THE INFLUENCE OF SUBSTITUENTS ON THE EASE AND DIRECTION OF RING OPENING IN THE LiAlH₄–AlCl₃ REDUCTIVE CLEAVAGE OF SUBSTITUTED 1,3-DIOXOLANES

Abstract

The room temperature hydrogenolysis by Li,llH,-AlC13 of ether solutions of a number of 1,3-dioxolanes has been studied.Electron donor substituents on the Cz atom of the ring accelerate while electron acceptor substituents on C2 retard the reductive ring cleavage. The same effect but to a lesser extent is observed when these substituents are attached to the C4 or Cg atom of the ring.When electron donor substituents are attached to C4, ring-cleavage occurs predominantly a t the Cz-O bond remote from the C 4 position. On the other hand, electron-withdrawing groups attached to C4 give predominantly scission of the Cz-0 bond closer t o the C1-substituted position. In contrast to this n1arl;ed control over the direction of ring cleavage exhibited by substituents on C4, those on C n generally have little or no elfect on the direction of ring opening.A mechallistic interpretation of the results is presented. IN'IRODUCTIONThe stability of acetals and lretals towards lithillin a l u~~i i n u~n hydride perillits their use a s a means of protecting a lcetone or aldehyde nioiety during hydride reduction of other functional groups in a molecule (1). However, in the presence of acidic reagents such as anhydrous aluminum chloride or boron trifluoride, lithium aluminum hydride reductively cleaves the acetal or lretal to a n ether (1-5). Eliel et al. (6) have recently examined the LiAlH4-AlC13 hydrogenolysis of a nuiiiber of cyclic acetals and ketals (1,3-dioxolanes and 1,s-dioxanes) and obtained a high yield of the corresponding hydroxyethers arising from the cleavage of either the Cz-O1 or C2-O3 bond according to equation (a).Two ~nechanisms for this reduction have been suggested (3). One i~lvolves hydride attaclr on the complex formed frotn the Lewis acid and the acetal, a conlplex which tends t o form the stabilized oxocarbo~iiuin ion (equation (b)). The other requires prior fornlation of the a-haloether, which is then reaclily reduced (equation ( 6 )

The major reducing species operative when an equimolar proportion of AlCl3 is added over a short period of time to an ether solution of equimolar proportions of LiAlH4 and acetal or ketal

“…There has been considerable interest in the use of the "mixed reagent" AlCl3-LiAlH4 as a means of selective reduction of cyclic acetals and ketals (1)(2)(3)(4)(5)(6)(7). The nature of the reducing species in this "mixed reagent" has been uncertain.…”

Retardation of AlH2Cl Hydrogenolysis of the Ethylene Ketals of 5-Norbornen-2-one and 2-Norbornen-7-one Due to the Presence of the Double Bond

“…was attributed to increased stabilization of the In view of the above information, we wished transition state leading to the intermediate 0x0-determine whether an O1efinic double bond carbonium ion, a species which is very rapidly would provide similar rate enhancement in the alld irreversibly attacked by hydride ion to hydrogenolysis of 5-norbornen-2-one ethyleneproduce ally1 P-hydroxyethyl ether. A similar ketal and 2-n0rb0rnen-7-one eth~leneketal rate enhancement of hydrogenolysis by A1H2C1 (2) A1H2C1 in ether. This paper rep0rts Our has been when a phenyl group is observations concerning this question.…”

“…This paper rep0rts Our has been when a phenyl group is observations concerning this question. attached to C-2 of the 1,3-dioxolane ring (1,2). Results Alkyl substituents on the 2-vinyl group of 2-vinyl-1,3-dioxolane further enhance the ease of…”

THE RELATIVE EASE OF REDUCTIVE CLEAVAGE OF 1,3-DIOXOLANES AND 1,3-DIOXANES IN ETHER SOLUTION BY LiAlH4–AlCl3

“…Reductions and analyses of products were carried out according to the general procedure described previously (6). Care was taken to ensure that the reductions of each of the pairs of cor:lpounds, the 1,3-dioxolane and the 1,3-dioxane, \\.ere carried out identically.…”

“…A similar, but much less pronounced effect occurs because of substituents on C4 of the 1,3-dioxolane system. However, in addition, C4 substituents have a inarked control on the direction of ring-opening during the reductive cleavage (6).…”