Photochemical reactions. 148th communication. Photochemistry of acylsilanes: Preparation,photolyses, and thermolyses of α,β-unsaturated silyl ketones

The syntheses, photolyses, and thermolyses of the α,β-unsaturated silyl ketones (E/Z)-7, (E)- 8 , and (E)- 9 are described. On n,π*-excitation (λ > 347 mm), the aforementioned compounds undergo (E/Z)-isomerization followed by γ-H abstraction. The intermediate enols are trapped intermolecularly by siloxycarbenes leading to the dimeric acetals 27A + B, 30A + B , and 31A + B . In addition, the acylsilanes (E/Z)- 7 undergo photoisomerization by δ-H abstraction furnishing the acylsilanes 29A + B . Flash vacuum thermolyses (FVT) of (E/Z)- 7 , (E/Z)- 8 , and (E)- 9 give rise to intramolecular reactions of the siloxycarbene intermediates. Thus, FVT (520°) of (E)- and (Z)- 7 selectively leads to the enol silyl ethers 32 and (E)- 33 , respectively, arising from carbene insertion into an allylic C–-H bond. FVT of (E/Z)- 8 (560°) and (E)- 9 (600°) affords the trienol silyl ethers 34A + B and the cyclic silyl ethers 37A + B , respectively, which are formed by CH insertion of the siloxycarbenes. As further products of (E)- 8 and (E)- 9 , the bicyclic enol ethers 35 and 36 are formed, presumably via siloxycarbene addition to the cyclohexene C?C bond.     

Photochemical Reactions. 153th communication. Photochemistry of acylsilanes: Photolyses and thermolyses of α,β-epoxy silyl ketones

The photolyses and thermolyses of the α,β-epoxy silyl ketones 5 and 6 are described. On n,π*-excitation, the silyl ketones 5 and 6 were transformed to the ketone 7 and the ketene 8 in quantitative yield. The formation of 8 may be explained by initial cleavage of the C(α)? O bond and subsequent C(1)→C(2) migration of the (t-Bu)Me₂Si group. In contrast to the acylsilanes 5 and 6 , the photolyses of the analogous methyl ketones 11 and 12 gave a very complex mixture of products. On thermolysis, 5 and 6 yielded the ketone 7 and the acetylenic compound 9 , which were probably formed via a siloxycarbene intermediate. In addition, the 1,3-dioxle 10 was formed via an initial C(α)? C(β) bond cleavage leading to the ylide g and subsequent intramolecular addition of the carbonyl group. The analogous 1,3-dioxole 13 was obtained on pyrolysis of the methyl ketones 11 and 12 .     

Photochemical reactions. 139th Communication. Photochemistry of acylsilanes: 1. Siloxcarbene formation versus γ-H-abstraction

The Syntheses and the photolyses of the acylsilane 1 and the corresponding methyl ketone 2 are described. On n,π*-excitation, the silyl ketone 1 as well as the methyl ketone 2 undergo a Norrish type II reaction involving γ-H-abstraction and fragmentation to the diene 12 , and acetone ( 20 ) or the acylsilane 26 , respectively. The methyl ketone 2 , but not the acylsilane 1 , isomerizes to cyclobutanols ( 21A - D ). Additionally, compound 1 shows photochemical behavior typical of acylsilanes undergoing rearrangement to the siloxycarbene intermediate c . Insertion of c into the O? H-ond of the enol 28 leads to compound 13 . Initial trapping of the siloxycarbene c by H₂O, however, gives rise to the formation of compounds 16 – 18 . As minor photolysis products of 1 , compounds 14 and (Z)- 15 were formed; however, on vapor phase thermolysis (520°) of 1 , compounds 14 and (E/Z)- 15 were obtained in 92% combined yield. To a small extent the acylsilane 1 also undergoes Norrish type I cleavage leading to the acid 19 .     

An efficient synthesis of cyclopropyl silyl ketones

The reaction of silylcyclopropyl bromides with dichloromethyl methyl ether in the presence of n-butyllithium is investigated. Under basic reaction conditions, the corresponding cyclopropylidene derivatives are exclusively obtained. The resulting cyclopropylidene compounds are subjected to protonolysis or trapping with electrophiles in a one-pot to give the cyclopropyl silyl ketone derivatives in good yields. Acidic treatment of derived cyclopropyl silyl ketone allows isomerization to give the thermodynamically favorable trans form exclusively.     

Synthesis of 1-hydrocarbon substituted cyclopropyl silyl ketones

The synthesis of cyclopropyl silyl ketones possessing a hydrocarbon group at 1-position of three-membered ring was investigated. The reaction of sulfoxonium ylide with α,β-unsaturated acylsilanes derived from α,β-unsaturated aldehydes did not afford the desired acylsilane derivatives. Instead, the corresponding silyl enol ethers were yielded exclusively. On the other hand, the Corey-Chaykovsky cyclopropanation of α-substituted α,β-unsaturated aldehydes proceeded well to give 1-substituted cyclopropyl aldehydes. The silyl substitution of formyl proton in the obtained aldehydes via umpolung of carbonyl group afforded the target acylsilanes.     

Photochemical reactions. 142nd communication. Photochemistry of a 6,7-epoxy-1,3-diene of the ionone series

On singlet excitation (λ = 254 nm), the epoxydiene (E)- 3 underwent (E)/(Z)-isomerization, electrocyclic ring closure of the diene side chain leading to the cyclobutenes 4A + B , and rearrangement to the cyclohexanones 5A + B . Compounds 5A + B were presumably formed in a series of processes including a 1,3-acyl shift of the homoconjugated ketone 8 , arising from (Z)- 3 by a 1,5-H-shift accompanied by cleavage of the C,O-bond of the oxirane.     

Dimerization of cyclopropyl ketones and crossed reactions of cyclopropyl ketones with enones as an entry to five-membered rings

An unexpected dimerization of cyclopropyl ketones was observed, and analysis of the reaction pathway led to development of a synthetically useful crossed reaction between cyclopropyl ketones and enones to afford densely functionalized cyclopentane products.     

Photochemical reactions. 140th Communication. Acid-catalyzed rearrangement and electron transfer photooxygenation of cyclopropanols and their silyl ethers

On acid-catalyzed hydrolysis, the tricyclic compounds 2 and 10 , incorporating cyclopropyl-silyl-ether moieties undergo rearrangement to the cis-decalones 3 and 7 , respectively. Hydrolysis of 2 and 10 in the presence of oxygen leads additionally to the formation of the 1,2-dioxolan-3-ols 9 and 13 , respectively, which involves an electron-transfer oxygenation process as could be demonstrated by photooxygenation of the silyl ether 10 and the cyclopropanol 14 in the presence of 9,10-dicyanoanthracene. The configurations of 3 and 9 were assigned by X-ray analysis of the latter compound as well as of the p-nitrobenzoate 8 of 3 .     

Photochemical reactions of ketones to synthesize gold nanorods

Photochemical synthesis of gold nanorods (NRs) in a micellar solution of hexadecyltrimethylammonium bromide (CTAB) was triggered by photoreactions of ketones. Photoexcitation of gold ions and silver bromide clusters ((AgBr)n) in the absence of ketones did not produce NRs. The initial products of the photoirradiation were probably ketyl radicals, which then initiated reactions to form NRs. NRs formed in darkness, if the reaction solutions were irradiated by UV light for a few minutes, thus indicating that the photochemical products catalyzed NR-formation.     

New Fe(III)-mediated radical cascade reactions of cyclopropyl silyl ethers

[reaction: see text] Fe(III)-mediated ring opening of cyclopropyl ethers bearing a phenyl-substituted butenyl side chain leads to the generation of beta-keto radicals that undergo 5-exo cyclization followed by a novel cascade sequence resulting in the formation of tricyclic ethers.     

Fluoride-catalyzed conversion of acylsilanes to aldehydes and ketones

Acylsilanes undergo efficient protodesilylation upon treatment with fluoride ion in moist DMSO, HMPT, or THF. The reaction is also catalyzed by methoxide ion in methanol or DMSO, but not by chloride ion or bromide ion. Fluoride-catalyzed desilylation in the presence of alkyl halides, aldehydes, or ketones results in modest yields of the corresponding alkylated products.     

Synthesis of silyl aziridines and alpha-amino acylsilanes with silyldibromomethyllithium

The reaction of silyldibromomethyllithium with aromatic imines provides alpha-amino acylsilanes via a bromo aziridine intermediate upon quenching the reaction with water. Alternatively, treatment of the bromo aziridine intermediate with various Grignard reagents or lithium aluminum hydride permits the nucleophilic displacement of the halogen to furnish substituted silyl aziridines.     

An efficient synthesis of 5-silyl-2,3-dihydrofurans via acid-catalyzed ring-enlargement of cyclopropyl silyl ketones and their functionalization

Treatment of cyclopropyl silyl ketones with trimethylsilyl trifluoromethanesulfonate as a strong acid having low nucleophilic counter anion gives the corresponding 5-silyl-2,3-dihydrofuran derivatives, exclusively, regardless of substituents on the cyclopropane ring or silicon atom. The resulting 5-silyl-2,3-dihydrofuran derivatives exhibit both reactivities of the vinylsilane and the cyclic enol ether in the subsequent reaction with electrophilic reagents or Heck type reaction.     

Diastereoselective aldol reactions of enolates generated from vicinally substituted trimethylsilylmethyl cyclopropyl ketones

[reaction: see text] Vicinally substituted trimethylsilylmethyl cyclopropyl ketones undergo facile desilylative ring opening with Lewis acids under mild conditions. The enolates, thus generated, undergo aldol addition with aldehydes and ketones. The diastereoselectivity of the reaction with aldehydes depends on the nature of the Lewis acid used. The resulting aldol product is easily converted into a substituted tetrahydrofuran derivative.     

Photochemical Reactions. 149th Communication. Photochemistry of 7,8-dihydro-4-hydroxy-β-ionone and derivatives

The photolysis of 7,8-dihydro-4-hydroxy-β-ionone ( 6 ) was investigated together with its acetate and isopropyl ether 7 and 8 , respectively. Irradiation (λ > 245 nm) of 6 in MeCN or i-PrOH at temperatures between 25° and ?65° leads to the tricyclic ethers 9 , 10 and 13A + B , and to the spirocyclic ethers 11 and 12 , which are all known types of photoproducts, previously obtained on photolysis of 7,8-dihydro-β-ionone ( 1 ). The same types of products are obtained on irradiation of the acetate 7 and the isopropyl ether 8 . On the other hand, irradiation of the hydroxy compound 6 in MeCN or i-PrOH at temperatures between ?35° and ?65° leads to the new tricyclic tertiary alcohols 14 and 15 as the major products. Their formation involves an intramolecular trapping of a carbocation by the neighbouring OH group, thus, supporting the previously proposed mechanism of the transformation 1 → 5. For structure proof, the tricyclic alcohol 14 and the pheny1 carbamate 42 , derived from 9 , were subjected to X-ray analysis.     

Cyclopropanation reactions with α, β-epoxy diazomethyl ketones and rearrangement of α, β-epoxy cyclopropyl ketones

The cyclopropanation reactions of α, β-epoxy diazomethyl ketones 1 with olefins using Pd(OAc)₂ as catalyst is described. Differently substituted epoxy diazo ketones 1a-1f give with cyclohexene exo-norcarane derivatives. 3, 3-Diphenyloxiranyl-2 diazomethyl ketone 1a reacts with olefins like isobutene, E- and Z- butene-2 to give epoxy cyclopropyl ketones. 3, 3-Diphenyloxiranyl-2 cyclopropyl ketones 2a and 9 undergo two consecutive rearrangement reactions with BF₃ as catalyst. In the first step an epoxide rearrangement of 9 takes place to give β-ketoaldehyde 10, which in a second step rearranges to enolester 12. The latter reaction is most likely restricted to β-ketoaldehydes which have a quaternary α-C atom. A rationale for this unusual reaction has been proposed.     

Photochemical Reactions 150th Communication. Wavelength Dependence of the Photochemistry of 7-Methyl-β-ionone

The wavelength dependence of the photolysis of 7-methyl-β-ionone ((E)- 1 ) was investigated. Irradiation of (E)- 1 with light of λ > 347 nm leads primarily to (E/Z)-isomerization followed by transformation to the tricyclic enol ether 3 as the only secondary photoproduct. On photolysis of (E)- 1 with light of shorter wavelength (λ > 280 nm or λ = 254 nm), however, a series of other products was formed (via a) photocyclization of the dienone chromophore (→ 5 ), (b) photo-enolization (→ 8 ), and (c) a 1,5-sigmatropic H-shift (→ (E/Z)- 7 ). For the structure elucidation of the new products, 7-[¹³C]methyl-β-ionone ((E)-[7-methyl-¹³C]- 1 ) was prepared and irradiated furnishing the corresponding ¹³C-labelled photoproducts.     

Photochemical reactions. 144th Communication. Photochemistry of 5,6-epoxy-1,3-dienes in the ionone series: Influence of a methoxy group in position 7

On triplet excitation (λ > 280 nm, acetone), the epoxydiene (E)- 5 undergoes initial cleavage of the C(5)? O bond of the oxirane and subsequent cleavage of the C(6)? C(7) bond leading to the diradical intermediate e which reacts by recombination furnishing the cyclic compounds (E/Z)- 6 , (E/Z)- 7,8 , and 9 . Alternatively, a H -shift leads to the aliphatic methyl-enol ether 10 which undergoes a photochemical [2+2]-cycloaddition to compounds 12 and 13 , the main products on triplet excitation of (E)- 5 . On singlet excitation (λ = 254 nm, MeCN), (E)- 5 undergoes cleavage to the carbene intermediates f and g . The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 14 as the main product. From the carbene intermediate g , the methyl-enol ether 15 arises by carbene insertion into the neighboring C? H bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)- 16 , and compounds 17A+B are formed via the ylide intermediate h . Finally, the cyclobutene 18 is the product of an electrocyclic reaction of the diene side chain.