subsequent cross-coupling provide an excellent method for introducing new carbon bonds on alkyne carbons with the free hydroxyl group located at the distal end (Scheme 7)18.

Scheme 7.Alkyne carbon that is in a remote position from a free hydroxyl group

Aladdin offers [Cp*Ru(MeCN)3]PF6, and a wide range of other catalysts for hydrosilylation.

Reference：

1. Speier, J. L.; Webster, J. A.; Bernes, G. H. J. Am. Chem. Soc. 1957, 79, 974. https://doi.org/10.1021/ja01561a054

2. Lewis, L. N.; Sy, K. G. ; Bryant, G. L.; Donahue, P. E. Organometallics 1991, 10, 3750. https://doi.org/10.1021/om00056a055

3. Denmark, S. E; Wang, Z. Org. Lett. 2001, 3, 1073. https://doi.org/10.1021/ol0156751

4. Itami, K.; Mitsudo, K.; Nishino, A.; Yoshida, J. J. Org. Chem. 2002, 67, 2645. https://doi.org/10.1021/jo0163389

5. Kettler, P. B. Org. Proc. Res. Dev. 2003, 7, 342. https://doi.org/10.1021/op034017o

6. Ojima, I.; Kumagai, M. J. Organomet. Chem. 1974, 66, C14. https://doi.org/10.1016/S0022-328X(00)93873-7

7. Dickers, H. M.; Haszeldine, R. N.; Mather, A. P.; Parish, R. V. J. Organomet. Chem. 1978, 161, 91. https://doi.org/10.1016/S0022-328X(00)80914-6

8. Takeuchi, R.; Tanouchi, N. J. Chem. Soc., Perkin Trans. 1 1994, 2909. https://doi.org/10.1039/P19940002909

9. Takeuchi, R.; Nitta, S.; Watanabe, D. J. Org. Chem. 1995, 60, 3045. https://doi.org/10.1021/jo00115a020

10. Esteruelas, M. A.; Herrero, J.; Oro, L. A. Organometallics 1993, 12, 2377. https://doi.org/10.1021/om00030a057

11. Na, Y.; Chang, S. Org. Lett. 2000, 2, 1887. https://doi.org/10.1021/ol0059697

12. Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2001, 123, 12726. https://doi.org/10.1021/ja0121033

13.Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2005, 127, 17644. https://doi.org/10.1021/ja0528580

14. Trost Barry M.,Ball Zachary T.. Intramolecular Endo-Dig Hydrosilylation Catalyzed by Ruthenium:  Evidence for a New Mechanistic Pathway[J]. J. Am. Chem. Soc.,2002,125(1).

https://doi.org/10.1021/ja028766h

15. Denmark SE, Pan W. 2002. Org. Lett.. 44163.

16. Chung LWea. 2003. J. Am. Chem. Soc.. 12511578.

17. Trost BM, Ball ZT, Jöge T. 2002. A Chemoselective Reduction of Alkynes to (E)-Alkenes. J. Am. Chem. Soc.. 124(27):7922-7923. https://doi.org/10.1021/ja026457l

18. Trost BM, Machacek MR, Ball ZT. 2003. Ruthenium-Catalyzed Vinylsilane Synthesis and Cross-Coupling as a Selective Approach to Alkenes: Benzyldimethylsilyl as a Robust Vinylmetal Functionality. Org. Lett.. 5(11):1895-1898. https://doi.org/10.1021/ol034463w