Thermal Decomposition of α‐Diazo‐α‐(trialkylsilyl)alkanones – Intramolecular C/H Insertion of Siloxyalkylidenecarbene Intermediates

Abstract

a-Diazo-a-(trialkylsilyl)alkanones 6 have been prepared by silylation of primary a-diazo ketones 4 with trialkylsilyl triflates 5. Diazo compounds 6 are decomposed in boiling benzene to give l-oxa-2-sila-4-cyclopentenes 7 and 9. Evidence is presented that this transformation proceeds via alkylidenecarbenes 17 which undergo 1,5-cyelization by carbcne insertion into a Sic-H bond.The synthetic potential of a-diazocarbonyl compounds rests to a marked extent on their lability towards UV light, elevated temperature, and certain transition metals, which makes them a convenient and easily accessiblc source of carbenes or carbenoids".However, there is an obvious lack of information concerning the chemistry of a-(silyldiazomethyl)arbonyl compounds. Silylated diazo esters (la) and some r*-(silyldiazomethyl) ketones of type 1 b represent the exceptions; the prime interest in their chemistry was directed towards the eventual formation of silenes from silylated ketocarbenes derived from 1 a, b2'. As the sole "simple" silylated diazo ketone, 1 -diazo-1 -(triethylsilyl)-2-propanone has been prepared, and its thermal decomposition in the presence of copper has been studied 31. Recently, we have reported that (aroyldiazomethy1)silanes 1 c easily sulTer thermal dediazoniation with transformation into siloxyalkynes 34! Siloxyalkylidene carbenes 2 are considered the immediate precursors of the alkynes. The 1,2-aryl migration which brings about the transformation 2+3 has precedent in earlier work of Stangs', Gilbert ", and their coworkers who have shown that in alkylidenecarbenes [Ar(R)C=C:, R = alkyl, aryl] the aryl shift is usually faster than any other intra-or intermolecular trapping reaction of this carbene. On the other hand, an alkyl group is not prone to a facile 1,2-migration to the vinylidene carbon, as several successful intermolecular trapping rcactions of alkylidenecarbenes bearing Balky1 groups indicate"". We have therefore synthesized (alkanoyldiazomcthyl)silanes 6 which, by thermal decomposition, should give access to alkylidenecarbenes analogous to 2. Since B aryl groups were no longer present. the way seemed to be paved for other intra-or intermolecular reactions of these reactive intermediates. R,Si-C-CO,R Me3SiMe2C-CR III1 Synthesis of u-Diazo-a-(trialkylsilyl)alkanones 6The silylated adiazo ketones 6 are obtained under mild conditions in satisfactory to good yields by an electrophilic diazoalkane substitution at primary a-diazo ketones 4 with highly electrophilic trialkylsilyl triflates 5 as silylating reagents. This method had been used before for C-silylation of diazomethane*), ethyl diazoacetate**", (diazomethy1)phosphoryl compounds"', and diazo ketones ArCOCHN;'. Other approaches to silylated a-diazo ketones are given by acylation of lithio(pentamethyldisilanyl)diazomethane'l) and by the reaction between bis(triethylsily1)mercury and mercury-bis(diazo ketones), Hg[C(N2)COR];'; the latter reaction made diazo ketone 6aA available3'. As in the case of (aroyldiazomethy1)silanes 1 c4), the high lability of the C(N2...

Aryl Trialkylsilyl Ketenes: Acid‐Catalyzed Synthesis from 1‐Aryl‐2‐diazo‐2‐trialkylsilylethanones and Their Conversion into 3‐Silyl‐1‐silyloxyallenes

“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) practical method to prepare α-silyl α-diazo ketones. [4,8,9] Typically, diethyl ethyl ether is used as the reaction medium, from which most (but not all) of the formed trialkylammonium triflate precipitates as the reaction proceeds. In order to avoid protonation/desilylation of the product by the acidic ammonium salt, Danheiser et al have used the less polar reaction medium diethyl ether/hexane (1:1), in which the ammonium salt is less soluble, and in fact obtained the silyldiazo ketones in higher yields.…”

Cycloaddition Behavior of 1,2‐Thiaphospholes: Reactions with Diazocumulenes and with Cyclopentadiene

“…(1-Diazo-2-oxoalkyl)silanes 1a, [31] 1b, [32] and 1c [31] were prepared as published. Yields higher than those reported were -syn-cis-Isomer 5aA), and 4-[(Z)-2,2-Dimethyl-1-(triisopropylsilyloxy)propylidene]-2,3a-diphenyl-3a,4-dihydrophosphireno[1,2-b][1,2]thiaphosphole (6a): A solution of thiaphosphole 3a (0.424 g, 1.67 mmol) and diazo ketone 1a (0.471 g, 1.67 mmol) in dichloromethane (15 mL) was stirred at 20°C for 15 h. The solvent was evaporated and the residue was subjected to column chromatography (silica gel, diethyl ether/petroleum ether, 1:14).…”

“…The 13 C NMR spectra were recorded as proton-decoupled, but phosphorus-coupled; thus, the signal multiplicities reported here indicate P,C coupling; pt ϭ pseudo-triplet. The following references were applied: internal TMS for the proton spectra, the solvent signal for the 13 C NMR spectra [δ(CDCl 3 ) ϭ 77.0], and external 85% H 3 PO 4 for the 31 P spectra. IR: PerkinϪElmer IR-1310 and IR-883 as well as Bruker Vector-22 spectrometers; relative intensities are given in parentheses.…”

“…In line with these symmetry properties and the expectation that all three positional isomers are formed with more or less the same probability (cf. mechanistic scheme), three closely spaced 31 P NMR signals are observed (δ ϭ 25.6, 26.6, and 27.5 ppm) that have relative intensities of approximately 1:2:1 and can therefore be assigned to meso, rac, and meso forms, respectively. The 1 H and 13 C NMR spectra do not readily allow us to distinguish the signal sets of the three isomers because of extensive signal overlap and the mentioned intensity ratio.…”

“…The bicyclic alkylidenephosphiranes 6 were identified by the high-field shifts of their 31 P resonance (δ P ϭ Ϫ113.3 to Ϫ116.6 ppm), which are characteristic for phosphirane rings. [22] A comparison with the related 2-alkylidenephosphirane [23] 8 can be made, which has a similar 31 P chemical shift (R 1 ϭ tBu: δ P ϭ Ϫ134.5 ppm).…”