Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1281 135

22. Radical anions and cations derived from CDC, CDO or CDN groups 1341

MeO			OMe
			MeO
			MeO	65%
	OMe	MeOH
			MeO
		Pt anode
OMe		OMe		H
1. MeOH			OMe
Pt anode			42% overall
OMe 2. TsOH
acetone
				H	O
		O
MeO
			MeO	OMe

SiMe3

MeOH

Pt anode

H

42% overall

O

		H
		OMe
MeO	MeO
	MeO
MeOH		70% (2 : 1)
OMe Pt anode	MeO

SCHEME 72

c. The vinylic SRN1 reaction. In 1970, Bunnett’s group found that several aryl halides react with various nucleophiles via a free radical chain process and suggested the name ‘SRN1’175. (A similar mechanism for nucleophilic substitution in aliphatic and benzylic systems had been discovered by Russell and Kornblum in 1966.)176 Evidence was subsequently presented which suggested that, in addition to aryl halides, several vinyl halides also underwent nucleophilic substitution by the SRN1 mechanism (Scheme 75)177.

1342

Daniel J. Berger and James M. Tanko

+2

+

−

•

DMV

e

DMV +

NC

H

NC

H

•

+

C

C

DMV+2 +

C

C

DMV +

•

H

CN

H

CN −

NC

H

NC

H

C

C

+

H+

•C

C

H

H

•

CN −

H

CN

NC

H

NC

H

+ e−

•C

C

H

H

C

C

H

+ H

+

H

CN

H

CN

SCHEME 73

	Ph	Ph
							K+DBB−			Ph
Ph			Ph				THF
	Ph
							+ −			Ph
	Ph	CH2 CH3					K DBB
							THF
				SCHEME 74
	R		R							R
			•
	R		X−							R
	R	R								R
		+	Nu−
	•									R
	R
R	R	R	R							R
	+
	•
R	Nu−	R	X							R

SCHEME 75

Ph

Ph

Ph

Ph

CH2 CH3

R

+ X−

•

R
•
Nu−
R	R	R
	+	•
Nu	R	X−

22. Radical anions and cations derived from CDC, CDO or CDN groups	1343
In 1994, it was found that several reactions thought to proceed via the	vinylic

SRN1 mechanism were contaminated by a nonradical, ˛, ˇ-elimination/addition pathway (equation 40)178. However, this elimination/addition pathway becomes inaccessible when substrates without ˇ (or ˇ0) hydrogens are utilized. Thus, the reaction of pinacolone enolate (t-Bu(CO)CH2 ) with 1-bromo-1,2,2-triphenylethylene was touted to be the first ‘unequivocal’ example of vinylic substitution exclusively by the SRN1 pathway.

R CH

CH

Nu

C

CH

NuH

CH

CH

Nu

40

D

X ! R

R

D

-NuH, X

!

V. RADICAL IONS OF >C=N− CONTAINING COMPOUNDS

In contrast to radical ions generated from alkenes or carbonyl compounds, substantially fewer recent reports have appeared which describe the chemistry of radical ions generated from the >CDN functional group. This situation likely results from the relative obscurity of the >CDN group (compared to >CDO and >CDC<), rather than specific problems with the chemistry, per se. Based upon the limited data available, and as might be anticipated, >CDN žC chemistry appears to be analogous to that of >CDC<žC , while >CDN ž chemistry is reminiscent of >CDOž .

A. >C=N− Radical Cations

1. Overview

Several reports appear in the more recent literature of syntheses using electrochemical or PIET oxidation of compounds containing >CDN bonds. These fall into three categories based upon a mechanism or presumed mechanism: Cycloadditions, nucleophilic attack on >CDN žC cation radicals and radical annulations. The latter will not be reviewed here179 as none of the annulations appears to involve >CDN žC cation radicals. It should be pointed out that it is by no means certain that the electronic structure of >CDN žC is that of a -cation radical rather than of an iminium cation radical (Figure 5). As will be seen below, reactivity appears sometimes in one guise and sometimes in the other.

2. Reactions of >CDN-žC

AzirenežC cation radicals (81) have proven useful as 1,3-dipole equivalents for cycloaddition reactions. Several heterocycles, such as pyrrolines, imidazoles, pyrroles and porphyrins, have been synthesized from azirenes in low to moderate yields, via PIET using DCN or DCA as electron acceptors (Scheme 76)163.

Cycloadditions of alkenes and alkynes onto imine cation radicals have been reported, with the cation radicals generated by either PIET mediated by DDQ (2,3-dichloro-5,6- dicyano-1,4-benzoquinone)180, or by TIET mediated by FeCl3106b. The reaction is shown in Scheme 77.

R	•	R	R	••	N	R
C	+	N	C			•
R		••	R
						+
π-radicalcation			σ-radicalcation

FIGURE 5. Possible electronic states of >CDN-žC

1344

••

N

Ph Ph

(81)

X Y = Ph

X Y = MeO2 C

Daniel J. Berger and James M. Tanko

	Ph		+	Ph				Ph
− e−
					X		Y
		•	N
				H
			(82)

•

Ph N+

N

82

Ph N

H

Ph N

•

N•

Ph

•

Ph N+

CO2 Me 82

MeO2 C

H

•

N+

X Y

products

H

Ph

Ph

H

Ph

CO2 Me

Ph

N

Ph

MeO2 C CO2 Me

SCHEME 76

H

Ph

22. Radical anions and cations derived from CDC, CDO or CDN groups 1345

				Ph
X			X
N	Ar			N	Ar
			50−90%
− e−
			Ph
X			•
			+
		N	Ar
	Ph
Ph			Ph
X		X
		+
N	Ar		N	Ar
			H
0−90%			100−0%

yield given for X = electron-donating to electron-withdrawing group

SCHEME 77

Keteneimines will also undergo electrochemical hole-catalyzed cycloaddition reactions, producing dimers and even trimers as shown below (Scheme 78)181. Adventitious water or the replacement of aryl ˛-hydrogens leads to somewhat different products (Scheme 79)182.

Dimers 83 and 84 will undergo electrochemical oxygenation, replacing ‘Ph2C’ with ‘O’ (Scheme 80)183.

Another recent report of ring closings involving CDN cation radicals, generated by anodic oxidation, appears to involve intramolecular nucleophilic attack (Scheme 81)184.

B. >C=N− Radical Anions

Because they are isoelectronic, it is reasonable to expect that imine radical anions (>CDN ž ) would exhibit chemistry analogous to that of >CDOž . Such does appear to be the case, based upon the limited information available.

Imine radical anions appear to be substantially more basic than their ketyl anion counterparts. In 1991, Zhan and Hawley reported that Ph2CDNHž (generated via the electrochemical reduction of benzophenone imine) was a sufficiently strong base to deprotonate weak carbon acids whose pKa values were as high as 33185.

Imamoto and Nishimura reported a SmI2-induced coupling of imines in direct analogy to the pinacol coupling of aldehydes and ketones (Table 12)186. However, no mechanistic

1346			Daniel J. Berger and James M. Tanko
							X = H, Me, OMe				83
XC6 H4 N		C		CPh2						(good yield)
						n-Bu4 NBF4
						CH2 Cl2					84 + 85
						anode
										(moderate yield)
							X = Br
								Ph
Ph				Ph
X					N						N
							Ph
				N						N
							X	Ar			X
			Ph			Ph				Ph	Ph
					(83)		Ph				(84)
						Ar
						N		Ph
						Ph
							N				BF4 −
						Ph	+
						N
						Ar					X

Ph Ph

(85)

SCHEME 78

O

Ph

Ar

N

p-NO2 C6 H4 N CCPh2 Ph

N

Ar

Ph

Ph

Ph

N Ar

							Ph
o-RC6 H4 N		C		CPh2			N
					Et2 NBF4
(R= Me or OMe)
						Ph	Ar
					CH2 Cl2
					anode		Ph

SCHEME 79

22. Radical anions and cations derived from CDC, CDO or CDN groups 1347

								N
83		or		84	anode
		(A)				O2		N
								O
Mechanism:								(86)
				A	− e	−	•
							A+
								N
	•		3				N	+	•
A	+	+		O2
									(B+)
							Ph		O
								Ph	O •

N

•

B + + A

N	NHCR3	NaOA c
		MeOH
		anode
	− e−
R1	O
+	•
	N NHCR3
R2	O
R1
R2

•		N	O
A+ +
		Ph	O
		Ph
		86 + Ph2 C = O
SCHEME 80
N	N		N	N
R1		OMe
		or
2	O	3	R1	O	R3
R		R
R2 ≠ H			R2 = H
•	+	H	A cO−
			MeOH
N	N		− e−
R1

O R3

R2

SCHEME 81

1348	Daniel J. Berger and James M. Tanko
	TABLE 12. SmI2-promoted coupling of
	iminesa				R1	NHR2
	2
	NR		Sml2			CH
	C
						CH NHR2
	R1	H			R1
	R1		R2			% Yield
	Ph	Ph				93
	Ph	p-CH3C6H4				84
	Ph	CH2Ph				38
	Ph	t-Bu				10

a Reference 186.

details were provided, nor was any data available regarding the diasteroselectivity of the process.

VI. CLOSING REMARKS

Hopefully, this chapter has provided the reader with an appreciation of the diverse range of chemical transformations which may be achieved based upon the chemistry of radical ions. Over the past fifteen years, neutral free radical processes have enjoyed a transition from ‘mechanistic curiosities’ to their present status as important tools in the synthetic repertoire. It is likely that the same will hold true in future years for radical ions.

A number of mechanistic challenges remain. Unlike neutral free radicals, radical ions also possess charge and thus their reactivity is sensitive to environmental effects (i.e. counterion, solvent). Thus, there remains much that needs to be learned both about this important class of intermediates, as well as about the role of these environmental factors, before this chemistry can be completely understood and exploited.

VII. ACKNOWLEDGMENT

We are pleased to acknowledge the National Science Foundation (CHE-9412814) for support of our research during the writing of this chapter.

VIII. NOTES AND REFERENCES

1.F. W. McLafferty, Interpretation of Mass Spectra, 3rd ed., University Science Books, Mill Valley, CA, 1980.

2.Y. Matsumura and T. Shono, Bull. Electrochem., 6, 89 (1990).

3.E. Beckman and T. Paul, Ann. Chem., 266, 1 (1891).

4.W. Schlenk and T. Weickel, Chem. Ber., 44, 1182 (1911).

5.Estimates of the pKa values of ketyl radicals have been reported: For Ph2C(ž)OH, the pKa is estimated to be approximately 9.2 in i-PrOH/H2O based upon results obtained from flash photolysis [A. E. J. Forno, M. E. Peover and R. Wilson, Trans. Faraday Soc., 66, 22 (1970)]. The pKa of PhC(ž)HOH in H2O has recently been estimated via pulse radiolysis to be around 8.2 8.5 [S. Solar, N. Getoff, J. Holcman and K. Sehested, J. Phys. Chem., 99, 9425 (1995)].

6.C. Amatore, J. Badoz-Lambling, C. Bonnel-Huyghes, J. Pinson, J.-M. Saveant´ and A. J. Thie´- bault, J. Am. Chem. Soc., 104, 1979 (1982).

7.For a review, see S. G. Cohen, A. Parola and G. H. Parsons, Jr., Chem. Rev., 73, 141 (1973).

8.For a recent review, see K. S. Peters, in Advances in Electron Transfer Chemistry, Volume 4 (Ed. P. S. Mariano), JAI Press, Greenwich, CT, 1994, pp. 27 52.

9.J. P. Dinnocenzo and T. E. Banach, J. Am. Chem. Soc., 111, 8646 (1989).

22. Radical anions and cations derived from CDC, CDO or CDN groups 1349

10.G. A. Russell, E. G. Janzen and E. T. Strom, J. Am. Chem. Soc., 86, 1807 (1964).

11.(a) Review: T. Holm, Acta Chem. Scand., Ser. B, 37, 567 (1983). See also T. Holm, Acta Chem

Scand., 45, 925 (1991); T. Holm and J. Ø. Madsen, Acta Chem. Scand., 46, 985 (1992); T. Holm,

J. Am. Chem. Soc., 115, 916 (1993).

(b)I. G. Lopp, J. D. Buhler and E. C. Ashby, J. Am. Chem. Soc., 97, 17 (1975); E. C. Ashby, and A. B. Goel, J. Am. Chem. Soc., 103, 4983 (1981).

(c)C. Walling, J. Am. Chem. Soc., 110, 6846 (1988).

(d)H. Yamataka, T. Matsuyama and T. Hanafusa, J. Am. Chem. Soc., 111, 4912 (1989).

(e)J. M. Tanko and L. E. Brammer, Jr., J. Chem. Soc., Chem. Commun., 1165 (1994).

(f)T. Lund, M. L. Pedersen and L. A. Frandsen, Tetrahedron Lett., 35, 9225 (1994).

12.M. L. D. Vona, and V. Rosnati, J. Org. Chem., 56, 4269 (1991); L. Luchetti and V. Rosnati, J. Org. Chem., 56, 6836 (1991).

13.(a) E. C. Ashby and J. N. Argyropoulos, J. Org. Chem., 51, 472 (1986).

(b)C. A. Palmer, C. A. Ogle and E. M. Arnett, J. Am. Chem. Soc., 114, 5619 (1992); E.M. Arnett and C. A. Palmer, J. Am. Chem. Soc., 112, 7354 (1990).

14.H. Yamataka, K. Nagareda, T. Takatsuka, K. Ando, T. Hanafusa and S. Nagase, J. Am. Chem. Soc., 115, 8570 (1993).

15. (a) E. C. Ashby, A. B. Goel and J. N. Argyropoulos, Tetrahedron Lett., 23, 2273 (1982); E. C. Ashby, and J. N. Argyopoulos, J. Org. Chem., 51, 3593 (1986).

(b)D. Nasipuri and A. Saha, Ind. J. Chem., 293, 471 (1990).

16.(a) H. Yamataka, Y. Kawafuji, K. Nagareda, N. Miyano and T. Hanafusa, J. Org. Chem., 54, 4706 (1989).

(b)K. S. Rein, Z.-H. Chen, P. T. Perumal, L. Echegoyen and R. E. Gawley, Tetrahedron Lett., 32, 1941 (1991).

17.(a) L. T. Scott, K. J. Carlin and T. H. Schultz, Tetrahedron Lett., 22, 4637 (1978).

(b)E. C. Ashby, A. B. Goel and R. N. DePriest, Tetrahedron Lett., 22 4355 (1981).

(c)M. Newcomb and M. T. Burchill, J. Am. Chem. Soc., 106, 8276 (1984).

18. D. D. Tanner, H. K. Singh, A. Kharrat and A. R. Stein, J. Org. Chem., 52, 2142 (1987); D. D. Tanner and A. R. Stein, J. Org. Chem., 53, 1642 (1988); D. D. Tanner and J. J. Chen,

J. Org. Chem., 54, 3842 (1989).

19.D. D. Tanner and C. M. Yang, J. Org. Chem., 58, 5907 (1993).

20.D. D. Tanner, G. E. Diaz and A. Potter, J. Org. Chem., 50, 2149 (1985).

21.D. Yang and D. D. Tanner, J. Org. Chem., 51, 2267 (1986).

22.(a) R. L. Blankespoor, D. L. Schutt, M. B. Tubergen and R. L. De Jong, J. Org. Chem., 52, 2059 (1987).

(b) A. J. Lasia, J. Electroanal. Chem., 102, 117 (1979).

23. N. Hirota, in Radical Ions (Eds. E. T. Kaiser and L. Kevan), Wiley, New York, 1968, pp. 35 85.

24.C. P. Andrieux, P. Hapiot and J.-M. Saveant,´ Chem. Rev., 90, 723 (1990).

25.T. Lund and H. Lund, Acta Chem. Scand., Ser. B, 40, 470 (1986); K. Daasbjerg, S. U. Pedersen and H. Lund, Acta Chem. Scand., 45, 424 (1991).

26.J. F. Garst and C. D. Smith, J. Am. Chem. Soc., 98, 1520 (1976); J. F. Garst and C. D. Smith,

J. Am. Chem. Soc., 98, 1526 (1976).

27.J.-M. Saveant,´ Acc. Chem. Res., 26, 455 (1993).

28.H. Lund, K. Daasbjerg, T. Lund and S. U. Pedersen, Acc. Chem. Res., 28, 313 (1995).

29.N. Kimura and S. Takamuku, J. Am. Chem. Soc., 116, 4087 (1994).

30.J. M. Tanko and R. E. Drumright, J. Am. Chem. Soc., 112, 5362 (1990).

31.N. R. Armstrong, N. E. Vanderborgh and R. K. Quinn, J. Electrochem. Soc., 122, 615 (1975).

32.M. M. Baizer, in Organic Electrochemistry, An Introduction and a Guide, 3rd. ed. (Eds. H. Lund and M. M. Baizer), Marcel Dekker, New York, 1991, pp. 433 464.

33.T. Shono, N. Kise, T. Fujimoto, A. Yamanami and R. Nomura, J. Org. Chem., 59, 1730 (1994).

34.Reviews: G. A. Molander, Chem. Rev., 92, 29 (1992); H. B. Kagan and J. L. Namy, Tetrahedron, 42, 6573 (1986).

35.J.-L. Namy, J. Souppe and H. B. Kagan, Tetrahedron Lett., 24, 765 (1983).

36.A. Lebrun, J.-L. Namy and H. B. Kagan, Tetrahedron Lett., 34, 2311 (1993).

37.G. A. Molander and C. Kenny, J. Org. Chem., 53, 2132 (1988).

38.J. H. Wagenknecht, R. D. Goodin, P. J. Kinlin and F. O. Woodard, J. Electrochem. Soc., 131, 1559 (1984).

1350	Daniel J. Berger and James M. Tanko

39.For example, for R D CH3, k D 5ð 103 s 1; for R D PhCH2, k D 4.8ð 104 s 1. See J. Masnovi,

J. Am. Chem. Soc., 111, 9081 (1989).

40.R. D. Webster, A. M. Bond and T. Schmidt, J. Chem. Soc., Perkin Trans. 2, 1365 (1995).

41.D. D. Tanner, J. J. Chen, L. Chen and C. Luelo, J. Am. Chem. Soc., 113, 8074 (1991).

42.D. O. Wipf and R.M. Wightman, J. Phys. Chem., 93, 4286 (1989).

43.N. Mathivanan, L. J. Johnston and D. D. M. Wayner, J. Phys. Chem., 99, 8190 (1995).

44.G. Molander and G. Hahn, J. Org. Chem., 51, 1135 (1986).

45. R. J. Kolt, D. D. M. Wayner and D. Griller, Tetrahedron Lett., 30, 4259 (1989); F. Fontana,

R.J. Kolt, Y. Huang, and D. D. M. Wayner, J. Org. Chem., 59, 4671 (1994).

46.R. J. Kolt, D. Griller and D. D. M. Wayner, Tetrahedron Lett., 31, 7539 (1990).

47.C. P. Andrieux, M. Robert and J.-M. Saveant,´ J. Am. Chem. Soc., 117, 9340 (1995).

48.W. G. Dauben and E. J. Deviny, J. Org. Chem., 31, 3794 (1966).

49.(a) R. V. Volkenburgh, K. W. Greenlee, J. M. Derfer and C. E. Boord, J. Am. Chem. Soc., 71, 3595 (1949).

(b)T. Norin, Acta Chem. Scand., 19, 1289 (1965).

(c)H. O. House and C. J. Blankley, J. Org. Chem., 33, 47 (1968).

50.(a) B. Maillard, D. Forrest and K. U. Ingold, J. Am. Chem. Soc., 98, 7024 (1976).

(b)R. J. Kinney, R. D. Jones and R. G. Bergman, J. Am. Chem. Soc., 100, 7902 (1978).

(c)A. L. J. Beckwith and G. Moad, J. Chem. Soc., Perkin Trans. 2, 1473 (1980).

(d)A. Effio, D. Griller, K. U. Ingold, A. L. J. Beckwith and A. K. Serelis, J. Am. Chem. Soc., 102, 1734 (1980).

(e)L. Mathew and J. Warketin, J. Am. Chem. Soc., 108, 7981 (1986).

(f)A. L. J. Beckwith, V. W. Bowry and G. J. Moad, J. Org. Chem., 53, 1632 (1988).

(g)M. Newcomb and A. G. Glenn, J. Am. Chem. Soc., 111, 275 (1989).

(h)A. L. J. Beckwith and V. W. Bowry, J. Org. Chem., 54, 2681 (1989).

51.J. M. Tanko and R. E. Drumright, J. Am. Chem. Soc., 114, 1844 (1990).

52.J. M. Tanko, R. E. Drumright, N. K. Suleman and L. E. Brammer, Jr., J. Am. Chem. Soc., 116, 1785 (1994).

53.V. W. Bowry, J. Lusztyk and K. U. Ingold, J. Chem. Soc., Chem. Commun., 923 (1990).

54.R. Hollis, L. Hughes, V. W. Bowry and K. U. Ingold, J. Org. Chem., 57, 4284 (1992).

55.J. M. Tanko, L. E. Brammer, Jr., M. Hervas and K. E. Campos, J. Chem. Soc., Perkin Trans. 2, 1407 (1994).

56.G. Molander and J. A. McKie, J. Org. Chem., 56, 4112 (1991).

57.J. W. Timberlake and T. Chen, Tetrahedron Lett., 35, 6043 (1994).

58.(a) R. A. Batey and W. B. Motherwell, Tetrahedron Lett., 32, 6649 (1991).

(b)T. Imamoto, T. Hatajima and T. Yoshizawa, Tetrahedron Lett., 35, 7805 (1994).

59. (a) A. G. Davies and M.-W. Tse, J. Organomet.	Chem., 155, 25 (1978); A. G. Davies and
B. Muggleton, J. Chem. Soc., Perkin Trans. 2, 589	(1979).

(b)D. H. R. Barton, R. S. H. Motherwell and W. B. Motherwell, J. Chem. Soc., Perkin Trans. 1, 2363 (1981).

(c)M. Cook, O. Hares, A. Johns, J. A. Murphy and C. W. Patterson, J. Chem. Soc., Chem. Commun., 1419 (1986); A. Johns and J. A. Murphy, Tetrahedron Lett., 29, 837 (1988).

60.E. Hasegawa, K. Ishiyama, T. Horaguchi and T. Shimizu, J. Org. Chem., 56, 1631 (1991); Correction: J. Org. Chem., 56, 5236 (1991).

61.E. L. Shapiro, M. J. Gentles, P. Kabasakalian and A. Magatti, J. Org. Chem., 46, 5017 (1981).

62.T. Inokuchi, M. Kusumoto and S. Torii, J. Org. Chem., 55, 1548 (1990).

63.G. A. Molander and G. Hahn, J. Org. Chem., 51, 2596 (1986).

64.K. Otsubo, J. Inanaya and M. Yamaguchi, Tetrahedron Lett., 28, 4437 (1987).

65.E. Hasegawa, K. Ishiiyama, T. Kato, T. Horaguchi, T. Shimizu, S. Tanaka and Y. Yamashita, J. Org. Chem., 57, 5352 (1992).

66.H. Stamm, T. Mall, R. Falkenstein, J. Weny and D. Speth, J. Org. Chem., 54, 1603 (1989). See also H. Stamm, A. Sommer, A. Woderer, W. Wiesent and T. Mall, J. Org. Chem., 50, 4946 (1985); T. Mall and H. Stamm, Chem. Ber., 121, 1349 (1988); R. Falkenstein, T. Mall, D. Speth and H. Stamm, J. Org. Chem., 58, 7377 (1993).

67.G. A. Molander and P. J. Stengel, J. Org. Chem., 60, 6660 (1995).

68.D. Liotta, M. Saindane and L. Waykole, J. Am. Chem. Soc., 105, 2922 (1983).

69.S.-R. Yeh and D. E. Falvey, J. Am. Chem. Soc., 113, 8857 (1991); D. J. Fenick and D. E. Falvey, J. Org. Chem., 59, 4791 (1994).