R substitute on expression with assignment -

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why these 2 cases behave differently?

>substitute(c1<-100,list(c1=100)) 100 <- 100

vs

> substitute(c1=100,list(c1=100)) [1] 100

as understand assignops operator = evaluates immediately. second expression equivalent substitute(100,list(c1=100)).
take in braces , result is

> substitute({c1=100},list(c1=100)) {     100 = 100 }

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