Trichlorosilane-Dimethylformamide (Cl3SiH-DMF) as an Efficient Reducing Agent. Reduction of Aldehydes and Imines and Reductive Amination of Aldehydes under Mild Conditions Using Hypervalent Hydridosilicates

Kobayashi, Shū; Yasuda, Manabu; Hachiya, Iwao

doi:10.1246/cl.1996.407

Cited by 164 publications

(71 citation statements)

References 13 publications

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“…The concept that neutral and anionic Lewis bases can promote the transfer of activated silyl nucleophiles to carbonyl substrates offers a useful alternative to the use of metal -based catalysts and was fi rst used by the group of Kobayashi for the reduction of aldimines with a trichlorosilane/DMF reagent [31] . This concept has also been widely used in the fi eld of asymmetric catalysis for various types of silyl nucleophiles, including allyl -, enol -, cyanide -and hydride -based reagents.…”

Section: Chiral Lewis Bases As Catalystsmentioning

confidence: 99%

Hydrosilylation of Imines

Riant¹

2008

Modern Reduction Methods

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13 IntroductionThe fi rst examples of the production of chiral secondary alcohols and amines by the reduction of a carbonyl or imino group using a silane as a reducing agent and a transition metal catalyst was reported during the mid 1970s by Kagan [1] . Following this fi rst report, the literature showed a growing interest in the discovery of effi cient catalytic systems for the hydrosilylation of ketones and aldehydes, while imines only started to show signifi cant advances much later . One of the main reasons is that most of the effi cient catalytic systems for the reduction of ketones were rhodium -based catalysts that usually required activated silanes such as bisaryl silanes. While these systems were active under very mild conditions, they suffered from strong economic drawbacks due to the cost of the catalysts, relatively poor catalytic effi ciency compared to hydrogenation and the cost of the silanes. Imines being poorer substrates than aldehydes and ketones suffered even more from these limitations and were neglected as substrates for hydrosilylation reactions until more effi cient systems were discovered during the 1990s. Indeed, imines are less electrophilic substrates compared to their carbonyl counterparts and are also susceptible to isomerization to enamines , when they posses one or more α -hydrogen. It is, however, possible to increase the electrophilic character of the imino group by the introduction of an aryl or an electron -withdrawing group on the nitrogen. The most common activating groups are tosyl, Boc or diphenylphosphinoyl. However, it is usually considered that only diphenylphosphinoyl combines ease of synthesis, stability to moisture and simplicity of removal for further transformations in synthesis. When designing a chiral catalyst for enantioselective addition of a nucleophile on an aldimine or a ketimine, another diffi culty arises from the possible cis -trans isomerization of the C = N double bond.However, nucleophilic addition to imines remains an attractive method for the synthesis of amines and increasing numbers of examples in the fi eld of asymmetric catalysis have been reported during the since the 1990s [2] . Concerning reduction processes, the costs of the catalytic systems and the silanes have often drawn chemists to develop more economic alternatives, such as hydrogenation.

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Section: Chiral Lewis Bases As Catalystsmentioning

confidence: 99%

Hydrosilylation of Imines

Riant¹

2008

Modern Reduction Methods

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“…The results obtained using 3a-f as activators are summarized in Table 1, which also shows the results using 1a-e for comparison. Although the yield of 5a in the absence of activators was low (entry 1), the reduction became more efficient by addition of a catalytic amount of N-formylpyrrolidines 1a-e (entries [2][3][4][5][6]. Similarly, compounds 3a-f were found to work as activators for Cl 3 SiH (entries 7-12).…”

Section: A 5amentioning

confidence: 99%

“…One of such reagents may be trichlorosilane (Cl 3 SiH), a liquid material easily available from silicon industry, 4 though some activator is necessary for Cl 3 SiH to efficiently reduce ketones and imines. 5 We already reported chiral N-formylpyrrolidine derivatives 1 as organic activators in the enantioselective reduction of ketones 6 and imines 7 with Cl 3 SiH. The reduction proceeds smoothly at room temperature with good yields and enantioselectivity of up to 43%ee…”

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confidence: 98%

New organic activators for the enantioselective reduction of aromatic imines with trichlorosilane

Onomura

Kouchi

Iwasaki

et al. 2006

Tetrahedron Letters

108

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Abstract-N-Picolinoyl-(2S)-(diphenylhydroxymethyl)pyrrolidine was found to work as an organic activator in the reduction of aromatic imines to the corresponding amines by Cl 3 SiH. The highest selectivity was 80%ee. This is the first data showing that N-formyl group is not always essential as N-protecting group of pyrrolidine derivatives for the reduction of imines by Cl 3 SiH.Enantioselective reduction of ketones 1 and imines 2 has been one of recent topics in asymmetric synthesis. 3 A variety of reducing reagents have been used in the reductions but it is still worthwhile to exploit new methods which can be carried out using inexpensive reducing reagents under mild conditions. One of such reagents may be trichlorosilane (Cl 3 SiH), a liquid material easily available from silicon industry, 4 though some activator is necessary for Cl 3 SiH to efficiently reduce ketones and imines. 5 We already reported chiral N-formylpyrrolidine derivatives 1 as organic activators in the enantioselective reduction of ketones 6 and imines 7 with Cl 3 SiH. The reduction proceeds smoothly at room temperature with good yields and enantioselectivity of up to 43%eefor the reduction of ketones and 66%ee for the reduction of imines. Recently, a new activator 2 for Cl 3 SiH in reducing imines with high enantioselectivity (up to 92%ee) was reported. 8 The noticeable point in those reductions was that the presence of N-formyl substituent was essential for those reductions. In our continuing effort to exploit new chiral organic compounds in place of 1 to activate Cl 3 SiH, 9 we found N-picolinoylpyrrolidine derivatives 3a-f to also work as organic activators in the reduction of aromatic imine 4 to amine 5 (Eq 1). This is the first data showing that N-formyl group is not always essential in the structure of organic activators for Cl 3 SiH.10,11

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“…Silanes are normally too unreactive for a direct hydride transfer to an unsaturated compound and the addition of a suitable mediator is necessary in order increase the reactivity. This is in particular true for the trichlorosilane (3) (see Figure 14.3 ) which requires activation by a Lewis base to release its hydride [23,24] . Matsumura and coworkers found that activating trichlorosilane with catalytic amounts of N -formylpyrrolidine derivatives resulted in the chemoselective reduction of imines [25,26] .…”

Section: Silanesmentioning

confidence: 99%