A dipole moment study of the structures of some ketene acetals

The dipole moments of several acyclic and cyclic ketene acetals have been determined in benzene solution at 293 K using the Halverstadt-Kumler method. For ketene dialkyl acetals (alkyl = Me, Et) the results point to a predominance of the s-cis,s-trans retamer, which disagrees with the conclusions drawn previously from ¹³C NMR chemical shift data, i.e. the s-cis,s-cis form is the more stable species. In the case of 2-methoxyfuran, the dipole moment measurements confirm the previous findings based on the ¹³C NMR spectra, viz the s-cis form is the predominating rotamer. The dipole moments and structures of some other ketene acetals are also discussed.     

The use of carbon black-supported sulfuric acid to initiate the cationic polymerization of cyclic ketene acetals

Stable polymers were made by the cationically initiated 1,2-polymerization of cyclic ketene acetals employing heterogeneous, activated carbon-supported sulfuric acid catalysts. A methodology has been established for the preparation of the carbon black of different acidic strengths. By adjusting either the acid strength or the amount of carbon black used, cyclic ketene acetals with different activities can be polymerized efficiently to form stable high molecular weight polymers. This methodology will be a useful tool for polymerization, copolymerization, and studies of the relative reactivities of the cyclic ketene acetals. The polymer structures were determined by FTIR, ¹³C-NMR, and ¹H-NMR studies. © 1996 John Wiley & Sons, Inc.     

Ketene acetal monomers: Synthesis and characterization

A new, facile, and high-yield synthesis of ketene acetals derived from readily available and inexpensive starting materials has been developed. For example, an α,β-unsaturated aldehyde can be condensed with an alkane diol to afford a 2-vinyl substituted cyclic acetal. This latter compound can be converted to the desired cyclic ketene acetal by isomerization of the double bond in the presence of tris(triphenylphosphine)ruthenium(II) dichloride. Good to excellent yields of cyclic ketene acetals were obtained employing this method. The novel monomers were fully characterized by IR, NMR, and by elemental analysis. © 1996 John Wiley & Sons, Inc.     

Assignment of ring size in isopropylidene acetals by 13C N.M.R.

Cyclic isopropylidene acetals containing 5,6 and 7-membered rings can be distinguished by measurement of the ¹³C chemical shifts of the acetal carbon and the methyl groups.     

Study of photocrosslinkable polysiloxanes bearing gem di-styrenyl groups: Synthesis and thermal properties

The synthesis of photocrosslinkable polysiloxanes containing gem di-oxaalkylene styrenyl groups and gem di-urethane-α-methyl styrenyl groups has been performed by copolycondensation of α,ω-dihydroxy polydimethyl siloxanes and dichlorosilanes bearing either cyclic acetal groups or Si-H groups (onto which the cyclic acetal groups are further added) and dichlorosilanes bearing alkyl groups. The introduction of styrenyl groups was then achieved by hydrolysis of the acetal groups into the corresponding alcohols followed by reaction with chloromethyl styrene or with 3-isopropenyl-α,α- dimethylbenzyl isocyanate.The structure of the different products synthesized was determined by IR, ¹H, ¹³C and ²⁹Si NMR spectroscopies. The thermal properties of the polysiloxanes bearing gem di-styrenyl groups have been studied at low and high temperatures.These products have been crosslinked under UV, in the presence of a cationic photoinitiator, and showed very good release paper properties.     

Stable polymers from cyclic ketene acetals: Cationic polymerization initiated by acid-washed glassware or acid-washed glass beads

Stable polymers were made by the cationic 1,2-polymerization of cyclic ketene acetals initiated by acid-washed glassware or acid-washed glass beads. Among several reactions possible for the very reactive cyclic ketene acetals, only the corresponding acetals of polyketene were formed. These structures were demonstrated by FTIR, ¹H-NMR, and ¹³C-NMR analyses. © 1996 John Wiley & Sons, Inc.     

The first synthesis of cyclopropanone acetals from the reaction of Fischer carbene complexes with ketene acetals

The reaction of iso-propoxy stabilized Fischer carbene complexes with ketene acetals gives moderate to excellent yields of cyclopropanone acetals when carried out under a carbon monoxide atmosphere. This is in contrast to the known reaction of methoxy substituted complexes which give cyclic ortho esters under the same conditions. A mechanism is proposed which involves a branch point between the two products as the zwitterionic intermediate resulting from nucleophilic addition of the ketene acetal to the carbene carbon. A 1,3-migration of the methoxyl group to the cationic center leads to the ortho ester and a ring closure by backside attack leads to the cyclopropanone acetal. A double-labeling experiment shows that the 1,3-migration occurs by an intramolecular process that is proposed to involve a bridging oxonium ion. The effect of the isopropoxy group is thus interpreted to be to sterically hinder the formation of a bridged oxonium ion.     

A Precautionary Note Regarding Derivatization of Secondary Amines with Dialkylacetals of Formamide Other Than the Diethyl Derivative

Abstract

We have extended our earlier work on alkylation of secondary amino compounds with dimethylformamide diethyl acetal to other alkyl acetals, such as dimethyl and dipropyl acetal. All the alkylating DMT-dialkyl acetal reagents tested gave only N-ethyl derivatives of the secondary amines. The reagent purity was tested by preparing the esters of carboxylic acids, where the appropriate alkyl (methyl or propyl) ester is obtained. It is concluded that all dimethylformamide dialkyl acetals yield a single N-ethyl derivative of secondary amine. The reaction mechanism is not understood.     

Features of Acetal Interchange of 1,1-Dialkoxyalkanes with 1,2-Propylene Glycol in Neutral Solutions

Reactions fo 1,1-dimethoxymethane, 1,1-dimethoxyethane, and 1,1-dimethoxypropane with 1,2-propylene glycol at 25°C in neutral solutions were studied by ¹³C NMR spectroscopy. Under these conditions, 1,1-dimethoxymethane does not react with the glycol. Only the primary OH group of the glycol participates in the acetal interchange. The reactions of 1,1-dimethoxyethane and 1,1-dimethoxypropane with the glycol, along with the linear acetals, yielded 2-alkyl-4-methyl-1,3-dioxolanes. The product ratio depends on the molar ratio 1,1-dialkoxyalkane : glycol. In the system 1,1-dimethoxypropane-1,2-propylene glycol, the equilibrium concentration of the cyclic acetal slightly increases with temperature in the range 25-80°C.     

Cyclohexanone Dimethyl Acetals: A 13C NMR, Thermodynamic, and Ab Initio Study

The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by ¹³C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the ¹³C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol^–1, while the entropy term favors the less stable isomer by 4 to 16 J K^–1 mol^–1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol^–1 in G _m at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations.     

Anodic cyclization reactions: probing the chemistry of N,O-ketene acetal derived radical cations

The chemical reactivity of radical cations derived from N,O-ketene acetals has been examined and compared with the reactivity of radical cations derived from both ketene dithioacetals and enol ethers. Synthetically, the N,O-ketene acetal radical cations lead to more efficient cyclization reactions than either the ketene dithioacetal or enol ether derived radical cations. Cyclic voltammetry experiments using allylsilane trapping groups show that the efficiency of these cyclizations is not due to the N,O-ketene acetal radical cations being more reactive but rather more stable to decomposition. Finally, cyclizations using chiral oxizolidinones were examined.     

Ketene dithioacetal derivatives of 1-phenyl-3,5-dioxopyrazolidine. Synthesis and NMR characterization

1-Phenyl-3,5-dioxopyrazolidine 1 reacts with carbon disufide and alkyl halides in presence of excess of sodium acetate in dimethylformamide to afford the ketene dithioacetals 3a-h . The ¹³C chemical shift assignments of these compounds were made on the basis of two-dimensional nmr studies performed on the N-methylketene dithioacetal derivative 4.     

N-alkylation of 4-chloro-5-cyano-1,2,3-triazole with orthoformic acid derivatives

The N-alkylation of 4-chloro-5-cyano-1,2,3-triazole with methyl and ethyl orthoformate, and the dimethyl, diethyl and ethylene acetals of DMF has been examined. Methylation gives all three N-methyl isomers, whereas ethylation and hydroxyethylation gives the 2-N-alkyl derivatives only. It has been shown for the first time that it is possible to use DMF ethylene acetal to obtain N-hydroxyethylazoles. The structures of the products were established by¹³C NMR spectroscopy.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 920–924, July, 1988.     

Etude de la reaction chlorocarbene-acetals de cetenes : II. Stereoselectivite de la reaction

We have studied the stereoselectivity of the addition reaction of chloro and chloromethyl carbenoids with ketene alkylsilyl acetals. The best stereoselectivity was generally observed with the dimethyl tertiobutylsilyloxy group. With the chlorocarbenoid, using an E ketene acetal we obtained in majority (8?0%) an E α,β-ethylenic ester and using a Z ketene acetal we obtained in majority (7?0%) a Z α,β-ethylenic ester. In the case of the chloromethyl carbenoid the two ketene acetal isomers led to the same E α-substituted α,β-ethylenic ester (8?8% of selectivity). With the chlorophenylcarbenoid, formation of 9?0% of E α phenyl α,β-ethylenic ester is observed.     

Experimental and theoretical studies on Mannich-type reactions of chiral non-racemic N-(benzyloxyethyl) nitrones

The nucleophilic addition of both silyl ketene acetals and lithium enolates derived from methyl acetate to chiral non-racemic N-(benzyloxyethyl)nitrones has been studied both experimentally and theoretically. Aromatic nitrones showed lower reactivity that aliphatic nitrones and the addition of the silyl ketene acetal led to lower selectivities than the addition of the corresponding lithium enolate. Whereas low selectivity was obtained for the addition of the silyl ketene acetal, only one diastereomer could be detected in all cases for the addition of lithium enolate to aliphatic nitrones. The synthetic utility of the two chiral auxiliaries employed lies in the preparation of enantiomeric compounds. DFT theoretical calculations confirmed the stepwise mechanism for the addition of silyl ketene acetals to nitrones and are in good agreement with the observed experimental results.     

Ring opening during the cationic polymerization of 2-methylene-1,3-dioxepane: Cyclic ketene acetal initiation with sulfuric acid supported on carbon

The stable cyclic ketene acetal, 2-methylene-1,3-dioxepane, 7, has been polymerized cationically in pentane, CH₂Cl₂ and THF at 25°C to form a polymer which is composed of both ring-opened (40–50%) and ring-retained (50–60%) structures. Initiation was catalyzed by using H₂SO₄-supported on activated carbon black. This unique outcome differs significantly from the cationic polymerization of several other five- and six-membered ring cyclic ketene acetals which gave 100% 1,2-vinylpolymerization under these conditions. As the polymerization temperature increased in cationic polymerization of 7 the ring-opened content increased and the molecular weight of the polymers decreased in such solvents as cyclohexane, 1,2-dichloroethane, dimethoxyethane, and bis-(2-methoxyethyl) ether. The mechanism of this polymerization is discussed. This research also illustrated the ability to initiate the cationic polymerization of cyclic ketene acetals by acidified carbon black while avoiding subsequent polymer decomposition. © 1997 John Wiley & Sons, Inc.     

Co2(CO)8-catalyzed carbonylation of acetals using N-silylamines

Formaldehyde dialkyl acetals and cyclic acetal, 1,3-dioxolane, are smoothly carbonylated using N-silylamines at 140–160 °C under 70–88 kg cm⁻² CO pressure in the presence of a catalytic amount of Co₂(CO)₈ to give the corresponding 2-alkoxyamides in moderate to good yields. In the carbonylation of formaldehyde dialkyl acetals using N-silylamines, addition of pyridine drastically enhances the catalytic activity.     

Vicinal alkylation-carboxymethylation of electron-poor alkenes by radical-chain reactions with O-alkyl O-silyl ketene acetals and their [3+2] annulation by reaction with O-cyclopropylcarbinyl O-silyl ketene acetals

O-Silyl ketene acetals of the type H₂CC(OR)OSiMe₂Bu^t, in which R is a tertiary or secondary alkyl group, react with electron-poor alkenes to bring about vicinal alkylation-carboxymethylation of the latter. When R is a cyclopropyldimethylcarbinyl group such reactions take a more complex course involving ring opening of the cyclopropylcarbinyl radical and lead ultimately to [3+2] annulation of the alkene.     

Exploring the Border between Concerted and Two‐Step Pathways of 1,3‐Dipolar Cycloadditions of Organic Azides to Cyclic Ketene N,X‐Acetals. – Synthesis and 15N‐NMR Spectra of Zwitterions and Spirocyclic Cycloadducts

Cyclic ketene N,X‐acetals 1 are electron‐rich dipolarophiles that undergo 1,3‐dipolar cycloaddition reactions with organic azides 2 ranging from alkyl to strongly electron‐deficient azides, e.g., picryl azide ( 2L ; R¹=2,4,6‐(NO₂)₃C₆H₂) and sulfonyl azides 2M – O (R¹=XSO₂; cf. Scheme 1). Reactions of the latter with the most‐nucleophilic ketene N,N‐acetals 1A provided the first examples for two‐step HOMO(dipolarophile)–LUMO(1,3‐dipole)‐controlled 1,3‐dipolar cycloadditions via intermediate zwitterions 3 . To set the stage for an exploration of the frontier between concerted and two‐step 1,3‐dipolar cycloadditions of this type, we first describe the scope and limitations of concerted cycloadditions of 2 to 1 and delineate a number of zwitterions 3 . Alkyl azides 2A – C add exclusively to ketene N,N‐acetals that are derived from 1H‐tetrazole (see 1A ) and 1H‐imidazole (see 1B , C ), while almost all aryl azides yield cycloadducts 4 with the ketene N,X‐acetals (X=NR, O, S) employed, except for the case of extreme steric hindrance of the 1,3‐dipole (see 2E ; R¹=2,4,6‐(^tBu)₃C₆H₂). The most electron‐deficient paradigm, 2L , affords zwitterions 16D , E in the reactions with 1A , while ketene N,O‐ and N,S‐acetals furnish products of unstable intermediate cycloadducts. By tuning the electronic and steric demands of aryl azides to those of ketene N,N‐acetals 1A , we discovered new borderlines between concerted and two‐step 1,3‐dipolar cycloadditions that involve similar pairs of dipoles and dipolarophiles: 4‐Nitrophenyl azide ( 2G ) and the 2,2‐dimethylpropylidene dipolarophile 1A (R, R=H, ^tBu) gave a cycloadduct 13 H , while 2‐nitrophenyl azide ( 2 H ) and the same dipolarophile afforded a zwitterion 16A . Isopropylidene dipolarophile 1A (R=Me) reacted with both 2G and 2 H to afford cycloadducts 13G , J ) but furnished a zwitterion 16B with 2,4‐dinitrophenyl azide ( 2I) . Likewise, 1A (R=Me) reacted with the isomeric encumbered nitrophenyl azides 2J and 2K to yield a cycloadduct 13L and a zwitterion 16C , respectively. These examples suggest that, in principle, a host of such borderlines exist which can be crossed by means of small structural variations of the reactants. Eventually, we use ¹⁵N‐NMR spectroscopy for the first time to characterize spirocyclic cycloadducts 10 – 14 and 17 (Table 6), and zwitterions 16 (Table 7).     

A Precautionary Note Regarding Derivatization of Secondary Amines with Dialkylacetals of Formamide Other Than the Diethyl Derivative

Abstract

In an earlier communication we reported GC-MS studies on the reaction products of secondary amino tricyclics and dimethylformamide.¹ The diethyl acetal reaction products were identified as N-ethyl derivatives on the basis of mass spectroscopic analysis. In a follow-up study, we reported the same reaction products were formed from other DMF -dialkyl acetals, such as dimethyl and dipropyl acetals.² In view of this unusual reaction we reinvestigated the structure of the reaction products utilizing alternative spectroscopic methods, viz. NMR and IR.