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41.
Bean A Bobbink GJ Brock IC Engler A Ferguson T Kraemer RW Rippich C Vogel H Bebek C Berkelman K Blucher E Cassel DG Copie T DeSalvo R DeWire JW Ehrlich R Galik RS Gilchriese MG Gittelman B Gray SW Halling AM Hartill DL Heltsley BK Holzner S Ito M Kandaswamy J Kowalewski R Kreinick DL Kubota Y Mistry NB Mueller J Namjoshi R Nordberg E Ogg M Perticone D Peterson D Pisharody M Read K Riley D Silverman A Stone S Yi X Sadoff AJ Avery P Besson D Bowcock T Giles RT Hassard J Kinoshita K Pipkin FM 《Physical review letters》1987,58(3):183-186
42.
Csorna SE Garren L Mestayer MD Panvini RS Yi X Alam MS Avery P Bebek C Berkelman K Cassel DG Copie T DeSalvo R DeWire JW Ehrlich R Ferguson T Galik R Gilchriese MG Gittelman B Halling M Hartill DL Holzner S Ito M Kadaswamy J Kreinick DL Kubota Y Mistry NB Nordberg E Ogg M Peterson D Perticone D Read K Silverman A Stein PC Stone S Kezun X Sadoff AJ Giles RT Hassard J Hempstead M Kinoshita K McKay WW Pipkin FM Wilson R Haas P Jensen T Kagan H Kass R Behrends S Gentile T Guida JM Guida JA Morrow F 《Physical review letters》1985,54(17):1894-1897
43.
Bhupendra P. Joshi 《Tetrahedron》2005,61(12):3075-3080
An ultrasound-assisted convenient method was developed for the conversion of toxic methoxylated cis-isomer of arylalkenes into its hypolipidemic active trans-isomer. Treatment of cis-isomer or mixture of all three isomers (1a-1j) with ammonium formate and 10% Pd/C gave arylalkanes (2a-2j), which upon oxidation with DDQ in anhydrous dioxane containing a little amount of silica gel, provided (E)-arylalkenes (3a-3g) in 42-72% yield depending upon the substituents attached at the aryl ring. The same method, upon addition of a few drops of water, provided hypolipidemic active arylalkanones (3h-3j) in 59-65% yield. 相似文献
44.
Here, we present a remarkably mild and general initiation protocol for alkyl-radical generation from non-activated alkyl-iodides. An interaction between a silane and an alkyl iodide is excited by irradiation with visible light to trigger carbon–iodide bond homolysis and form the alkyl radical. We show how this method can be developed into an operationally simple and general Giese addition reaction that can tolerate a range of sensitive functionalities not normally explored in established approaches to this strategically important transformation. The new method requires no photocatalyst or other additives and uses only commerical tris(trimethylsilyl)silane and visible light to effectively combine a broad range of alkyl halides with activated alkenes to form C(sp3)–C(sp3) bonds embedded within complex frameworks.Here, we present a remarkably mild and general initiation protocol for alkyl-radical generation from non-activated alkyl-iodides.The efficient and straightforward construction of C(sp3)–C(sp3) bonds is a crucial process in organic synthesis. Over the past 80 years, the polar conjugate addition reaction has become a powerful method to forge a variety of C(sp3)–C(sp3) bonds.1 Alongside two-electron nucleophiles, alkyl-radicals – neutral yet nucleophilic species – have emerged as alternatives to organometallic reagents for additions to electron deficient alkenes.2 Since the 1960s, a variety of methods have been reported for the formation of alkyl-radicals; early examples include the decomposition of in situ generated organomercurial hydrides, the fragmentation of xanthate or Barton esters, or the UV-mediated homolysis of alkyl halides, amongst many others.3 Although these strategies tolerate a broad range of functionalities, the initiation processes can be complicated by the need for aggressive reaction conditions and frequently require toxic reagents such as tributyltin hydride, with notable exceptions.4,5The emergence of photoredox catalysis has obviated many of the potential drawbacks to the generation and use of alkyl-radicals. The exploitation of the multifaceted reactivity of visible light excited transition metal or organic-photocatalysts, whose properties can be tuned through modification of the ligand, metal and/or scaffold, facilitates optimization of the single electron transfer event towards alkyl-radical generation from a wide range of functionalized alkyl groups.6 In addition, the reactivity of electron donor–acceptor (EDA) complexes has also provided a straightforward means to form alkyl-radicals from a variety of precursors.7 As such, a plethora of methods have been developed for the generation of C(sp3)-centred radicals from a variety of commercially available native functionalities, which dramatically expand the scope of alkyl-radical chemistry†. In this context, the single electron reduction of non-activated alkyl halides provides a useful means to generate alkyl radicals.8 As an example, Leonori and co-workers recently developed a method wherein halogen atom abstraction pathways were leveraged using radical species forged through photocatalyst-mediated oxidation event leading to a general alkyl-radical generation.9 Related to the current study, Jørgensen and co-workers published a visible-light mediated reduction of alkyl halides under very mild conditions. Accordingly, there remains a need for further innovation towards orthogonal, general and benign methods of alkyl-radical generation that tolerate a broad range of functionalities, thereby enabling the construction of a greater variety of C(sp3)–C(sp3) bonds.10Recently, we reported a general reaction to form tertiary alkylamines via the addition of alkyl-radicals (generated from non-activated alkyl-iodides) to in situ-generated all-alkyl iminium ions.11a This carbonyl alkylative amination (CAA) reaction was promoted by the action of blue LEDs and tris(trimethylsilyl)silane ((Me3Si)3Si–H). No photoredox catalyst is required. We believe that the alkyl-radical formation step, devoid of traditional initiating reagents, proceeds through the visible-light excitation of a transient ternary EDA complex, which stimulates homolysis of the carbon–iodide bond that would be otherwise stable under such irradiation conditions (Fig. 1B). The presence of an enamine was important to the initiation pathway, as revealed by an absorption band in the UV/vis spectrum of its mixture with an alkyl-iodide and (Me3Si)3Si–H.11a Gouverneur and co-workers have also reported an elegant example of visible-light mediated addition of more functionalized alkyl halides, such as iodofluoromethane, to electron deficient alkenes.12 They proposed that light mediated homolytic cleavage of iodofluoromethane was responsible for radical initiation prior to a classical chain process.Open in a separate windowFig. 1(A) Selected visible-light mediated methods for the generation of alkyl-radicals; (B) previous work – a method for tertiary amine formation exploiting a visible-light activation of a ternary EDA complex to promote alkyl-radical formation. (C) Previous work from Gouverneur & Gaunt labs on radical fluoromethylation. (D) This work – alkyl-radical formation promoted solely by visible light and tris-trimethylsilyl silane demonstrated through a remarkably practical and straightforward Giese reaction.Gouverneur et al. also showed methyl iodide was only efficient as a radical source under these conditions when an organic photocatalyst was present and the reaction of other simple non-activated alkyl iodides was only demonstrated in the presence of iodofluoromethane, which was presumably responsible for the initiation pathway (vide supra). Our prior work in this area also identified iodofluoromethane as a visible-light activated source of fluoromethyl radical and its addition to iminium ions and electron deficient alkenes (Fig. 1C).11b Taken together, these works reveal that the use of visible light and (Me3Si)3Si–H to initiate radical formation from non-activated alkyl halides has not been achieved in an unbiased transformation without the requirement of an initiation process via of the reaction components or a photocatalyst. Accordingly, we questioned whether a pathway mediated by visible-light and (Me3Si)3Si–H alone might facilitate alternative modes of radical initiation from non-activated alkyl halides, and therefore enable the general coupling of unbiased alkyl fragments with a wider range of acceptors under practical, straightforward reaction conditions.Herein, we report the successful realization of this idea through the development of a remarkably straightforward visible-light mediated method for alkyl-radical generation from non-activated alkyl iodides using only non-toxic tris(trimethylsilyl)silane as a reagent (Fig. 1D). While we are not certain of the precise pathway for the radical initiation, it seems likely that excitation of a species resulting from the interaction of tris(trimethylsilyl)silane and the alkyl iodide, leading to carbon–iodide bond homolysis. The utility of this activation mode is demonstrated through a broad and chemoselective Giese addition to electron deficient alkenes and is notable by its tolerance to a range of synthetically valuable functionalities in both alkyl iodide and alkene components. In comparison to other methods for Giese-addition,2,3,8,9,12 the conditions are mild and do not require expensive catalysts or cocktails of additives.Our studies were stimulated from an observation arising from the development of the visible light mediated carbonyl alkylative amination (shown in Fig. 1B). High yields of the tertiary amine product, arising from the union of alkyl-radical, aldehyde and secondary amine were maintained when using a 455 nm long-pass filter, which discounted UV-mediated carbon–iodide bond homolysis as the initiation pathway for alkyl-radical formation.11a To explore the formation of an alkyl-radical independently from the enamine component, the reaction conditions were simplified to comprise a representative alkyl halide and (Me3Si)3Si–H, which allowed us to first assess any impact solvent might have on the radical forming process. As shown in 13 However, 47% of 5 was still obtained after visible-light irradiation of a reaction mixture from which air had been rigorously excluded (entry 10), suggesting an alternative initiation pathway excluding oxygen could also operate.14 A reaction at 80 °C in the absence of light showed no conversion to 5. This data shows the nature of the solvent is not relevant for the initiation step and suggests a straightforward radical initiation process that results from visible-light excitation of an intermediate arising from an interaction between the alkyl halide and (Me3Si)3Si–H.Effect of different parameters on radical initiationa
Open in a separate windowaYields of 5 were calculated by 1H NMR using 1,1,2,2-tetrachloroethane as internal standard.With the operationally simple and mild reaction conditions for the homolysis of non-activated alkyl halides, we next focussed on benchmarking the process against existing transformations: namely the Giese addition reaction of alkyl-radicals to electron deficient alkenes. Therefore, using acrylamide 2a (as a representative alkene acceptor), 3.0 equivalents of iso-propyl iodide 1a (as a representative non-activated alkyl halide) and 1.5 equivalents of (Me3Si)3Si–H in MeOH at 0.1 M, we were pleased to find visible light irradiation of this reaction mixture led to the formation of alkylamide 3a in 59% assay yield ( Entry Solvent (Me3Si)3Si–H Alkyl-iodide Conc. Yield 3aa (%) 1 MeOH 1.5 equiv. 3.0 equiv. 0.1 M 59 2 MeOH 2.0 equiv. 3.0 equiv. 0.2 M 66 3 EtOH 2.0 equiv. 3.0 equiv. 0.2 M 79 4 EtOH 2.0 equiv. 2.0 equiv. 0.2 M 77 5 EtOH 2.0 equiv. 1.5 equiv. 0.2 M 70 6 EtOH 1.5 equiv. 1.5 equiv. 0.2 M 47
Entry | Solvent | Deviation in conditions | Yield 5 (%) |
---|---|---|---|
1 | CH2Cl2 | — | 33 |
2 | THF | — | 68 |
3 | MeOH | — | 85 |
4 | EtOH | — | 55 |
5 | C6H12 | — | 84 |
6 | PhH | — | 41 |
7 | PhMe | — | 34 |
8 | EtOH | 16 h | 86 |
9 | EtOH | 16 h, 455 nm filter | 82 |
10 | EtOH | 16 h, degassed | 47 |
11 | EtOH | 80 °C, dark | 0 |