Benzeneselenenyl chlorides, reactions with

A solvent-dependent chemoselectivity, pointing to a dependence of the relative reactivities of the 1,2- and 1,1-disubstituted double bonds on solvent polarity and nucleophilicity, has been observed in the reaction of benzeneselenenyl chloride with 2-methylenebicyclo[2.2.1]hept-5-ene (159) which gives products 160-163140. In methylene chloride the reaction occurs with a moderate chemoselectivity, attack on the endocyclic bond being preferred over that on the exocyclic one in a 60 40 ratio. In methanol, the addition is completely chemoselective and the attack occurs exclusively on the endocyclic double bond (equation 132). It may be further noted that 162 and 163 isomerize and solvolyze at high temperatures, leading to the homoallylic products 160 and 161. 

Pyrrolidines piperidines.7 Reaction with benzeneselenenyl chloride converts A4-and As-unsaturatcd ethyl urethanes into phcnylseleno-substituted pyrrolidines and piperidines, respectively. Yields are considerably improved by the presence of silica gel, which presumably facilitates the ring closure subsequent to addition of the reagent to the double bond (equation I). 

The final step in a recent synthesis of cannabichromene (2) is the aromatization of the cyclohexenone ring of 1. Reagents used for this purpose also attack the double bond in the side chain, but the desired reaction was effected by treatment of the lithium enolate of 1 with benzeneselenenyl chloride followed by selenoxide elimination in the presence of 3,5-dimethoxyaniline.5... 

Oxidative contraction of glycols.1 Reaction of 3,4-dihydro-2//-pyran (1) with benzeneselenenyl chloride and then with methanol provides the adduct 2, which is oxidized by m-chloroperbenzoic acid to the ring-contracted aldehyde 3 (cf., 12, 120). Application of this sequence to the tribenzyl-D-glucal (4) furnishes the adduct... 

Selenium and tellurium reagents have been used for stereoselective halogenations of alkenes. For example, trans addition of benzeneselenenyl chloride to alkenes followed by the displacement of the seleno moie with chloride can lead to dr-1,2-dichlorides (equation 25). The addition of 2-naph-thyltellurium trichloride proceeds in an anti stereospecific manner (equation 26), whereas tellurium tetra-diloride gives a mixture of syn and anti adducts. The reaction of allyl esters with tellurium tetrachloride accompanies acyl migration to give the l-(trichloiottlluro)-3-chk>io adduct (54 equation 27). ... 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

The addition of selenium electrophiles to activated ir-bonds (i.e. enol ethers) occurs readily. Enol ethers react with benzeneselenenyl chloride to produce cis- and tranj-a-chloro-P-phenylseleno adducts. These adducts can be transformed into a,p-dichlorides or allylic chlorides. If the reaction is carried out in the presence of alcohols, stereoisomers of P-seleno mixed ketals are isolated (equation 12). °... 

Reaction with p,y-unsaturated acids. Phenylselenolactonization (8, 26-28) is the usual reaction of benzeneselenenyl chloride with 7,6- or 6,e-unsaturated acids. The reaction with acyclic /3,y-unsaturated acids results in adducts that are converted by base into allyl phenyl selenides or, phenylselenolactones. The first reaction is favored by a /3-substituent, and the second by a y-substituent. ... 

Reaction with a,fi-unsaturated ketones, a-Phenylseleno-a,/3-unsaturated ketones can be prepared by addition of the enone to a methylene chloride solution of benzeneselenenyl chloride and pyridine (about 1.10 equiv. of each). The reaction at room temperature proceeds slowly (1-6 hours for unhindered enones). As the number of substituents in the y- and S-positions is increased, the reaction time... 

Cleavage of a cyclopropyl ketone. Reaction of 1 with benzeneselenenyl chloride at room temperature gives a mixture of 2 and 3. The latter compound was converted into ( )-erysotramidine (4) by reaction with CHjOH and AgNOs followed by desulfurization with Raney nickel (507o overall yield). 

Ketones react slowly with benzeneselenenyl chloride in the presence of HCl to yiw a-phenylseleno ketones. Propose a mechanism for this acid-catalyzed a-substitui reaction. (See Unusual Elements in Organic Chemistry at the end of this chapt%]... 

More recently, these concepts have been extended to include a range of elegant amidoselenation processes. Alkenes react with benzeneselenenyl chloride and nitriles to yield 3-amido phenyl selenides in excellent yield. - Oxidative elimination of the latter provides the corresponding allylic amides (Scheme 60). Since the reaction proceeds via an episelenonium intermediate, Ae amido selenides are formed with tram stereospecificity. However, when applied to nonsymmetrical alkenes (equation 50) it only proceeds with regioselectivity. 

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