Si-Containing Amides of Phosphoric Acid

The first representative of the N-silylmethylamides of phosphoric acid O=P[NMe(CH₂SiMe _n (OEt)_3-n ]₃ have been synthesized by interaction of MeNHCH₂SiMe _n (OEt)_3-n (n = 2, 3) with POCl₃. The interaction of the N,N′,N″-trimethyl-N,N′,N″-tris[(ethoxydimethyl- silyl)methyl]triamide phosphoric acid with BF₃·Et₂O or BCl₃ results in the formation of the N,N′,N″-trimethyl-N,N′,N″-tris[(fluorodimethyl-silyl)methyl]triamide phosphoric acid or N,N′,N″-trimethyl-N,N′,N″-tris[(chlorodimethylsilyl)methyl]triamide phosphoric acid. NMR data show on the tetracoordinate state of silicon in these products.     

Asymmetric hydrosilylation of prochiral ketones in the presence of N-benzyl-N-methylephedrinium halometallates

N-benzyl-N-methylephedrinium hexachloroplatinate(IV), bromotrichlororhodate(III), and dibromodichlorozincate(II) have been synthesized by reacting (—)-N-benzyl-N-methylephedrinium bromide with K₂PtCl₆, RhCl₃ · 4H₂O and ZnCl₂, respectively. The above halometallates have been found to catalyse the asymmetric hydrosilylation of acetophenone and 3-acetylpyridine with diphenylsilane. The hydrosilylation of 3-acetylpyridine in the presence of (—)-N-benzyl-N-methylephedrinium zincate followed by silyl ether hydrolysis gives 1-(3-pyridyl)ethanol in ca 50% optical yield.     

Chemoselective Reduction of Tertiary Amides to Amines Catalyzed by Triphenylborane

Triphenylborane (BPh₃) was found to catalyze the reduction of tertiary amides with hydrosilanes to give amines under mild condition with high chemoselectivity in the presence of ketones, esters, and imines. N,N‐Dimethylacrylamide was reduced to provide the α‐silyl amide. Preliminary studies indicate that the hydrosilylation catalyzed by BPh₃ may be mechanistically different from that catalyzed by the more electrophilic B(C₆F₅)₃.     

Si-Containing Amides of Phosphoric Acid

Summary. The first representative of the N-silylmethylamides of phosphoric acid O=P[NMe(CH₂SiMe _n (OEt)_3-n ]₃ have been synthesized by interaction of MeNHCH₂SiMe _n (OEt)_3-n (n = 2, 3) with POCl₃. The interaction of the N,N′,N″-trimethyl-N,N′,N″-tris[(ethoxydimethyl- silyl)methyl]triamide phosphoric acid with BF₃·Et₂O or BCl₃ results in the formation of the N,N′,N″-trimethyl-N,N′,N″-tris[(fluorodimethyl-silyl)methyl]triamide phosphoric acid or N,N′,N″-trimethyl-N,N′,N″-tris[(chlorodimethylsilyl)methyl]triamide phosphoric acid. NMR data show on the tetracoordinate state of silicon in these products. Professor Vadim Aleksandrovich Pestunovich, our chief, teacher and friend died on July 4^th, 2004     

Bis­[N,N′‐diiso­propyl‐N‐(tri­methyl­silyl)­benzamidinium] di‐μ‐chloro‐bis­[tetra­chloro­zirconate(IV)] di­chloro­methane disolvate

The structure of the salt of the di‐μ‐chloro‐bis­[tetra­chloro­zirconate(IV)] anion and the N,N′‐iso­propyl‐N‐(tri­methyl­silyl)benzamidinium cation, (C₁₆H₂₉N₂Si)₂[Zr₂Cl₁₀]·2CH₂Cl₂, is reported. The anion lies about an inversion centre and shows a substantially octahedral coordination around Zr, while the structure of the cation is unequivocally assigned as that of a benzamidinium ion.     

Mechanism of BPh3-Catalyzed N-Methylation of Amines with CO2 and Phenylsilane: Cooperative Activation of Hydrosilane

BPh₃ catalyzes the N-methylation of secondary amines and the C-methylenation (methylene-bridge formation between aromatic rings) of N,N-dimethylanilines or 1-methylindoles in the presence of CO₂ and PhSiH₃; these reactions proceed at 30–40 °C under solvent-free conditions. In contrast, B(C₆F₅)₃ shows little or no activity. ¹¹B NMR spectra suggested the generation of [HBPh₃]⁻. The detailed mechanism of the BPh₃-catalyzed N-methylation of N-methylaniline ( 1 ) with CO₂ and PhSiH₃ was studied by using DFT calculations. BPh₃ promotes the conversion of two substrates (N-methylaniline and CO₂) into a zwitterionic carbamate to give three-component species [Ph(Me)(H)N⁺CO₂⁻⋅⋅⋅BPh₃]. The carbamate and BPh₃ act as the nucleophile and Lewis acid, respectively, for the activation of PhSiH₃ to generate [HBPh₃]⁻, which is used to produce key CO₂-derived species, such as silyl formate and bis(silyl)acetal, essential for the N-methylation of 1 . DFT calculations also suggested other mechanisms involving water for the generation of [HBPh₃]⁻ species.     

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

A new catalytic asymmetric tandem α‐alkenyl addition/proton shift reaction of silyl enol ethers with ketimines was serendipitously discovered in the presence of chiral N,N′‐dioxide/Zn^II complexes. The proton shift preferentially proceeded instead of a silyl shift after α‐alkenyl addition of silyl enol ether to the ketimine. A wide range of β‐amino silyl enol ethers were synthesized in high yields with good to excellent ee values. Control experiments suggest that the Mukaiyama–Mannich reaction and tandem α‐alkenyl addition/proton shift reaction are competitive reactions in the current catalytic system. The obtained β‐amino silyl enol ethers were easily transformed into β‐fluoroamines containing two vicinal tetrasubstituted carbon centers.     

IPr3Si3Cl5+: A Highly Reactive Cation with Silanide Character

The reaction of a metastable SiCl₂ solution with the sterically less‐demanding carbene N,N‐diisopropylimidazo‐2‐ylidene (IPr) yields the salt [(IPr₃Si₃Cl₅)⁺]Cl^? ( 1 ‐Cl), containing a silyl cation with a Si₃ backbone. Salt 1 is highly reactive, but it can be used as a reagent in deuterated dichloromethane, whereby dehalogenation with Me₃SiOTf (OTf=O₃SCF₃) gives the dicationic silyl halide [(IPr₃Si₃Cl₄)]²⁺ 2 . Quantum chemical calculations show that the HOMO is localized at the negatively charged central silicon atom of 1 and 2 , and thus although both compounds are cations they are better described as silanides, which was also corroborated by NMR investigations.     

The extension of the mechanistic concept of the nucleophilic catalysis in the silicon chemistry to some reactions of the P(III) center: Analogies between silylation and phosphorylation

Many observations prove that a number of silylation reactions of a trialkylsilyl halide-uncharged base system occur with the transient formation of a 1:1 tetrahedral silicon ionic complex of the silyl halide with the base. Some catalytic processes of phosphorylation of protonic substrates with tricoordinate phosphorus halides in mixture with an uncharged base show similar features to these silylation reactions, implying that a similar mechanism may operate. It was demonstrated that Ph₂PCl phosphorylates t-BuOH faster under catalysis with 4-N,N-dimethylamino pyridine or N-methylimidazole than in the presence of Et₃N by a factor of 400 and 33, respectively. The catalytic phosphorylation process exhibits a very low activation energy and a high negative value of entropy of activation. The interaction of the uncharged bases with model tricoordinate phosphorus halides was demonstrated to lead to the formation of ionic 1:1 complexes without changing the coordination number of phosphorus, in full analogy to the silyl halide complex formation. Finally, the interaction of phosphorous tris(dimethylamide) with a silyl iodide and a phosphorous iodide results in both cases in the formation of the ionic 1:1 complex, which also leads to analogous reactions of exchange of the amide group with iodide. These close similarities imply that some phosphorylation reactions with tricoordinate phosphorus halides catalyzed with uncharged bases occur via a tricoordinate phosphorus cation intermediate.     

A monomeric three-coordinate magnesium bis(amide)

The metallation reaction between di­butyl­magnesium and 2,6-diiso­propyl-N-(tri­methyl­silyl)­aniline gives the unusual monomeric three-coordinate complex (diethyl ether-κO)­bis­[2,6-diiso­propyl-N-(tri­methyl­silyl)­anilido-κN]­magnesium(II), [Mg(C₁₅H₂₆NSi)₂(C₄H₁₀O)] or [Mg{(Me₃Si)(2,6-ⁱPr₂C₆H₃)N}₂(Et₂O)]. This low-coordinate species has a distorted trigonal-planar coordination environment, with an additional short Mg—C_ipso contact of 2.799 (2) Å.     

1,2 → 1,1-Migration of Silyl Group. Formation of 1,1-Bis(dichloromethylsilyl)-2-chloroethane in Catalytic Hydrochlorination of 1,2-Bis(dichloromethylsilyl)ethylene

Hydrochlorination of 1,2-bis(silyl)ethylenes of the general formula Me_nCl_3-n SiCH = CHSiMe_n· Cl_3-n , where n = 0-3, in the presence of FeCl₃ was carried out. By NMR spectroscopy it was shown that in the case of the MeCl₂Si derivative the major product is 1,1-bis(silyl)chloroethane and the minor product is its isomeric 1,2-bis(silyl)chloroethane (4:1). A reaction scheme is proposed, that includes isomerization of the intermediate bis(silyl)ethyl cation.     

Synthesis of N-acyl-N,O-acetals from N-aryl amides and acetals in the presence of TMSOTf

Secondary amides undergo in situ silyl imidate formation mediated by TMSOTf and an amine base, followed by addition to acetal acceptors to provide N-acyl-N,O-acetals in good yields. An analogous, high-yielding reaction is observed with 2-mercaptothiazoline as the silyl imidate precursor. Competing reduction of the acetal to the corresponding methyl ether via transfer hydrogenation can be circumvented by the replacement of CY₂NMe with 2,6-lutidine under otherwise identical reaction conditions.     

13C n.m.r. studies of organosilanes: II—substituent chemical shift parameters for alkoxysilanes

The ¹³C n.m.r. spectra of forty alkoxysilanes of the general type X_nSi(OR)_4–n (X = CH₃, C₆H₅, H; R = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, i-C₄H₉, s-C₄H₉, n-C₅H₁₁, CH(CH₃)(C₆H₅), C₆H₅) have been recorded and assigned. The chemical shifts of the α-carbon resonances of the alkoxy groups are shown to depend on both the nature of the alkoxy group and the number and type of substituents on the silicon. Regression analyses of the data give empirical substituent chemical shift (SCS) parameters for the silyl substituents. The β-carbon resonances are shown to be dependent on the presence of the silyl group, but not the specific silyl substituents.     

Modification of olefin polymerization catalysts. I. Mechanism of the interaction between AIEt3 and silyl ethers

The interaction between AlEt₃ and silyl ethers, Ph_nSi(OMe)_4-n (n = 0–3), was followed by ¹³C- and ²⁹Si-NMR techniques in conditions close to those typical for an olefin polymerization reaction with supported Ziegler–Natta catalysts (A1Et₃:silyl ether ratios from 1 to 10, temperature range 25–75°C). A1Et₃ and silyl ethers form instantaneously at ambient temperature a donor-acceptor complex, which is stable at a 1:1 molar ratio. In the presence of excess A1Et₃ the complex decomposes via a mechanism consisting, in the case of PhSi(OMe)₃, of five consecutive steps: alternating complexation and ether reductions with the formation of alkylated silyl ethers, Ph(Et)_nSi(OMe)_3-n (n = 1,2), and dialkyl-aluminum alkoxides, (Et₂A1OMe₃)_n (n = 2,3). The rate of decomposition was enhanced by the increasing number of methoxy groups present in the silyl ether, heating, or a high A1Et₃:silyl ether ratio. The decomposition was not inhibited by the presence of 1-hexene.     

Preparative and 1H NMR Investigation on Regioselective Silylation of Starch Dissolved in Dimethyl Sulfoxide

Reaction of starch 1 dissolved in dimethyl sulfoxide (DMSO) with bulky thexyldimethylchlorosilane (TDSCl) in the presence of pyridine leads to regioselectively functionalized silyl ethers with a degree of substitution (DS) up to 1.8. The control of the DS_Si, of the regioselectivity, and of the reaction pathway is described in detail. The reaction proceeds homogeneously up to DS_Si of 0.6. With ongoing silylation the polymers form a separate phase incorporating the silylating agent to form TDS‐starches with DS_Si values higher than 1.0. After peracetylation of the silyl starches, the substitution pattern has been characterized not only in the anhydroglucose repeating units (AGU) but also in the non‐reducing terminal end groups (TEG) by means of two‐dimensional ¹H NMR techniques. Up to DS_Si 1.0, a very high regioselective functionalization of the primary 6‐OH groups in the AGU as well as in the TEG is detectable. With increasing silylation (DS_Si > 1.0), the subsequent silylation takes place at the 2‐OH groups of the AGU and at the 3‐OH groups of the TEG. These results are compared with our own investigations on the silylation of starch in the reaction system N‐methylpyrrolidone (NMP)/ammonia and on the silylation of cellulose in N,N‐dimethylacetamide (DMA)/LiCl/pyridine solution.     

Synthesis,Properties, and Crystal Structure of Silyl Nitronates (Silyl Esters of aci-Nitroalkanes): Towards the SN2 Reaction Path with Retention of Configuration at Silicon

An efficient and flexible method for the preparation of silyl nitronates is described (see 1–10 ). NMR. spectral investigations indicate a rapid 1,3-silyl migration process, with an activation energy of about 10 kcal mol^?1. X-ray crystallographic studies on the silyl nitronates 3 and 8 show structures that lean towards an S_N2 retention pathway at silicon.     

Coupling Reaction of Enol Derivatives with Silyl Ketene Acetals Catalyzed by Gallium Trihalides

A cross‐coupling reaction between enol derivatives and silyl ketene acetals catalyzed by GaBr₃ took place to give the corresponding α‐alkenyl esters. GaBr₃ showed the most effective catalytic ability, whereas other metal salts such as BF₃?OEt₂, AlCl₃, PdCl₂, and lanthanide triflates were not effective. Various types of enol ethers and vinyl carboxylates as enol derivatives are amenable to this coupling. The scope of the reaction with silyl ketene acetals was also broad. We successfully observed an alkylgallium intermediate by using NMR spectroscopy, suggesting a mechanism involving anti‐carbogallation among GaBr₃, an enol derivative, and a silyl ketene acetal, followed by syn‐β‐alkoxy elimination from the alkylgallium. Based on kinetic studies, the turnover‐limiting step of the reaction using a vinyl ether and a vinyl carboxylate involved syn‐β‐alkoxy elimination and anti‐carbogallation, respectively. Therefore, the leaving group had a significant effect on the progress of the reaction. Theoretical calculations analysis suggest that the moderate Lewis acidity of gallium would contribute to a flexible conformational change of the alkylgallium intermediate and to the cleavage of the carbon?oxygen bond in the β‐alkoxy elimination process, which is the turnover‐limiting step in the reaction between a vinyl ether and a silyl ketene acetal.     

Heterosubstituted cyclopropanation of alkenes with organochromium reagents derived from heterosubstituted dihalomethanes,CrCl2, and tetraalkylethylenediamine

Iodocyclopropanes of trans configuration are produced stereoselectively from terminal alkenes by treatment with a reagent derived from iodoform, chromium(II) chloride, and TEEDA (N,N,N′,N′-tetraethylethylenediamine) in THF. Similarly, cyclopropylsilanes and cyclopropylboronic esters are obtained by using R₃SiCHI₂, and a combination of Cl₂CHB(OR)₂ and LiI instead of iodoform, respectively. The heterocyclopropanation occurs selectively at terminal double bonds, and di- and trisubstituted double bonds in the same molecules remain unchanged. Such functional groups as alcohol, ether, silyl ether, ester, tertiary amine, and amide groups are compatible with the reaction conditions.     

One‐Pot Synthesis of (−)‐Oseltamivir and Mechanistic Insights into the Organocatalyzed Michael Reaction

The one‐pot sequential synthesis of (?)‐oseltamivir has been achieved without evaporation or solvent exchange in 36 % yield over seven reactions. The key step was the asymmetric Michael reaction of pentan‐3‐yloxyacetaldehyde with (Z)‐N‐2‐nitroethenylacetamide, catalyzed by a diphenylprolinol silyl ether. The use of a bulky O‐silyl‐substituted diphenylprolinol catalyst, chlorobenzene as a solvent, and HCO₂H as an acid additive, were key to produce the first Michael adduct in both excellent yield and excellent diastereo‐ and enantioselectivity. Investigation into the effect of acid demonstrated that an acid additive accelerates not only the E–Z isomerization of the enamines derived from pentan‐3‐yloxyacetaldehyde with diphenylprolinol silyl ether, but also ring opening of the cyclobutane intermediate and the addition reaction of the enamine to (Z)‐N‐2‐nitroethenylacetamide. The transition‐state model for the Michael reaction of pentan‐3‐yloxyacetaldehyde with (Z)‐N‐2‐nitroethenylacetamide was proposed by consideration of the absolute configuration of the major and minor isomers of the Michael product with the results of the Michael reaction of pentan‐3‐yloxyacetaldehyde with phenylmaleimide and naphthoquinone.     

Silylhydrazine und dimere N,N′‐Dilithium‐N,N′‐bis(silyl)hydrazide – Synthesen,Reaktionen, Isomerisierungen

Silylhydrazines and Dimeric N,N′‐Dilithium‐N,N′‐bis(silyl)hydrazides – Syntheses, Reactions, Isomerisations Di‐tert.‐butylchlorosilane reacts with dilithiated hydrazine in a molar ratio to give the N,N′‐bis(silyl)hydrazine, [(Me₃C)₂SiHNH]₂, ( 5 ). Isomeric tris(silyl)hydrazines, N‐difluorophenylsilyl‐N′,N′‐bis(dimethylphenylsilyl)hydrazine ( 7 ) and N‐difluorophenylsilyl‐N,N′‐bis(dimethylphenylsilyl)hydrazine ( 8 ) are formed in the reaction of N‐lithium‐N′‐N′‐bis(dimethylphenylsilyl)hydrazide and F₃SiPh. Isomeric bis(silyl)hydrazines, (Me₃C)₂SiFNHNHSiMe₂Ph ( 9 ) and (Me₃C)₂‐ SiF(PhMe₂Si)N–NH₂ ( 10 ) are the result of the reaction of di‐tert.‐butylfluorosilylhydrazine and ClSiMe₂Ph in the presence of Et₃N. Quantum chemical calculations for model compounds demonstrate the dyotropic course of the rearrangement. The monolithium derivative of 5 forms a N‐lithium‐N′,N′‐bis(silyl)hydrazide ( 11 ). The dilithium salts of 5 ( 13 ) and of the bis(tert.‐butyldiphenylsilyl)hydrazine ( 12 ) crystallize as dimers with formation of a central Li₄N₄ unit. The formation of 12 from 11 occurs via a N′ → N‐silyl group migration. Results of crystal structure analyses are reported.