Chemistry Reference
In-Depth Information
available by successive transformation of tert-butanol with potassium and carbon
dioxide followed by an in situ acylation with pivaloyl chloride.
The reaction of 350b with mesitylamine 348 in the presence of DMAP provided
isocyanate 349 in 17% yield, along with 64% of urea 351. It is important to note
that the urea is formed exclusively from the isocyanate under these reaction con-
ditions and not from the corresponding urethane. The isolation of larger amounts
of the urea gives direct evidence for the intermediacy of the isocyanate, as shown
in the pathway outlined above. While dimethyl carbonate, di-tert-butyl carbonate,
and ethylene carbonate did not react, the reaction of diphenyl carbonate 350c with
mesitylamine and DMAP in acetonitrile under reflux yielded urea 351 in a moder-
ate 29% yield after 15 days. Under these reaction conditions, complete conversion
of the isocyanate to the respective urea derivative is not unexpected. Because of the
milder reaction conditions when bis(4-nitrophenyl) carbonate 350d is used, iso-
cyanate 349 could be isolated along with the secondary product 351. The combined
yield of 95% at 82
C indicates that the DMAP-catalyzed reaction pathway involv-
ing the isocyanate is virtually the exclusive route in this case [221].
Di-tert-butyl tricarbonate (BocO)
2
CO Recently, di-tert-butyl tricarbonate has been
reported as a versatile and mild reagent for the synthesis of unusual mono- and
multi-isocyanates (232) within minutes at room temperature [230, 231].
NH
2
NCO
(BocO)
2
CO
CH
2
Cl
2
, rt, 5 min
87 %
352
232
The synthesis of the reagent has been optimized and described in detail [232].
The facile synthesis of aliphatic [n]polyurethanes by using di-tert-butyl tricar-
bonate to prepare the appropriate monomers has yielded a general class of poly-
mers. These structures are especially interesting in the context of biodegradable
polymers, since the synthetic procedure is applicable to all amino alcohols with a
spacer of at least four carbon atoms between the two functionalities. The reagent
is the key element for the selective formation of the a,o-isocyanato alcohol 355
intermediates for polymeric urethanes.
(BocO)
2
CO
HO
(CH
2
)
x
NH
2
HO
(CH
2
)
x
NCO
-2 CO
2
, -2 tBuOH
CHCl
3
, 20°C
354
355
x = 4-12
The reaction is accompanied by the formation of two equivalents of carbon dioxide
and tert-butyl alcohol. The former escapes from the solution, while the latter stays
in the reaction mixture. Under the conditions employed, tert-butyl alcohol is un-
