Carbonyl compounds with nitrosobenzene

Regiochemical reversals in reaction of carbonyl compounds with nitrosobenzene, to form a-aminoxy ketone rather than a-hydroxyamino ketone products in the presence of Lewis acid or sterically hindered solvent, have been attributed to initial... 

Another organocatalyst proposed as altemative to proline was the 4-silyloxy proline derivative 25, which was able to catalyze the enantioselective a-anfinoxylation of ketones very efficiently, yielding the final products after only 1 min and with good chemical yields and excellent enantioselectivities [97]. Ranaikably, in this case neither a-amino ketones nor dihydroxy ketone derivatives could be detected in the crude reaction mixture. Moreover, this catalyst could perform the reaction with substrates, which failed with the initial proline protocol, such as cycloheptanone (50%, >99% ee). Also, the triflamide 21b (R=CF3) has been used as organocatalyst in the reaction of ketones, as well as aldehydes, with nitrosobenzene to give the expected hydroxy-lated carbonyl compounds with high level of enantioselectivity [98]. 

Chiral non-racemic 0-(2-ketoalkyl) A-phenylhydroxylamines such as 115 (equation 84) can be prepared through catalytic enantioselective a-aminoxylation of carbonyl compounds catalyzed by proline. This reaction proceeds with a variety of ketones and aldehydes although it has been tried only with a nitrosobenzene component ... 

Carbonyl compounds and other dipolarophiles containing heteroatoms react with miinch-nones. Thus benzaldehyde forms the betaine (245) which suffers ring-cleavage to yield the enamine (246 equation 65). Thiocarbonyl compounds and nitrosobenzene behave in an analogous manner. The action of dipolarophiles containing cumulative double bonds results in the formation of new mesoionic systems. Thus carbon disulfide and phenyl isothiocyanate afford a zwitterionic thiazole and imidazole, respectively (Scheme 25). 

It is obvious here to think of an interaction of the Debye type of the appreciable moment of the nitro or carbonyl group with the readily polarizable unsaturated C — C bond in the unsaturated hydrocarbons or with the likewise readily polarizable free electron pair in the amines, etc. Actually the stability increases with increasing polarizability, compare for example, the heat of formation of j-trinitrobenzene with, respectively, benzene, naphthalene and anthracene AH 0.6, 3.4 and 4.4 kcal. In the series mono-, di- and tri-nitro-benzene with acenaphthene AH also increases with increasing number of partial moments, namely AH — o, 1.35 and 2.45 kcal. Nevertheless this explanation is inadequate, nitromethane with a moment [x = 3.54 D gives very much less stable compounds than nitrobenzene with (X = 4,22 D, while nitrosobenzene with [x =3.2 D gives stronger colour effects than nitrobenzene. 

The Regioselectivity ofHetero Diels-Alder Reactions. In a few cases, carbonyl, nitrosyl, cyano, and other double bonds with one or more electronegative heteroatoms have acted as dienophiles in Diels-Alder reactions. The carbonyl group has a HOMO and a LUMO as shown in Fig. 1.51. The energies of both orbitals are relatively low, and most of their Diels-Alder reactions will therefore be guided by the interaction between the HOMO of the diene and the LUMO of the carbonyl compound. This explains the regioselectivity in the cycloaddition of dimethylbutadiene 6.176 and formaldehyde, and between 1-substitituted butadienes 6.177 and nitrosobenzenes. 

Acid-catalyzed electron transfer plays an important role in reduction of not only carbonyl compounds but also other substrates such as O2 [90, 91], N02 [92], nitrobenzene derivatives [93, 94], nitrosobenzene derivatives [93, 94] and sulfoxides [95, 96], The ket value for the photoinduced electron transfer from [Ru(bpy)3] + to nitrobenzene increases parabolically with increase in [HCIO4] [94]. This indicates that PhN02 is doubly protonated in the photoinduced electron transfer reaction to give PhN02H2 + (Eq. 9) ... 

Ylides may react with nitrosobenzene in a similar fashion to their reactions with carbonyl compounds (equation 12). Sulphonium ylides give nitrones phosphonium... 

In analogy t 0 the Cu(II) complex systems, the silver(I) -catalyzed aldol reaction is also proposed to proceed smoothly through a Lewis acidic activation of carbonyl compounds. Since Ito and co-workers reported the first example of the asymmetric aldol reaction of tosylmethyl isocyanide and aldehydes in the presence of a chiral silver(I)-phosphine complex (99,100), the catalyst systems of sil-ver(I) and chiral phosphines have been applied successfully in the aldol reaction of tin enolates and aldehydes (101), Mukaiyama aldol reaction (102), and aldol reaction of alkenyl trichloroacetates and aldehydes (103). In the Ag(I)-disphosphine complex catalyzed aldol reaction, Momiyama and Yamamoto have also examined an aldol-type reaction of tin enolates and nitrosobenzene with different silver-phosphine complexes (Scheme 15). The catalytic activity and enantioselectivity of AgOTfi(f )-BINAP (2 1) complex that a metal center coordinated to one phosphine and triflate were relay on solvent effect dramatically (Scheme) (104). One catalyst system solves two problems for the synthesis of different O- and AT-nitroso aldol adducts under controlled conditions. 

The reactions between nitrosobenzene and enamines as activated carbonyl compounds are known to provide aminoxylation or hydroxyamination products, depending on the catalyst used. In the absence of relatively strong acids such as carboxylic acids, the organocatalytic reaction of aldehydes with nitrosobenzene led to hydroxyamination products exclusively. This phenomenon was checked by Maruoka and Kano by performing the precedent reaction in the presence of a closely related catalyst which did not bear a carboxylic... 

Another way to carry out the electrophilic amination of a carbonyl compound is by the reaction of an enamine or enolate with a nitroso derivative, the so-called hydroxyamination, oxyamination, or N-nitroso aldol reaction. The regio- and enan-tioselectivity of the transformation have been studied by DFT calculations for the reaction between achiral enamines and nitrosobenzene (70a) promoted by a chiral Brmsted acid catalyst, such TADDOL [64]. The results suggested a transition state that involves the enamine, nitrosobenzene, and two or more organic acid molecules, forming a cluster-like structure, where hydrogen-bonds play an important role. 

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