J. Org. Chem., Vol. 40, No. 7, 1975 969
Notes
Scheme I
Photochemistry of Dimethylamine in Hydrocarbon Solvents. Striking Differences between Solutionand Gas-Phase Photochemical Reactivity
Me,”
2 Me,N*
2
[Me,N@ + He]
Kenneth G. Hancock* and David A. Dickinson
Department of Chemistry, University of California, Davis, Davis, California 95616
-
-t Ha
(2)
6
H, + MeN=CH,
(34
7
and/or 2Me,N* 6
Received August 12,1974
2
--t
+
MeN=CH, 7
(3b)
and/or MeZN. Generation of dimethylamino radicals both thermally RH-R*-MeN=CH, -t RH (3cl and photochemically from a variety of chemical precursors 7 has produced discordant results.’-* However, one widely Me,” + CH,=NMe Me,NCH,NHMe (4) accepted generalization has been that dimerization to 2 7 4 tetramethylhydrazine is a prominent reaction of dimethyMe,NCH,NHMe + Me,” Me,NCH,NMe, + Me”, lamino radical^;'^,^,^^ indeed, the reaction has even been 4 2 1 3 touted as a practicable synthetic route to the h y d r a ~ i n e . ~ ~ , ~ (5) Complicating interpretation of dimethylamino radical reactivity have been numerous other reaction products, apdisproportionation (eq 3b), and/or a hydrogen transfer separently varying with the precursor, medium, and method quence involving solvent (eq 3 ~ ) (iii) ; ~ addition of 2 to 7, for generating the dimethylamino m ~ i e t y . ~Among -~ these, giving 4 (eq 4);and (iv) transamination of 4 with 2 (eq 5 ) to N,N,N’,N’- tetramethylmethanediamine (1) has often been give 1 and 3, the observed products. f o ~ n d , ’ , ~but ~ , never ~ , ~ adequately explained. An alternative a priori explanation would have the seWe wish to report the results of our study of the soluquence of events as primary photochemical conversion of 2 tion-phase photochemistry of dimethylamine (2) in hydroto hydrazine 5, followed by secondary photolysis of 5 to the carbon solvents, which (a) contrast sharply with the gasobserved products. However, the failure to detect (GLCphase photochemistry of d i m e t h ~ l a r n i n e ;(b) ~ ~ ~indicate NMR monitoring) short-term accumulation of 5 in the that dimerization of dimethylamino radicals may be almost photochemical reaction of 2 suggests that hydrazine 5 is totally suppressed under some reaction conditions; (c) offer not involved as a cul-de-sac for dimethylamino radicals. an extremely clean, easy, and high-yield synthesis of the inIn the proposed mechanism the photochemical cleavage teresting6 diamine 1; and (d) support for the first time a and disproportionation steps are well d ~ c u m e n t e d . The ~,~ viable mechanism for the formation of 1 from dimethylamsubsequent addition and transamination steps follow not ino radicals. only from isolation of 4 from reaction mixtures after irraHydrocarbon (most conveniently, n- nonane; cf. Experidiation of 2, but also from the observed stoichiometry and mental Section) solutions of 2 (ca. 1 M ) were degassed and general absence of other by-products. However, conversion irradiated a t 35’ with a Vycor-filtered mercury arc; reacof independently synthesized 4 to 1 under the conditions tion was monitored by GLC (Carbowax 20M on firebrick). employed for photosynthesis of 1 from 2 needs to be demAt about 70% conversion, 100 mmol of 2 gave (eq 1) 22 onstrated before the transamination step is unequivocally hv trace Me,” Me,NCH,NMe, + Me”, + products (1) established. The smaller than theoretical yields of 3 may be ‘gH20 attributed both to analytical limitations and to the known 2 1 3 photolability of 3. Irradiation of 3 gives 7,11 which would mmol of diamine 1, 7 mmol of methylamine (3), and traces then give 4 and, ultimately, 1. (
