Brown s reagent

Reactions n>itb alknes. Caserio et at. have reported that partial hydroboration of racemic 1,3-dimethylallene and 1,3-diphenylallene with ( + )-Pn4B2H2 [from (-)- -pincnc] gave recovered (—)-allcne of moderate activity. The method has been studied in greater detail by Moore et al. and they report that in every case examined the recovered alicne is enriched in the R enantiomer. Brown s reagent is thus useful for preparation of optically active allenes however, only moderate activities result and. of course, some of the alIcne is irrevocably lost in the process. 

Of these reagents, Brown s reagent 229 and Marshall s reagents 230 and 233 have been used most extensively for appending diol units in natural product. syn-... 

Using Keck s original catalytic allylation procedure, Danishefsky and co-workers converted aldehyde 474 to the homoallylic alcohol 475 (conditions A, Scheme 11-38, 60% yield, >95% ee) used in their total synthesis of epothilones A and B [314], Asymmetric allylation with a stoichiometric amount of Brown s reagent, [(-)-lpc]2BAll (195), however, was higher yielding and required a shorter reaction time (conditions B, Scheme 11-38, 83% yield, >95% ee). 

The synthesis began with Yb(OTf)3-catalyzed oxymercuration [88] of ho-moallyUc alcohol 204 [89], prepared by asymmetric allylation with Brown s reagent, to generate organomercury chloride 205. Rh(I)-catalyzed formy-lation [90] of 205 in the presence of DABCO (0.5 equiv.) furnished aldehyde 206. Stereoselective crotylation of 206 via Brown s reagent, followed by regioselective Rh(I)-catalyzed hydroformylation provided lactol 203. Dia-... 

Nessler s reagent An alkaline solution of Hglj in KI used for detecting and estimating ammonia (brown colour or precipitate formed). 

All ammonium salts evolve ammonia on heating with alkali. Ammonia may be delected by (a) its smell, (b) its action in turning red litmus blue and (c) the orange-brown colour produced with Nessler s reagent. This is a very sensitive test. 

In cases where Noyori s reagent (see p. 102f.) and other enantioselective reducing agents are not successful, (+)- or (—)-chlorodiisopinocampheylborane (Ipc BCl) may help. This reagent reduces prochiral aryl and tert-alkyl ketones with exceptionally high enantiomeric excesses (J. Chandrasekharan, 1985 H.C. Brown, 1986). The initially formed boron moiety is usually removed hy precipitation with diethanolamine. Ipc2BCl has, for example, been applied to synthesize polymer-supported chiral epoxides with 90% e.e. from Merrifield resins (T. Antonsson, 1989). 

Discussion. J. Nessler in 1856 first proposed an alkaline solution of mercury(II) iodide in potassium iodide as a reagent for the colorimetric determination of ammonia. Various modifications of the reagent have since been made. When Nessler s reagent is added to a dilute ammonium salt solution, the liberated ammonia reacts with the reagent fairly rapidly but not instantaneously to form an orange-brown product, which remains in colloidal solution, but flocculates on long standing. The colorimetric comparison must be made before flocculation occurs. 

In Brown s classification a diazonium ion is a reagent of very low reactivity and correspondingly high substrate selectivity and regioselectivity. This follows from the fact that benzenediazonium salts do not normally react with weakly nucleophilic benzene derivatives such as toluene. More reactive heteroaromatic diazonium ions such as substituted imidazole-2-diazonium ions will even react with benzene (see Sec. 12.5). 

They also showed that if Olivier s data183 for this system was treated as three-halves-order instead of second-order as he had assumed, then the initial rate coefficients in his kinetic runs were satisfactorily consistent, contrary to his observation (Table 50). It was necessary to use the initial values since Olivier found a decrease in the rate coefficients with time, contrary to the observations of Jensen and Brown, who assumed, therefore, that Olivier s reagent may have contained impurities. 

It has been suggested that the transformations accomplished by the brown-rot fungus Gleophyllum striatum may involve hydroxyl radicals, and this is supported by the overall similarity in the structures of the fungal metabolites with those produced with Fenton s reagent (Wetzstein et al. 1997). 

Brown s synthesis of 34 and 35 utilized -2 -isoprenyldi-Z-isopinocam-pheylborane B as a reagent for asymmetric isoprenylation of aldehydes as shown in Scheme 50 [76]. 

Occupational air Air exposed to polystyrene strip with adsorbed cholinesterase assayed in cuvette with Ellman s reagent and acetylthiocholine UV absorbance (412 nm) 20 ppb (1-min exposure) No data Brown et al. 1984... 

A useful reagent for asymmetric hydroboration. See Brown s Hvdroboration Reaction. 

Identification of Amatol, a) Place about 0.05 g of previously pulverized unknown material in a 5-cc beaker, add 2 to 3 cc of distilled water, stir for 5 mins and observe the color of aliquot. It is colorless in case of amatols b) Test the aliquot with a strip of Universal pH indicator paper-.there shall be no change in color c) Add a drop of Ness-ler s reagent — brown ppt in case of amatol. 

To a 25 nil of condensate in a 50 ml Nessler tube was added from a pipette 5 ml of Nessler s reagent and distd w to the mark. Tbe presence of ammonia was indicated by the appearance of brown turbidity due to the formation of NHg,I - HjO and the intensity of this turbidity was detd colori-metrically using the Nalco Phototester, which was previously calibrated by using freshly prepd standards... 

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