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Cationic Telomerization of Olefin with Isocyanide As Telogen

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Cationic Telomerization of Olefin with Isocyanide As Telogen Polymer Journal Vol. 1, No. 5, pp 532-536 (1970) Cationic Telomerization of Olefin with Isocyanide as Telogen Takeo SAEGUSA, Naotake TAKA-ISHI, and Yoshihiko ITO Department of Synthetic Chemistry, Faculty of Engineering, Kyoto University, Kyoto, Japan. (Received May 2, 1970) ABSTRACT: Telomerization of vinyl ether as taxogen with !-butyl isocyanide as the telogen was carried out at room temperature using BFaOEt2 catalyst. Telomers were obtained along with the isomer and isomerized dimer of !-butyl isocyanide. On the basis of a previous study on the cationic isomerization and oligomerization of tertiary alkyl isocyanide, the formations of the telomers of the present reaction were schematized. a-Methylstyrene and 4,5-dihydropyran were also found to react with !-butyl isocyanide to produce the corresponding l : 1 adducts. KEY WORDS Telomerization / Vinyl Ether / !-Butyl Isocyanide / Boronfluoride / Telogen / Cationic Isomerization / p-Scission / Imidoyl Cation/ a-Methylstyrene / 4,5-Dihydropyran / Isocyanide (1) is a nucleophile due to the addition of an imidoyl cation to isocyanide (pro­ lone-pair electrons of the terminal carbon atom. pagation) and the ,B-scission of the propagating Primary and secondary alkyl isocyanides are imidoyl cation (chain transfer). polymerized by Lewis acid to produce an in­ This paper reports on the cationic telomeri­ soluble and infusible polymer1• Tertiary alkyl zation of olefins with tertiary alkyl isocyanide isocyanide, on the other hand, is isomerized as the telogen. In this reaction, tertiary alkyl and oligomerized by Lewis acid catalyst. 2 The isocyanide is a powerful chain transfer agent. courses of cationic isomerization and oligomeri­ zation of tertiary alkyl isocyanide are well­ RESULTS AND DISCUSSION explained by assuming imidoyl cations (3, 5 When a mixture of !-butyl isocyanide and and 6) as the key intermediates. isobutyl vinyl ether was treated with BF3 OEt2 The j3-scission of imidoyl cation 3 produces the in methylene dichloride at room temperature, isomer cyanide 4 and the cation of the iso­ the following six products (7-12a) were isolated cyanide's alkyl 2. The reaction of 1 with 2 from the reaction mixture (Table I). prcducing 4 and 2 corresponds to the isomeri­ zation. In the j3-scission of the second imidoyl t-BuCN t-Bu-<;:=N-t-Bu t-Bu-(-CH2<;:H--)-CN n cation 5, the dimer is produced and the cation 7 CN Oiso-Bu of the isocyanide's alkyl is regenerated. Thus 8 9a n= 1 10a 11=2 the oligomerization of tertiary alkyl isocyanide 11a n=3 is formulated as being a consecutive series of t-Bu-CH2CH~~C=N-t-Bu I I three elemental reactions, the addition of a terti­ Oiso-Bu CN ary alkyl cation to isocyanide (initiation), the 12a ----;,RCN + R+ 4 2 + R-N;;'C: + R+ ---+ R-N=C-R-- 1 2 3 -----+R-C-C+---+ R-C-C-C+ II II II II II NRNR NRNRNR 5 6 532 Cationic Telomerization of Olefin with Isocyanide Among these products, 7 and 8 are the isomer and isomerized dimer, both of which are pro­ _,,,,/ duced from t-butyl isocyanide with BF30Et2 in t-Bu-CH2-7H-CN the absence of vinyl ether. The products of 9a, OBu-1 10a and Ua are the 1 : 1, 1 : 2 and 1 : 3 telomers, respectively, which consist of the vinyl ether taxogen and the t-butyl isocyanide telogen. It is of interest to note that the direction of bond­ 1-Bu-f CH 2-fH t, CN ing of the CN group in the telomers is opposite 08"-i to that of the isocyanide telogen. The telomers having the same back-bone structure were formed also in the free-radical telomerization of olefins 3600 2800 2000 1600 1200 with t-butyl isocyanide. 3 The IR spectra of these telomers are shown in Figure 1. The Figure 1. The IR spectra of telomers. product 12a may be regarded as a 2: 1 telomer; i.e., it consists of one mole of vinyl ether and two the six products of the present reaction are moles of t-butyl isocyanide. schematized in Diagram I. The elementary re­ With reference to the mechanistic scheme of actions in this diagram are the additions of a the cationic isomerization and oligomerization cation to one of two nucleophiles, isocyanide of tertiary alkyl isocyanide, 2 the formations of and vinyl ether, and the /3-scissions of imidoyl Diagram I t-BuNC BE30Et2 1 Scheme t-Bu+ / t-BuNC Scheme 2 1CH2=CHOR Scheme 2.2 + t-BuCH2CH 1 I t-BuC=N-t-Bu OR + 13 t-BuNC 15 -t-Bu+ ff Scheme 2.1 t](n-l)CH2=CHOR Scheme 2.3 t-BuCN t-BuCI-hCHOR t I + 7 C=N-t-Bu t-Bu(CH2CH)n-1CH2CH I I + t-BuC0 ·N-t-Bu OR OR I 18 C=N-t-Bu -t-Bu+ 16 + 1t-BuNC 14 1t-BuNC t-BuCH2CHOR 1-t-Bu+ J I t-Bu(CH2CH)n-1CH2CHOR C=N-t-Bu I I t-BuCH2CHOR I I OR C 0 -N-t-Bu t-BuC=N-t-Bu C=N-t-Bu I CN + CN 17 9 19 8 9a R=iso-Bu 1-t-Bu+ 1-t-Bu+ 9b R=Et t-Bu(CH2CH)nCN t-BuCH2CHOR I I C=N-t-Bu OR I 10 n=2 11 n=3 CN 10a R=iso-Bu lla R=iso-Bu 12 10b R=Et 12a R=iso-Bu 12b R=Et Polymer J., Vol. 1, No. 5, 1970 533 T. SAEGUSA, N. TAHAISHI, and Y. lTo Table I. Cationic telomerization of isobutyl vinyl affording a cation 15. Successive additions of ether with t-butyl isocyanide as telogen 15 to one and two molecules of vinyl ether using BF30Et2 catalysta followed by the addition of 16 (n=2 and 3) to t-butyl isocyanide give rise to two telomers of Products Yield, % 10 and 11. The formation of 9 is explained by t-BuCN (7) 47 the sequence of 15 to 18 to 9. The sequence t-Bu-C=N-t-Bu of 15 to 18 to 19 ends with the production of I CN 31 12. t-Bu-CH2CHCN Scheme I and 2 bifurcate according to the I Oiso-Bu (9a) 3 additions of t-butyl cation to isocyanide and t-Bu-(CH2CH)2-CN vinyl ether, respectively. The combined yield I Oiso-Bu (10a) 4.5 of 7 and 8 is higher than that of the products t-Bu-(CH2CH)3-CN from 9 to 12. This fact suggests that t-butyl I Oiso~Bu (lla) 4.5 isocyanide is more nucleophilic than vinyl ether. CN Styrene is also a cationically polymerizable I t-Bu-CH2CH-C=N-t-Bu (12a) 5 monomer, which, however, does not enter into I Oiso-Bu the reaction with isocyanide. Only 7 and 8 were produced in the treatment of a mixture a Telomerization conditions: t-BuNC, 112 mmol; of t-butyl isocyanide and styrene with a catalyst CH2=CHOiso-Bu, 76 mmol, BFsOEt, 7 mmol; of BF 0Et . Also in the case of p-methoxystyrene, CH2Cb, 50 ml; One day's duration, at room temp. 3 2 no telomer was formed. However, a-methyl­ styrene was reacted with t-butyl isocyanide to cations regenerating the t-butyl cation. Every produce an 1 : 1 adduct of 20. The formation scheme is a combination of these elementary of 20 is rationalized by scheme 2.2 and 2.3 in reactions, which start with the addition of a Diagram I. t-butyl cation to a nucleophile and end with the regeneration of a t-butyl cation. As to the CHa initial generation of the t-butyl cation complex, two possible processes may be assumed, i.e., < J-~-CH2 t-Bu CN the direct interaction of t-butyl isocyanide with 20 BF30Et2 (eq 1) and the participation of a pro­ tonic acid due to a small amount of water impurity (eq 2). EXPERIMENTAL t-BuNC+BF 0Et (t-BuNC->BF )+0Et2 } 3 3 Reagents t-BuNC-BF3 t-Bu+[BF3CNr !-Butyl isocyanide was prepared according to ( 1 ) Ugi's procedure. 4 Commercial methylene chlo­ ride, isobutyl vinyl ether, ethyl vinyl ether, 4,5- BF30Et+H20 H'[BF30Hr +0Et2 f H+[BFPHr +t-BuNC dihydropyran and a-methyl styrene were purified + (2) by the usual methods. Commercial borontriflu­ t-BuN=CH[BF3 0Hr oride etherate (BF3 0Et2 ) was used without purifi­ t-Bu+[BF30Hr +HCN cation. In scheme 1, the addition of a t-butyl cation to Telomerization of Isobutyl Vinyl Ether with !­ t-butyl isocyanide produces an imidoyl cation 14 Butyl Isocyanide as Telogen whose decomposition gives 7. The addition of Under nitrogen atmosphere at room temper­ 13 to the second molecule of t-butyl isocyanide ature, 7 mmol of BF30Et2 in 7 ml of methylene yields another imidoyl cation 14 which decom­ choride was added dropwise to a mixture of poses into 8 and t-butyl cation. Scheme 2 is the 10 ml (112 mmol) oft-butyl isocyanide and 10 ml course of telomerization, which is initiated by (76 mmol) of isobutyl vinyl ether in 50 ml of the attack of the t-butyl cation on vinyl ether methylene chloride. An exothermic reaction 534 Polymer J., Vol. 1, No. 5, 1970 Cationic Telomerization of Olefin with Isocyanide began immediately, and the reaction mixture mm)), the 1 : 3 telomer, 2,4,6-triisobutoxy-8,8- turned red-brown. The mixture was allowed to dimethylnonanenitrile (Ila), was isolated by stand at room temperature for one day, and means of preparative GLPC in a yield of 4.5%. then was quenched with aqueous sodium bicar­ NMR (CDC13): similar to 1 : 2 adduct (10a).
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