α-Selective Allylation of Azo Compounds Using Allylic Barium Reagents

Abstract

The addition of allylic barium reagents to azo compounds was achieved with high α-regioselectivity. The double-bond geometry of allylic barium reagents was retained throughout the reaction at -78°C and E-or Z-enriched allylic hydrazines were selectively obtained from the corresponding allylic barium reagents. An allylic hydrazine was efficiently converted into an allylic amine by reductive N-N bond cleavage.Organobarium reagents prepared from reactive Rieke barium 1 and allylic or propargylic halides are known to react with electrophiles with α-regioselectivity. 2 We have previously reported that a Barbier-type propargylation of azo compounds with propargylic halides proceeds smoothly in the presence of the reactive barium as a lowvalent metal to afford propargylic hydrazines (α-adducts) not only from azobenzenes (diaryldiazenes) but also from dialkyl azodicarboxylates. 3 We report herein an α-selective Grignard-type allylation of azo compounds using allylic barium reagents (Scheme 1). In addition, studies of the transformation of the products, allylic hydrazines, into the corresponding allylic amines by reductive N-N bond cleavage of the former compounds are also described. Scheme 1 α-Selective allylation of azo compounds using allylic barium reagentsSeveral methods to synthesize allylic hydrazines starting from azo compounds have been developed so far, including allylation with allylic organometallics, 4 ene reaction, 5 and the Mitsunobu reaction. 6 Among them, the first method seems to be the most synthetically useful from the viewpoints of high reactivity of allylic metal reagents and broad applicability to azo compounds; however, as far as we know, there are no examples of use of the procedure for the regioselective α-allylation of azo compounds. First, we tried to perform a Barbier-type reaction of azobenzene with geranyl chloride promoted by reactive barium at -78°C; however, the major product was 1,2-diphenylhydrazine and the yield of the desired adduct was low. We then took a Grignard-type approach in which azobenzene was treated with a preformed geranylbarium reagent. As a result, the targeted geranylated hydrazine (α-adduct) was obtained in 77% yield and the corresponding γ-adduct was not detected at all (Scheme 2). In addition, the double-bond geometry of the geranylbarium reagent was retained at the reaction temperature.

Synthesis and Structural Characterization of Butadienylcalcium‐based Heavy Grignard Reagents and a Ca4[O] Inverse Crown Ether Complex

“…[1] However,a st he congener derivatives in Group 2, heavy Grignard reagents (RAeX, Ae = Ca, Sr, and Ba, X = halides) and the related chemistry are still in their infancy. [3] Thus,i ts till remains great demand and challenge for the structure elucidation of heavy Grignard reagents or corresponding reaction intermediates in this area. [3] Thus,i ts till remains great demand and challenge for the structure elucidation of heavy Grignard reagents or corresponding reaction intermediates in this area.…”

Chemoselective Ketone Synthesis by the Strontium‐mediated Alkylation or Arylation of N,N‐Dimethylamides or Urea

“…In our laboratory, we have been investigating the synthetic applications of strontium compounds as alkaline‐earth‐based organometallic reagents . However, apart from our work, few other studies on the preparation and reactivity of organostrontium compounds may be found in the literature …”

Selective Propargylation of Diaryl Azo Compounds Using Metallic Barium

“…2 We have previously reported that a propargylation of azo compounds with propargylic halides occurs via a Barbier-type procedure using reactive barium as the low-valent metal to yield propargylic hydrazines (-product). 3 In addition, a Grignard-type -allylation of azo compounds with allylic barium reagents 4 and a Barbier-type benzylation of azo compounds with benzylic chlorides 5 have been achieved. We report herein a metallic-barium-promoted Barbier-type propargylation of azo compounds with propargylic tosylates (Scheme 1).…”