Chromium, allylic reactions

Zr(allyl)3Br gives mainly, but not exclusively, a center of type (XV). This follows from the observation that reaction of Zr(allyl)3Br with silica gives two molecules of propene per metal atom and no halogen is liberated. Addition of excesses of n-butanol to the SiCb/Zrfallyl Br reaction product however gives one molecule of propene per metal atom and one molecule of HBr per metal atom is liberated with excess benzoic acid solution. The structure of (XVI) was determined in a similar manner. Chromium allyls give transition metal centers with structure (XVII). 

The chromium-mediated reaction of allylic phosphates with aldehydes constitutes an important advance in stereoselection [143]. The reaction of y-disubsti-tuted and y-disubstituted allylic phosphates with aldehydes proceeds with good to excellent diastereoselectivity. All of the reactions examined were stereodiver-gent, with the stereochemical outcome dependent upon the configuration of the starting allyl phosphate (Table 10-25). The dilference in behavior of the allylic phosphates is attributed to a very slow isomerization of the intermediate y-disub-... 

Bade et at postulate that the primary reaction products formed from the reaction of chromium allyls with the surface hydroxyl groups on dehydrated silica are shown in Figure 3.41 A and B. In addition, chromium allyls may also react with a siloxane linkage with silica dehydrated at 800°C, and this reaction is shown in Figure 3.41C. 

In the chromium-mediated reaction of 1-fluoro-l-bromo-l-alkene with an aldehyde, the ( )-isomer reacts more quickly than the (Z)-isomer. Therefore, when a mixture of stereoisomer 42a was subjected to the reaction with benzaldehyde in the presence of CrCl2 and Ni catalyst, a (Z)-isomer of allylic alcohol 51 was selectively formed [77, 78] (Scheme 21). The reaction proceeds through an alkenylchromium species 52, and the formation of the ( )-alkenylchromium species is much faster than that of the (Z)-isomer. Actually, the reactimi of a pure (Z)-isomer of 1 -fluoro-1 -bromo-1-alkene with aldehyde is sluggish, and the corresponding ( )-isomer of the product was obtained in low yield or not obtained at all. Therefore, a good method for the synthesis of ( )-51 has not yet been reported. 

Xia G, Yamamoto H. Catalytic enantioselective Nozaki-Hiyama allylation reaction with tethered bis(8-quinolinolato) (TBOx) chromium complex. J. Am. Chem. Soc. 2006 128 2554-2555. 

Diastereoselective Cr-mediated allylation reactions have been documented in numerous applications. In a classic synthesis, Kishi demonstrated the synthetic utility of crotyl chromium couplings to access the C15-C29 fragment (136) of the antibiotic rifamycin S (Scheme 5.22) [97]. The allylation of aldehydes 132 and 134 proceed smoothly to afford the corresponding products 133 and 135, respectively, in dr >20 1. 

The formation of an epoxyketone (1) is generally favoured when the expected product of oxidation of an allylic alcohol is a cisoid enone. This type of reaction is promoted by acid conditions and may be prevented by using the chromium trioxide-pyridine reagent which gives only the unsaturated ketone (2) corresponding to the starting alcohol. ... 

Some data allow the realization of the second possibility to be proposed (8, 140) at least in the case of catalysts formed with the use of 7r-allylic compounds of chromium. In the reaction of Cr(7r-C3H3)2 with the Si02 surface, complexes are formed that may have vacant coordination sites... 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

Homoallylic amines result from the reaction of aldimines, previously activated by boron trifluoride etherate, with allylic bromides in the presence of chromium(II) chloride, e.g. equation 68194. 

Haloarene chromium tricarbonyl complexes are activated to nucleophilic attack by thiolate anions [58, 59]. High yields of the thioethers are obtained under liquiddiquid two-phase conditions, but optimum yields are achieved under soliddiquid conditions. In many cases the thioether is produced directly but, where the reaction mixture contains thioether and its chromium complex, the thioether can be isolated by degradation of the complex with iodine or an excess of the thiol. Both 1,2- and 1,4-dichlorobenzenes yield only monothioethers, even when an excess of thiolate anion is used. In contrast, 1,3-dichlorobenzenes produce a mixture of the mono- and dithioethers [59]. Aryl allyl thioethers have been produced under catalysed Heck reaction conditions from S-allyl thiocarbamates and iodobenzene [60]. 

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

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