Selenocyanates alkyl

OH - Br. A primary or secondary alcohol can be converted to the alkyl phenyl selenide by reaction with phenyl selenocyanate and tri-n-butylphosphine (7, 252-253). Reaction with Br2 and triethylamine (7, 34-35) replaces the SeC6H5 group by Br. 

Alkyl phenyl setenides selenol esters. N-PhcnylscIcnophthalimidc is superior to aryl selenocyanates for conversion of alcohols to alkyl phenyl selenides (6, 252-253) and of carboxylic acids to selenol esters (7, 396-397). When conducted in the presence of an amine the latter reaction provides amides in high yield (equation l). ... 

A similar reaction yielding alkyl selenocyanates occurs between trialkylboranes and sodium selenoiso-cyanate in the presence of iron(ni) ions. There is a strong preference for the transfer of secondary or tertiary as compared to primary alkyl groups. This allows selective transfer reactions to be carried out in yields of 57-68% (equation 71) and 67-78% (equation 72). ... 

Alkyl selenides are most conveniently prepared by the dialkylation of Na2Se or by the monoalkylation of selenolates. Common routes to aryl derivatives include the reaction of a selenolate with a diazonium salt, the SrnI reactions described earlier, and the reaction of aryUithiums with aryl selenocyanates (ArSeCN). A different approach employs the reaction of alcohols with aryl selenocyanates or N-(phenylseleno)phthalimide (see Section 6) in the presence of tri-n-butylphosphine. Similar conditions can also be used to convert carboxylic acids to selenoesters (8). These reactions are illustrated in Scheme 3. 

Alkyl selenocyanates diselenldes. Primary alcohols are converted by the reagent into alkyl selenocyanates in about 80% yield. If the reaction mixture is treated with 5% KOH in CH3OH for 2 hours the alkyl selenocyanates are transformed to dialkyl diselenides (about 70% yield). The reagent converts secondary alcohols into selenocyanates and to a lesser extent into isoselenocyanates. It does not react with tertiary alcohols. 

ALKYL SELENOCYANATES Triphenyl-phosphine-Diethyl azodicarboxylate. ALKYNES Acetic anhydride-Pyridine. Dimethyl) diazomethyl)phosphonate. OrganoUthium reagents. 

Cyanoselenation of aldehydes with phenyl selenocyanate in the presence of BusP gave a-(arylsele-no)nitriles. Selenylation of nitriles aSiST treatment with 2 equiv. of base also led to the formation of a-selenonitriles. Their lithiated derivatives undergo smooth alkylation with methyl iodide (80%) and Michael addition to cyclohexenone in 90% yield (Scheme 30). ... 

Conversion of O-alkyl esters to their corresponding mefhylselenoesters by treatment with dimethylaluminum methylselenolate has also been reported [29]. Preparation of selenoesters directly from carboxylic acids is accomplished by reaction with selenocyanates [30] or N-PSP (N-phenylselenophthalimide) [31] in the presence of trialkylphosphine. Yields of selenoesters prepared from N-PSP and BujP are usually higher than those obtained by use of ArSeCN and BU3P (Scheme 15.9). 

Selenocyanates (RSeCN), which are employed for the synthesis of selenides and selenoesters, are usually derived from selenenyl halides and cyanide ion (or silyl cyanate) [38 k, 40]. Alkylation of potassium selenolate (KSeCN) with alkyl halides is an alternative means of preparation of aliphatic selenocyanates (Scheme 15.13)... 

Displacement of OH by an ArSe group. Reaction of aryl selenocyanates with alcohols, aldehydes or carboxylic acids to give alkyl aryl selenides, homologation of aldehydes or esters of arylselenols. 

In addition to the wide variety of primary, secondary and tertiary alkyl aryl sulfides, selenides and tellurides which can be prepared by this method, the Barton group also developed concise syntheses of two of the most important selenoamino acids, L-selenomethionine and L-selenocystine using this approach [18] (Scheme 13). The use of crystalline dicyanogen triselenide for introduction of the selenocyanate moiety is also very useful. 

Allyl selenides. AUylsamarium bromide reacts with alkyl selenocyanates to. give RSeCH2CH=CH2 (7 examples, 87-95%). 

Dialkyl diselenides. A convenient access to RSe-SeR from RBr is by reaction with KSeCN in aq DMF in the presence of K2CO,. Alkyl selenocyanates are hydrolyzed in situ and the resulting alkyl selenide ions undergo aerial oxidation. 

Olefin synthesis. The preparation of olefins by decomposition of alkyl phenyl selenoxides (Diphenyl diselenide, 5, 272-276) is very useful, except in the case of primary alkyl phenyl selenoxides, which usually give low yields of terminal olefins on decomposition. However, the presence of electron-withdrawing substituents on the benzene ring increases both the rate of elimination and the yield of olefins. In instances where use of diphenyl diselenide results in low yields. Sharpless and Young recommend use of o-nitrophenyl selenocyanate (1) or 4,4 -dichlorodiphenyl diselenide, both of which are converted into the corresponding ArSe Na" reagents on reduction with sodium borohydride in ethanol. 

Alkylation reactions affording substituted 1,3-diselenanes have been discussed in the previous section. The desired precursor 1,3-diselenanes can be prepared by modifications of standard synthetic procedures (see CHEC-II(1996), 6.24.2.3.2). Treatment of bis-selenocyanates 45 with excess H3PO2 in the presence of aldehydes (or aldehyde equivalents) and ZnCl2 provides a convenient route to the target compounds (Scheme 6) <1996TL2667>. 

Reaetion with triaikylboranes. These reagents react with trialkylboranes to form alkyl thiocyanates and alkyl selenocyanates - ... 

The use of methyl or ethyl alcohol is usually suggested however, these solvents react with the selenocyanate to form volatile, malodorous alkyl selenides, which do not allow the preparation of an odorless product, as acetone does. 

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