BF3·Et2O-Promoted Decomposition of Cyclic α-Diazo-β-Hydroxy Ketones: Novel Insights into Mechanistic Aspects

Francesco Venturoni,Antimo Gioiello,Emidio Camaioni,Maura Marinozzi,Roberto Pellicciari,Bruno Cerra

doi:10.3390/catal8120600

Francesco Venturoni, Antimo Gioiello + Show 4 more

Open Access

https://doi.org/10.3390/catal8120600

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Abstract

We report novel insights into the cascade rearrangement of destabilized vinyl cations deriving from the BF3·Et2O-induced decomposition of cyclic α-diazo-β-hydroxy ketones in turn prepared by aldol-type condensation of cycloalkanones with diazoacetone. Complexation of the hydroxy group of the α-diazo-β-hydroxy compound with the Lewis acid is the first event, followed by the generation of the cycloalkanylidenediazonium salt that, after nitrogen loss, produces the highly reactive vinyl cation. The subsequent ring expansion results in the formation of a cycloalkenyl vinyl cation that affords the allylic cation by 1,2-methylene shift and ring contraction. The cation can then trap the solvent, the fluoride or the hydroxide released from the [BF3OH]− to afford different reaction products. The effect of both solvent and substrate ring size on products types and ratios were analyzed and discussed from a mechanistic point of view.

Highlights

IntroductionSince Curtius’s pioneering work on the diazotization of glycine in 1883 [1], α-diazocarbonyl compounds have represented versatile reagents for myriad chemical transformations including the homologation reaction of carbonyl compounds, cyclopropanation of olefins, aziridination, bond insertion reactions, sigmatropic rearrangements, 1,3-dipolar and Staudinger cycloaddition [2,3,4,5,6,7]
It is worth noting that the main differences in terms of reactivity between α-diazo-β-hydroxy esters and ketones are related to the different stabilizing/destabilizing effects of carbonyl in comparison to carboxyl group in the vinyl cation cascade [28]
In line with previous findings on α-diazo-β-hydroxy esters [28], vinyl cation formation, rearrangement and carbenium ion trapping accommodate both products structure and distribution deriving from the different reaction pathways

Summary

Introduction

Since Curtius’s pioneering work on the diazotization of glycine in 1883 [1], α-diazocarbonyl compounds have represented versatile reagents for myriad chemical transformations including the homologation reaction of carbonyl compounds, cyclopropanation of olefins, aziridination, bond insertion reactions, sigmatropic rearrangements, 1,3-dipolar and Staudinger cycloaddition [2,3,4,5,6,7]. In 1981, we reported the Rh2 (OAc)4 -catalyzed decomposition of α-diazo-β-hydroxy ketones 3a, prepared from diazo lithioacetone (LiDAA, 1a) and aldehydes 2a, affording β-diketones 4 in 68–81% isolated yield (Scheme 1A) [9]. Βof Rh2 (OAc) in CH2 Cl2 at 0 ◦ C leads to only the 1,2-thio group migration products (Z)-10 and trimethylsiloxy α-diazo carbonyls 3e are reacted with a stoichiometric amount of trimethylsilyl (E)-11 [18]. In 1981, Miyauchi et al reported the acid-catalyzed decomposition of 3-aryl-2-diazo-3-hydroxy-1ketones rearrangements. The effect of solvent and ring size on products distribution and mechanism paths were analyzed and discussed

Results and Discussion

40. Rearrangement

Carbocation

Materials and Methods

Conclusions