B-alkyl group

However, conjugate additions with trialkylboranes are inefficient in that only one of the three organic groups is transferred. The inefficiencies of group transfer are surmounted by the use of either the B-alkylboracyclanes (55)37 37b or B-alkyldiphenylboranes (56),37c in which the B-alkyl groups arc preferentially transferred under radical conditions. The B-alkylboracyclanes are obtained by sequential... 

Due to their ready isomerization simple cyclopentenones present a particular challenge in the Nazarov cyclization. In all of the cases studied in a- and p-monosubstituted and a, -disubstituted systems the cy-clopentenone product contained the double bond in the less substituted position, as required by loss of the silicon electrofiige (Scheme 17). The relative configuration of substituents in disubstituted cases is controlled by kinetic protonation and weakly favors the cis isomers. Substituent effects in rate were particularly noted in these cases where substitution with a- and B-alkyl groups greatly accelerated and decelerated the reactions, respectively. 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

Fig. 1. Stmctures of alkyllithium tetramers and hexamers (a) tetrahedron of lithium alkyl groups (b) arrangement of alkyl groups around octahedron of...

The nitrosation of pyrroles and indoles is not a simple process. The 3-nitroso derivatives (84) obtained from indoles exist largely in oximino forms (85) (80IJC(B)767). Nitrosation of pyrrole or alkylpyrroles may result in ring opening or oxidation of the ring and removal of the alkyl groups. This is illustrated by the formation of the maleimide (86) from 2,3,4 -trime thylpyrrole. 

However, other studies on the nitration of a series of 3-methyl- and 3-ethyl-1,2-benzisoxazoles have shown that a mixture of the 5-nitro and 5,7-dinitro derivatives is formed (77IJC(B)1058, 77IJC(B)1061). The effect of substituents in the benzene ring is also of interest. If the 5-position is blocked, e.g. by a chloro group or by alkyl groups, nitration then occurs at the 4-position. 3-Alkyl-7-chloro and 3,7-dialkyl derivatives result in the formation of the appropriate 5-nitro derivative. The isomeric 3-alkyl-6-chloro- and 3,6-dialkyl-1,2-benzisoxazoles yield a mixture of the 5-nitro and 5,7-dinitro compounds. Both H NMR measurements and alternate syntheses were used in establishing the structures of these substitution products. 

Structures of (a) [Cr2(CO)6-it- f) -cycZo(AsMe)9 ], and (b) [Mo2(CO)6-it- f) -cfltentz(AsPr")8 ]. In both structures the alkyl group attached to each As atom has been omitted for clarity. 

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